Chapter 11

Friction Theory

After studying this chapter, you will be able to: D Define . D Explain the relationship of weight and speed to kinetic energy. Define coefficient of friction. D Define static and kinetic friction. D Define brake fade. D Explain the relationship of friction to heat development. o Identify and describe the factors affecting stopping power. Identify outside factors affecting coefficient of friction. o Describe materials and construction.

Important Terms

Friction Static friction Weight transfer Metallic

Kinetic energy Kinetic friction Inertia Rotm Momentum Brake fade Non-metall ic Drum Coefficient of friction Heat dissipation Semi-metallic

179 180 Auto

Before stu dyi ng the operation of the brake friction possible. This is why engines and parts have members, you must have an understanding of the basic elaborate lubrica ti on sys tems. principles of friction. These principles explain what friction In the brake sys tem, friction is put to work to over­ is, how it is used to overcome motion, and various come the vehicle's momentum; in other words, to stop the factors affecting brake material construction. Thi s chapter vehicle. What momentum is, and how it is overcome is is an introducti on to bl'ake friction principles w hich will explained in th e following paragraphs. prepare you for the disc and theory and se rvice chapters. Momentum and Kinetic Energy

What Is Friction? Kinetic energy is the total amount of energy the brake system must convert to stop the vehicle and is measured in When two things move against each other, there is a foot-pounds (ft-Ibs.) or Newton-meters (N. m). You w ill re sista nce to the movement between th em. This is caused often hear the term momentum used in pl ace of the term by microscopic imperfections (high spots) th at exist on kinetic energy. Although th e two terms are closely related even th e smoothest surfaces, Figure 11-1. Th e imperfec­ mathematica lly and both are functions of the ve hicle's tions on one surface contact the imperfections on the other weight and velocity, kinetic energy is the measurement surface as they move against each other. The resistance you should use when discussing the vehicle's braking sys­ caused by this contact is called friction. tem. As a tec hni cian, you will not be asked to calculate if there was no friction, would have no traction kinetic energy. However, for youl' own understa nding of against the road. Without friction, bolts wou ld not tighten, the basic principles of brake operation, you should and doors would not stay closed . However, in many remember the following concepts. instances, it is desirable to minimize friction as much as At any given speed, kinetic energy increases directly with the weight of the vehicle. For example, a 4000 lb. (1812 kg) vehicle traveling at a given speed w ill have tw ice th e kinetic Imperfections in the surfaces energy of a 2000 lb. (906 kg) vehicle trave ling at the same resist movement speed. A 6000 lb. (2722 kg) will have three times the kinetic Two smooth energy of th e 2000 lb. (906 kg) vehicle, Figure 11-2. Speed has much more effect on kinetic energy th an weight does. For example, if a travels twice as fast as an identical car, it has four times the kinetic energy of the slowel' car. If the faster car is moving at three times the speed of the slower car, it has nine times the kinetic energy. If the fastel' ca r is traveling four times the speed of the slower car, it w ill have sixteen times the kinetic energy. Brakes perform more work stopping a light vehic le going at a high rate of speed th an a hea vier ve hicle going slowly. You would think that a 4000 lb. vehic le moving at 25 mph (40 kph) would have th e same amount of kinet ic energy as a 2000 lb. (906 kg) ve hi c le moving at Figure 11-1. A magnifying glass showing the surface imperfec­ 50 mph (80 kph). However, the 4000 lb. (1812 kg) vehi­ tions which cause friction between the two moving surfaces. cle has only 86,000 ft-Ibs. of kinetic energy at 25 mph, These imperfections act like two pieces of sandpaper being while the 2000 lb. vehicle has over 172,000 ft-Ibs. of rubbed together. kineti c energy at 50 mph.

