Politecnico di Torino

Automotive Engineering

MASTER DEGREE THESIS

Feasibility study of an Electric Racing Motorcycle

for the Motostudent Competition

Advisor: Student:

Prof. Lorenzo Peroni Leandro Lasciato

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TABLE OF CONTENTS

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ABSTRACT

The team 2WheelsPoliTO of Politecnico di Torino decided to concur in the

Motostudent Competition, in “Electric Category”. It will be the first experience in

Electric Racing Motorcycle Design and for this reason a Feasibility Study is required

in order to define the design approach and the quantity of components necessary to

the correct functioning of the vehicle. In order to fulfil the request, an analysis of

Regulations and how scoring assignment works is required. This first step helps to

understand which the focus will be to reach best results. Old Regulations will be

confronted with new ones to understand how the other teams worked in the past.

Then a benchmark analysis becomes useful to define best practices, design

particularities and best in class motorcycles. Next step will be the analysis of the most

important necessary components. Particular attention will be given to Battery Pack

and all its auxiliaries, because its design will hardly affect the dynamic behaviour of

the vehicle, the power transferred to wheels and maximum range at racing working

conditions. This last argument is crucial: will be analysed creating a script based on

best time lap performance and requires temporary assumption from previous

experiences and benchmark analysis. It will follow the design of the Transmission. It

depends not only by electrical and physical characteristics of Electric Motor but also

by dynamic necessities, the most important one is the squat ratio. The design will not

be definitive and is done trying to create a lightweight system, the simplest and most

effective possible, looking both to theory and past experiences of other teams, not

only participating to Motostudent. The thesis will end with the Bill of Material, and

the setup of an efficient strategy to understand which ones should be designed

internally by the team, which ones requires design outsourcing and collaborations,

and the ones worthless to design and bought by external suppliers.

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AKNOLEDGEMENTS

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1ST CHAPTER:

MOTOSTUDENT EVENT AND TEAMS

OVERVIEW

1.1: MOTOSTUDENT EVENT

MotoStudent is a university challenge between student teams, that must design and

develop a competition motorbike project (“Electric” or “Petrol”) which will be

evaluated and tested in a Final Event to be held at the MotorLand Aragón (in MotoGP

World Championship version) facilities in Alcañiz (Teruel), Spain [RB49].

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This is the 6th edition and the Final Event will be on October 2020, with sign-up

opened from mid-2019.

The two different classes compete in two different categories. For each category

there are different prizes not only symbolic but of economic and material nature,

always remembering that the main objective of the competition is didactical:

During MS1 teams will summarize the evolution of the project, showing the design

process, the theoretical process of Racing Team creation and all aspects related to

real or theoretical business activities that the Team faced during all the event. During

this phase the minor prizes of “Best Design Project” (best documentation related to

structural, thermodynamic, manufacturing/purchasing and design validation) and

“Best Innovation” (most creative solutions, formal aspects of solution and feasibility

of manufacturing) are assigned. The remaining part of score is mostly related to

Business Plan: it requires a summary, Market analysis, Internal analysis and SWOT

analysis. The presentations in front of the jury will have a moderate influence on the

final score.

MS2 is characterized by a list of dynamic tests: Braking (from a minimum speed of 80

km/h, there are two attempts, the lowest score is the result), Gymkhana (description

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of the circuit is showed below), Acceleration (from 0 km/h in 150 m, there are two

attempts), Maximum Speed at Speed Trap, Regularity (measured in a given sector)

and warm up sessions. At the end there will be a race with a qualifying session. All of

this (except warm ups) give a score. Criteria are fixed by the organization and comes

from tables:

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The full participation to Main Event requires the accomplishment of defined

milestones and a Scrutineering phase. Milestones are set during development

process, don’t assign points but can be a source of penalties if not fully respected.

Scrutineering consists in an Administrative check (verification of enrolment

conditions), a Static check (application of Static Forces on saddle and front wheel,

plus a braking test and specific tests depending on category) and a series of dynamic

tests (in which is included a full Lap by the tester). The lack of relative approval

stickers means the exclusion from MS2 phase.

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1.2: 2WHEELSPOLITO AND MOTOSTUDENT

2wheelsPolito Team participated at all editions except the 5th, achieving through

these years very satisfying results. At the 1st edition team was awarded with “Best

Design” prize, two years later won the MS1 event and reached an impressive 3rd place

at MS2 event. At 3rd edition the motorcycle presented by the team was very

competitive, winning MS1 Event and constrained to renounce to MS2 Win due to

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engine failure during Race. In the following edition, the last one before hiatus, the

team ended the event at overall 2nd place winning the MS2 Event for the first time

[RB69]. All these accomplishments were reached concurring in “Petrol” category.

The 6th edition will be the first in which the team will concur in “Electric” Category.

The team lacks the know-how from past years that other teams have achieved: the

first step will be bringing the project at a similar level respect the opponents, from

there the brand new motorcycle will be perfectioned trying to replicate the glories of

previous editions. A Benchmark analysis becomes very useful to reach these targets

in a short time.

1.3: CONFRONTATION WITH OLD RULES

MotoStudent Regulations slightly change in each edition. On the previous, for

example, Regularity Test was not complained and replace a Mechanical Test

consisting in the rapid disassembly and reassembly of fairing and front assembly. But

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the most important difference was on points assignment. For the new event all points

are derived from tabs and depends by the position only.

On previous edition almost all the MS2 Tests were characterized by a point

assignment strategy that took into account the differences in performance in the

specific test. An example related to Gymkhana test, replicable for almost all the other

test, is the following:

Where 𝑀𝑡 represent the best time, 𝑃𝑡 the worst and 𝑇 the time scored by the

examined team. It’s clear that the gap between most performing motorcycles and

motorcycles with good but not outstanding scores was steep. For instance, a 2.5

seconds gap between the 1st and the 4th motorcycles corresponds to more than 30

points of difference, while looking to previous tab related to Gymkhana is possible to

see a difference of 20 points between the same positions.

Race Event assignment points can be food for thought:

𝑃 =𝑀𝑡 − 100 ∗ 𝑃𝑡 + 99 ∗ 𝑇

𝑀𝑡 − 𝑃𝑡

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What immediately jumps on eyes is that on previous edition the 2nd place and 4th

place gives the same points than the 3rd place and 9th place (respectively) on next

edition. This means that while in the past the design of a very performing motorcycle

was practically an almost mandatory requirement to reach podium results, in the new

edition the pure performances will lose importance at favour of design rationality,

innovation and functionality. The teams must be sure that their motorcycles will be

able to participate to all the tests and that MS1 documentation will be praised by

organization jury.

This is a very important point to state before that Benchmark activity starts: it will

help the team to understand where the Design Focus should be.

Another huge difference which will determine the energy installed in the Battery Pack

is the increase of Race Laps to 6 respect to 5 of last edition.

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1.4: BENCHMARK ANALYSIS SETUP

To identify main targets of this benchmark activity and exclude from the analysis least

useful targets, a statistical approach is necessary.

Analysis starts regrouping all the data necessary for score calculation, related to each

Test of the Event [RB06].

For sake of simplicity and just for statistical study, Event is subdivided by the author

in two different Days: the first Day contains all the Static Tests, the second Day all the

Dynamic Tests. For each Static Test scores were assigned by the commission, while

for each Dynamic Test scores are calculated through different interpolation formulas,

considering performance indicators such as Time or Speed. Scores can be subject to

penalization.

In the first Day these main Awards were assigned:

-Best Design, to the team with the highest score in the Design Test;

-Best Innovation, to the team with the highest score in the Innovation Test;

-Best Industrial Project, to the team with the overall highest score of the Day.

While in the second Day main Awards assigned were:

-1°, 2° and 3° Best Day2, to teams with 1°, 2° and 3° highest score of the Day;

-Best Event, to the team with highest overall score, in practice the winner of the

Event;

-Best Rookie, to the team at its first participation with highest overall score.

Scores, their experimental Medium Value 𝑚𝑥 and Standard Deviation (transformed

in unbiased estimator) 𝜎𝑐𝑜𝑟𝑟, and Awards are useful to create different Classes.

Calculation of standard deviation 𝜎𝑐𝑜𝑟𝑟 takes into account the limited number of

samples 𝑛, applying a correction:

𝜎𝑐𝑜𝑟𝑟 =𝜎

𝑐4=

1

𝑐4∗ √

∑ (𝑥𝑖 − 𝑚𝑥)2𝑛𝑖=1

𝑛 − 1

Theoretical value of 𝑐4 [RB01] is:

𝑐4 = √2

𝑛 − 1∗

𝛤(𝑛2⁄ )

𝛤[(𝑛 − 1)

2⁄ ]

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But it can be taken from specific tables or approximated to:

𝑐4 =4 ∗ (𝑛 − 1)

4𝑛 − 3

If 𝑛 > 25. [RB02].

Classes are defined in the following way:

-one class (Class 1) regroups “best teams” in the event. Each one scored the best

result in at least 2 different Tests, won at least 2 main Awards, and reached very high

scores in most of the tests, with few exceptions;

-one class (Class 2) regroups “underdogs”. Excluding “best teams”, this class is

created considering teams with scores exceeding 0.5 ∗ 𝜎𝑐𝑜𝑟𝑟 from 𝑚𝑥, verified for

each Day, and adding to them teams which scored best result in at least 1 Test or won

at least 1 main Award. With few exceptions, all the teams in this Class reached high

scores respect to the others in at least one Day, most of times in Day2;

-one class (Class 3) regroups “middle rank” teams. Excluding the team previously

grouped, all the teams belonging to this class reached scores between 𝑚𝑥 and 𝑚𝑥 −

0.5 ∗ 𝜎𝑐𝑜𝑟𝑟, verified for each Day. Sometimes interesting scores are reached in single

Tests, and in few cases they were comparable even to Class 1, but overall scores were

farer from the optimal;

-all the remaining teams (Class 4) were “excluded” for statistical reasons. These

teams could be disqualified or didn’t participate to Day2 for various reasons or

reached very bad scores, especially in Day1. The majority of Rookie teams belong to

this class.

An example related to Brake Test is showed below. Team belonging to Class 1 and

Class 2 are labelled, respectively, in Green and in Yellow. Team labelled in White are

part of Class 3 while in Rosé and Grey are indicated Class 4 teams (the ones in Grey

didn’t participate to MS2 Event):

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1.5: BENCHMARK ANALYSIS RESULTS

Once that Class 3 and Class 4 teams are excluded from the analysis, each team is

compared with the others trying to define technical datasheets for each motorcycle.

Particular attention is given to chassis design, battery technology and, when

available, to performance data declared by teams.

These are the main results of the analysis:

-Chassis: with an important exception (Multi-Tubular together with composites),

Geometry is somehow attributable to a twin spar for all the motorcycle analysed. This

is a very common choice on Motorsport because the elimination of down-tubes and

lower cradle means easy access to work on motor and saving of valuable space in that

zone [RB10]. Twin Spar layout is not particularly efficient looking to stiffness to weight

ratio, as consequence different solutions could be evaluated, not only by structural

behaviour but also looking to placement of electronic devices.

Talking about materials, the winner of MS1 declared 25CrMo45 steel as structural

material, a choice awarded maybe due to the enhanced workability of the material,

which is easy to weld, to extrude, tends to self-hardening if temperature at which

casting process starts increases and has not tempering related fragility [RB09]. In

general, excepting a Class 1 motorbike with Carbon Fibre structure, material choice

varies between Steel Alloys and Aluminium Alloys.

Talking about mounts, being a PM motor, it isn’t requested a particular tendency to

damping. All teams showed simple metallic clamp as the following showed:

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-Caster Angle: with few exceptions at 25°, motorcycles are characterized by very little

caster angles, ranging from 17,5° to 22°. These values are slightly lower respect to

Moto3 Honda NSF250R with a Caster Angle equal to 22° 36” [RB03] but very different

weight. A good reason to decrease Caster Angle could be the necessity to reach better

placements on Gymkhana Test, because if it decreases also normal trail will decrease.

This means that also the ratio between front and rear normal trail will decrease.

Assuming 𝑎𝑛 as front normal trail, 𝑏𝑛 as real normal trail and the ratio between 𝑁𝑓

and 𝑁𝑟 as the weight distribution:

𝑅𝑛 =𝑎𝑛

𝑏𝑛∗

𝑁𝑓

𝑁𝑟

If 𝑅𝑛 decreases, the motorcycle will have better manoeuvrability at low speeds, at

cost of lower stability at high speeds [RB04].

