Test rig for Shock absorber

8/11/2019 Test rig for Shock absorber

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DESIGN OF SHOCK ABSORBER TEST RIG FOR

UNSW@ADFA FORMULA SAE CAR

By

SBLT Carl Heritier1

May 2008

Supervisor: Mr Alan Fien

1Mechanical Engineering: Project, Thesis and Practical Experience ZACM 4051

Initial Thesis Report 2008, UNSW@ADFA

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Abstract: This paper describes research into the design of a shock absorber test rig suitable for use by the School of

Aeronautical, Civil & Mechanical Engineering (ACME) Racing team. The shock absorbers currently being used on

the ACME Racing vehicles are adapted from downhill mountain bikes are combined in a double wishbone

suspension. As there is currently no infrastructure available to test the shock absorbers performance, this project

aims to build a test rig which will allow the performance characteristics of the various damper settings to beexplored in order to further optimize the performance of the vehicles. Furthermore, a simple impulse test rig will be

developed to verify that the hysteresis loop produced by the sinusoidal testing on the Instron correlates with the

impulse rig results.


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Disclaimer: This thesis report has been written in partial fulfillment for the requirements for the degree for

Aeronautical Engineering. It is the result of a period of research and analysis by the author while a student of the

University of New South Wales. Views expressed do not represent the views of the University College, the

University of the Australian Defence Force Academy.


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TableofContents

DESIGN OF SHOCK ABSORBER TEST RIG FOR UNSW@ADFA FORMULA SAE CAR..................................1

I.

Introduction...........................................................................................................................................................5

II. Background Information.......................................................................................................................................5

A. F-SAE Competition..........................................................................................................................................5

B. Instron Material Testing Machine ....................................................................................................................5

C. ACME Shock Absorbers ..................................................................................................................................6

D. Literature Review.............................................................................................................................................6

III. Design Task....................................................................................................................................................10

A. Outline............................................................................................................................................................10

B. Aims ...............................................................................................................................................................11

C. Development ..................................................................................................................................................11

IV. Future Work ...................................................................................................................................................13

A. Design Considerations and Construction........................................................................................................13

V. Summary.............................................................................................................................................................14

References ...................................................................................................................................................................14

Annex A Client Brief................................................................................................................................................16

Annex B Milestone Chart .........................................................................................................................................17

Annex C Gantt Chart ................................................................................................................................................18

Annex D Risk Assessment........................................................................................................................................19


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I. Introduction

Shock absorbers are one of the most important elements in a vehicles suspension system, and they play an

important role in determining vehicle handling and ride characteristics. Depending on the application of the vehicle,

such as if it is to be used as a race car, an off-road landrover or a luxury vehicle, the priorities will dictate the

different damping characteristics. Therefore, it is necessary to properly understand the effect different damping

settings will have on a vehicle.

In racing vehicles, the effect of different damping settings is of even greater importance due to the competitive

nature of the race. Any enhancement in vehicle cornering and handling will give the competitive edge over opposing

vehicles. Therefore, the ACME Racing team requires a shock absorber test rig which will allow the examination ofcurrent shock absorbers with the intent on improving the cars performance.

II. Background Information

A. F-SAE Competition

Each year ADFA ACME Racing takes part in an automotive design and build competition hosted by the Society

of Automotive Engineers (SAE). This competition, titled Formula SAE (F-SAE), involves universities from aroundthe world and aims to encourage engineering learning in a practical environment. Over a year teams are required to

conceive, design, fabricate and compete with small formula-style racing cars [1].By setting few limitations, theorganizers encourage innovation by the teams with each seeking the competitive edge.

The project culminates in an annual meet in December where vehicle designs are evaluated by a panel of judges

and compete against each other in a range of events. The static events allow evaluation of the merits of the vehicle

designs and a range of dynamic events reveal the speed, acceleration, fuel economy, performance and

manoeuvrability of the race cars.

B. Instron Material Testing Machine

ACME currently has no machines suitable for use as a shock absorber test rig. However, it has several Instronmaterial testing machines available for projects, such as the one pictured in figure 1. These machines use servo-

hydraulic systems for high capacity fatigue and static testing of materials and manufactured components. These

systems allow great control with variable parameters of frequency, applied force and amplitude of actuator

movement. These variable parameters and customization abilities have raised the possibility of using the Instron foruse as a shock absorber test rig.