1x kinetic energy ... 1x kinetic energy ... 20 mph (32 kph) 20 mph (32 kph) 4x kinetic energy ... 40 mph (64 kph)

2x kinetic energy ... 9x kinetic energy ... 20 mph (32 kph) 4000 Ibs. (1812 kg) A 8 60 mph (92 kph) 2000 Ibs. (906 kg)

Figure 11-2. A vehicle's kinetic energy is a combination of its mass (weight) and velocity (speed). A-Kinetic energy increases in direct proportion to the weight. B-A vehicle 's kinetic energy increases exponentially as the speed increases. Chapter 11 Friction Brake Theory 181

shoes or pads become glazed, they do not create as much Putting Friction to Work friction against the drum or rotor surface. This is similar to making the box out of a slicker material. The job of the brake system is to overcome momen­ tum and stop the vehicle. To do this, it uses hydraulics, pneumatics, mechanical leverage, and friction. Note: The coefficient of friction will always Hydraulics, pneumatics, and mechanical leverage were be less than one. discussed in earlier chapters. The following sections explain how friction is put to work to stop the vehicle. Static and Kinetic Friction Coefficient of Friction The two basic types of friction are stationary or static As stated earlier, friction is always present between friction and kinetic friction, sometimes called sliding or two matel'ials that slide against each other. The coefficient dynamic friction. Keep in mind that static friction is a of friction is the amount of friction that can be produced holding action that keeps a stationary object in place, as two materials sl ide across each other. The coefficient of while kinetic friction slows a moving object by converting friction is determined by a simple calculation. In the exam­ momentum to heat. Note that static friction is always ple shown in Figure 11-3, the coefficient of friction is cal­ higher than ki netic friction. culated by measuring th e force required to slide a block The most obvious use of static friction is the parking over a surface and then dividing it by the weight of the brake, Figure 11-5. When the parking brake is appl ied, block. static friction between the applied brake components If it takes 10 Ibs. (22 kg) of force to sl ide a 10 lb. resists movement. To move the vehicle, static friction must (16 kg) block over a flat surface, the coefficient of friction be eliminated by releasing the brakes. Since the vehicle for the block is 1. Another 10 Ib, (22 kg) object, made from has no movement, there is no momentum to overcome, a different material, may only require Sibs. (8 kg) of force and no heat is generated. to slide it across the same surface. In this case, the coeffi­ As discussed earlier, vehicle weight times vehicle cient of friction for that block is .5. See Figure 11-4. speed equals momentum. Applying the brakes on a mov­ In the previous example, the way the box slides ing vehicle causes the stationary friction members (pads or across the floor was altered by changing the material that shoes) to be forced into contact with the rotating friction the box was made of. When the box was made out of a members (rotors or drums). This contact, Figure 11-6, slicker material, it became easier to slide. However, if you causes friction and heat, which results in the rotating parts look at the basic relationships between the box, the floor, slowing and eventually stopping. Since the momentum of and the pulling force, you will see th at the same effect the rotating parts is called kinetic energy, the friction used could have been achieved by decreasing the box weight or to stop the rotating parts is called kinetic friction. by using a different material on the floor. Now that you understand the basic relationships measured by the coefficient of friction, you can apply these relationships to the operation of a vehicle's brake system. Imagine the box is the bl'ake pad or shoe, the Bilk 1 weight of the box is the hydraulic pressure used to apply the brakes. The shop floor is drum or rotor surface, and the If pull required pulling force is the momentum in the turning drum or rotor. weight is 10 pounds, 10 Ibs. By applying more pressure to the brakes, you can slow the C.O.F. =100% or 1.0 momentum of the spinning drum or rotor. This is the same as increasing the weight of the box, making it more diffi­ A ~ Shopfloor cult to pull across the shop floor. Similarly, when brake

Block 2

1 If pull required If pull required Weight is 5 pounds, I Weight is 100 pounds, 101bs. C.O.F. =.5 t 100 Ibs. C.O.F. = 100% or 1.0

B Shopfloor Shop floor ~

Figure 11-3. A 100 pound (45,359 kg) weight being pulled Figure 11-4. Two examples illustrating the coefficient of friction. across the shop floor. The coefficient of friction (C.OF) is 1.0. A-C.OF at 10 pounds of pull. 8-G.OF at five pounds. (TRW Inc.) (TRW Inc.) 182 Auto Brakes

