1. Braking System Principles
chapter4
Braking System Principles

2. Objectives Discuss the energy principles that apply to brakes.
Discuss the mechanical principles that apply to brakes.
Discuss the friction principles that apply to brakes.

3. Objectives Describe how brakes can fade due to excessive heat.
Describe how deceleration rate are measured.
This chapter will help you prepare for the Brakes (A5) ASE certification test.

4. FIGURE 4–1 Energy ,which is the ability to perform work , exists in many forms.

5. Energy Principles: Kinetic Energy
Kinetic energy is a fundamental form of mechanical energy.
It is the energy of mass in motion.
The job of the brake system is to dispose of that energy in a safe and controlled manner.

6. Energy Principles: Kinetic Energy
Every moving object possesses kinetic energy, and the amount of that energy is determined by the object’s mass and speed.
The greater the mass of an object and the faster it moves, the more kinetic energy it possesses.
Even at low speeds, a moving vehicle has enough kinetic energy to cause serious injury and damage.

7. FIGURE 4–2 Kinetic energy increases in direct proportion to the weight of the vehicle.
FIGURE 4–3 Kinetic energy increases as the square of the increase in vehicle speed.

8. Energy Principles: Kinetic Energy and Brake Design
The relationships between weight, speed, and kinetic energy have significant practical consequences for the brake system engineer.
If vehicle A weighs twice as much as vehicle B, it needs a brake system that is twice as powerful.
But if vehicle C has twice the speed potential of vehicle D, it needs brakes that are, not twice, but four times more powerful.

9. FIGURE 4–4 Inertia creates weight transfer that requires the front brakes to provide most of the braking force.

10. FIGURE 4–5 Front-wheel -drive vehicles have most of their weight over the front wheels.

11. FIGURE 4–6 A first-class lever increases force and changes the direction of the force.

12. FIGURE 4–7 A second-class lever increases the force in the same direction as the applied force.

13. FIGURE 4–8 A third-class lever reduces force but increases the speed and travel of the resulting work.

14. FIGURE 4–9 A brake pedal assembly is a second-class lever design that provides a 5 to 1 mechanical advantage.

15. Mechanical Principles: Mechanical Advantage
Leverage creates a mechanical advantage that, at the brake pedal, is called the pedal ratio.
For example, a pedal ratio of 5 to 1 is common for manual brakes,
Which means that a force of 10 lb at the brake pedal will result in a force of 50 lb at the pedal pushrod.

16. Mechanical Principles: Mechanical Advantage
In practice, leverage is used at many points in both the service and parking brake systems to increase braking force
While making it easier for the driver to control the amount of force applied.

17. Friction Principles: Coefficient of Friction
The amount of friction between two objects or surfaces is commonly expressed as a value called the coefficient of friction.
It is represented by the Greek letter mu (μ).

18. Friction Principles: Coefficient of Friction
The coefficient of friction, also referred to as the friction coefficient, is determined by dividing tensile force by weight force.
The tensile force is the pulling force required to slide one of the surfaces across the other.
The weight force is the force pushing down on the object being pulled.

19. FIGURE 4–10 The coefficient of friction in this example is 0.5.

20. FIGURE 4–11 The type of friction material affects the coefficient of friction, which is just 0.05 in this example.

21. Friction Principles: Friction Contact Area
For sliding surfaces, such as those in wheel friction assemblies,
The amount of contact area has no effect on the amount of friction generated.
This fact is related to the statement that brake friction materials always have a friction coefficient of less than 1.0.

22. Friction Principles: Friction Contact Area
To have a friction coefficient of 1.0 or more, material must be transferred between the two friction surfaces.
The amount of contact area does not affect the coefficient of friction.
It does, however, have significant effects on lining life and the dissipation of heat that can lead to brake fade.

23. Friction Principles: Static and Kinetic Friction
There are actually two measurements of the coefficient of friction: the static friction coefficient and the kinetic friction coefficient.
The static value is the coefficient of friction with the two friction surfaces at rest.
The kinetic value is the coefficient of friction while the two surfaces are sliding against one another.

24. FIGURE 4–12 The static coefficient of friction of an object at rest is higher than the kinetic (dynamic) friction coefficient once in motion.

25. Friction and Heat
The function of the brake system is to convert kinetic energy into heat energy through friction.

26. Friction and Heat
It is the change in kinetic energy that determines the amount of temperature increase
And kinetic energy increases proportionately with increases in weight, and as the square of any increase in speed.

27. Friction and Heat
If the weight of the vehicle is doubled to 6,000 Ib, the change in kinetic energy required to bring it to a full stop will be 180,602 ft-Ib.
The temperature increase computed with this equation is the average of all the friction-generating components.

28. Brake Fade
The temperature of a brake drum or rotor may rise more than 100°F (55°C) in only seconds during a hard stop,
But it could take 30 seconds or more for the rotor to cool to the temperature it was before the stop.

29. Brake Fade
If repeated hard stops are performed, the brake system components can overheat and lose effectiveness, or possibly fail altogether.
This loss of braking power is called brake fade.

30. Brake Fade
The point at which brakes overheat and fade is determined by a number of factors
Including the brake design, its cooling ability, and the type of friction material being used.

31. Brake Fade There are four primary types of brake fade: Mechanical fade
Lining fade affects
Gas fade
Water fade

32. FIGURE 4–13 Mechanical fade occurs when the brake drums become so hot that they expand away from the brake lining.

33. FIGURE 4–14 Some heat increases the coefficient of friction, but too much heat can cause it to drop off sharply.

34. FIGURE 4–15 One cause of GAS brake fade occurs when the phenolic resin, a part of the friction material, gets so hot that it vaporizes. The vaporized gas from the disc brake pads gets between the rotor (disc) and the friction pad. Because the friction pad is no longer in contact with the rotor, no additional braking force is possible.

35. Summary Energy is the ability to do work.
A vehicle in motion represents kinetic energy, which must be absorbed by the braking system during a stop.
The front brakes must provide a higher percentage of the braking force due to weight bias and weight transfer.

36. Summary
The brake pedal uses mechanical advantage to increase the force applied by the driver to the master cylinder.
Coefficient of friction represents the amount of friction between two surfaces.

37. Summary
Friction creates heat during a stop, and the braking system must be able to absorb this heat.
Deceleration rates are expressed in feet per second per second, or ft/sec2.

38. Summary
Brake fade results when the heat generated by the brakes causes changes in the friction materials that reduce the braking force
Or when water gets between the brake drum and the linings.