Chap68

[object Object],[object Object],[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 68, the reader should be able to:

[object Object],KEY TERMS: Continued

ENERGY PRINCIPLES ,[object Object],Figure 68–1 Energy, which is the ability to perform work, exists in many forms. Chemical, mechanical, and electrical energy are the most familiar kinds involved in the operation of an automobile. When the ignition key is turned to the “ Start ” position, chemical energy in the battery is converted into electrical energy to operate the starter motor. Continued The starter motor converts the electrical energy into mechanical energy to crank the engine.

KINETIC ENERGY ,[object Object],Continued

[object Object],Continued See these formulas on Pages 818 and 819 of your textbook.

Figure 68–2 Kinetic energy increases in direct proportion to the weight of the vehicle. Continued

[object Object],Continued The results show that when the weight of a vehicle is doubled from 3,000 to 6,000 lb, its kinetic energy is also doubled from 90,301 ft-lb to 180,602 ft-lb. In mathematical terms, kinetic energy increases proportionally as weight increases. In other words, if the weight of a moving object doubles, its kinetic energy also doubles. If the weight quadruples, the kinetic energy becomes four times as great.

Figure 68–3 Kinetic energy increases as the square of any increase in vehicle speed. Continued

[object Object],Continued In mathematical terms, kinetic energy increases as the square of its speed . In other words, if the speed of a moving object doubles (2), the kinetic energy becomes four times as great (2 2  4). And if the speed quadruples (4), say from 15 to 60 mph, the kinetic energy becomes 16 times as great (4 2  16). This is the reason speed has such an impact on kinetic energy.

INERTIA ,[object Object],Continued

Figure 68–4 Inertia creates weight transfer that requires the front brakes to provide most of the braking power. Continued

[object Object],Figure 68–5 Front-wheel-drive vehicles have much of their weight over the front wheels. Continued When brakes are applied, weight transfer and bias increase the load on the front wheels; the load on the rear is substantially reduced. The front brakes provide 60% to 80% of the total braking force.

[object Object],Brakes Cannot Overcome the Laws of Physics To deal with the extra load, the front brakes are much more powerful than the rear brakes. They are able to convert more kinetic energy into heat energy.

MECHANICAL PRINCIPLES ,[object Object],Continued

Figure 68–6 This brake pedal assembly is a second-class lever design that provides a 5 to 1 mechanical advantage.

FRICTION PRINCIPLES ,[object Object],Continued

[object Object],See these formulas on Pages 820 and 821 of your textbook. The friction coefficient is determined by dividing tensile force by weight force. Tensile force is pulling force required to slide one of the surfaces across the other. Weight force is the force pushing down on the object being pulled. The equation: Continued

[object Object],Continued ,[object Object],[object Object],[object Object],For reasons that will be explained later, the friction coefficient of the wheel friction assemblies of vehicle brake systems is always less than one.

[object Object],Figure 68–7 The coefficient of friction ( μ ) in this example is 0.5. Continued

[object Object],Continued The friction coefficient drops by half, and it would decrease even further if the surface finish of the floor were changed from rough concrete to smooth marble. It is obvious that the surface finish of two connecting surfaces has a major effect on their coefficient of friction.

[object Object],Figure 68–8 The types of friction materials affect the friction coefficient, which is only 0.05 in this example. Continued The equation: In this case, it requires only a 10-lb force to pull the block across the concrete. Further reductions would be seen if the floor surface were changed to polished marble or other smooth surface.

Figure 68–9 The static friction coefficient of an object at rest is higher than its kinetic friction coefficient once in motion. ,[object Object],Continued

Figure 68–10 Every combination of materials has different static and kinetic friction coefficients. See the table on Page 822 of your textbook. Continued

FRICTION AND HEAT ,[object Object],Although there are too many variables to obtain exact temperature increase of any specific component, the average temperature rise of the brakes during a single stop can be computed as follows: See this formula on Page 822 of your textbook. Continued

[object Object],Figure 68–11 Brake temperature increase is determined mostly by vehicle weight, drum and rotor weight, and the change in kinetic energy. Continued We calculated this vehicle has 90,301 ft-lb of kinetic energy, and since the vehicle is coming to a full stop, the change in kinetic energy during the stop will equal the entire 90,301 ft-lb. Based on this information, the equation for computing the rise in brake temperature reads:

The temperature increase computed with this equation is the average of all the friction generating components. Some of the heat is absorbed by the brake drums and rotors, some goes into the shoes and pads, and some is conducted into the wheel cylinders, calipers, and brake fluid. NOTE: The possibility of brake fade caused by heat can be reduced if the transmission gear selector is placed in a lower gear when descending a steep or long hill to reduce amount of braking needed to maintain a safe road speed. Engine braking will help keep vehicle speed under control.

HEAT-CAUSED BRAKE FADE ,[object Object],Continued

[object Object],Figure 68–12 Mechanical fade occurs when the brake drums become so hot they expand away from the brake lining. Continued ,[object Object],[object Object],[object Object],Mechanical fade occurs when a brake drum overheats and expands away from the brake lining. To maintain braking power, the brake shoes must move farther outward, which requires additional brake pedal travel.

Figure 68–13 Some heat increases the coefficient of friction, but too much heat can cause it to drop off sharply. ,[object Object],Partial braking power can sometimes be restored by increasing pressure on the brake pedal. Extra pressure increases the amount of heat and fade. Disc brakes lining fade is possible, but less a problem because of disc brakes’ ability to dissipate heat. Continued

[object Object],Figure 68–14 One cause of 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. In most cases brake fade is a temporary condition. Brakes will return to normal once they have all been allowed to cool in all except extreme situations where the heat has been so great it has damaged the friction material or melted rubber seals within the hydraulic system.

WATER-CAUSED BRAKE FADE ,[object Object]

DECELERATION RATES ,[object Object],Continued

[object Object],[object Object],[object Object],[object Object],An average deceleration rate of 15 ft/sec 2 (3 m/sec 2 ) can stop a vehicle traveling at 55 mph (88 km/h) in about 200 ft (61 m) in less than 4 seconds. During a standard brake system test, a vehicle is braked at this rate 15 times. Temperatures at the front brake pads can reach 1300°F (700°C) or higher, sometimes reaching as high as 1800°F (980°C). Brake fluid and rubber components may reach 300°F (150°C) or higher.

SUMMARY ,[object Object],[object Object],[object Object],[object Object],Continued

SUMMARY ,[object Object],[object Object],[object Object],( cont. )

Chap68

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Chap68