Stopping Distance Parking brake shoes are expanded inside hub The horsepower developed by the engine to move the vehicle from 0-60 mph (0-95 kph) must be absorbed by the braking system when slowing the vehicle from 60 mph to O. Floating To prevent accidents, the brake system must be capable of caliper stopping the vehicle in a much shorter distance than it takes to accelerate it. The engine in a modern vehicle is often capable of Parking brake moving the vehicle from 0-60 mph in one-eighth of a mile, cable or 660 feet. The brake system of the same vehicle can take the vehicle from 60 mph to 0 in about '150 feet. The distance taken to get from 60 to 0 is about one-quarter the distance taken to get from 0-60 mph, Figure 11-7. The design of the brake system will vary depending on the demands that w ill be placed on it. A

Factors Affecting Friction Development

Many factors affect the development of friction by the brake system. While some may seem obvious, others will seem unlikely. However, they all have their part to play in support the proper development of friction . Intermediate adapter Rotor/drum B assembly Apply Pressure The more pressure applied to the brake friction mem­ bers, the more they resist movement, and the more friction Figure 11-5. Static friction (not moving). ~With the parking brake shoes expanded into contact with the inside hub (drum), is developed. More friction means more braking action. the rotor is prevented from turning. B-Exploded view of the Pressure is created by a con,-bination of mechanical lever­ various brake components. (DaimlerChrysler) age and hydraulic pressure, plus the action of th e power assist unit. These factors of pressure development were dis­ cussed in earlier chapters.

t., Heat Friction M aterial Temperature The temperature of the friction materials has a great !/J~ effect 011 the amount of friction developed. As the friction material gets hotter, its ability to stop the vehicle is .-.-r- reduced. Not only must friction materials be designed to -"-- '-- operate under greatly va rying temperatures, th ey must ""'-" have roughly the sa me coefficient of friction, both cold JV' "'­ and hot. Too much variation means the brake pedal feel JV '-.. ...r/ and required pressure would change drastically as the .r-/' 0 ~ brakes become heated. ~ ~ ~ ~ ,~ Friction Material Contact Area J0; Rotating Although a small braking surface could produce as 1/ ) \\\\2 "\ \ drum much friction as a larger surface by being applied harder, it would quickly overheat and become use less. Brake fric­ tion members must be large enough to absorb and spread Figure 11-6. The brake shoes are being forced into contact with the revolving brake drum producing kinetic friction and the frictional heat out. For this re aso n, the larger the vehi­ heat. This will cause the rotating parts to slow down and come cle, the larger th e brake friction components. to a stop. (Bean) Chapter 11 Friction Brake Theory 183

· 1 0-60 mph in 660 feet (20 meters) I ------~------~

A Bumper to touch this line

Vehicle .....------Brakes applied here stops here

Stopping distance 150 feet (4.6 meters) 60 mph to 0 B

Figure 11-7. Stopping distance. ~Vehicle accelerates from 0 to 60 mph in 660 feet. B-The same vehicle being brought to a stop with its brake system, while traveling from 60 mph to 0 in 150 feet. (Parker Automotive)