When the weight increases and the weight distribution is higher on front wheel,

decreasing the Caster Angle will be less impactful on stability decrease, in fact heavier

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motorcycles have lower Caster Angle respect to the others. Going under 17,5° will be

probably dangerous due to higher wobble effect caused by the greater deformation

at which the fork will be subjected.

As previously stated, few motorcycles had a Caster Angle of 25°. In these cases,

results related to Best Lap and Race were excellent if related to the overall position

at the event. Maybe the choice was successful due to the conformation of the Circuit,

characterized by a limited number of tight curves and several curves at open throttle.

Further investigations could be useful;

-Wheelbase: data related to wheelbase of the various motorcycles are not declared

by most of the teams. The few data at disposition suggest that value could be

between 1250 mm and 1350 mm. This range of values is derived in the following way:

a) tires and rims are given by MotoStudent, in the case of the front wheel:

95 70⁄ 𝑅17

From here it’s possible to calculate the front wheel radius. Assuming, by definition

[RB05]:

𝑊 = 95 𝑚𝑚

𝐻 𝑊⁄ = 70%

𝐷 = 17 ∗ 2,56

Front wheel radius will be:

𝑟𝑓 = (𝑊 ∗ 𝐻 𝑊⁄ ) +𝐷

2= 282,4 𝑚𝑚

b) considering a Caster Angle range between 17,5° and 25°, and assuming null fork

offset, normal front trail will be [RB4]:

𝑎𝑛 = 𝑟𝑓 ∗ 𝑠𝑖𝑛𝜀

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c) From (FORMULA 1) it’s possible to derive the ideal 𝑏𝑛 assuming different values for

weight distribution and 𝑅𝑛, and then calculate wheelbase as follows:

𝑝 =𝑏𝑛 − 𝑎𝑛

𝑐𝑜𝑠𝜀

It can be applied a reverse engineering approach: if data about wheelbase are

available, it’s possible to calculate the optimal weight distribution, given 𝑅𝑛.

The low Caster Angle of heavier motorcycles decreases the normal trail. As

consequence, if a good level of manoeuvrability is requested, 𝑅𝑛 must decrease and,

as consequence, the wheelbase will increase;

-Battery Pack Position/Technology/Energy and Inverter: Even if the Regulations allow

the possibility to use multiple battery packs [RB06], none of the examined

motorcycles presents this solution.

Battery pack takes always place in between the main beams. For Li-ion batteries with

cylindrical cells it must be inclined to fit in. Adopting pouch cells, it’s possible to see

that best in class teams opted for almost vertical battery packs positioned in a raised

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position, raising up centre of gravity. This assured success in Braking tests for sure: in

particular, there are some videos in which is possible to see how it’s easy for these

motorbikes to stoppie. However, it helps also on Gymkhana test because it decreases

the lean angle necessary to accomplish the same turn [RB05].

Cell Types are various, from the classical Cylindrical (mainly on 18650 format), to the

Pouch format, which is more compact and more affordable thinking from a Six-Sigma

quality perception [RB07]. Most of teams bought or received battery cells from

sponsors, in few cases all the battery pack. Technology is always Li-ion. It is important

to underline that often the nomenclature Li-ion-Po is used. This is not related to the

choice of a cell with a solid electrolyte but is related to the Pouch cell polymer

packaging. The misconception, that causes often confusion, is even known and stated

in literature [RB07, RB08].

However, the choice of Pouch format requires strategies to overcome two main

problems: the swelling, which is the generation of 𝐶𝑂2 and 𝐶𝑂 related to small

internal shorts happening during aging [RB08] and, due to the non-rigid construction,

cell can increase volume. An increase in volume between 8 and 10% over 500 cycles

is considered normal and must be taken into account during design phase. The

second is the so called Lithium Plating: means that lithium-ions remain stuck inside

the cell, increasing battery resistance and wearing, and happens when external

pressure is not distributed uniformly on the cell [RB07]

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Energy installed is function of weight, both of battery pack and of motorcycle. Higher

values installed means that the battery pack can be able to deliver higher current,

allowing to the motor to produce higher power, maintaining the same autonomy.

Energy installed depends also on efficiency of each component (BMS, motor,

Inverter, etc.) and higher values allow a higher safe margin. Best in class motorcycles

show values of energy installed between 5,5 kWh and 7 kWh.

About Inverter, some teams which have Cylindrical Cells Li-ion batteries decided to

position it under the seat of the motorcycle, due to space. This changes the weight

distribution shifting it slightly to the rear. Other motorcycle with more compact

battery packs have Inverter hidden inside the fairings, maybe integrated with battery

pack itself.

-Weight: a lot of different technologies applied on chassis design and battery packs

means a wide range of different weight for all the motorcycles analysed. Considering

only motorcycles with declared weight, range goes from 116 kg to 152 kg. Must be

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noted that the heaviest motorcycle is placed on Class 1 and obtained excellent

placements in all the dynamic tests which could be negatively affected by weight

(Gymkhana, Brake, Acceleration), while motorcycles weighting less than 130 kg

obtained overall good results, but never excellent and sometimes disappointing. In

author opinion, the choice of the rider and its adaptability and knowledge of the

motorcycle played the most important role, except in Acceleration test where power

delivered by battery pack is the main objective.

-Swing arm: none of the examined motorcycles present the heavier single arm swing

arm. Double arm design, both in high truss and low truss variants, is variable: it’s

possible to see Tubular with Rectangular section, Tubular with Cylindrical section,

Trellis design or Asymmetric design. Materials ranges from Aluminium Alloy, chosen

by all Class 1 teams, to Steel Alloy. In certain cases the production process is

underlined: in particular for Aluminium Alloy swing arms, due to Innovation or

Business reasons, very different processes are presented, which goes from Selective

Laser Melting (the alloy is AlSi7Mg0.6, characterized by high corrosion resistance and

high static/dynamic strength) if the swing arm is designed through Topological

Optimization, to machining and extrusion.

However, doesn’t seems to exist a trend between design choice and performances in

dynamic tests.

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-Transmission: different layouts are presented by the teams, but information about

them are few. This suggests that in most cases, except when differently declared,

most of motorcycles have not a real gearbox, but a single gear directly attached to

motor shaft. It is possible and easy to actuate due to the Transversal Layout of the

Motor always chosen. To obtain better results during all the events, teams could have

at least two different final gears.

-Battery case cooling: for all motorcycles analysed, except one, seems that cooling of

battery case is accomplished by air. In certain cases cooling is forced by one or more

fans, this is true in particular for Li-ion Cylindrical cells packs, which compartments

are bulkier, longer and very crowded, so create artificially a pressure jump becomes

necessary. In other cases cooling can be facilitated by conveyors. Motorcycles

equipped with conveyors demonstrate excellent maximum speed values, so further

investigation related to Internal Aerodynamics consequent to the choice of this

solution is required.

-Brakes: All teams opted for a single brake disk on the front.

All Spanish Teams chose NG Brake Disc as producer of discs, but no one declared

dimensions of disks. Looking to the only one team that declares dimensions and uses

Brembo brake disks, and the very good result reached in Braking test, it’s possible to

state that the choice of a Ø310 for front brake and Ø220 for the rear brake is quite

conservative considering the medium weight of all the motorcycles analysed. Further

investigations and calculations are requested.

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2ND CHAPTER:

BATTERY PACK DEVELOPMENT

2.1: PREVIEW

From the definition given by Joseph Beretta [RB23], an Electric Motorcycle with a

single EM and a single Battery Pack can be considered as a Single-Energy Chain and

Single-Energy System vehicle.

This system can be modelled as follows [RB35]:

The Recharging Module allows to recharge the battery from the power distribution

grid and can be external. The Battery System, mounted on chassis, stores the electric

energy that will be converted in mechanical power by the Electric Machine, which is

driven by the Power Converter unit. This last element converts the fixed DC voltage

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available from the source into a variable voltage, variable frequency source

controlled to maintain the desired operating point of the vehicle [RB24]. It can also

allow regenerative braking, if appositely designed.

The aim of the thesis, as a feasibility study, is the definition of what should be bought

and, more important, what should be designed and built by the team, or outsourced;

then the integration of the system inside the actual chassis.

Cause the EM is selected by regulations, and what concerns most of Power Converter

and Recharging Module will be bought from external suppliers, main focus of the

thesis will be the definition of Battery Pack (in detail in this chapter), Transmission

and their synergy with actual chassis.

2.2: BATTERY SYSTEM BLOCKS

As general rule, Battery System (BS) can be divided in 4 principal operative blocks:

-Battery Pack (BP): the core of the system, which contains cells and their connection, defining

energy installed, power and voltage of the system;

-Battery Management System (BMS): it optimizes the usage, avoiding abuse of the pack,

detecting state of the battery cells and maximizing performance and efficiency [RB29];

-Thermal Management System (TMS): in communication with the cooling system allows to

maintain, inside the BS, a temperature range between -20 °C and 60 °C, avoiding detrimental

side reactions such as Lithium Plating and Corrosion or Gas generation [RB29];

-Power Interface (PI): it can be considered as the communication centre between the BS and

the rest of the system, allowing current flow from Recharging Module and to (or also from in

Regenerative Braking phase) Power Converter.

Functions of BMS and TMS can be managed by a single system. In this case it takes the name

of BTMS.

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2.3: ABOUT BASIC FUNCTIONS AND LI-ION CELLS

Doesn’t matter which type of technology and materials are used, the basic

energy/power module follows the same working principles and has a common

structure.

Without going, for now, too much in depth regarding chemical reactions and

technologies, it’s possible to detect four different components inside a cell [RB24]:

-Anode: the negative electrode, through Oxidation process it releases electrons

during discharge;

-Cathode: the positive electrode, through Reduction process it acquires electrons

released by the Anode during discharge;

-Electrolyte: it is a positive ions selector, allowing passage between Cathode and

Anode;

-Separator: it physically divides Anode from Cathode.

Notice that main chemical transformations inside the cell are exothermal [RB07].

Today, Li-ion cells are the most performing and widespread in almost all utilization

fields. Advantages respect to other type of cells are related to position of Lithium in

the periodic table: it is the lightest of all metals and one of the smallest atoms, which

means highest specific energy (considering pure elements) and electrochemical

potential [RB18, RB36] equal to -3.065 V [RB24]. Other noticeable advantages related

to utilization in a racing motorcycle are low internal resistance, relatively short charge

times and high Coulombic Efficiency (CE), intended as the ratio between the

discharged capacity respect the charged capacity.

In the case of Li-ion cells, Anode and Cathode are composed by the active material (a

lot of types, which will be discussed later) and current collector material, almost

always Aluminium for Cathode side and Copper for Anode side, due to their high

conductivity and potential stability respect to active materials in each side. A BP

composed by Li-ion cells is often called, in literature, LIB.

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The final BP will be created connecting, in series and in parallel, multiple type of cells.

Ideally, through cells connected in series will flow the same current, while single (or

grouped) cells connected in parallel will have the same voltage.

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As we will see, due to regulations and energy/power request, this aspect will be the

core of the problem. In fact, the voltage requirements (highest as possible, but under

126 V in operating conditions) will be the primary importance condition about the

choice of the single cell and, depending on the nature of connection, will require

advanced methods to balance the system, guaranteeing highest possible

performances together with safety and efficiency of the pack itself.

2.4: POUCH FORMAT

In commerce there exist three cell main formats: Cylindrical, Prismatic and Pouch.

Packs mounted on motorcycles analysed and participating at MotoStudent adopt

Cylindrical or Pouch formats. The Prismatic format, used mainly on automotive field,

is not suitable for racing motorcycles due to its bulkiness except rare cases such as

Mugen Shinden Nana.

Talking about Cylindrical cells, the most common type is 18650. Main advantages on

application are mechanical stability (the presence of metal shell simplifies the

internal geometry of the battery case, facilitating also cooling), cheapness and

calendar life. It can allow higher specific energy respect to Pouch cells [RB37]. Ideally

it should allow a lot of freedom about the shape of the pack, but the huge number of

cells requested to reach satisfying energy installed value makes difficult the

construction and the control by a Team, requiring outsourcing (together with

increased costs). Last but not least, the necessity of alleviate cell-to-cell heat

propagation, reinforced by the low heat transport from inner cell parts to the outside

[RB38], requires a minimum of 2 mm spacing between each cell [RB30]. Together

with the lower packing density of the Cylindrical cell [RB37], this means that Battery

Case will be easier to cool, but bulkier. As result, a look to photographs of concurrent

BPs adopting this layout underlines the necessity to place the pack in a lower position.

