Section 8 BRAKES CHAPTER 48 BRAKE SYSTEMS OBJECTIVES ■ Explain the basic principles of braking, including kinetic and static friction, friction materials, application pressure, and heat dissipation. ■ Describe the components of a hydraulic brake system and their operation, including brake lines and hoses, master cylinders, system control valves, and safety switches. ■ Perform both manual and pressure bleeding of the hydraulic system. ■ Briefly describe the operation of drum and disc brakes. ■ Inspect and service hydraulic system components. ■ Describe the operation and components of both vacuum-assist and hydraulic-assist braking units. t is commonly believed that the purpose of a brake FRICTION system is to slow or halt the motion of a vehicle. I There are two basic types of friction that explain how However, that is really not true. The friction of the brake systems work: kinetic, or moving, and static, or tires against the road is what slows down and stops a stationary (Figure 48–1). The amount of friction, or vehicle. The brake system slows or stops the rotation resistance to movement, depends on the type of of the wheels. This is a minor point but one that materials in contact, the smoothness of their rubbing extends the responsibility for braking to the tires as surfaces, and the pressure holding them together well as the brake system. (often gravity or weight). Friction always converts The brake system converts the momentum of the moving, or kinetic, energy into heat. The greater the vehicle into heat by slowing and stopping the vehi- friction between two moving surfaces, the greater the cle’s wheels. This is done by causing friction at the amount of heat produced. wheels. The application of the friction units is con- As the brakes on a moving automobile are applied, trolled by a hydraulic system. This chapter looks at rough-textured pads or shoes are pressed against the basics of all brake systems and gives a detailed rotating parts of the vehicle—either rotors (discs) or look at the hydraulic systems required to stop a drums. The kinetic energy, or momentum, of the vehicle. vehicle is then converted into heat energy by the kinetic friction of rubbing surfaces and the car or truck slows down. Chapter 8 for a detailed discussion on When the vehicle comes to a stop, it is held in place friction and its effects. by static friction. The friction between the surfaces of the brakes and between the tires and the road resists any movement. 1424 Copyright 2009 Cengage Learning, Inc. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 11491_48_ch48_p1424-1456.indd 1424 11/26/08 6:06:51 PM CHAPTER 48 • Brake Systems 1425 WEIGHT TRANSFER BRAKING TENDS TO BRAKING TENDS TO LIFT THE REAR OF APPLY MORE WEIGHT THE VEHICLE ON THE FRONT WHEELS KINETIC FRICTION STATIC FRICTION STATIC FRICTION DIRECTION OF TRAVEL Figure 48–1 Braking action creates kinetic friction in the brakes and static friction between the tire and road to slow the vehicle. When brakes are applied, the vehicle’s weight is trans- ferred to the front wheels and is unloaded on the rear wheels. Factors Governing Braking dividing the force required to pull an object across a Four basic factors determine the braking power of a surface by the weight of the object (Figure 48–2). For system. The first three factors govern the genera- example, if it requires 100 pounds (455 N) of pull to tion of friction: pressure, coefficient of friction, and slide a 100-pound (45.4 kg) metal part across a con- frictional contact surface. The fourth factor is a crete floor, the COF is 100 Ϭ 100 or 1. To pull a 100- result of friction. It is heat or, more precisely, heat pound (45.4 kg) block of ice across the same surface dissipation. may require only 2 pounds (9 N) of pull. The COF then An additional factor influences how well a vehicle would be only 0.02. will stop when the brakes are applied, that being As it applies to automotive brakes, the COF weight transfer. When the brakes are applied while expresses the frictional relationship between pads the vehicle is moving forward, the weight of the vehi- and rotors or shoes and drums. The required COF cle shifts forward. This causes the front of the vehicle depends on the vehicle and other factors and is to drop or “nose dive.” It also means that the front carefully chosen by the manufacturer to ensure safe brakes will need the most stopping power. If the vehi- and reliable braking. Therefore, when replacing cle is overloaded or if the front suspension is weak, more weight will be thrown forward and the brakes will need to work harder. Pressure The amount of friction generated between moving surfaces in contact with each other depends in part on the pressure exerted on the surfaces. For example, if you slowly increase the downward pres- 100 sure on the palm of your hand as you move it across a 100 ÷ 100 = 1 desk, you will feel a gradual increase in friction. 