Friction Material Finish The bui Id-up of heat ca n lower the coefficient of fric­ tion in the brake pads m shoes to the point where the The finish, or smoothness of the fri ction materials has brakes begin to fade. Brake fade is the term given to grad­ an effect on the vehicle's braking ability. A rough brake ual brake failure caused by brake overheating. As the surface would have a higher coefficient of friction, but brakes begin to fade, it takes more pedal pressure to stop would grab and wear quickly. Modern brake lining mate­ the vehicle. After a certain point, the brakes will have very rial develops a smooth finish as it is used. As the top layer I ittle effect, regardless of the pres su re appl ied to the brake of material wears away, the underlying surface maintains pedal. the smooth finish. Sometimes, when low quality is used, or the fluid is water contaminated, the fluid will boil due to Type of Friction Material excessive frictional heat. When this happens, pressing on th e bl'ake pedal will compress the vaporized fluid instead The materials used in a brake friction unit have a great of applying the bl'akes. effect on its stopping ability. More force is needed to move Not only is brake fade dangerous, the heat generated some materials over a surface than others, even when by excessive braking can wear the linings, overheat and apply press ure, contact area, and finish are the same. The warp the drums and rotors, and cause premature failure of fr iction characteristics of a brake material make up its coef­ the hydraulic system, bearings, and seals. ficient of friction. If the coefficient of friction is too high, the brakes will work too well and cause the to lock up. If the coefficient of friction is too low, the brake pedal Brakes Are Designed to Fade would require excessive force to stop the vehicle. Types of friction materials are discussed in more detail later in this Brakes are designed to fade at a certain temperature. If chapter. this temperature is passed, the frictional materials melt and the brake linings will no longer stop the vehicle. While this seems to be a poor brake design, it makes sense when con­ sidel'ing the potential damage from allowing brake tempera­ Heat Removal tures to continuously ri se. If temperatures were allowed to rise without dissipation, the wheel bearing seals, lubricating Friction always causes heat. The more friction needed grease, CV boots, and any other nearby parts would be to stop the veh icle, the greater the amount of heat gener­ destroyed. In extreme cases, nearby combustible compo­ ated during braking. Therefore, the temperature of the nents, including the tires, could catch fire. brake components rises as the brakes are applied. If a To prevent this, a built-in fade point is designed into vehicle weighs 4000 Ibs (1812 kg), one emergency stop the lining material. This point is high enough to allow for from 60 mph (96 kph) can raise brake lining temperatures all but the most extreme braking situations. However, this by 160°F (71°C), Repeated hard stops ca n continue to raise emphasizes the importance of removing heat as efficiently the temperature by equal amounts, Figure 11-8. as pOSSible. 184 Auto Brakes

Vehicle speed: Mph - 60 (96 km/h)

Stop- 1 ~ Brakes are cool

Stop-2 -=:J Stop-3~

Stop- 4 _ '-­ __--'

Stop- 5

Stop- 6

Stop- 7

Stop- 8

Stop- 9 I - _ ~ ~ -­ -- ­ _ _- , Brakes at fade

\~------~V r------~/ increasing as brake temperatures grow

Figure 11-8. Chart illustrating a brake temperature range from cool to the paint of brake fade. At Stop 9, the brakes have begun to fade severely. Notice as the brake temperatures go up, stopping distances become longer. (FMC)

Heat Dissipation Weight Transfer Heat is removed from the brake friction surfaces by When a vehicle is at rest, most of the vehicle weight is direct transfer to the surrounding air. This process is called over the front wheels, since the engine and are heat dissipation, Figure 11-9. Modern brake systems are at the front of the vehicle. As much as 60% of the total weight designed to provide the best possible heat dissipation. The of a rear-wheel drive vehicle is suppolied by the front wheels. size of the friction surfaces is designed for maximum heat On a front-wheel drive vehicle, the figure is closer to 80%_ dissipation. The larger the friction al-ea, the better the heat In addition, more weight is placed on the front wheels dissipation. Rotors and drums are made from cast iron, during braking, due to weight transfer. When the brakes which can do an efficient job of absorbing heat and tl-ans­ are appl ied, extra veh icle weight is transferred from the ferring it to the outside air. rear of the vehicle to the front. This is caused by inertia_ The rotor is exposed to outside air Inertia is the resistance to any change in momentum, in throughout its diameter, except where it contacts the this case, the tendency of the moving vehicle to keep mov­ brake pads. In addition, the dust shields and sometimes ing. Although the brakes are slowing the wheel assemblies, the wheel covers are designed to direct air over the rotor. the body, drivetrain, and frame try to continue moving for­ Disc brake rotors on heavier contain internal fins ward_ See Figure 11-10. to allow air to flow through the rotor. The design of the The vehicle's original weight distribution and the fins actually pumps air through the rotor internals. In effects of weight transfer cause the rear wheels to have less some cases, the rotor braking surface is drilled to weight and the front wheels to have more weight. Since increase airflow. more braking must be done by the front wheels than the Brake drums used on heavier vehicles are finned on rear wheels, the front brake systems have a larger friction their outside diameter. Air is directed ovel' the drum by area than the rear brake system. vehicle movement. Some brake drums are made of alu­ minum, with a cast iron lining that contacts the shoes. and Road Conditions If the ti res do not grip the road properly, the brake sys­ tem will not work. Traction between the tires and the road Other Factors Affecting Braking must be maintained for proper stopping. If the brake sys­ tem works so well that the tire stops rotating, it is said to be Other factors affect the operation of the brake system, skidding. When the road is wet or icy, the tire is skidding and therefore vehicle braking ability. While these are not on a layer of water or ice. On dry pavement, a skidding tire parts of the brake friction system, they directly affect its causes so much frictional heat that the tire rubber melts. operation. The tire then skids on a layer of I iqu id rubber. Chapter 11 Friction Brake Theory 185

