SAE Road Show IISAE Road Show II

Mini Baja

BRAKE SYSTEMS 101

Energy Conversion ManagementEnergy Conversion Management

Presented by Paul S. GrittPresented by Paul S. Gritt

3BRAKE SYSTEMS 101Topics To Be PresentedTopics To Be Presentedu The Basic Conceptsu Hydraulic layoutsu Component functionsu Brake Balanceu Stopping Distance and Fadeu Formula SAE vs. Mini Baja u Lessons Learned u The Rulesu Questions

4BRAKE SYSTEMS 101

The Basic ConceptsThe Basic Concepts

Kinetic energy = heatF = maNewton is always right!Do the calculations firstWhen all else fails see rule 3.

5BRAKE SYSTEMS 101

Energy ConversionEnergy Conversion

The brake system converts the kinetic energy of vehicle motion into heat

6BRAKE SYSTEMS 101

Energy ConversionEnergy ConversionA vehicle weighing 290 kg. (639 lbs.)

At 90 kph (55.9 mph) has kinetic energy of:

Stopping the vehicle at .9G takes 2.9 Seconds

This is equal to 31 kilowatts (42 HP).

OR 90,770 N-M.

7BRAKE SYSTEMS 101Kinetic Energy as a Function of Speed and Mass

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

30 60 90 120Speed kph

Ene

rgy

N-M

200 kg

250 kg

290 kg

8BRAKE SYSTEMS 101

F = maF = ma

The Key Question!The Key Question!

How do you calculate F?

9BRAKE SYSTEMS 101

Basic System Model

)(2 FbfRpAmAw

RrF +

Brake Force

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BRAKE SYSTEMS 101Hydraulic SystemHydraulic System ConfigurationsConfigurationsThere are two layouts of hydraulic brake systems used in cars and light trucks.

Front/Rear hydraulic split:Also called axle by axle, vertical, and some times black and white.

Diagonal Split:Also called criss-cross.

The type of split is only significant in the event of a hydraulic system failure.

11

BRAKE SYSTEMS 101Front/rear Hydraulic SplitFront/rear Hydraulic Split

Front AxleRear Axle

Primary SystemPrimary System

Secondary SystemSecondary System

12

BRAKE SYSTEMS 101

Diagonal Split System

In a diagonal split system, one brake line is run toeach rear brake and one to each front brake.

The connections are such that the left front and the right rear brake are on one circuit and the right front and left rear are on the other circuit

