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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.
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BRAKE SYSTEMS 101Front/rear Hydraulic SplitFront/rear Hydraulic Split
Front AxleRear Axle
Primary SystemPrimary System
Secondary SystemSecondary System
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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
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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
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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
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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
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BRAKE SYSTEMS 101Secondary System FailureSecondary System Failure
Bottoms at End of Cylinder Bore
Pressure for Normal Primary System Function
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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
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BRAKE SYSTEMS 101
Adjustable Proportioning valvesAdjustable Proportioning valves
Wilwood Tilton
Only the split points are adjustable
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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
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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.
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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.
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BRAKE SYSTEMS 101
Brake BalanceBrake Balance
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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
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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
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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
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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%
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BRAKE SYSTEMS 101Calculating Dynamic Weight TransferCalculating Dynamic Weight Transfer
fsWbHtW
df WDWcgt +=
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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
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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
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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
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
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BRAKE SYSTEMS 101
Formula SAE vs.
Mini Baja
The brake system design must match the vehicle objectives
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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
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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?
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BRAKE SYSTEMS 101
Lessons Learned Lessons Learned
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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
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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)
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BRAKE SYSTEMS 101Lessons LearnedLessons Learned
Lesson Number 1 - Keep it SIMPLE
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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.
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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.
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BRAKE SYSTEMS 101
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BRAKE SYSTEMS 101
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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
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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
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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