brakes

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SAE Road Show II:

SAE Road Show II Mini Baja

PowerPoint Presentation:

BRAKE SYSTEMS 101 Energy Conversion Management Presented by Paul S. Gritt

Topics To Be Presented:

3 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

The Basic Concepts:

4 The Basic Concepts Kinetic energy = heat F = ma Newton is always right! Do the calculations first When all else fails see rule 3.

Energy Conversion:

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

Energy Conversion:

6 Energy Conversion 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 ,770 N-M.

PowerPoint Presentation:

7

PowerPoint Presentation:

8 F = ma The Key Question! How do you calculate F?

PowerPoint Presentation:

9 Basic System Model Brake Force

Hydraulic System Configurations:

10 Hydraulic System Configurations 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 “black and white”. Diagonal Split: Also called criss-cross. The type of split is only significant in the event of a hydraulic system failure.

Front/rear Hydraulic Split:

11 Front/rear Hydraulic Split Front Axle Rear Axle Primary System Secondary System

PowerPoint Presentation:

12 Diagonal Split 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

Typical Diagonal Split System:

13 Typical Diagonal Split System

PowerPoint Presentation:

14 Brake Component Function

Four Sub-systems:

15 Four Sub-systems Actuation sub-system Foundation sub-system Parking brake sub-system ABS & ESP ( electronic stability program) sub-system

Actuation Sub-system:

16 Actuation Sub-system Brake Pedal Master Cylinder Proportioning Valves Brake Lines 16

The Brake Pedal:

17 The Brake Pedal Driver Input Output to master cylinder 100 N and 144 mm 400 N and 36 mm 4:1 Nominal Pedal Ratio

Master Cylinders:

18 Master Cylinders Output Pressure Input Force A master cylinder is just a simple piston inside a cylinder

M/C Unapplied:

19 M/C Unapplied

M/C Applied:

20 M/C Applied

Primary System Failure:

21 Primary System Failure Operated Mechanically Bottoms Against Secondary Piston Pressure for Normal Secondary System Function

Secondary System Failure:

22 Secondary System Failure Bottoms at End of Cylinder Bore Pressure for Normal Primary System Function

Proportioning Valves:

23 Proportioning Valves 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

Adjustable Proportioning valves:

24 Adjustable Proportioning valves Wilwood Tilton Only the split points are adjustable

PowerPoint Presentation:

25 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.

Foundation Brake Sub-system :

26 Foundation Brake Sub-system Disc Brakes Linings

Front Disc Brake:

27 Front Disc Brake

Front Disc Brake:

28 Front Disc Brake

Brake Linings:

29 Brake Linings Brake 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 (µ) between the lining and the disc or drum. The challenge is knowing what the instantaneous value of µ is during any given stop. Any design calculations you do, go right out the window if the lining you use does not have the µ value you assumed.

Brake Linings:

30 Brake Linings Remember the equation for a disc brake The best method for determining the actual value of µ for a given lining is from a dynamometer test.

Brake Balance:

31 Brake Balance

Both Front Wheels Locked::

32 Both Front Wheels Locked: Not good if you are on a curved road You can’t steer OK, if you must hit something The vehicle goes straight

Both Rear Wheels Locked::

33 Both Rear Wheels Locked: The front wheels track straight ahead Then the rear wheels deviate to the side Until the vehicle can’t track straight any longer and the rear starts to spin around the front

PowerPoint Presentation:

34 µ vs. % Wheel Slip 0 10 20 30 40 50 60 70 80 90 100 0 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

Front Lock:

35 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

Rear Lock:

36 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

Optimum Braking:

37 Optimum Braking Optimum braking is achieved when brake torque distribution matches dynamic weight distribution Weight Distribution No Braking Hard Braking 40% 60% 20% 80%

Calculating Dynamic Weight Transfer:

38 Calculating Dynamic Weight Transfer

Ideal Vs Actual Torque:

39 Ideal Vs Actual Torque 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Front Torque Rear Torque Ideal 60/40 Actual 70/30 Actual With a prop valve

Stopping Distance:

40 Stopping 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

PowerPoint Presentation:

41 µ vs. % Wheel Slip 0 10 20 30 40 50 60 70 80 90 100 0 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 Fade:

42 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

Formula SAE vs. Mini Baja:

43 Formula SAE vs. Mini Baja The brake system design must match the vehicle objectives

PowerPoint Presentation:

44 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

PowerPoint Presentation:

45 Mini Baja, Hostile Environment Will your master cylinder fill up with water? How does you brake lining work when it is wet?

Lessons Learned :

46 Lessons Learned

Don’t Do Something Because::

47 Don’t Do Something Because: The other guys do it. All the race cars have it. It looks really cool

Design to a Specific Objective::

48 Design to a Specific Objective: Do the math first Verify your assumptions Take the easy solution (KISS)

PowerPoint Presentation:

49 Lessons Learned Lesson Number 1 - Keep it SIMPLE

PowerPoint Presentation:

50 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 things can happen.

PowerPoint Presentation:

51 Too 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.

PowerPoint Presentation:

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PowerPoint Presentation:

53

PowerPoint Presentation:

54 Lessons Learned Lesson Number 3 - Packaging and Integration drive 90% of design Lesson Number 4 - Read the rules Lesson Number 5 - Allow enough time for details and testing

PowerPoint Presentation:

55 Brake 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

Any Questions:

56 Any Questions