13

BRAKE SYSTEMS 101

Typical Diagonal Split SystemTypical Diagonal Split System

Right frontleft rear

Left frontright rear

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BRAKE SYSTEMS 101

Brake Component FunctionBrake Component Function

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BRAKE SYSTEMS 101

Four Sub-systemsFour Sub-systemsu Actuation sub-system

u Foundation sub-system

u Parking brake sub-system

u ABS & ESP (electronic stability program) sub-system

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BRAKE SYSTEMS 101

Actuation Sub-systemActuation Sub-system

Brake Pedal

Master Cylinder

Proportioning Valves

Brake Lines

16

17

BRAKE SYSTEMS 101

The Brake PedalThe Brake Pedal

Driver Input

Output to master cylinder

100 N and 144 mm

400 N and 36 mm4:1 NominalPedal Ratio

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BRAKE SYSTEMS 101

Master CylindersMaster Cylinders

Output Pressure

Input Force

A master cylinder is just a simple piston inside a cylinder

19

BRAKE SYSTEMS 101M/C UnappliedM/C Unapplied

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BRAKE SYSTEMS 101

M/C AppliedM/C Applied

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BRAKE SYSTEMS 101

Primary System FailurePrimary System Failure

Operated Mechanically Bottoms Against

Secondary Piston

Pressure for Normal

Secondary System

Function

22

BRAKE SYSTEMS 101Secondary System FailureSecondary System Failure

Bottoms at End of Cylinder Bore

Pressure for Normal Primary System Function

23

BRAKE SYSTEMS 101

Proportioning ValvesProportioning ValvesTYPICAL PROP VALVE PERFORMANCE CURVE

0 200 400 600 800 1,0000

200

400

600

800

Front Brake Pressure

Rea

r B

rake

Pre

ssur

e

Split Point

Slope

Hard Stops

Reduce the pressure to the rear brakes

Diagonal systems require two

Split and slope are changed to create proper balance

24

BRAKE SYSTEMS 101

Adjustable Proportioning valvesAdjustable Proportioning valves

Wilwood Tilton

Only the split points are adjustable

25

BRAKE SYSTEMS 101

Brake LinesBrake Lines

Double wall steel tubing (Bundy Tubing) is industry standard.

3/16 o.d. is standard size.

Very robust, can take a lot of abuse

Use SAE 45 inverted flare (J533 and J512) joints if you can.

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BRAKE SYSTEMS 101

Foundation Brake Sub-system Foundation Brake Sub-system

Disc Brakes Linings

27

BRAKE SYSTEMS 101Front Disc BrakeFront Disc Brake

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BRAKE SYSTEMS 101

Front Disc BrakeFront Disc Brake

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BRAKE SYSTEMS 101

Brake LiningsBrake LiningsBrake linings are probably the most mis-understood part of a brake system.The output of any brake is directly related to the coefficient of friction () between the lining and the disc or drum.The challenge is knowing what the instantaneous value of is during any given stop.Any design calculations you do, go right out the window if the lining you use does not have the value you assumed.

30

BRAKE SYSTEMS 101

Brake LiningsBrake Linings

)(2 fRpAmAw

RrF

Remember the equation for a disc brake

The best method for determining the actual value of for a given lining is from a dynamometer test.

31

BRAKE SYSTEMS 101

Brake BalanceBrake Balance

32

BRAKE SYSTEMS 101Both Front Wheels Locked:Both Front Wheels Locked:

u Not good if you are on a curved road

u You cant steer

u OK, if you must hit something

u The vehicle goes straight

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BRAKE SYSTEMS 101Both Rear Wheels Locked:Both Rear Wheels Locked:

u The front wheels track straight aheadu Then the rear wheels deviate to the sideu Until the vehicle cant track straight any

longer and the rear starts to spin around the front

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BRAKE SYSTEMS 101 vs. % Wheel Slip

0 10 20 30 40 50 60 70 80 90 1000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

% Wheel Slip

Mu

(Dec

ele r

atio

n)

Typical Dry Surface

Steering

BrakingBraking

35

BRAKE SYSTEMS 101

Front LockFront LockIf there is more front brake torque than dynamic front weight

The front wheels will lock up before the rears

20%80% 40% 60%

Brake torquedistribution

Dynamic weight distribution

36

BRAKE SYSTEMS 101Rear LockRear Lock

If there is more rear brake torque than dynamic rear weight;

The rear wheels will lock up before the fronts

40% 60% 20% 80%

Brake torquedistribution

Dynamic weight distribution

37

BRAKE SYSTEMS 101

Optimum BrakingOptimum Braking

Optimum braking is achieved when brake torque distribution matches dynamic weight distribution

Weight Distribution

No Braking Hard Braking

40% 60% 20% 80%

38

BRAKE SYSTEMS 101Calculating Dynamic Weight TransferCalculating Dynamic Weight Transfer

fsWbHtW

df WDWcgt +=

39

BRAKE SYSTEMS 101Ideal Vs Actual TorqueIdeal Vs Actual Torque

0.0

0.1

0.2

0.3

0.4

0.5

0.60.7

0.8 0.9 1.0 1.1

Front Torque

Rea

r To r

que

Ideal60/40 Actual70/30 Actual

With a prop valve

40

BRAKE SYSTEMS 101

Stopping DistanceStopping Distance

Does not Depend on:

Type of brakes

Size of brakes

Does Depend on:

Tire to road friction

Vehicle balance

Skill of driver

System Reaction Time

41

BRAKE SYSTEMS 101 vs. % Wheel Slip

0 10 20 30 40 50 60 70 80 90 1000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

% Wheel Slip

Mu

(Dec

ele r

atio

n)

Typical Dry Surface

BrakingBraking

42

BRAKE SYSTEMS 101

Brake FadeBrake FadeBrake fade is the loss of performance resulting from the lining friction decreasing as the lining

and rotor or drum rises in temperature

43

BRAKE SYSTEMS 101

Formula SAE vs.