Direction of travel >II

Braking tends to force down front

Greater braking Less braking power here power here

Brake rotor Figure 11-10. Braking forces create weight transfer when stopping the vehicle. Note how the front end is going down Cool air Air duct and the rear end is rising. A

the closed throttle plate. This creates a drag on the drive­ train which helps to slow the vehicle. The lower the trans­ mission gear and final drive gear ratios, the faster the engine is turning in relation to road speed, and the more the engine braking effect. This is why drivers are advised to switch to a lower gear when descending a steep hill. Older vehicles with large, high compression engines and lower gearing were able to get considerable engine braking to assist the wheel brakes. Newer vehicles with small engines and overdrive gears have only a small amount of engine bl'aking in top geal', and the wheel brakes must do more of the braking. A vehicle with an also has less engine braking capac­ ity, since some frictional drag is lost inside of the when the lockup is not applied.

Water on Brake Linings In many parts of the world, It IS not uncommon to travel through roads that are partially or completely cov­ ered with water. In addition to the increasingly common use of off-road vehicles, passenger sometimes enter Figure 11-9. A-Cool air flowing through an air duct and being directed onto the brake rotor and other parts. B-Arrows areas where the pavement is covered by water. If water indicate the airflow through and around a typical disc brake reaches the brake linings, it acts as a lubricant, causing assembly Heat, generated by braking, is being carried away by brake fade. There is no way to design bt'ake linings to over­ the airflow. (Bendix, Mercedes-Benz) come brake fade due to water. The only cure, other than avoiding flooded roads, is to allow the brakes to dry out. Disc brake systems dry out more quickly, since the In addition to loss of braking ability, one or more skid­ exposed rotor wi II spin off water once the vehicle is on dry ding tires can cause loss of steering control. Even if skid­ pavement. Drum brakes are enclosed, and water takes a ding is not severe, it can cause flat spots on the tire surface. longer time to exit the assembly. Therefore, both the tires and brakes must be in good con­ dition for adequate braking. On modern vehicles, anti-lock brake systems are able to assist in braking and preventing skids. ABS is covered in Chapters 21 and 22. Friction Members

There are two friction surfaces in any brake assembly. Engine Size and D rive Train Gearing The brake pad or shoe lining, which are made of a mixture The size and compression ratio of the engine has an of heat resistant materials, and the cast iron rotor or drum. effect on braking. When the accelerator pedal is released, These two frictional surfaces are discussed in the following the engine is trying to draw air into the cylinders against paragraphs. 186 Auto Brakes

Pad and Shoe Linings shape. Semi-metallic linings are harder and last longer. They are also more fade re sistant than completely non­ Materials used on modern brake shoes and pads must metallic linings. However, semi-metallic linings increase have about the same coefficient of friction when either hot brake pedal effort, may squeak on application, and cause or cold. They must also resist fading at high temperatures, some wear to the rotors and drums. but fade when there is enough heat to cause damage to Metallic brake linings are used on high performance other vehicle parts. Brake lining materials must also be and competition vehicles only. They are made from sin­ able to stop when the linings are wet, and recover quickly tered metal (powdered metal that is formed into linings). when dried out. Th ey must stop the vehicle smoothly and Metallic brake linings resist brake fade very well, but quietly, la st for tens of thou sa nds of miles, and not cause require high pedal pressure, are noisy, and severely wear excessi ve wear to the rotor or drum. rotors and drums.