The Instron machine is able to produce very large loads at a controllable frequency and is also able to apply

these large loads with large actuator movement at low frequencies. However, this project requires a load to beapplied to a shock absorber at both frequencies and amplitudes, which is beyond the capabilities of the Instron.

Therefore, this project aims to adapt this machine to meet the requirements of a shock absorber test rig.


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Figure 1. Instron material testing machines

C. ACME Shock Absorbers

The shock absorbers being used on current ACME Racing vehicles, the Fox DHX 5.0, are adapted from

downhill mountain bikes. They are used due to their lightweight and compact design as well as being cheap. Asindicated in figure 2 [2],this style of shock absorber is highly adjustable and therefore versatile. However, due to

their not being used in their intended role, there is little performance information provided for these adjustments.

Figure 2. Breakdown of Fox DHX 5.0 Shock Absorber (excl. spring)

D. Literature Review

In its nature, design is highly individualistic due to dictated limitations and requirements set by the problem. Thedesign of a test rig to test mountain bike shock absorbers is no different, and as ACME doesnt have a large budget

for such a project, unlike other researchers and SAE teams, attempts must be made to adapt current equipment to

suit our objectives.

A search of literature in the databases of the Institution of Mechanical Engineers (IMechE), SAE and the

International Journal of Vehicle Design resulted in many journals related to shock absorber design and adaptation

yet few journals on how the resultant shock absorber was tested. Dixons Shock Absorber Handbook [3]details acomprehensive literature review of shock absorber journals from 1929-2006 and also states that there is very little

useful information on dampers.


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The approaches of the useful journals have been detailed below and it seems that these researchers have also

adapted existing machines for the purpose of testing the shock absorbers, despite the machine capabilities limiting

the testing range of velocities and frequencies.

Although mechanical methods were used to test dampers for many decades such as the crank/scotch-yokedesign, structural testing rigs capable of waveform variations were not developed until the 1960s, using servo-

hydraulic technology [4].This technology is to be used indirectly in this project, through the Instron, due to the

controllability of variables it allows. While servo-hydraulic machines are more flexible they are also more expensiveand complex. For this reason we are going to test the shock absorbers using both a servo-hydraulic machine and a

simple mechanical rig and then compare results.

D J Purdy from the Royal Military College of Science in the United Kingdom investigated adjustable automotive

dampers in 1999 [5], resulting in theoretical and experimental results. Using the test rig in figure 3 Purdy adjustedthe valves on the provided shock absorber and was able to produce some force-velocity curves for various

adjustments. The work of Surace found that under continuous operation, the effects of temperature can cause the

characteristics of the damper to vary. Therefore Purdy used a sinusoidal input rather than random testing to enablethe results to be closely analyzed so that the influence of temperature could be discovered and disregarded.

Purdys approach is similar to the approach to be taken in this project, which is to use a hydraulic actuator to

accurately test the ACME Racing teams shock absorbers. However, as we do not have this machine or any detailed

descriptions on his method, this journal is only useful in understanding another researchers approach to thisproblem.

Figure 3. Test Rig used by Purdy

In 2002 W H Li submitted a journal investigating the use of a Magnetorheological (MR) steering damper for

motorcycles [6]. Although the damper is different from the one to be tested, Li also used an Instron machine to testhis steering damper, as shown in figure 4. Although he used a bracket to overcome the grip limitations of the

Instron, Li only tested the damper at a stroke velocity of 6mm/s, much lower than the 100mm/s the damper was

designed to take. The approach for my thesis project will allow the testing of the Fox DHX 5.0 damper at the typicalfrequencies experienced whilst fitted on the ACME Racing vehicles.


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Figure 4. Test Rig (incl. Instron) used by Li

In his journal article of 1963 [7], B O Dowsett discussed the development of a dimpled sheet shock absorbing

device for use in a nuclear reactor. Although Dowsett was investigating the use of a shock absorbing device toprevent damage to the reactor caused by accidently dropped items, the approach is similar to that which will be used

in the impulse tester. That is, a weight is attached to a guide wire or tube and dropped from a height on to a

plate/rubber which will transfer the force to the shock absorber as an impulse, thus allowing us to measure its

reaction.

