Brakes must not only be capable of stopping a vehicle but must stop in as short a distance as possible. Because brakes are expected to decelerate a vehicle at a faster rate than the engine can accelerate, they must be able to control a greater power than that developed

Figure 7-2.- Braking requirements.

by the engine. This is the reason that well-designed, powerful brakes have to be used to control the modern high-speed vehicle.

It is possible to accelerate an average vehicle with an 80 horsepower engine from a standing start to 80 mph in about 36 seconds. By applying the full force of the brakes, such a vehicle can be decelerated from 80 mph to a full stop in about 4.5 seconds. The time required to decelerate to a stop is one eighth of the time required to accelerate from a standing start. Therefore, the brakes harness eight times the power developed by the engine. Thus about 640 (8 x 80) horsepower has to be spent by the friction surfaces of the brakes of an average vehicle to bring it to a stop from 80 mph in 4.5 seconds.

Vehicle Stopping Distance
Operator reaction time is the time frame between the instant the operator decides that the brakes should be applied and the moment the brake system is activated. During the time that the operator is thinking about applying the brakes and moving his or her foot to do so, the vehicle will travel a certain distance depending on the speed of the vehicle. After the brakes are applied, the vehicle will travel an additional distance before it is brought to a stop.

Total stopping distance of a vehicle is the total of the distance covered during the operator's reaction time and the distance during which the brakes are applied before the vehicle stops. Figure 7-3 shows the total stopping distance required at various vehicle speeds, assuming the average reaction time of 3/ 4 second and that good brakes are applied under most favorable road conditions.

Braking Temperature
Brakes are devices that convert the energy of a moving vehicle into heat whenever the brakes are applied. This heat must be absorbed and dissipated by the brake parts. Unless the heat is carried away as fast as it is produced, brake part temperatures will rise.

Since the heat generated by brake applications usually is greater that the rate of heat dissipation, high brake temperatures result. Ordinarily, the time interval between brake applications avoids a heat buildup. If, however, repeated panic stops are made, temperatures become high enough to damage the brake linings, brake drums. brake fluid, and, in some extreme cases, even tires have been set on fire.

Figure 7-3.- Total vehicle stopping distance of an average vehicle.

Factors that tend to increase brake temperatures include the following:

Load on the vehicle
Operator abuse
Speed of the vehicle
Maladjustment of brakes
Incorrect installation of brake parts
Unbalanced braking
If road speeds are increased and/ or more weight is placed in the vehicle, brake temperatures increase. In fact, under extreme conditions of unbalanced brakes on a heavy truck making an emergency stop from high speed, enough heat is generated to melt a cube of iron weighing 11.2 pounds.

Braking Ratio
Braking ratio refers to the comparison of front-wheel to rear-wheel braking effort. When a vehicle stops, its weight tends to transfer to the front wheels. The front tires are pressed against the road with greater force. The rear tires lose some of their grip on the road. As a result, the front wheels do more of the braking than the rear.

For this reason, many vehicles have disc brakes on the front and drum brakes on the rear. Disc brakes are capable of producing more stopping effort than drum brakes. If drum brakes are used on both the front and rear wheels, the front shoe linings and drums typically have a larger surface area.

Typically, front-wheel brakes handle 60 to 70 percent of the braking power. Rear wheels handle 30 to 40 percent of the braking. Front-wheel drive vehicles, having even more weight on the front wheels, have even a higher braking ratio at the front wheels.