Lining Materials Temperature Markings Brake linings use various metals and high temperature It is very important that brake lining materials ha ve synthetic fibers such as Kevlar and other heat resistant about the same coefficient of friction when cold as when compounds. Asbestos is no longer used in new brake lIn­ hot. Brake pads and shoes have makings on the edges that in gs, but may be found in some brake linings currently give the approximate coefficient of friction at low and insta lied on veh icles. high temperatures. The markings are in letters, with lining Some vehicles use linings made from organic or materials marked A being the lowest, and Improving compl etely non-metallic materials. These materials are through the alphabet. Typi ca l letter markings and their mixed from va rious compounds an d molded into the relationship to tempel·ature are shown in Figure 11-11. proper shape. Non-metallic lining materials are quieter The best linings have markings that indicate a high coeffi­ (less prone to squeaking) and do not damage the cast iron cient of fri ction that stays the sa me at high and low tem pera­ drums or rotors. However, th ey provide the lowest coef­ tures. The more variation in the high and low temperature ficient of friction, and therefore, the least amount of brak­ letters, the poorer the qual ity. ing power. Most brake linings, especially those sold as lifetime gua ranteed brakes, are semi-metallic linings . Semi-metal­ Lining Attachment Methods lic materials are made from a combination of non-metallic Disc brake linings are bonded (glued) or riveted to a material s and iron, mixed and molded into the proper metal backing pad, Figure 11-12. The attachment method is not an indicator of quality, since both methods are w idely used. As a general rule, most pads used on heavier cars and are riveted to the backing plate, while pads used on smaller vehicles are bonded. Drum brake linings are also bonded or riveted to a metal shoe, Figure 11-13. Attachment methods vary, but modern shoes tend to be bonded rather than ri veted .

A A .

Friction Material Code Coefficient of Friction

C <.15 0 .15-.25 E .25-.35 B F .35-.45 G .45-.55 H >. 55 B Figure 11-12. Brake lining to shoe attachment methods. Figure 11-11. A- A disc brake pad illustrating the various edge A- Disc brake pad that has been riveted to the shoe. 8- DISC code markings. B-Chart showing friction matenal codes used brake pad which is bonded (glued) to the metal shoe. on brake linings. (Jack Klasey) (Jack Klasey) Chapter 11 Friction Brake Theory 187

Brake shoes away heat more rapidly than cast iron. The other materials in the composite give the drum or rotor adequate durability.

Rotors Both sides of the brake rotor Jre machined smooth where they contact the brake pads, Figure 11-14. This con ­ ta ct area is extremely smooth for smooth stops. The rotor's cast iron is designed to last through several sets of linings. Rotors are heavy for maximum heat absorption, and have enough extra metal so they ca n be resurfaced if they become slightly damaged.

Drums The inner surface of the brake drum is machined smooth w here it contacts the brake shoes, Figure 11-15. Bonded Riveted The weight of the drum helps to absorb heat and transfer it A lining B lining to the outside air. Drums are also built with extra metal so that they can be resurfaced if they become slightly dam­ Figure 11-13. A-Brake lining which has been bonded (glued) aged. A drum should outlast several sets of shoes. to the shoe. ~Lining that was riveted to the shoe. Both attach­ ment methods are currently used. (General Motors)

Smooth Rotor and Drum Construction braking surface The basic job of drums and rotors is to provide a con­ tact surface for the brake linings, and to absorb hea t. Most modern brake rotors and drums are made from cast iron. A few veh icles are equipped with aluminum drums having cast i ron liners. Some high-performance imported vehicles have aluminum metal matrix composite (AI-MMC) drums and rotors. These drums and rotors al'e made from an alloy of aluminum and other materials. The aluminum alloy carries Brake drum

Figure 11-15. A brake drum depicting a smooth braking surface, (FMC)

Summary

When the microscopic imperfections of two moving surfaces contact each other, they create resistance. This resistan ce is ca lled friction. Friction should be avoided in some vehicle systems, but is vital to the operation of the brakes. Momentum is a combination of vehicle weight and speed. The job of the brake system is to overcome momen­ tum and stop the vehicle. The brakes use friction to stop the vehicle. Fri ction is always present between two mate­ rials that slide aga inst each other. The coefficient of friction is the amount of friction that can be produced as the two surfaces slide aga inst each other. The coefficient of friction is determined by dividing sliding force by the weight of the Figure 11-14. A brake rotor with a smooth braking surface. object. 188 Auto Brakes