This is detrimental for the agility of the motorcycle, performance necessary in

Gymkhana test. This reason (forgetting all the problems related to optimization usage

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of a high number of cells) is sufficient to not consider the Cylindrical format for this

thesis.

What remains is the Pouch format. The Pouch cell is in a vacuum-packed thin plate

shape in which many stacking layers of thin cathode and anode electrodes are

alternately winded by a long ribbon of polyolefin separator. Each current collector,

except the outermost stacks, is coated with electrode films [RB19]. This

separator/electrode assembly including widespread solid-state electrolyte between

each layer are sealed inside a flexible thin plastic or aluminium pouch cover. [RB33]

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The main advantage of Pouch format is the elevate packing efficiency, between 90-

95%, and the high degree of design freedom. Thanks to its production process, the

dimensions of each cell are easy to customize. These are the main reason for which

Pouch format is widespread in EV field: they simplify the definition of the shape of

battery pack in order to find best position in the vehicle.

Exactly as other formats, talking about terminals position, a single Pouch cell can have

both electrodes on the same side (Type A), allowing for simplified electrical wiring in

the final pack structure, or can have terminals on opposite side (Type B): cells

adopting this last layout shows improvement on current and heat distribution, hence

their use on high-power applications such as modern HEVs [RB30]. Type B cells could

be more indicated for this specific project, but opponent teams often adopted Type

A cells, so this will be a secondary importance element of choice.

2.5: LI-ION POUCH CHEMISTRIES

Pouch cells are available in a wide range of technologies. Name of the technology is

related to Cathode chemistry, which is usually composed by a Transition Metal (TM)

[RB34] but exist also batteries with Cathode composed by Sulphur [RB18].

Mechanisms of deterioration are different for each chemistry, but are all amenable

to structural changes during cycling, chemical decomposition or dissolution, and

surface film modifications [RB41].

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A panoramic of technologies is now reported:

-LCO: active material is 𝐿𝑖𝐶𝑜𝑂2, with optimal hysteresis cycle, low self-discharge, high

discharge voltage (3.7 V) and good practical specific charge [RB18]. This is by far the

most common technology in consumer electronics. Cost is high, related to Cobalt

presence, and there are limits concerning thermal stability and specific power [RB40].

LCO is one of the few technologies which can allow Surface Coating, increasing

maximum voltage of the cell;

-LFP: active material is 𝐿𝑖𝐹𝑒𝑃𝑂4. Best characteristic is the safety related to thermal

runaway. Other advantages are related to very low internal resistance, the possibility

to reach high values of power density [RB12] and the excellent cycling stability. On

the other side, LFP Cells are bigger respect their counterparts, and their lower

potential (3.5 V and lower), together with lower ionic conductivity and carbon

coating, significantly reduces energy density [RB18]. For this reason, it could be a

possible choice in case of speed record attempts [RB39] but an analysis for

motorcycle racing adoption must be done;

-NCA: active material is 𝐿𝑖𝑁𝑖0.8𝐶𝑜0.15𝐴𝑙0.05𝑂2. This is the technology used by Tesla

for its battery packs. It can be considered as a LNO technology stabilized by Cobalt

and Aluminium [RB32]. Pure LNO is not viable because ions 𝑁𝑖2++ and 𝐿𝑖+ have

similar ionic radius and the first one tends to substitute the second during synthesis

and de-lithiation, making impossible to use the technology [RB18]. Advantages are

related to high specific energy and good specific power, while cost is still high (but

lower respect LCO) and tendency to thermal runaway is sadly famous [RB40]. NCA is

the second technology that allow Surface Coating, even if more critical respect to LCO

due to Manganese presence;

-NMC: it can be considered as a “family”. Formula of the Cathode chemistry is

standard: 𝐿𝑖𝑁𝑖𝑥𝑀𝑛𝑦𝐶𝑜𝑧𝑂2, where 𝑥 + 𝑦 + 𝑧 = 1 [RB17]. The adding of Tetravalent

Manganese limits reactivity of Nickel with most of electrolytes used today, so this

technology is better respect to NCA [RB18]. NMC cells shows very high energy density

and low internal resistance. Cost is a lot lower respect LCO technology, due to the

low presence of Cobalt in most of chemistries, but performances are similar. Today,

𝑥 > 0.6 [RB19] and the state of the art is represented by 𝑁𝑀𝐶622 and 𝑁𝑀𝐶811:

specific capacities are similar but the first shows a strong dependence on electrode

31

loading, worsening performances at higher loads [RB20]. 𝑁𝑀𝐶811 is considered as

the near future by most EV designers. Another member of the family is the so called

LR-NMC, characterized by very interesting values related to specific charge and lower

costs respect NMC, but a permanent loss on capacity after activation [RB18];

-LMO: active material is 𝐿𝑖𝑀𝑛2𝑂4. It could be viable as power buffer, combining it

with NMC as already done by EV manufacturers such as Nissan, Chevrolet and BMW.

This because LMO batteries could keep a high discharge efficiency at the high

discharging rate, showing good rate discharging performance [RB29], but specific

energy values are limited [RB40];

-LNMO: active material is 𝐿𝑖𝑀𝑛1.5𝑁𝑖0.5𝑂4. They are still in prototype phase because

the extremely high voltage reachable (4.7 V) is too high for the electrolytes today in

commerce [RB18];

-Li-S: this is a promise for the future, thanks to the theoretical specific energy

achievable, around 900 Wh/kg, while today values are around 300 and 350 Wh/kg.

Cost is similar to LR-NMC but production process is not reliable enough [RB18]. To be

competitive, strategies to increase the Sulphur-cathode density, such as roll pressing

and high-content Sulphur design, are crucial [RB19].

Talking about Anode, in general is made in Graphite. The reason is the following: in

Anode, during each cycle, a small amount of active Li reacts to thicken a passivating

layer on the surface of the electrode, universally known as SEI [RB22]. The presence

of SEI is related to irreversible loss of active Lithium and can also disconnect,

increasing impedance of cell [RB41]. The usage of Graphite helps creating a protective

layer upon SEI [RB34]. Graphite is also cheap and has good cycle life. However,

gravimetric capacity and recharging current are limited. Usually creation of Anode is

helped by binders: historically PVDF is used, but in recent years new aqueous or

polymeric materials like SBR+CMC show better performances and reduced toxicity

[RB21].

The best material for Anode should be Lithium, together with other materials that

can increase its Specific charge. Another reason for which it shouldn’t be used alone

is the easy formation of dendrites, related to short-circuits and heavy exothermal

activity in contact with air [RB17].

32

In recent years, and only for NMC and LMO, in order to overcome limits of Graphite

anodes, there exist some cells with 𝐿𝑖4𝑇𝑖5𝑂12 as anode material. It shows wide

working temperature range and long cycle life [RB29], and very high discharge

currents, but also low specific energy, the lowest in Li-ion cells [RB40]. Cells like this

can be used as power buffers.

Also Silicon and Tin are valid candidates for the utilization for Anodes, actually the

limit is fixed by the swelling related to their utilization [RB32].

Talking about electrolyte, typical formulations are based on constituents such as

ethylene carbonate (EC), diethyl carbonate (DEC), ethyl-methyl carbonate (EMC) and

𝐿𝑖𝐹𝑃6 and are generally stable up to around 4.5 V vs. Li+/Li (or lower at elevated

temperatures). Notice that an excess of electrolyte can negatively affect internal

resistance of the cell [RB31].

In the end, separator is usually made in polymeric material, permeable to ions,

sometimes it is designed to support electrolyte. It can be also designed as a fuse

[RB26]: at high temperatures, related to an excess of current, pores in the polymer

start to melt, prohibiting motion of li-ions [RB07]

2.6: PERFORMANCES SPECIFICATIONS

One of the basics about choosing the best cells is that a battery cannot be designed

in order to deliver high electric power, still guaranteeing highest specific energy

possible. Ragone’s Relationship can summarize this:

(𝑆𝑃)𝑛 ∗ (𝑆𝐸) = 𝜆

Where 𝑛 and 𝜆 are constants [RB24].

33

Ragone’s Relationship is a direct consequence (or, if preferred, an alternative way to

express the same concept) of Peukert’s Law: under the condition of high discharge

current, the maximum available capacity of the battery is not fully released [RB29].

In a simplified way [RB27] Peukert’s Law states that:

𝑡 = 𝑡𝑟𝑎𝑡𝑒𝑑 (𝐼𝑟𝑎𝑡𝑒𝑑

𝐼)

𝑘

Where 𝑡, measured in hours, is the actual discharge time at current 𝐼, and 𝑘, which is

always higher than 1, has no dimension and depends on technology and chemistry.

𝑡𝑟𝑎𝑡𝑒𝑑 is related to 𝐼𝑟𝑎𝑡𝑒𝑑, the one declared in datasheet.

Once that this point is clarified, main specifications which will guide to the choice of

the correct cell for the battery are reported [RB24, RB25]:

-Capacity: it is measured in Ah or mAh and, as can be expected by Peukert’s Law, is

related to specific modalities of discharge;

-C-Rate: it is one of the most interesting parameter. It is related only to the chemistry

chosen, and represent the normalized current respect declared battery capacity. In

practice, if a datasheet reports a declared battery capacity of 100 Ah, if C-Rate is equal

to 1C the continuous discharge current is equal to 100 A. High C-Rates are welcome

in the utilization with an EM;

34

-E-rate: represent the correspondent of C-rate, but related to Electric Power;

-OCV (Open Circuit Voltage): it’s the voltage between the two electrodes when there

isn’t any load applied. It is function of SOC (which decreases with OCV) and

Temperature. Models progressively more precise were developed to define how

these parameters have influence [RB42];

-Terminal V: it’s the voltage between the two electrodes when a load is applied. It is

not equal to OCV because dynamic events have an influence on inductance of the

system. It is function of SOC, Temperature and Discharge current: in particular,

increases when the discharging current increases [RB29];

-Internal Resistance: which is different between charge and discharge, and is function

of SOC. In particular, if SOC increases, Internal Resistance decreases. It is an important

parameter to define Efficiency;

-Nominal (or Installed) Energy: it is related to the single cell and will define the total

Installed Energy of the battery pack. It is function of the C-Rate. Depending on cell

selection and usage profile, energy calculated before represents the 80-90% of the

effective Energy used [RB07] and this must be taken into account during design

phase;

-Max Continuous Discharge Current: simply C-Rate multiplied for Capacity. It will

define the maximum speed reachable by the motorcycle;

-Max 30s Discharge Pulse Current: a battery is able to discharge higher current, but

only for a limited amount of time. This will define the Acceleration performance of

the motorcycle during Acceleration Test. There are reasons to think that at least one

team used Discharge Pulse Current to reach maximum speed only during Qualifying

phase.

-Efficiency: it is related to Internal Resistance, but also on unavoidable parasitic

currents [RB22]. There are 4 main processes which cause parasitic losses: SEI growth

and repair (which causes Li-ion losses), electrolyte oxidation at positive electrode,

dissolution of TM ions from positive electrode and deposition on negative one (TM

leaching, directly related to Surface Coating and more severe in NCA cells), lithium

trapping on positive electrode. In general, the battery has high discharging efficiency

with high SOC and a high charging efficiency with low SOC. The net cycle efficiency

has a maximum in the middle range of the SOC [RB28].

35

2.7: NECESSITY OF BMS AND TMS

In such a high-voltage, high-power system, with different connection in series and in

parallel, both these systems are strictly necessary.

Values of current and voltage of a series/parallel connection of cells are defined and

measurable over time. This doesn’t mean that every cell is at the same conditions of

the others. As already stated, in a series connection will flow the same current, but

in the same series connection can happen (or better say, it is always the case) that,

at the same SOC, voltages for each cell aren’t equal due to manufacturing tolerances

and different boundary conditions. This means that, when state of charge is 100% or

0%, voltage of singular cells can be out of the boundaries allowed by the technology.

At maximum SOC some cells can show Overcharge, while at minimum SOC different

cells can show Overdischarge. Both, the second more than the first, are very

dangerous because they lead to unwanted side reactions: there are risks of internal

short circuits [RB29] and Lithium Plating (it depends also on Temperature), which will

cause permanent lithium-ions losses and increase of internal resistance [RB07].