Mini Baja

The brake system design must match the vehicle objectives

44

BRAKE SYSTEMS 101

Formula SAEu Absolute reliabilityu High Speedsu Maximum possible decel without lockingu Consistent balance with changing temperatures

Mini Bajau Absolute reliabilityu Low Speedsu Very hostile environmentu Brake must work when wet and muddy

45

BRAKE SYSTEMS 101Mini Baja, Hostile EnvironmentMini Baja, Hostile Environment

Will your master cylinder fill up with water?How does you brake lining work when it is wet?

46

BRAKE SYSTEMS 101

Lessons Learned Lessons Learned

47

BRAKE SYSTEMS 101

Dont Do Something Because:Dont Do Something Because:

The other guys do it. All the race cars have it. It looks really cool

48

BRAKE SYSTEMS 101

Design to a Specific Objective:Design to a Specific Objective:

Do the math first Verify your assumptions Take the easy solution (KISS)

49

BRAKE SYSTEMS 101Lessons LearnedLessons Learned

Lesson Number 1 - Keep it SIMPLE

50

BRAKE SYSTEMS 101Lessons LearnedLessons Learned

Lesson Number 2 - Keep it light; but not too light.

90% of the braking energy goes into the rotor If the rotor is too light it gets very hot If the temperatures get too high very nasty things can happen.

51

BRAKE SYSTEMS 101

Too lightToo light

2006 DCAE program involved a Hemi powered Grand Cherokee.

One team used much smaller rear brakes and rotors to save a few pounds on a 3200 lb vehicle with 500

hp.

The vehicle went off the end of the main straight at over 100 mph on the third lap because of brake failure.

52

BRAKE SYSTEMS 101

53

BRAKE SYSTEMS 101

54

BRAKE SYSTEMS 101Lessons LearnedLessons Learned

Lesson Number 3 - Packaging and Integration drive 90% of design

Lesson Number 4 - Read the rulesLesson Number 5 - Allow enough time for details and testing

55

BRAKE SYSTEMS 101Brake rulesBrake rules

-The car must have four wheel brakes operated by a single control. -It must have two independent hydraulic circuits with independent fluid reserves.-The brake system must be capable of locking all four (4)

wheels, and stopping the vehicle in a straight line

The braking systems must be protected with scatter shields from failure of the drive train (FSAE only)-A brake pedal over-travel switch must be installed. This switch must kill the ignition and cut the power to any electrical fuel pumps. (FSAE only)-The car must be equipped with a red brake light that illuminates when ever the brakes are applied

56

BRAKE SYSTEMS 101

Any QuestionsAny Questions

SAE Road Show IISlide 2Topics To Be PresentedThe Basic ConceptsEnergy ConversionSlide 6Slide 7Slide 8Slide 9Hydraulic System ConfigurationsFront/rear Hydraulic SplitSlide 12Typical Diagonal Split SystemSlide 14Four Sub-systemsActuation Sub-systemThe Brake PedalMaster CylindersM/C UnappliedM/C AppliedPrimary System FailureSecondary System FailureProportioning ValvesAdjustable Proportioning valvesSlide 25Foundation Brake Sub-system Front Disc BrakeSlide 28Brake LiningsSlide 30Brake BalanceBoth Front Wheels Locked:Both Rear Wheels Locked:Slide 34Front LockRear LockOptimum BrakingCalculating Dynamic Weight TransferIdeal Vs Actual TorqueStopping DistanceSlide 41Brake FadeFormula SAE vs. Mini BajaSlide 44Slide 45Lessons Learned Dont Do Something Because:Design to a Specific Objective:Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Slide 55Any Questions