The two kinds of friction are static friction and kinetic 13. Linings are attached to the pad or shoe by __ or friction. Static friction is friction that keeps a stationary object in place. Kinetic friction slows a moving object by 14. Modern rotors and drums are made from converting movement to heat. Static friction is always higher than kinetic friction. 15. A drum or rotor should outlast several sets of The brake system can absorb much more horsepower than the vehicle engine can produce. For this reason , stop­ ping distances from a given speed are much shorter than the distances needed to accelerate to th e same speed. Many factors affect the ability of a frictional material to function. Brake apply pressure, temperature, contad area, 1. Tech nician A says that a vehicle that has weight has and type of material all affect the braking ability of a material. momentum, even when it is stopped. Technician B Friction always causes heat, and this heat must be says that momentum is a combination of speed and removed to prevent fading. Brake materials are designed to weight. Who is right? fade at a certain temperature to avoid starting a fire. Heat (A) A only. is absorbed by the metal of the drums and rotors and dis­ (B) B only. sipated to the outside air. Other factors that affe ct braking (e) Both A & B. ability are weight transfer, tire and road conditions, engine braking ability, and water on the brake linings. (D) Neither A nor B. All brake assemblies have two frictional surfaces. The 2. If it takes 2 pounds of force to move an 8 pound matching frictional surfaces are brake pads and rotors and weight across the floor, the coefficient of friction is brake shoes and drums. Brake pad and shoe linings are a mixture of heat resistant composition materials. The linings (A) 4 are marked with a letter series indicating their hot and cold (B) .5 braking performance. Rotors and drums are made of cast iron, and always turn with the wheel and tire. (e) .25 (D) .125 3. Technician A says that static friction turns movement Review Questions-Chapter 11 into heat. Technician B says that static fl·iction is a form of kinetic friction. Who is right? Please do not write in this text. Write your answers on (Al A only. a separate sheet of paper. (B) B only. 1. Define friction. (e) Both A & B. (D) Neither A nor B. 2. Define momentum. 4. Technician A says that a vehicle's kinetic energy 3. A material with a low coefficient of friction generates increases in direct relation to its weight. Technician B ___ fri ction than a material with a high coefficient says that as a vehicle's speed increases, its kinetic of friction. energy increases exponentially. Who is right? 4. The type of fri ction that keeps a stationary object in (A) A only. place is _ __ friction. (B) B only. 5. Friction that stops a moving object is ___ friction. (e) Both A & B. It does this by converting ___ into _ _ _ . (D) Neither A nor B. 6. If the speed of a vehicle doubles, its kinetic energy 5. Which of the following affect the braking ability of a increases by a factor of ___. brake materia I? 7. What five factors affect friction development? (A) Its temperature. (B) What it is made of. 8. When heat causes the coefficient of friction to decrease dramatically, this is called brake _ __. (C) How hard it is applied. (D) All of the above. 9. Excessive brake heat may cause substandard brake fluid to _ _ 6. Brake linings are designed to fade a certain _ _ _ . (A) temperature 10. How is brake heat removed ? (B) humidity 11. Which set of brakes do the most braking, front or rear? (e) speed 12 . After driving through water, _ _ brakes dry off (D) momentum sooner than brakes. Chapter 11 Friction Brake Theory 189

7. in which direction is 9. Technician A that non-metallic linings make most noise. Technician B says that non-metallic lin­ (A) ings will wear rotors and drums. Who is Back to A Left to B Right to Both A & B. (D) A nor B. 8. If a tire skids on it is skidding on a layer 10. A great variation in (A) melted I indicate well when melted (A) cold water vapor hot gas vapor hot or cold (D) None the above. 190 Auto Brakes

Disc brakes are used on the front and rear of many vehicles. This is one of the front disc brake assemblies on a vehicle with four-wheel disc brakes. (DaimlerChrysler)