Another aspect to take into account is the current discharged by each parallel branch.

The worst cell in the series branch will limit the functioning of the others, so, during

discharge, it’s possible to say that a cell 𝑖 will have the minimum discharge capacity:

𝑄𝑑𝑐ℎ_𝑚𝑎𝑥[𝑖] = 𝑚𝑖𝑛{𝑄𝑚𝑎𝑥[1] ∗ 𝑆𝑂𝐶[1]; 𝑄𝑚𝑎𝑥[2] ∗ 𝑆𝑂𝐶[2]; … 𝑄𝑚𝑎𝑥[𝑛] ∗ 𝑆𝑂𝐶[𝑛]}

= 𝑄𝑚𝑎𝑥[𝑖] ∗ 𝑆𝑂𝐶[𝑖]

When discharge capacity reaches 𝑄𝑑𝑐ℎ_𝑚𝑎𝑥[𝑖], cell 𝑖 will finish its discharging first,

practically prohibiting to other cells on the same branch to release current. Obviously,

a similar concept can be applied on recharging [RB29]. Parallel branches can so

consistently define the BP voltage, but cannot help on define charge/discharge

properties and ageing of each series branch.

Other issues are caused by the Temperature. As already stated, temperature range

allowed by most Li-ion technologies is between -20 and 60 °C, with optimal

temperature between 20 and 40 °C [RB43]. Due to the location of Aragon circuit, low

temperatures aren’t a real issue, so attention is required in order to maintain

temperature far from the upper limit. A temperature equal to 60 °C is sufficient to

36

cause Thermal Runaway if it is maintained for a prolonged period of time [RB47]. High

temperatures are also related to Corrosion and Gas formation inside the cell [RB34],

which cause swelling, reinforced by the high rates [RB45]. The problem, impossible

to avoid but at least controllable, will define some challenges relating the design of

the case, argument which will be discussed later. Obviously, also Temperature

between cells will not be consistent, meaning different aging. Some manuals [RB07]

advise to maintain a difference of 2-3 °C between the coolest and the warmest cell.

The aim of BMS and TMS (or BTMS if the system is unique) is to solve all these

inconsistencies, allowing optimal usage: no abuse and irrational use, clear definition

of states of cells, maximize performance during driving and increase of efficiency and

battery usage.

Advanced BTMS must accomplish seven basic functions [RB29]:

-Online monitoring of battery external parameters, such as voltage, temperature,

current…

-Battery fault analysis and alarm;

-Starting the cooling fan when the battery temperature is high;

-BP SOC estimation;

-BP Estimation, SOC + SOE (Energy) + SOF (Function) + SOH (Health);

-Battery Equalization;

-Battery Safety Management.

A BTMS can be Passive or Active: a Passive BTMS tends to discharge the most charged

cells through a resistive load, in order to balance voltages (see the example in the

image showed below), while the Active BMS shows theoretically the highest

efficiency because it transport charge from the most charged cell to the least. There

have been a lot of evaluations of both systems over the past few years; however,

current studies do not show the long-term benefits of using an Active balancing

system. In short, with the current level of technology, there does not appear to be

any major system benefits that can be achieved, and those minimal benefits do not

outweigh the added cost for an Active balancing system [RB07].

37

To manage temperature, production designs will usually include two to three

temperature sensors per module, more or less mounted one of the first cells in the

module, one in the middle, and one near the end. The proper method to determine

exactly how many sensors are required and where to locate them should be based

on module characterization testing and CFD analysis. It is also good practice to install

temperature sensors to monitor the temperature of the incoming fluid (liquid or air)

as well as the temperature of the outgoing fluid (liquid or air) [RB07].

38

2.8: CASE DESIGN

The Case must be safe, compact, with good NVH, EMC and EMI performances,

lightweight and thermally efficient, still guaranteeing optimal working conditions for

every single cell.

It’s comfortable to divide the BP in “system interfaces”:

Inside the system interfaces more attention is required on “control factors”. A rule of

thumb for identification of control factors is: any factor that lies outside the system

boundary is not regarded as a control factor [RB30]. The most critical are cells, cell

spacers’ type, number and location of exhaust gas nozzles, cooling system and

insulation coating system.

Talking about the structure of the case, the material should be lightweight and rigid:

two conflicting goals because more the Battery Case is heavy, more it is stiff. Carbon

Fiber would be the best choice, without considering the cost, and was chosen by a

Class 1 team. Other teams opted for Aluminum or Composite Cases, or a mix between

them. Pure metal cases require that the battery and all connectors are sealed and

insulated, or that design provides painting or sealing the case to render it non-

conductive [RB08]. If Polymeric material is used, the choice must be done considering

39

the Flame Retardant Rate. The case must ensure an Ingress Protection which is at

least IP69K, corresponding to “sealed against both liquid and dust intrusion” [RB07].

Vibration frequencies should be suppressed to avoid resonances at typical natural

frequencies of the system: critical values are between 0 and 7 Hz for suspensions and

sprung masses, from 7 to 20 Hz for drivetrain, and from 20 to 40 Hz for the chassis

[RB30]. Modal Analysis can be a strong tool to test frequencies during design phase.

Another characteristic highly recommended is the fast-swap. This is related to the

entire battery pack, not only single modules or cells. Due to the short time available

for recharging between sessions and the intrinsic difficulty of fast-charging, build two

equal BPs and swap them between events will help to maintain battery healthy and

to decrease with safety the BP energy. If the same chassis is maintained, the only

viable dismount solution is vertical.

The utilization of pouch cells requires the facing of strict challenges for what concern

the design choices about cell spacers, which perform a dual role in the case of pouch

cells. Besides their primary function (providing cell-holding functionality), they

provide the binding pressure necessary to counteract the internal spring forces and

to prevent the cell windings from expanding as a result of it. Battery cell spacers

should create sufficient binding on the cell sides without covering so much of the cell

surface area that cooling becomes ineffective (according to NASA-Battery Safety

Requirements Document [RB44], cell spacing is more critical for pack designs

employing battery cells of gravimetric energy density greater than 80 Wh/kg).

Spacers should also avoid delamination of electrodes, the alternative is to choose, as

assembly tech for the cells, clamping.

Even if not convenient for what concern cooling design, BP should be compact for

two reasons: the first is the possibility to place it in an optimal position (argument

more in depth in the chapter “Design Hypothesis”), the second is that has an

appreciable impact on thermal performance of the battery pack. Research shows that

increasing the cell-to-cell spacing for a battery pack from 1 to 10 mm can lead to a

loss of approximately 1 °C in the steady-state cell core temperature, for all the three

physical formats.

An aspect extremely important for what concern safety is the presence of exhaust

nozzles. Almost all cells have a system which limits the gas pressure inside it, venting

40

it outside. Up to half of the gas venting from lithium battery cells is made up of highly

flammable gases including hydrogen, methane and ethylene gas [RB08]. In the case

of pouch cells, it’s possible to identify a weak spot across the seals close to the tabs,

which will collapse when pressure rises dramatically during, for example, an

Overcharge event [RB46]. Packing location and battery chemistry type will influence

whether a gas vent routing system to the vehicle exterior is necessary in the event of

a cell vent during a malfunction [RB30].

The choice of Forced Air Cooling for most of teams is amply justified by weight

reduction and simplicity in the design. Efficiency of the system, eventual synergies

with Aerodynamics of the motorcycle and Dimensions and position of cooling fans

must be carefully evaluated. This last process can be done at the end, analysing

pressure drop inside the pack. Only one motorcycle of Class 2 is equipped with Liquid

Cooling, in this case also the lower fairing must be chosen in order to contain at least

41

half of the cooling fluid, with a minimum value of 2.5 litres [RB06]. Natural Air Cooling

is not recommended because cells cannot have enough time to cool efficiently at

lower speeds reached in the middle part of the circuit, exacerbating the loss of range.

Efficiency of the thermal design can be further increased by using perforated battery

compartments. Effluents generated by the battery cells enter the hollow guideways

formed within the battery pack through these perforations. The guideways direct the

effluents to a gas exhaust nozzle, which releases it out of the battery pack.

2.9: BATTERY PACK DIMENSIONING

Correct choice of the technology requires the definition, in an approximate way, of

energy stored inside the pack, before to choice the best assembly method.

At disposal there are old data from telemetry of the Petrol motorcycle already

designed by the university team. This motorcycle was able to reach a speed higher

than 180 km/h in the main straight, and perform a time lap better than 2:30:000: if

compared to scores done during last race event, this motorbike would be one of the

best of Class 1.

By the way, foreseeing a performance improve of the other teams, the target time

lap is set to a value under 2:15:000.

Data from telemetry, together with altimetry data from the circuit, are sufficient to

define the energy requirements (in the next section will be see how). Once that a first

attempt on technology is done, weight of this simulated pack will be, simply:

𝑚𝐵𝐴𝑇𝑇,𝐸 =𝐸𝐵𝐴𝑇𝑇

𝐸𝑠𝑝𝑒𝑐

And will be compared to weight required to satisfy maximum power requirement:

𝑚𝐵𝐴𝑇𝑇,𝑃 =𝑃𝑀𝐴𝑋

𝑃𝑠𝑝𝑒𝑐

The highest value will satisfy both energy and power requirements [RB12]. If the last

one is chosen, energy stored in will be calculated again.

42

As already said, the author had at disposal old telemetry data. Inside this .xls file there

are, defined for fraction of time, instantaneous velocity and throttle conditions.

The author programmed a script, written with MATLAB 2014a, using this process:

-first of all telemetry data are cleaned up with the objective of remove redundant

data, tending to increase drastically the value of energy requested;

-the speeds from .xls file are artificially modified identifying moments in which the

motorcycle is accelerating. The total time will be reduced by 15 seconds, in this way

we can define a coefficient:

𝑝𝑟𝑜𝑝𝑜𝑟𝑡𝑎𝑐𝑐 =𝑇𝑎𝑐𝑐 − 15

𝑇𝑎𝑐𝑐

The instantaneous speed will be defined as:

𝑉𝑐𝑜𝑟𝑟 =𝑉

𝑝𝑟𝑜𝑝𝑜𝑟𝑡𝑎𝑐𝑐

43

Notice that with this approach speeds during cornering and braking are ignored. As

direct consequence, this time lap is created ignoring shifting of braking points, so

calculations about power will be conservative due to higher speeds than real. A speed

limit is fixed at 56 m/s, also acceleration values will have a maximum value fixed to

be sure that values after modification of the data are coherent with reality;

-total mass of the system motorbike + driver, 𝐶𝑥 and rolling coefficient are taken by

hypothesis and the instantaneous force is derived as:

𝐹𝑖𝑛𝑠𝑡 = 𝑚 ∗ 𝑎𝑐𝑜𝑟𝑟 +1

2𝐶𝑥𝜌𝑉𝑐𝑜𝑟𝑟

2 + 𝑚𝑔𝑠𝑖𝑛𝛼 + 𝑚𝑔𝐶𝑟𝑜𝑙𝑙𝑐𝑜𝑠𝛼

Where 𝜌, 𝑔 and 𝛼 are, respectively, the air density at ambient conditions, gravity

acceleration and instantaneous slope of the circuit;

44

-power is simply derived using the established formula:

𝑃𝑖𝑛𝑠𝑡 = 𝐹𝑖𝑛𝑠𝑡 ∗ 𝑉𝑐𝑜𝑟𝑟

Cause the new engine has a maximum power declared of 42 kW, thanks to the higher

voltage assured by the BP it was reasonable to limit the script to a power equal to 50

kW.

-mean power is calculated and the energy spent on a lap is simply equal to:

𝐸𝑙𝑎𝑝 = 𝑃𝑚𝑒𝑎𝑛 ∗ 𝑇𝑙𝑎𝑝

-total energy spent on the event is the value obtained from previous step multiplied

for a constant, taking into account that the race will consist of 6 laps plus recognition

and exiting lap, which together will consume, depending on driver’s behaviour, more

than 70% of energy of a regular lap [RB16]. For safety reason during calculation is

45

assumed that energy spent is 80%. Real value will be surely lower, especially if

regenerative braking is considered:

𝑐𝑜𝑛𝑠𝑡 = 6.8

A further safety factor is taken into account, because calculation is done considering

an ideal battery. In reality, internal resistance and transient current will negatively

afflict the total energy consumed by the motor. This safety factor is in line with other

works in the same field [RB16]:

𝑆𝐹 = 1.05

Depending on cell selection, usage profile, and efficiencies of Transmission and

Inverter, energy at disposal can be the 80% of the required in the worst case scenario

[RB07]. This means that Installed Energy must be, at least:

𝐸 =𝐸𝑟𝑒𝑞𝑢𝑒𝑠𝑡

0.8

At the end, total energy necessary to end the race, calculated with this method, is

equal to 7.7622 kWh for the version without regenerative braking. Script can be

adapted to study impact of regenerative braking, estimated by previous analysis on

about the 10% of energy saved. The premises are good, but adaptation on the motor

must be carefully evaluated.

