brakes by paul s gritt

7/13/2019 Brakes by Paul s Gritt

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BRAKE SYSTEMS 101

Energy Conversion Management

Presented by Paul S. Gritt


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Topics To Be Presented

The Basic Concepts

Hydraulic layouts

Component functions

Brake Balance Stopping Distance and Fade

Formula SAE vs. Mini Baja

Lessons Learned

The Rules

Questions


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The Basic Concepts

Kinetic energy = heat

F = ma

Newton is always right!

Do the calculations first

When all else fails see rule 3.


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Energy Convers ion

The brake system converts the kineticenergy of vehicle motion into heat


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Energy Convers ion

A 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 90,770N-M.


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Kinetic 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

EnergyN-M

200 kg

250 kg

290 kg


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F = ma

The Key Question!

How do you calculate F?


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Basic System Model

Brake Force


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Hydrau l ic SystemConf igurat ions

There 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 blackand white.

Diagonal Split:Also called criss-cross.

The type of spl i t is only s igni f icant in theevent o f a hyd raul ic system fai lure.


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Fron t/rear Hyd raul ic Sp l i t

Front Axle

Rear Axle

Primary System

Secondary System


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DiagonalSplit System

In a diagonal split system, one brake line is run to

each 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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Typ ical Diagonal Sp l i t Sys tem

Right front

left rear

Left front

right rear


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Brake Component Funct ion


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Fou r Sub-systems

Actuation sub-system

Foundation sub-system

Parking brake sub-system

ABS & ESP (electronic stabilityprogram) sub-system


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Ac tuat ion Sub -sys tem

Brake Pedal

Master Cylinder

Proportioning Valves

Brake Lines

16


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The Brake Pedal

Driver Input

Output to master cylinder

100 N and 144 mm

400 N and 36 mm4:1 Nominal

Pedal Ratio


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Master Cy l inders

Output Pressure

Input Force

A master cylinder is just a simple piston

inside a cylinder


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M/C Unapp l ied


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M/C App l ied


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Primary System Fai lu re

Operated

Mechanically Bottoms Against

Secondary Piston

Pressure for

Normal

Secondary

System

Function


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Secondary Sys tem Fai lure

Bottoms at End of Cylinder Bore

Pressure for Normal Primary System Function


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Proport ion ing ValvesTYPICALPROPVALVEPERFORMANCEC

02004006008001,00

200

400

600

800

FrontBrakePressure

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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Adjustable Proport ion ing valves

Wilwood Tilton

Only the split points are adjustable


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Brake 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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Foundat ion B rake Sub-system

Disc Brakes

Linings


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Fron t Disc B rake


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Fron t Disc B rake


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Brake Lin ings

Brake linings are probably the most mis-understood partof a brake system.

The output of any brake is directly related to thecoefficient of friction () between the lining and the disc

or drum.The challenge is knowing what the instantaneous valueof 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 youassumed.


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Brake Lin ings

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 Balance


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Both Front Wheels Locked:

Not good if you are on a curved road

You cant steer

OK, if you must hit something

The vehicle goes straight


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Both Rear Wheels Locked:

The front wheels track straight ahead Then the rear wheels deviate to the side

Until the vehicle cant track straight anylonger and the rear starts to spin around the

front


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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(Deceleration)

Typical Dry Surface

Steering

Braking


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Front Lock

If there is more front brake torque than dynamic front weight

The front wheels will lock up before the rears

20% 80% 40% 60%

Brake torque

distribution

Dynamic weight

distribution


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Rear 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 torque

distribution

Dynamic weight

distribution


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Opt imum Brak ing

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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Calcu lat ing Dynam ic Weigh t Trans fer


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Ideal Vs Actual Torque

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.70.8

0.9 1.0 1.1

Front Torque

RearTorque

Ideal

60/40 Actual

70/30 Actual

With a prop valve


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Stopping Distance

Does notDepend on:

Type of brakes

Size of brakes

Does Depend on:

Tire to road friction

Vehicle balance

Skill of driver

System Reaction Time

A S S S 101


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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(Deceleration)

Typical Dry Surface

Braking

BRAKE SYSTEMS 101


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Brake Fade

Brake fade is the loss of performance resulting

from the lining friction decreasing as the lining

and rotor or drum rises in temperature

BRAKE SYSTEMS 101


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Formula SAE

vs.

Mini Baja

The brake system design mus t

match the vehic le ob ject ives

BRAKE SYSTEMS 101


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Formula SAE

Absolute reliability

High Speeds

Maximum possible decel without locking

Consistent balance with changing temperatures

Mini Baja

Absolute reliability

Low Speeds Very hostile environment

Brake must work when wet and muddy

BRAKE SYSTEMS 101


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Mini Baja, Host i le Env ironment

Will your master cylinder fill up with water?

How does you brake lining work when it is wet?

BRAKE SYSTEMS 101


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Lessons Learned

BRAKE SYSTEMS 101


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Dont Do Something Because:

The other guys do it.

All the race cars have it. It looks really cool

BRAKE SYSTEMS 101


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Design to a Spec i f ic Object ive:

Do the math first

Verify your assumptions

Take the easy solution (KISS)

BRAKE SYSTEMS 101


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Lessons Learned

Lesson Number 1 - Keep it SIMPLE

BRAKE SYSTEMS 101


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Lessons 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 thingscan happen.

BRAKE SYSTEMS 101


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Too l igh t

2006 DCAE program involved a Hemi poweredGrand Cherokee.

One team used much smaller rear brakes and rotors

to save a few pounds on a 3200 lb vehicle with 500hp.

The vehicle went off the end of the main straight atover 100 mph on the third lap because of brakefailure.

BRAKE SYSTEMS 101


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BRAKE SYSTEMS 101


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BRAKE SYSTEMS 101


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Lessons Learned

Lesson Number 3 - Packaging and Integration drive90% of design

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

details and testing

BRAKE SYSTEMS 101


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Brake rules

-The car must have four wheel brakes operated by a singlecontrol.

- It must have two independent hydraulic circuits with independent fluidreserves.

-The brake system must be capable of locking all four (4)

wheels, and stopping the vehicle in a straight lineThe braking systems must be protected with scatter shields fromfailure of the drive train (FSAE only)

-A brake pedal over-travel switch must be installed. This switch mustkill the ignitionand cut the power to any electrical fuel pumps. (FSAE

only)-The car must be equipped with a red brake light that illuminates whenever the brakes are applied

BRAKE SYSTEMS 101


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BRAKE SYSTEMS 101

Copyrigh t 2011 Paul S. Gri tt.

A l l r igh ts reserved.

brakes by paul s gritt

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