Brakes have three functions: (a) maintaining a car at rest, (b) reducing the speed of a vehicle or bringing it to a stop and (c) holding a vehicle to a constant speed on a descending grade. The kinetic energy of a moving vehicle is directly proportional to the weight of the vehicle and to the square of its speed. The amount of heat produced in the braking surfaces of a vehicle descending a given grade for a given distance will be the same whether the speed be high or low, but the rate of heat production will vary inversely as the speed. In addition to the retarding effect of the braking system a braking effect is constantly present that depends on the tractive resistance of the vehicle at various speeds and on the engine itself. Wind resistance and the resistance of the engine when the throttle is closed also produce retarding effects that assist in the work of braking.
Among the desirable features that should be kept in mind in designing brakes are (a) the maximum retarding effect with reasonable physical effort on the part of the driver, (b) smoothness of retardation, (c) a retarding effect proportional to the pedal pressure, (d) no tendency to be self-locking, (e) durability or long life, (f) ease and simplicity of adjustment, (g) strength, easy operation and thorough dependability and (h) brake-operating levers that may be easily and quickly reached in emergency.
The power available for operating brakes is a pedal movement of 4 to 5 in. coupled with a foot pressure that varies with the person operating the brake; consequently, brakes should be designed for the weakest driver. A properly designed mechanism will sometimes accomplish the same result with one-half the pedal movement that is required by another linkage of the same type but of inferior design. The linkage should be simple and direct, and the relation of the centers of the connections that operate the brakes on the axles and the wheels should be carefully studied so that the relative motions of the axle and the frame will not affect the linkage. If the major part of the multiplication of leverage takes place at the brake-drum the strains in the operating mechanism will be reduced to a minimum and the longitudinal motion will be increased to a maximum. In any design the two points to be kept in mind are a low rate of wear and a high rate of heat dissipation. In general, large linear dimensions are desirable but better air-circulation sometimes makes smaller sizes preferable. In light cars weight is of importance, while in heavy cars size is the controlling consideration. Unit pressures depend largely on the materials used; a brake-drum with hard surface with a lining of suitable material may enable higher unit pressures to be used than a soft surface with a lining inherently weak. Most brakes depend on either the external or internal application of shoes or bands. The features of the band type are light weight and a large friction-area. The external band is easier to design as the anchor point can be placed in any position desired. To obtain smooth action and long wear from a shoe-type brake, rigidity of both the drum and the shoes is essential. The balance of advantage appears to be in favor of the contracting-band brake, especially when the travel is over hard-surfaced roads. The requirements of braking materials are that the drum be as hard as is commercially possible and retain its hardness when heated, while the brake-lining should be close-grained and hard, with a uniform coefficient of friction throughout its life.