2.10: CHOICE BETWEEN SINGLE OR DOUBLE PACK

Rules [RB49] allow the possibility to create more modules, and the possibility to place

them in different places in the motorcycle. This choice can be done in order to satisfy

power request without renounce to range capability and to increase Regenerative

Braking Efficiency, creating a Series Hybrid Electric Vehicle [RB52].

46

The storage system will take the name of HESS (Hybrid Electric Storage System)

[RB53]. Usually the Power Buffer, as the name can suggest, is created connecting in

series cells with high specific power (such as LTO, LFP, LMO or Supercapacitors) that

can supply it under request, but can be created also adopting same chemistry of Main

BP but at higher voltage. It shows also excellent performances in Regenerative

Braking because such chemistries show interesting C-Charge rates.

However, no one of the other teams chose a layout like this and there aren’t

examples of commercial and racing motorcycles with such technology. The reason is

simple: as calculated in the cycle, regenerative braking will have a non negligible but

small role on BP dimensioning and, except when Pulse Discharge Current is required

(during Acceleration test and in very rare cases during Race and Qualifying events),

Battery will run almost in Continuous Discharge Current conditions. On the other side,

there is the necessity of an interface between the two packs, because they cannot be

connected directly in parallel due to safety issues [RB53]. Whatever is the

architecture (between the ones viable on Racing, so semi-active or fully active), this

corresponds to an increase of number of components (and as direct consequence,

weight) and increase of design requirements. For example, it could be necessary to

design a further case, which means more complex thermal management and analysis,

to add other BTMS boards and there is the risk to denaturalize weight distribution on

the vehicle. Furthermore, the setup of a robust control strategy is required. These

47

reasons are enough to state that, for high performance lightweight motorcycles, is

more convenient to focus on a single and compact BP.

2.11: BP LAYOUT

Once that requirements of the pack (in terms of Power, Energy and Voltage) are set,

the research of a suitable cell and layout can start. Approach will follow this scheme:

-Data about different cells (and modules) related to Electrical Characteristics

(Voltages, Capacity, C-Rates) and Physical Characteristics (Dimensions and Weight)

are collected.

-As further safety factor, Capacity is corrected as follows:

𝐶𝑐𝑜𝑟𝑟 = 0.9 ∗ 𝐶𝑑𝑒𝑐𝑙𝑎𝑟𝑒𝑑

This takes into account that cells aren’t available for testing. Capacity declared by

manufacturers is related usually to low values of current (from less than 1C to around

2C), but it’s known by bibliography [RB48] that, at ambient temperature, battery

maintains good performances until maximum continuous C-Discharge, then losing a

huge amount of its discharge capacity until maximum pulse C-Discharge. Value

obtained is expressed in Wh.

48

-Number of cells belonging to a series branch is chosen imposing that the maximum

voltage admissible for the competition, equal to 126 V [RB49], must be lower respect

the maximum voltage of the series branch. Even if it could seems disrespectful of

rules, this strategy takes advantage of charging and discharging characteristics of

various technologies: only a little part of energy is stored at around maximum

voltage, and time required to recharge at maximum voltage is higher. This means that

it’s convenient to increase Total Capacity and decrease recharge time limiting

artificially the total voltage to regulation request, even if this means renounce to a

part of maximum energy (this is more true for NMC and LCO, looking to their very

similar discharging law, both from testing and modelling [RB51]).

Another possible trick is to charge the battery at its maximum voltage and then

unload it before utilization, being sure that voltage of the BP will be the maximum

allowed by regulation and not a lower value depending on charger characteristics

and, as consequence, decreasing current necessary to obtain maximum power. For

these, and other reasons often related to BMS and specific rules, the same tactic was

chosen by other teams participating to Motostudent [RB50] and FSAE [RB12].

49

-Number of required parallel branches is calculated with the objective of satisfy the

Installed Energy Request. This means that Installed Energy must be, at least:

𝐸 = 7762 𝑊ℎ

And the number of Parallel Branches must be, at least:

𝑃𝑎𝑟𝑎𝑙𝑙 =𝐸

𝑆𝑒𝑟𝑖𝑒𝑠 ∗ 𝐶𝑐𝑜𝑟𝑟

-If the BP virtually built satisfies the minimum power request it can be considered

valid. Minimum power is fixed at:

𝑃𝑚𝑖𝑛 = 50000 𝑊

As declared by other teams in previous edition. Notice that there is a high probability

that value of Power is related to C-Pulse current. For lack of this information, a very

safe approach is chosen:

𝑃 = 𝑃𝑎𝑟𝑎𝑙𝑙 ∗ 𝑉𝑚𝑎𝑥 ∗ 𝐶𝑐𝑜𝑟𝑟 ∗ 𝐶𝐷𝐼𝑆𝐶𝐻

Where 𝑉𝑚𝑎𝑥 = 126 𝑉 and 𝐶𝐷𝐼𝑆𝐶𝐻 is the C-Rate relative to Continuous Discharge

Current.

On the following table there will be results of the cell research. Packs that don’t

satisfy minimum power request (as chosen by the author) are labelled in Red. The

packs labelled in Green show very high power and a good energy margin, together

with the lowest weight:

50

51

2.12: BP GEOMETRY AND CONNECTIONS (CASE 1)

Data related to dimensions of a single module are showed below:

Each module should be already equipped with a fuse (further investigations are

required).

Considering the 1st model, a 16S3P BP is required to satisfy Energy Request.

The total volume occupied by the cells will be:

𝑉 = 𝑆𝑒𝑟𝑖𝑒𝑠 ∗ 𝑃𝑎𝑟𝑎𝑙𝑙 ∗ 𝑙 ∗ 𝑡 ∗ 𝑤 = 0,0199 𝑚3

For the 2nd model, with a configuration 11S3P:

𝑉 = 𝑆𝑒𝑟𝑖𝑒𝑠 ∗ 𝑃𝑎𝑟𝑎𝑙𝑙 ∗ 𝑙 ∗ 𝑡 ∗ 𝑤 = 0,0202 𝑚3

Traducing in a very compact shape. This means that there will be enough space to

place other components such as BTMS and Inverter in the same case, decreasing

wiring length and thus increasing efficiency of the whole system. This will have also

positive effects on motorcycle dynamics and will decrease number of components

that must be designed. However, special care about EMC, EMI and electrical isolation

must be taken into account.

Now, it must be decided how to connect the batteries. Two solutions are viable: in

both cases, if a module stop to function, the maximum current available will be

limited by BTMS in order to save the others.

52

In this case, the “Solution c” should be better for modularity because only two

terminals (positive and negative) are required for its functioning, while the “Solution

b” requires two positive and negative terminals, plus connections between each

parallel stack for a total of 15 or 10 depending on the module chosen. However, the

same layout can be risky in the case of a broken module: it means that all the series

branch will be disconnected from the circuit, losing 1/3 of overall installed energy and

higher electrical work on the functioning branches.

It's clear that, due to the difficulty to separate the single cells of the module,

connection will be done through connectors.

53

2:13 BP GEOMETRY AND CONNECTIONS (CASE 2)

In order to have better control on the manufacturing process of the pack, looking to

experiences of other teams (competing also in old MotoE championship) and

considering performances of the Motenergy MS1718, it’s possible to decrease the

power requirements assuming that is sufficient to deliver Continuous Discharge

Power in order to satisfy the request:

𝑃𝑚𝑖𝑛 = 32000 𝑊

In this way another cell, produced by Melasta, will be a potential candidate:

Let’s see all the advantages related to this choice:

-very strong brand in EV racing;

-better chemistry;

-slightly minor battery cell volume:

𝑉 = 𝑆𝑒𝑟𝑖𝑒𝑠 ∗ 𝑃𝑎𝑟𝑎𝑙𝑙 ∗ 𝑙 ∗ 𝑡 ∗ 𝑤 = 0,0195 𝑚3

-possibility to choose different technologies to connect cells each other;

-better BTMS management (it could be done on single cell and not on single module);

-potentially better efficiency (due to the possibility to avoid the utilization of

connectors);

-easier to cool.

Disadvantages are:

-weight increase of the whole BP (42.1 kg against 38.5 or 39.8 kg);

-increase of complexity due to the higher number of cells, this requires a very robust

approach on connections and further considerations about BTMS;

54

-range capability and reliability must be verified in case of Pulse Discharge Current

utilization (especially if is intended to set a map in order to apply during the race);

-dimensioning of fuses becomes necessary.

In this case, “Solution b” is always better for what concern robustness of the system,

even if the increased number of parallel branches decreases the previous problem of

installed energy for “Solution c” (1/7). Another clear advantage is that if a suitable

connecting method is chosen, like for example clamping, it will give clear advantages

on modularity because will allow a direct dismount of the broken cell. Another

advantage to consider is that balancing issues can be managed, under certain limits,

by cells themselves, because current can flow in between branches. However, this

means that parallel connections must be carefully dimensioned. It’s worth to

consider that for “Solution c” more boards are necessary because parallel group cells

could have different voltages but reading errors can be an issue in “Solution b”

[RB50].

Differently respect the Case 1, the connection between the cell can be done in

different ways. In order to understand which is the most convenient, the author setup

a Pugh Matrix inspired by application studies on Automotive Field [RB54]. The Criteria

considered particularly important are grouped and a Weight is arbitrarily given,

considering the application in a racing environment:

55

Even if there isn’t an evident advantage looking to points only, the author suggests

the design of a mechanical assembly through clamping. At the cost of a slight increase

in weight and dimensions and a reduction of efficiency, safety related to null heat

transfer during assembly, strength of the joint and, in particular, the total modularity

of the system, are advantages impossible to ignore in a racing environment. To

support this idea, other teams working on way more powerful motorcycles [RB55]

used successfully this technology, obtaining a quite acceptable value of efficiency of

97% at maximum charge and 96% at minimum charge. These values can be reached

sanding and degreasing the cell tabs and the copper in order to remove oxidation and

grease.

2.14: FURTHER ANALYSIS REQUIRED

There are enough information to start the development of the pack, starting from

purchasing the cells.

56

The pack must be physically created, tested, together with its case and then placed

in a suitable position:

-behaviour of the single cell under specific current charge and discharge (suitable for

the real requirement of a single branch) can be validated with an automated tester,

interfaced through MATLAB/Simulink. The tester should guarantee a way higher

current respect the single cell capability and relays to simulate the various conditions

(rest, charge, discharge), and should be equipped with a DC/DC converter to comply

with typical portable computer direct current (DC) power supply [RB50]. It’s

suggested to buy more cells in order that during testing the ones with lower internal

resistance can be selected for the construction of the pack;

-quality of connection between different cells must be evaluated. Once that the stack

is designed, by using a programmable DC load it is possible to control the drawn

current very accurately and indeed, when the load is enabled, it is observed that the

total voltage on the terminals of the stack drops slightly. Using simply Ohm’s law, the

internal resistance of the stack can be calculated straightforward. The linearity of this

equation even allows to extrapolate the obtained resistance to the complete battery

pack [RB55]. It’s suggested to not go under values of 95% of Efficiency of the stack;

-it’s possible to design BTMS connections in order to have a single board for all the

BP. Usage of CAN Cell Group Modules makes a BTMS virtually scalable to an infinite

number of cells [RB56]. CAN protocol is widespread in Automotive Industry, and

already used in Formula E to the same purpose as presented here [RB57]. In case the

decision about BTMS will be towards the utilization of multiple boards, in order to

increase precision controlling directly the cells, an estimation of the number of

necessary boards must be done. The number of boards can directly affect thermal

analysis and dimensioning of the case;

Talking about the case, important requirements to consider are:

-Ingress Protection: at least IP69K. It must be considered that the case will have a

water/air filter necessary to comply both to cooling requirements and waterproof

tests required by regulations. If there are time and conditions available, waterproof

effectiveness can be tested using specific sensors like the WaterSwitch M158

produced by Kemo;

57

-Modal Analysis: natural frequencies of the case must be at least higher than natural

frequencies of the chassis. If possible, it should be higher respect shimmy, derived

from tolerances related to the front assembly. Virtual tests have to be done both in

straight and in cornering conditions. Last but not least, it should be as light as

possible;

-Thermal Analysis: the pack must be designed in order to never reach temperatures

higher than 60 °C, however as previously stated should be convenient to dimension

it having in mind that optimal range is between 20 and 40 °C. Also, temperature

between different cells should be as similar as possible. Differences of about 3 °C

between coolest and warmest cell are acceptable. In order to reach these results, due

to the lower heat transfer coefficient of air [RB59] can be useful the design of a Vortex

Generator, facilitating the mixing of air inside the pack. While results related to

Cylindrical cells are encouraging [RB58] the effectiveness related to Pouch cells

should be investigated. There are a lot of virtual methods useful to perform the

analysis [RB60], but one which is absolutely necessary is CFD in order to calculate the

pressure drop inside the pack, in order to dimension cooling fan and to analyze

aerodynamic interactions with side flow and wake. This is valid for each

compartment;

-Other requirements: the case must be not pierceable, acid resistant, and

compartment dedicated to Inverter should be electromagnetically shielded in order

to not interfere with functioning of the BP. It is suggested to place the Inverter in

downward position.

