chapter 4 Braking System Principles Automotive Brake Systems

chapter 4 Braking System Principles Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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 (A 5) ASE certification test. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 1 Energy , which is the ability to perform work , exists in many forms. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 2 Kinetic energy increases in direct proportion to the weight of the vehicle. Automotive Brake Systems James D. Halderman FIGURE 4– 3 Kinetic energy increases as the square of the increase in vehicle speed. © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 4 Inertia creates weight transfer that requires the front brakes to provide most of the braking force. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 5 Front-wheel -drive vehicles have most of their weight over the front wheels. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 6 A first-class lever increases force and changes the direction of the force. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 7 A second-class lever increases the force in the same direction as the applied force. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 8 A third-class lever reduces force but increases the speed and travel of the resulting work. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 9 A brake pedal assembly is a secondclass lever design that provides a 5 to 1 mechanical advantage. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Mechanical Principles: Mechanical Advantage Braking System Principles • 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Mechanical Principles: Mechanical Advantage Braking System Principles • 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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). Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 10 The coefficient of friction in this example is 0. 5. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 11 The type of friction material affects the coefficient of friction, which is just 0. 05 in this example. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Friction Principles: Static and Kinetic Friction Braking System Principles • There 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 12 The static coefficient of friction of an object at rest is higher than the kinetic (dynamic) friction coefficient once in motion. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles Friction and Heat • The function of the brake system is to convert kinetic energy into heat energy through friction. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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 frictiongenerating components. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles Brake Fade • There are four primary types of brake fade: – Mechanical fade – Lining fade affects – Gas fade – Water fade Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 13 Mechanical fade occurs when the brake drums become so hot that they expand away from the brake lining. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles FIGURE 4– 14 Some heat increases the coefficient of friction, but too much heat can cause it to drop off sharply. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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, or ft/sec 2. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ

Chapter 4 Braking System Principles 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. Automotive Brake Systems James D. Halderman © 2014 Pearson Higher Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