2 In a brake system, hydraulic systems provide application pressure. Hydraulic force is used to move brake pads or brake shoes against spinning rotors or drums mounted to the wheels. The amount of pres- sure is determined by the pressure on the brake pedal and the design of the brake system. Coefficient of Friction The amount of friction gen- 2 ÷ 100 = 0.02 erated between two surfaces is expressed as a coeffi- Figure 48–2 Coefficient of friction is equal to the cient of friction (COF). The COF is determined by pounds of pull divided by the weight of the object. Copyright 2009 Cengage Learning, Inc. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 11491_48_ch48_p1424-1456.indd 1425 11/26/08 6:06:58 PM 1426 SECTION 8 • Brakes pads or shoes, it is important to use replacement Brake Lining Friction Materials parts with similar COF. If, for example, the COF is Brake linings are made up relatively soft but tough too high, the brakes will be too sticky to stop the car and heat-resistant material with a high coefficient of smoothly. Premature wheel lockup or grabbing friction. The lining is typically attached to a metal would result. If the coefficient is too low, the friction backing with rivets or high-temperature adhesives. material tends to slide over the surface of the drum For many decades, asbestos was the standard brake or rotor rather than slowing it down. Most automo- lining material. It offers good friction qualities, long tive friction materials are engineered with a COF of wear, and low noise. But new materials are being used between 0.25 and 0.55. because of the health hazards of breathing asbestos dust. Asbestos has not been used in brake linings or Frictional Contact Surface The third factor is the pads since 2003. Many different materials are used as amount of surface area that is in contact. Simply put, lining material and the type of lining is defined by its bigger brakes stop a car more quickly than smaller composition. Each type has different heat dissipa- brakes used on the same car. For the most part, the tion, fade resitance, rotor wear, noise generation, and vehicle’s weight and potential speed determines the braking force characteristics. size of the friction surface areas. Also, the greater the surface areas of the wheel brake units, the faster heat Nonasbestos Organic Nonasbestos organic (NAO) can be dissipated. linings are installed on many vehicles by the OEM. Organic linings are made of nonmetallic fibers bonded Heat Dissipation Any braking system must be able together to form a composite material. Today’s to effectively handle the heat created by friction organic brake linings contain the following types of within the system. The tremendous heat created by materials: the rubbing brake surfaces must be conducted away from the pad and rotor (or shoe and drum) and be ■ Friction materials and friction modifiers. Some absorbed by the air. Brakes that do not effectively dis- common examples of these are graphite, pow- sipate heat experience brake fade during hard, con- dered metals, and even nut shells. tinuous braking. ■ Fillers. Fillers are secondary materials added Brake fade is a condition where the stopping for noise reduction, heat transfer, and other power of the brakes has been drastically reduced. purposes. This is commonly caused by excessive heat buildup. ■ Binders. Binders are glues that hold the other With brake fade, the brake pedal seems normal but materials together. there is reduced stopping ability. Brake fade may ■ Curing agents. These accelerate the chemical reac- become worse as heat builds up; this may be due to tion of the binders and other materials. outgassing. As the shoes or pads become extremely hot, they can generate a gas. This gas can become an Organic linings have a high COF, and they are eco- air bearing between the frictional material and the nomical, quiet, wear slowly, and are only mildly abra- rotor or drum. Rather than clamp on the wheel brake, sive to drums. However, organic linings fade more the friction elements will slip on the air (gas buildup). quickly than other materials and do not operate well Fade can also be caused by overheating the brake at high temperatures. High-temperature organic lin- fluid because gases form in the fluid. ings are available for high-performance use but they The friction materials must be able to dissipate do not work as well at low temperatures. They also heat and the system must be designed to allow the wear faster than regular organic linings.