58

3RD CHAPTER:

HYPOTHESIS ON TRANSMISSION AND

MOTORBIKE DESIGN

3.1: TRANSMISSION BASIC DESIGN

In the past edition of Motostudent there were various solutions related to

Transmission Design. The dominant one consists on a single gear ratio without any

clutch, and final pinion often directly mounted on EM shaft, sometimes with an

intermediate transmission shaft. In general, different solutions (like for example the

utilization of a 5-gear gearbox) came from a different philosophy, oriented more on

MS1 competitiveness or derived from unique design solutions.

Ideally, a good way to understand if the EM needs a multi-gear gearbox is observe its

torque and power characteristics. In the study case of the engine that will be

mounted on the motorbike, produced by ENGIRO, characteristics are showed below.

59

60

Of particular interest is the ratio between Maximum Rotation Speed and Nominal

Rotation Speed, which takes the name of “speed ratio”:

𝑥 = 1.4

This is a quite standard value in PMEM due to its intrinsic difficulty on flux weakening

range [RB28]. This means, as clearly shown by the graph, that constant torque region

is larger than constant power region and that maximum torque can’t be significantly

high. Looking to efficiency map, it is clear also that motor efficiency will be quite low

during all the utilization range.

61

If this would be a heavier racing car (like FormulaE) or a multi-purpose road vehicle

the utilization of a multi-gear gearbox would be advisable. However, in our case study

it’s important to notice that:

-efficiency will be higher due to higher voltage of BP;

-slopes in the circuit are steep only in few parts;

-total weight of the motorcycle has a dominant importance, especially related to BP

dimensioning;

-a lot of Road Racing Electric Motorcycles still adopt a single gear transmission even

with PMEM motors, like Energica Ego Corsa, Mugen Shinden Nana and Victory RR;

-the choice of a single gear ratio doesn’t require the compilation of a complicated

script to identify correct gear shifting patterns [RB60].

Last, but not least, the examined motor is very bulky in all reference planes (xy, xz

and yz). This means that the presence of a bulky transmission would make the

problem worse. In practice, the EM is wide enough to constrain designers to the

utilization of an intermediate transmission shaft in order to have the chain parallel to

x axis. This will give, as direct advantage, the possibility to reach a satisfying Squat

Ratio for two reasons: the secondary shaft can be positioned in a rearward and along

swingarm axis position, and the presence of an intermediate reduction stage allows

to decrease the final gear ratio [RB05].

3.2: SQUAT RATIO CONSIDERATIONS

To understand Squat Ratio and its link with dynamic behaviour of rear suspension, it

should be considered the intersection point A: it is the intersection between upper

branch of the chain and swingarm axis. The straight-line connecting contact point of

rear wheel and point A is called squat line, characterized by an angle 𝜎. The point A

represent the Instantaneous Force Centre [RB10]. With load transfer line is intended

the direction of force obtained summing load transfer force and thrust force and is

characterized by an angle 𝜍 (usually 𝜏 in bibliography [RB05], 𝜍 is used to avoid

confusion with gear ratios).

62

Assuming 𝛷 as swingarm angle respect the x axis, 𝐿 as its length, and 𝜂 as the angle

between upper branch of chain and same reference axis, we can define the Squat

Ratio as:

ℜ =𝑁𝑡𝑟 ∗ 𝐿 ∗ 𝑐𝑜𝑠𝛷

𝑆 ∗ 𝐿 ∗ 𝑠𝑖𝑛𝛷 + 𝑇 ∗ 𝐿 ∗ sin (𝛷 − 𝜂)

It’s possible to demonstrate that expressing the load transfer as a function of the

driving force, the ratio is a function of geometric characteristics only:

ℜ =ℎ ∗ 𝑐𝑜𝑠𝛷

𝑝 ∗ [𝑠𝑖𝑛𝛷 +𝑅𝑟

𝑟𝑐sin (𝛷 − 𝜂)]

=𝑡𝑔𝜍

𝑡𝑔𝜎

Squat Ratio is of crucial importance in dynamic behaviour: it says how the swingarm

will rotate due to thrust force, and as consequence will influence shock absorber

behaviour. In particular, if ℜ < 1 suspension will tend to extend, and this is a

detrimental result that will decrease torque utilization and stability in curve. Normally

63

the objective of designers is to obtain a neutral Squat Ratio equal to 1 or slightly

higher.

It’s worth to say that the Squat Ratio depends also on bumps and other obstacles

which can vary these geometric values. In the following examples is possible to see

how the suspension state can condition this situation:

In Drawing A we can see that Instantaneous Force Centre can be behind the wheel.

In this case the suspension will compress if the point is below the ground and expand

in the opposite case.

The other drawings are related (exaggerating) to extended suspension (B) and

compressed suspension (C). We can see that a progressive suspension compression

will condition negatively the instantaneous Squat Ratio, while during extension Squat

Ratio will tend to values higher than 1.

Let’s return to previous formula. Notice that, considering wheel dimensions constant:

-a bigger crown gear tends to decrease the squat ratio;

-swingarm inclination angle and difference between the arm inclination angle and

the chain inclination angle have a direct influence. This last value depends on position

64

and radial dimensions of pinion gear. In particular, a smart positioning along

swingarm axis, or concentric with swingarm pivot (like in some early Bimota models

[RB10]) will help to reach Squat Ratio values near to Neutral.

3.3: POSSIBLE LAYOUT

Considering the absence of the clutch, design of intermediate stage can be done

taking as reference camshaft activations systems:

-bevel gears: this is an old-fashion solution, in the ‘50s was very common in Racing

motorcycles with single camshaft like Honda RC141 and Honda RC160. Later on also

Ducati used this technology as trademark and in 1971 equipped its 500 GP engine

with this system [RB64]. Today this solution is abandoned in Motorcycle industry, due

to heaviness, cost, necessity of dimension at least 2 intermediate shafts and the lower

efficiency (around 93%) respect spur gears [RB65];

-timing belt or chain: simple and widespread solutions, the first has the advantage of

easier adjustability and no lubrication requirement. There are in the market double

sided timing belts, which can be used to create a counter rotating system without the

utilization of a further intermediate shaft. Timing chain is way more resistant and can

decrease weight of the system if carefully chosen. Both assure liberties about

secondary shaft positioning;

-gear train: this solution is typical of high revving racing engines of our days due to

best power/weight ratio, high precision of the system [RB66] and very high efficiency.

Another advantage is the opportunity to position the EM in a way that it rotates in

the opposite verse. This strategy is widespread on modern SBKs motorcycles because

in dry road conditions allows to decrease roll angle (given a curvature radius) and

limit wheeling during acceleration. Handling is improved during chicanes and curve

approach but worsen during exit [RB68]. Advantages of counter rotating engine are

however limited in our specific case due to centre of gravity shifting towards the front

and low power and inertia, so the objective of make transmission compact comes

first. As disadvantages, the number of components can increase drastically with

65

number of intermediate gears required and it’s by far the most expensive solution. It

is also very noisy, even if noise is rarely a concern in Racing.

The team opted for the design of a gear train with a secondary shaft. One side of the

shaft will define the intermediate reduction gear ratio and on the other side there

will be the final pinion directly connected to the final crown gear through a chain.

3.4: GEAR TRAIN DIMENSIONING

The reduction ratio is derived as follows:

-final gear ratio is set looking to common commercial setups. Usually, the factor

which affects more negatively the squat ratio, given the position of the pinion, is the

diameter of the crown gear. However, to maintain the drivetrain compact, also the

gear connected to intermediate shaft (rotating at same speed of the pinion gear)

should be enough compact. Four possible final gear ratios are selected: 46/15, 42/17,

42/14 and 40/16. Remember that these values are not definitive, they are useful just

to understand final dimensions;

-a final speed equal to 190 km/h is requested, during the race, in order to be

competitive with Class 1 Motorcycles. Given dimensions of rear wheel and maximum

rotational speed of the EM:

𝑟𝑤ℎ𝑒𝑒𝑙 = 0.298 𝑚

𝜔𝑀𝐴𝑋 = 8000 𝑟𝑝𝑚

Final gear ratio can be calculated as follows:

𝜏𝑇𝑂𝑇 = 𝜏1 ∗ 𝜏𝐹𝐼𝑁𝐴𝐿 =3.6

60∗

2𝜋𝜔𝑀𝐴𝑋 ∗ 𝑟𝑤ℎ𝑒𝑒𝑙

𝑣𝑀𝐴𝑋= 4.73

Is immediately clear how it is necessary a further stage to decrease final crown gear

dimensions. This stage will have a gear ratio of:

𝜏𝑠𝑡𝑎𝑔𝑒 =𝜏𝑇𝑂𝑇

𝜏𝐹𝐼𝑁𝐴𝐿

-talking about rotation verse of the motor, to obtain a synchronous motion of EM

respect wheel the intermediate stage requires an odd number of gears; if counter

66

rotating motion is required the number of gears will be even. Assuming a

synchronous motion, and assuming as hypothesis that we want to place the

intermediate shaft more or less along swingarm axis, is assumed that the

intermediate stage will have 5 gears, with a total of 7 gears considering final

reduction stage.

In this way:

𝜏𝑠𝑡𝑎𝑔𝑒 = 𝜏1−2 ∗ 𝜏2−3 ∗ 𝜏3−4 ∗ 𝜏4−5

It’s easy to demonstrate that the gear ratio depends exclusively by first gear and last

gear of the gear train;

-the dimensions of EM shaft pinion are assumed imposing as minimum primitive

diameter 40 mm. Assuming (for now) pitch equal to 12.7 mm, number of teeth is

equal to (value rounded to excess):

𝑧1 =2𝜋

arcsin (𝑝𝑖𝑡𝑐ℎ

𝑑𝑝𝑟𝑖𝑚_1)

= 20

Corresponding to a primitive diameter of:

𝑑𝑝𝑟𝑖𝑚 =𝑝𝑖𝑡𝑐ℎ

sin (2𝜋𝑧1

)= 41.1 𝑚𝑚

-calculation of number of teeth of the last intermediate gear is trivial. The value

obtained is rounded down in order to be sure that the maximum speed requirement

is satisfied and reached at rotational speed lower than 𝜔𝑀𝐴𝑋:

𝑧5 = 𝜏𝑠𝑡𝑎𝑔𝑒 ∗ 𝑧1

𝑑𝑝𝑟𝑖𝑚_5 =𝑝𝑖𝑡𝑐ℎ

sin (2𝜋𝑧5

)

-the remaining gears are dimensioned estimating that the distance between centre

of EM Shaft and ideal final pinion position should be around 340 mm. This must be

the minimum value of the sum of all intermediate gear diameters and radius of first

and last gear of the stage. Assuming that all intermediate gears should be equal:

67

𝑑𝑝𝑟𝑖𝑚_2 = 𝑑𝑝𝑟𝑖𝑚_3 = 𝑑𝑝𝑟𝑖𝑚_4 =𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 −

𝑑𝑝𝑟𝑖𝑚_1 + 𝑑𝑝𝑟𝑖𝑚_5

23

𝑧2 = 𝑧3 = 𝑧4 =2𝜋

arcsin (𝑝𝑖𝑡𝑐ℎ

𝑑𝑝𝑟𝑖𝑚_2)

Results of the dimensioning are showed in the following table:

Layout 𝜏𝐹𝐼𝑁𝐴𝐿 𝜏𝑠𝑡𝑎𝑔𝑒 𝑧5 𝑑𝑝𝑟𝑖𝑚_5 𝑧2

1 46/15 1.5425 30 61.1 48

2 42/17 1.9146 38 77.2 47

3 42/14 1.5767 31 63.1 48

4 40/16 1.8921 37 75.1 47

What jumps endured to the eye is the difference between primitive diameters of last

gear. It will condition space in the swingarm area and it’s convenient to maintain it

the smallest possible. In contemporary is important to maintain final crown gear the

smallest possible to control squat ratio as discussed before. Analysing differences

between Layout 1 and Layout 3 shows that intermediate gears differ for just one

tooth, meaning that weight difference is almost negligible: it’s clear that Layout 3

should be preferable.