A common approach to shock absorber testing is to use a quarter car model. These are explained thoroughly in

Weispfennings journal of 1997 [8] which discusses fault detection and diagnosis of vehicle vertical dynamics.Weispfenning highlights that worn dampers cause longer breaking distances, lead to faster wear of the tires and

deteriorate the handling of the car, especially during cornering. A quarter car model is a simplified model useful for

investigations into suspensions, which allows modeling and development of concepts such as semi-active

suspension with varying parameters. This allows researchers to obtain a force-velocity curve for the cars

suspension. For my project we are focusing only on the damper, and therefore, although this approach is interesting

and useful, it does not suit our purposes due to the high costs involved in making a quarter car test rig as discussedin Weisfennings journal and shownfigure 5.

Figure 5. Quarter Car Test Rig


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The difficulty involved in making a shock absorber test rig may be the reason for a lack of relevant literature.

Another approach SAE teams have is to send their shock absorbers to professional shock absorber testers who will

give a detailed analysis in a short time. Penske [9] and Koni [10]both offer these services at a cost. However, this

project aims to have a simple attachment to our current machines which will minimize this ongoing cost for shock

absorber testing. It will also allow other analysis, such as the effect of wear on the SAE cars handling. As Penskeoffers a personalized dynamometer sheet showing the shocks characteristics throughout the adjustment range,

ACMEs objective is to eventually be able to replicate this service.

The typical force-position curves for a sine wave test are shown in figure 6. This was produced by complicated

software from the company Dynamic Suspensions [11]. This is the shape of curves we are expecting from our test

rig. Infigure 7is an example of a force-displacement curve for different activities [3].

Figure 6. Typical force-position curves for a sine wave input

Figure 7. Force-Displacement graph for different activities


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KONI is a worldwide engineering company which provides shock absorbers for passenger cars, racings cars,

bus, truck and railway applications. Due to the wide range vehicles, KONI uses the shock absorber test rig infigure8which allows testing of all their types of shock absorbers. Figure 9 shows an example force-displacement graph

for a Koni shock absorber and what you would expect from different adjustments i.e. heavier and lower

adjustments in the shock absorber.

Figure 8. KONI Test Rig Figure 9. Force-Displacement for adjustments

III. Design Task

A. Outline

Racing car suspension aims to keep the tires in contact with the road surface and allow the generation of thegreatest lateral load from corner entry to corner exit, thus allowing greater speed and control through corners.

Professional racing teams use the latest technology and products to enhance their suspension characteristics in order

to gain an advantage. In the FSAE competition most of the racing teams use mountain bike shock absorbers due to

their light and compact design as well as being cheap. One of the most common styles, the Fox DHX 5.0 pictured infigure 10 [12], is utilized by the ACME Racing team. The drawback of these dampers is that they are not intendedfor FSAE racing applications, and hence it is desired for them to be adjusted to enhance the performance of the

vehicle.

The availability of a shock absorber test rig will enable the variable controls of the shock absorber to be properlyinvestigated, hopefully enabling greater performance in the SAE car. The rig will also allow the constant testing of

the dampers and to monitor the wear effects. Worn dampers cause longer braking distances, lead to faster wear of

tires and deteriorate the handling of the racing car, especially during cornering. Environmental effects such as the

heat caused during damper operation will also be able to be monitored.


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Figure 10. Fox DHX 5.0 Shock Absorber

B. Aims

This project focuses primarily on designing a suitable testing rig for the shock absorbers currently utilized by the

ACME Racing team. By use of collected data and detailed analysis, the aim is to design and manufacture a rig

suitable for testing the currently employed Fox DHX 5.0 shock absorbers, thus enabling the performancecharacteristics to be analysed. Furthermore, an impulse test rig will be developed to investigate whether the results

obtained from this much simpler test correlate with the hysteresis results produced using the Instron.

C. Development

To ensure that a logical and well thought out process is applied to this design project, the main focus thus far has

been to thoroughly explore every foreseeable aspect of the design problem. This has involved preliminary

examination of the Instrons capabilities versus the testing rig requirements, concept design and extensivediscussions with the subject matter experts.

The Instron machine has limitations in relation to being used as a shock absorber rig. By running the Instron atvarious frequencies, a table of results and suitable plots of resulting amplitudes were able to be constructed. These

plots are shown infigure 11 and the results are in table 1.