The Gear Train case can be done in Aluminium alloy. Lightness of the component

should be the primary requirement, so alloys with good performances on tensile

strength (obtained by precipitation hardening) are preferable. Another welcome

characteristic is toughness, in order to obtain the component by mechanical process.

So the author suggest an analysis of 6000 series alloys to identify the best [RB73].

68

3.5: TRANSMISSION CHAIN DIMENSIONING

Chain will be chosen from the Regina Chain catalogue, long-time partner of the team

since previous incarnations. In particular it will be examined section about Regina

Chain Extra, dedicated to racing motorcycles:

69

In past editions, 2wheelsPolito chose for their Petrol motorcycles the Regina Chain

415, mounted even in more powerful motorcycle. We want to verify if this same chain

is good for the Electric Motorcycle The approach is the following:

-the final pinion is the same chosen in previous pages, with 14 teeth, which

correspond to a primitive diameter equal to:

𝑑𝑝𝑟𝑖𝑚 =𝑝𝑖𝑡𝑐ℎ

sin (2𝜋𝑧6

)= 29.3 𝑚𝑚

-running conditions of the chain require that power transmitted should be corrected

before utilization [RB63]:

𝑃𝑐𝑜𝑟𝑟 =𝑃𝑀𝐴𝑋

𝜇 ∗ 𝜑 ∗ 𝜒

Coefficients 𝜇, 𝜑 and 𝜒 are related to lubrication, mechanical characteristics of chain

and power, respectively. 𝜇 = 1 assuming “perfect lubrication” due to the racing

utilization, 𝜑 = 0.8 as safety coefficient because chain analysed is different from

competitor catalogue and 𝜒 is derived by the following table:

70

Transmission ratio is lower than 2, while 𝑌 = 1.5 < 2 because it will be connected to

an EM [RB63], from here:

𝜒 = 0.92

𝑃𝑐𝑜𝑟𝑟 =𝑃𝑀𝐴𝑋

𝜇 ∗ 𝜑 ∗ 𝜒= 57065 𝑊

-transmission chain must withstand a force equal to:

𝐹𝑐ℎ𝑎𝑖𝑛 = 𝐹𝑝𝑜𝑤𝑒𝑟 + 𝐹𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑎𝑙

Where:

𝐹𝑝𝑜𝑤𝑒𝑟 =𝑃𝑐𝑜𝑟𝑟

𝑣𝑐𝑖𝑟𝑐𝑢𝑛𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙

𝐹𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑎𝑙 = 𝑞 ∗ 𝑣𝑐𝑖𝑟𝑐𝑢𝑛𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙2

With 𝑞 equal to the weight of the chain per unitary meter, and:

𝑣𝑐𝑖𝑟𝑐𝑢𝑛𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙 =𝑑𝑝𝑟𝑖𝑚 ∗ 𝜔𝑀𝐴𝑋 [𝑟𝑝𝑚]

19100

At the end:

𝐹𝑐ℎ𝑎𝑖𝑛 = 4745 𝑁

The model analysed, with an average tensile strength of 21000 𝑁, should be

sufficient to sustain chain shocks and efforts.

71

3.6: DESIGN HYPOTHESIS AND CHASSIS

CONSIDERATIONS

Last step is the placement of core components to the main chassis. In order to fulfil

requirements:

-EM is place in a low and forward position, and slightly inclined to decrease inverter

wires length but maintaining a good cooling efficiency;

-Inverter can be placed in the BP Case. This choice is done intending to maintain a

forward centre of gravity position and requires an adequate EMC shield between the

two components;

-BP Case will be inclined respect the vertical and will have an irregular shape. The

reason is that author intends to assure to the rider a comfortable position and to gain

space useful to place wirings, BTMS boards and other stuff, at expense of a lower

centre of gravity: the case previously dedicated to airbox, under that BP will find

place, will be the same of last editions.

72

73

The team should work on increase chassis length in order to gain BP space, not

sufficient to assure the Energy Requirement. Increase chassis width is detrimental for

torsional stiffness, which should be the highest possible to have good driving feeling,

as demonstrated by different testers trying chassis with different torsional stiffness

values [RB56]. Increasing length, lateral stiffness of the chassis will decrease, this

means that a list of tests and requirements must be satisfied in order to have a stable,

safe and with a good lateral stiffness value chassis. As tradition in 2wheelsPolito

team, it will be done by bonded components made in Aluminium Foam surrounded

by very thin Aluminium sheets.

-Twist Axis must be recalculated, which is defined in the following way: chassis is

constrained in steering hub, and a plane is defined drawing three points around rear

wheel hub. A force is applied perpendicularly on wheel hub, making this plane

rotating. The Twist Axis is the intersection between the plane in the new position and

the plane in the original. The axis will never be perpendicular to x axis and, together

with their intersection, will define the stiffness matrix [RB58];

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-the new chassis will be tested through simulation [RB59]. It must perform during

Panic Brake Stop, Impulsive Wheeling Force (Safety Factor sufficient equal to one

because the rider tends to make a soft impact and is unrealistic that the wheel is

locked while in mid-air), Impulsive Stoppie Force (Safety Factor = 1) and exceptional

forces just as step impact. This last calculation is done considering this kind of loads

as static or quasi-static loads, then multiply the results for dynamic factors:

𝐹𝑑𝑦𝑛𝑎𝑚𝑖𝑐 = 𝐹𝑠𝑡𝑎𝑡𝑖𝑐 ∗ 𝐷𝐹

𝐹𝑠𝑡𝑎𝑡𝑖𝑐 is calculated assuming, as simplification, that resultant reaction force will pass

through centre of wheel:

𝑋 = 𝑁𝐴 ∗ 𝑡𝑔𝜃

With:

𝜃 = 𝑎𝑟𝑐𝑠𝑖𝑛 (1 −𝐻

𝑅)

𝐷𝐹 will be derived confronting the quasi-static calculation with real dynamic

equivalent case and:

2.3 < 𝐷𝐹 < 3

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Last notes are related to swingarm. It has a low truss geometry, solution usually

preferred in Racing because respect to the high truss, it is possible to lower the shear

axis, about which is applied the torsion moment to the swingarm. The movements in

a point closer to the ground, resulting in a lower movement of the contact point tire-

ground. All this is translated in terms of rider feeling. Moreover, the system results

very compact and near to the centre of gravity [RB57].

Respect to chassis, where is desired a certain grade of lateral flexibility, swingarm

should be the stiffest possible. Flexibility is related to difficult and slow manoeuvring

and is dominant respect to eventual swingarm asymmetry and neutral axis [RB56].

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4TH CHAPTER:

“MAKE OR BUY” STRATEGY AND

PRODUCT ARCHITECTURE

4.1: PRODUCT ARCHITECTURE

This is a very important reflexion useful both for MS1 settlement and for effective

success on design final result. Differently respect to a manufacturer, inside the

University don’t exist any machinery or tool to build any component: the Team is able

to design a huge number of components, perform FEM and CFD analyses, and

assembly the motorbike and small subparts related mainly to GLVS (like for example

the Dashboard).

In this specific case, with “make” is intended a component that will be designed by

the team (or by an external supplier following team’s specifications) before getting

manufactured by a supplier (if design is done by an external supplier, manufacturing

can be done by a different company), with few exceptions; while with “buy” is

intended a component already in commerce and bought “as it is”, considering

obviously their relations with “made” components. It’s worth to underline that we

are speaking of “Component” Make or Buy, while “Strategic” (investment on facilities

and manufacturing tools) and “Tactical” (related to sudden changes on demand) are

less interesting for this specific competition [RB54].

While for certain components the choice can be trivial due to previous experiences

and MotoStudent Regulations [RB49], is still important to define a strategy in order

to obtain low purchasing and design cost (intended as money and time) remembering

that is extremely important to create a performing motorcycle in track but, after last

Regulations changes and the growing importance of MS1, also simple and cheap to

build and (possibly) simple to setup and repair intended as investment and as

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dexterity.

First of all, is important to underline that the Team will be considered as a Company

that wants to compete in a fictitious championship that will involve the following 6

races:

Inside Regulations isn’t specified how many motorcycles of the same Company can

compete in the Championship and if it’s possible to create Satellite Teams. However,

the possibility to create more than one prototype will be taken into account. Part

related to logistics organization related to transport of the Team and their

motorcycles is out of the scope of this thesis.

At this point settlement of the strategy can start. For most companies, business

strategy starts by having a top-level definition of what the business is aiming for. This

usually takes the form of a Company Mission Statement, possibly followed by some

specific goals [RB54]. Goal statement is a job for the top management team and will

define the most crucial components on which work will be focused.

At this point is possible to set up the list of all components. Keeping in mind Company

Mission Statement it will be possible to define the Make or Buy Strategy. A common

tool useful to breaking down the product architecture is the utilization of tree

diagrams as the one showed below, built considering main design areas and then

doing the assessment of all components related to each area (as example, only the

branch related to Fairing – Front is deepened):

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4.2: STRATEGY SETTLEMENT

A rule of thumb for outsourcing is described here. It prescribes that a firm outsource

all items that do not fit one of the following three categories:

1) the item is critical to the success of the product, including customer perception of

important product attributes;

2) the item requires specialized design and manufacturing skills or equipment, and

the number of capable and reliable suppliers is extremely limited;

3) the item fits well within the firm's core competencies, or within those the firm

must develop to fulfil future plans.

Items that fit under at least one of these three categories are considered strategic in

nature [RB61].

At this point is easy to see the extreme importance of Mission Statement. It gives an

answer to questions related to distinction between critical items and the others.

There are a lot of items belonging both to Category 1) and 2): examples are Chassis,

Swingarm, EM Supports and BP. At this stage it isn’t known who will design and/or

make the component. However, it’s advisable to choose suppliers with Competence

Trust (when possible), which means that each partner can actually do what they say

they can do, i.e. that they can provide the service to the specified level of capability.

Usually this is a matter of visiting the site, investigating manufacturing capability,

running trials and finally monitoring performance [RB54]. Geographic location of

Team and contacts from previous experiences, even if not the “best in class” respect

other teams (mainly due to location), surely helps on verify the Competence Trust.

In Regulations are reported the items which the Organization will give to the team,

and obviously all these items will be bought “as it is”. Doubts about “who” will design

and/or make the other components can be solved using a flow chart which takes the

name of Decision Tree. Utilization of this tool assumes that team already did the

strategic decisions, but often it helps more facing up the strategic decision through

the route [RB54]. For this project the author used the following Decision Tree:

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The main problem of the Team is the impossibility to use directly machinery tools in

order to produce in house, remembering always the educative purposes of the

project. For this reason, if the item isn’t strategic at all, it should directly be bought

“as it is”, except if it is of engineering interest and so it can be worth to be designed.

For example, design Footrests Supports could be a good opportunity to learn the

utilization of a CAD and a FEM program and it could be built by the same supplier

which machines a way more strategic item just as the Steering Plates. What concern

screws and bolt instead will be bought without doubts. An item where know-how is

stated, with good engineering interest and easy to assembly can be made in house.

An example can be the GLVS Wirings.