Table 1: Instron Amplitude Range for various Frequencies


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Figure 11. Plot of Instron Amplitude vs Frequency

As predicted, by increasing the frequency at which the Instron actuator moves, the amplitude greatly decreases.As the test rig is required to test the shock absorbers at an amplitude of 2 (~50mm), the collected data from the

Instron proves that to satisfy these requirements the Instron machine must be modified.

The desired frequency for testing of the shock absorbers is 2.5 Hz. This figure for small racing cars was stated by

the racecar engineering firm OptimumG [13]and also by Dixon [3]. For passenger cars the ride frequency ranges

from 0.5Hz-1.5 Hz.

The Instron machine to be used has many controls to allow complex testing of materials. One of these,

Amplitude Control, attempts to modify the function to make sure that the output is the same as the input. This iscrucial for a testing project such as this as it means that we can accurately set the sine wave behavior of the actuator.

Experimenting with the inputs of the Instron established that at the desired frequency, 2.5Hz, the maximum actuator

movement with amplitude control was +- 2.5mm. Therefore, a device must be constructed to convert this 5mm totalamplitude to 50mm total amplitude in order to enable accurate shock absorber testing.

Modifications to the Instron could be done in many different ways. However, workshop limitations and budget

restrictions narrowed the range of concepts to be investigated for the project. The two concepts which are currently

being considered to increase the amplitude range of the Instron actuator are to use a hydraulic amplifier and to use asimple lever attached to the Instron grips.

The hydraulic amplifier would use two different sized pistons in a specially constructed container with hydraulicfluid between them. By designing the different diameter pistons we would be able to move the smaller piston a

further distance for each stroke of the large piston, thus enabling us to increase the amplitude over which we could

test our shock absorber. Although this sounds like a simple and effective method, it is rather complex and expensive

and would need to be precisely designed and constructed, taking into account fluid flow principles and friction.

Lever systems are a simple and cost effective method of gaining mechanical advantage. In the second conceptwe aim to use levers to multiply the distance over which the shock absorber is testing by placing a fixed fulcrum a

distance along the scissor lever. An example of a simple lever is infigure 12[14].


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Figure 12. Simple Lever System

A well designed and constructed lever system should be able to overcome the Instron limitations and suitablyallow the testing of shock absorbers at various frequency and forces. At present this concept is being actively

pursued due to the above stated advantages of simplicity and cost effectiveness. As 50mm of total travel is required,

the required lever would be in the ratio 10 to 1. The concept currently being developed involves a mechanism which

uses a bias spring so chosen that the load on the Instron is always a tension load, thus avoiding any non-linearity dueto backlash.

The ultimate desire of the testing rig is to provide a reliable method of testing a shock absorber and producing

force-velocity and force-displacement plots with parameters of frequency and amplitude for variations in thedamper. This will allow the SAE team to attempt to improve the cornering of the car, which is of great concern. Theexpected plots should look something similar to the plots infigure 13 [3].

Figure 13. Theoretical Force Diagrams for a Sinusoidal Input with Constant Damping Coefficient

Whilst the shock absorber test rig is being manufactured, a simple impulse test rig will be developed to test the

shock absorbers with sudden force application in compression and tension. This will be developed to produce a rigsimilar to a slide hammer, but in the vertical direction. It is similar to the Drop Test, as detailed by Dixon [3]. By

varying the masses used and the distance over which they act, it will enable creation of force vs velocity plots and

hence correlation with the Instron Shock Absorber Rigs results.

IV. Future Work

A. Design Considerations and Construction

Design of the test rig will involve:

careful selection of materials

creation of a suitable structural configuration to fix the fulcrum

structural analysis of the final rig

consideration of placement of bias spring, shock absorber and load cells

safety considerations for test rig use

testing of the rig prior to shock absorber insertion


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Upon completion of the design, this design will be manufactured in the workshop. Whilst the design is being

manufactured, the development of the impact tester will be conducted with the objective of getting a force-velocity

curve for the Fox DHX 5.0 shock absorbers.

V. Summary

The requirement for shock absorber testing on the ACME Racing car has necessitated a design project for atesting rig. A literature review has shown that although some test rigs have been designed previously, they are not

feasible designs and the Instron machine available must be adapted for our purposes.