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If under analysis there is a strategic component, team members must understand if

their know-how is sufficient to design. Since this is the first experience with an Electric

Racing Motorcycle, it is always better to hear what a specialist has to say about

techniques and “tricks” in order to increase, at maximum, the necessity to be

competitive. For this reason, what concern BP (discussed in previous chapter) was

useful to understand where the team has to act to fit the system in the chassis and

which are the components necessary to make it work (with a rough estimation of Cell

Number), but it’s worth to outsource the effective design of the system (or, in this

case, allow a team member to collaborate and participate actively in this phase with

supplier). In future competitions, after that some experience is matured and network

of contacts is created and validated, the Team can decide to work with more

autonomy, buying directly cells, dimensioning HVS wirings, managing an aftermarket

BTMS (or, why not, design it directly), and so on.

Where instead the know-how is verified, if the assembly of the item can be done in

house (for example, the Dashboard) the Team should make it, otherwise the Team

should focus with attention on design and choose then a competent supplier (an easy

example is the Chassis).

4.3: BOM

Component Subsystem N. Strategy

Brake Lever Protection Accessories 1 buy

Chain Guard Accessories 1 make

Footrest Pads Accessories 2 buy

Footrest Supports Accessories 2 make

Footrests Accessories 2 buy

Handlebar supports Accessories 2 buy

Handlebars Accessories 2 buy

Knobs Accessories 2 buy

Left Protection Cap Accessories 1 make

Potentiometer for Shock Absorber Support Accessories 1 buy

Rear Brake Assembly Accessories 1 buy

Rear Brake Lever Accessories 1 buy

Right Protection Cap Accessories 1 make

Swingarm Protection Caps Accessories 2 make

Air Filter Battery 1 buy

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Battery Case Battery 1 make

Battery Cells Battery TBA buy

Battery Cells Clamps Battery TBA make

BMS Boards Battery TBA buy

BMS Cell Sensors Battery TBA buy

BMS Control Unit Battery 1 buy

Charging Connector (IP65) Battery 1 buy

Cooling Fans Battery TBA buy

Current Fuses Battery TBA buy

Escape Valve Battery TBA buy

Inverter EMC Shield Battery 1 make

Temperature Sensors Battery TBA buy

Vortex Generator Battery 1 make

Water Sensors (Waterswitch) Battery TBA buy

Brake Levers Bearings Brakes 2 buy

Front Brake Caliper Brakes 1 buy

Front Brake Disc Brakes 1 buy

Front Brake Oil Tank Brakes 1 buy

Front Brake Oil Tank Support Brakes 1 make

Front Brake Pump Brakes 1 buy

Front Brake Tube Brakes 1 buy

Rear Brake Caliper Brakes 1 buy

Rear Brake Caliper Support Brakes 1 make

Rear Brake Disc Brakes 1 buy

Rear Brake Oil Tank Brakes 1 buy

Rear Brake Pump Brakes 1 buy

Rear Brake Tube Brakes 1 buy

Chain Tensioner Chassis 1 make

Chassis Chassis 1 make

Duct Support Chassis 1 make

Motor Spacers Chassis 2 make

Rear Frame Chassis 1 make

Rear Wheel Hub Chassis 1 make

Rear Wheel Nuts Chassis 2 make

Suspension Hub Chassis 1 make

Swingarm Chassis 1 make

Swingarm Bearings Chassis 4 buy

Swingarm Bearings Spacers Chassis 2 make

Swingarm External Spacers Chassis 2 make

Swingarm Hub Chassis 1 make

Swingarm Internal Spacers Chassis 2 make

Wire Fixing Points Chassis TBA make

Airduct Fairing 1 buy

Dashboard Support Fairing 1 buy

Fairing Plastic Washer Fairing 12 buy

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Fairing Screws Fairing 12 buy

Fairing Support Fairing 2 make

Front Fairing Fairing 1 buy

Front Fender Support Fairing 2 make

Front Mudguard Fairing 1 buy

Left Fairing Fairing 1 buy

Low Fairing Fairing 1 buy

Rear Mudguard Fairing 1 buy

Right Fairing Fairing 1 buy

Saddle Fairing 1 buy

Seat Unit Fairing 1 buy

Tail Fairing 1 buy

Windshield Fairing 1 buy

Axial Bearings Steering Group Front Suspension 2 buy

Axial Flat Washer/Fifth Wheel Steering Group Front Suspension 4 buy

Bottom Steering Plate Front Suspension 1 make

Fork Front Suspension 1 buy

Fork Plates Ferrules Front Suspension 2 make

Front Brake Caliper Spacers Front Suspension 2 make

Front Rim Front Suspension 1 buy

Front Wheel Hub Front Suspension 1 make

Radial Bearings Steering Group Front Suspension 2 buy

Steering Damper Front Suspension 1 buy

Steering Hub Front Suspension 2 make

Top Steering Plate Front Suspension 1 make

Trasponder Zipties Front Suspension 1 make

Dashboard GLVS 1 make

Front Wheel Speed Sensor GLVS 1 buy

GLV Master Switch GLVS 1 buy

GLVS Wirings GLVS TBA make

Potentiometer for Fork Suspension GLVS 1 buy

Potentiometer for Shock Absorber GLVS 1 buy

Pressure Sensor Front Brake GLVS 1 buy

Rear Light GLVS 1 buy

Rear Wheel Speed Sensor GLVS 1 buy

Secondary Battery GLVS 1 buy

Trasponder GLVS 1 buy

Battery/Inverter Wirings HVS TBA make

Electric Ignition Button HVS 1 buy

Electric Machine HVS 1 buy

Emergency Shut-Down Button HVS 1 buy

Fast Throttle Command HVS 1 buy

IMD HVS 1 buy

Inverter HVS 1 buy

Inverter/Motor Wirings HVS TBA make

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Motor Microcontroller HVS 1 buy

NO-line Contactor HVS 1 buy

Throttle Potentiometer HVS 1 buy

Tractive System Master Switch (TSMS) HVS 1 buy

Flexible Coupling Rear Suspension 1 make

Flexible Coupling Bearing Rear Suspension 1 buy

Rear Rim Rear Suspension 1 buy

Rear Suspension Triangle Rear Suspension 1 make

Rubber Tips Rear Suspension 3 buy

Shock Absorber Rear Suspension 1 buy

Shock Absorber Hub Rear Suspension 1 buy

Small Rod for Rear Suspension Rear Suspension 1 make

Small Rod Hub Rear Suspension 1 buy

Triangle Hub Rear Suspension 1 buy

Outer Casing Transmission 1 make

Crown Gear (acc) Transmission 1 buy

Crown Gear (race) Transmission 1 buy

Drive Shaft Transmission 1 buy

Drive Shaft Sprocket Transmission 1 buy

Final Chain Transmission 1 buy

Intermediate Gears Transmission 3 buy

Intermediate Gears Bearings Transmission 3 buy

Pinion EM Shaft Transmission 1 make

Pinion Gear (acc) Transmission 1 buy

Pinion Gear (race) Transmission 1 buy

Shaft radial bearing Transmission 1 buy

Shaft axial bearing Transmission 1 buy

Some clarifications about most important components are showed below:

-about the transmission, the Pinion EM Shaft will be created ad hoc due to the unique

internal broach. Material suggested is 18NiCrMo5 hardened and carburized, in order

to have high teeth hardness and resiliency. All the other components (with, as unique

exception, casing) can be bought or are already in house: for final crown gear is

recommended the utilization of 7075-T6 Aluminium Alloy due to market diffusion,

high teeth resistance and lightness;

-the fairing will be the same of previous editions, which gave good results in air

penetration and pressure at air intake. New studies about Internal Aerodynamics are

required. Design of an internal fairing can be required if presence of EM wirings

results in not acceptable aerodynamic performances;

85

-separation between HVS and GLVS is assured physically. For this reason, there will

be a secondary battery, mounted under tail;

-with TBA are intended all the components which surely will find a place in the

motorcycle, but for which isn’t already known the final number. It’s related mainly to

Battery and GLVS, the first will be outsourced and the second isn’t already defined in

this design phase;

-speed sensors nature isn’t already defined. The author suggests the utilization of Hall

Sensors similar to the one mounted on Aprilia RSV4 competing in SuperStock Italian

Championship [RB55], which don’t use a magnetic exciter mounted coaxial with

wheel hub but performs using braking discs nuts without punching and flathead. The

result will be a very compact system;

4.4: COST ANALYSIS

There are different ways to classify costs related to production process: they can be

defined:

-as nature;

-depending on reference period;

-depending on attribution modality;

-depending on variability;

-evaluating their level of controllability;

-as programming modality.

In this analysis BOM will be analysed considering costs as nature. Precisely, only

production cost will be considered (ignoring distribution, administration and eventual

financial costs), and limiting the analysis on direct manpower and materials (no

indirect manpower and depreciation will be considered). These costs cannot be

definitive but can give a rough and conservative esteem during Design Phase.

First of all, MotoStudent gives to each team a standard kit composed by:

-EM;

-IMD;

-1 set of front and rear slick tires;

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-for what concern braking system there are Front Caliper, Rear Caliper, Front Hand

Master Cylinder and Rear Foot Master Cylinder.

These components are already labelled as “buy” and are given by organization for

free. Thus, they will be ignored in this phase.

With TBA is intended that the system isn’t already defined so it’s difficult to state an

approximate cost, except for some parts (for example, HVS will contain Inverter,

which the cost is already known). In the price even parts given by sponsors or already

available in warehouse are considered:

-the Entry Fee is fixed on a base of 3140 € sufficient to the participation of 7 members.

For each further member the Team must pay 325 € [RB49];

-about Battery subsystem, it’s possible to divide the cost in 2 parts. One part is related

to cells, and it’s already derivable even if cells aren’t defined simply dividing Installed

Energy for Cost per kWh [RB70]. The other part is assumed and it’s related to all the

contour (Case, Fans, BTMS, Wirings, Testing, etc.);

-data for Brake subassembly come from older experiences in MotoStudent

competition [RB72];

-in Front (and Rear) Suspension subassembly are considered both the Fork (or Rear

Shock Absorber) [RB71] and front (or Rear) Rim;

-in Transmission subassembly the only one component for which is premature to talk

about costs is the Outer Casing. The other prices (Final Gears, Chain, Intermediate

Gears and Shaft) are fixed after some research.

Subsystem Cost [€]

Entry Fee 3140+

Accessories

Battery 5000

Brakes 450

Chassis

Fairing 600+

Front Suspension 2700+

GLVS TBA

HVS TBA

Rear Suspension 1200+

Transmission 500+

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CONCLUSIONS

For the 6th edition 49 teams confirm their signup to participate in “Electric” category,

almost the double respect previous edition. In this year the number of Rookie Teams

increased drastically and some of them have an interesting history in “Petrol”

category. There are reasons to think that these elite teams from “Petrol” category

will not belong to Class 4 but have the capability of design a Class 2 (or even Class 1)

motorcycle. 2wheelsPolito is in the same position and hard work is required to shine

in these conditions.

On 1st chapter, Benchmark analysis gave as result the necessity to maintain the

prototype simple, with high and forward centre of gravity, with a small caster angle

(the possibility to vary this value can be taken into account designing bushing inside

the inserts of the steering tube [RB58]), and a wheelbase higher than 1300 mm. High

consideration will be given to preparation of MS1 documentation and Innovative

solutions.

On 2nd chapter, the author introduced the BS, studying basics of battery, state of the

art of technology and control methodologies. A script was created to estimate energy

requirements of the pack, after that it was possible to choose some cells from

catalogues. The solution gives an LCO or NMC BP, potentially under 40 kilograms and

an installed energy around 8 kWh. Matching with BTMS, Inverter, and electric scheme

related to HVS and GLVS are still under definition and will be done in collaboration

with local suppliers.

3rd chapter is dedicated to Transmission Design and Design Hypothesis: Transmission

chosen consists in a gear train and an intermediate shaft and it is dimensioned to

reach speed higher than 190 km/h. The final stage is dimensioned assuming the final

ratio and calculating static and dynamic forces on chain. Design Hypothesis is done

having in mind the desire to maintain a comfortable driving for the rider, together

with high and forward centre of gravity. A central attention is required to chassis

design, which will require a lot of work in the future for what concern design,

dimensioning and validation.

88

Then, in the last chapter, Product Architecture is defined and Make or Buy strategy is

set, considering that “Make” capabilities of the team are limited and so it will focus

mainly on Design aspect before to develop Manufacturing in collaboration with local

suppliers. The final result is the BOM composed by 131 labels, accompanied by an

approximate cost analysis. Cost analysis requires further investigations (and the

setting of a sub-team involved in this aspect) not only to purely economic reasons,

but also for its importance on MS1 scores.

89

REFERENCES