The design of the shock absorber test rig has progressed well upon analysing the limitations of the Instron test

machine and the requirements of the shock absorber. Further detailed design will enable a manufactured structure to

be adapted to the Instron machine and therefore facilitate the desired sinusoidal testing. A further objective has been

recently added which involves the development of an impact tester to compare the hysteresis loop for both methods.This impact tester will be developed whilst the primary test rig is being manufactured. The desired objective is to get

two test rigs designed and manufactured in order to enable comparisons of sinusoidal and impact inputs.

References

1. SAE Australasia, Formula SAE-A, Available: http://www.sae-a.com.au/fsae/about_fsae.htm [13April 2008]

2. Fox Racing Shocks, Technical Centre, Owners manuals, Available:http://www.foxracingshox.com/fox_tech_center/owners_manuals/05_RearShox_en.pdf [05April 2008]

3. Dixon, J.C. 2007, The Shock Absorber Handbook, 2nd edn, Professional Engineering Publishing Ltd & JohnWiley and Sons Ltd, England.

4. Plummer, A. 2007, "Control techniques for structural testing: a review", Proceedings of the Institution ofMechanical Engineers, Part I: Journal of Systems and Control Engineering,vol. 221, no. 2, pp. 139-169.

5. Purdy, D. 2000, "Theoretical and experimental investigation into an adjustable automotive damper",Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering,vol. 214,

no. 3, pp. 265-283.

6. Li, W H. Du, H. Guo, N Q. 2003, "Design and testing of an MR steering damper for motorcycles", The

International Journal of Advanced Manufacturing Technology,vol. 22, no. 3, pp. 288-294.

7. Dowsett, B.O. 1963, "Development of the dimpled sheet shock absorber for use in a nuclear reactor",ARCHIVE: Proceedings of the Institution of Mechanical Engineers, Conference Proceedings 1964-1970 (vols

178-184), Various titles labelled Volumes A to S, vol. 178, no. 310, pp. 116-129.

8. Weispfenning, T. 1997, "Fault Detection and Diagnosis of Components of the Vehicle Vertical Dynamics",Meccanica, vol. 32, no. 5, pp. 459-472.

9. Penske Racing Shocks UK, Available: http://www.penskeshocks.co.uk/partsandservices/dynotesting.htm

10. Koni 2008, Koni: Damper Testing. Available: http://www.koni.com/222.html[13 April 2008].

11. Dynamic Suspensions, Available: http://www.dynamicsuspensions.com/popups/graph3.htm[12 April 2008].


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http://www.sae-a.com.au/fsae/about_fsae.htm%20%5B13http://www.foxracingshox.com/fox_tech_center/owners_manuals/05_RearShox_en.pdf%20%20%5B05http://www.koni.com/222.htmlhttp://www.dynamicsuspensions.com/popups/graph3.htmhttp://www.dynamicsuspensions.com/popups/graph3.htmhttp://www.koni.com/222.htmlhttp://www.foxracingshox.com/fox_tech_center/owners_manuals/05_RearShox_en.pdf%20%20%5B05http://www.sae-a.com.au/fsae/about_fsae.htm%20%5B13


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12. Fox Racing Shocks, Technical Centre, Owners manuals, Available:

http://www.foxracingshox.com/fox_tech_center/owners_manuals/07/eng/2007_om_eng.htm [05April 2008]

13. Optimum G, Tech Tip: Spring & Dampers, Episode 4. Available:

http://www.optimumg.com/documents/techtips/Springs%20&%20Dampers_Tech_Tip_4.pdf[12 April 2008] .

14. Howstuffworks, Available: http://science.howstuffworks.com/pulley1.htm [24April 2008]


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http://www.foxracingshox.com/fox_tech_center/owners_manuals/07/eng/2007_om_eng.htm%20%5B05http://www.optimumg.com/documents/techtips/Springs%20&%20Dampers_Tech_Tip_4.pdfhttp://science.howstuffworks.com/pulley1.htm%20%5B24http://science.howstuffworks.com/pulley1.htm%20%5B24http://www.optimumg.com/documents/techtips/Springs%20&%20Dampers_Tech_Tip_4.pdfhttp://www.foxracingshox.com/fox_tech_center/owners_manuals/07/eng/2007_om_eng.htm%20%5B05

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