A heavy high-speed vehicle, such as a modern motor-coach, combines weight and speed, requires frequent brake-applications and necessitates the dissipation of an increased quantity of heat. As it weighs about four times as much as a passenger car approximately four times the braking-effort is required; consequently four times as much pedal-pressure must be exerted. To supplement the driver's muscular equipment, some outside force, such as compressed air, the vacuum of the intake-manifold or the inertia of the moving vehicle, must be utilized. The author limits his present discussion to the use of the last-named force which he terms the use of “self-actuation,” and also to its application to the rigid-shoe type of internal wheel-brake. Through mathematical analysis he determines the effect of self-actuation measured in terms of the increase or decrease of the cam pressure required to sustain the same normal pressure before and after an outside torque has been applied to the brake drum. The term “actuation” is used in preference to “energization,” because the action of actuation is said to be precisely the same as the action of the cam pressure, namely, to rotate the shoe about its anchor-pin; and this action is usually spoken of as one of actuation rather than of energization.
After defining the terms “actuation factor” and “actuation constant,” the author, by further mathematical analysis, determines their values under different conditions. Among other points considered are the determination of the limits of the safe arc of contact without the possibility of grabbing; the division of pressure between the shoes when journaled and when floating cams are used; a comparison of the life of two sets of floating-cam brakes of unequal length with that of conventional side-by-side floating-cam brakes of equal width, fitted into a brake drum of the same width; the method used in floating the cam; the use of aluminum for shoes; and a comparison of the braking-efforts with the journaled and with the floating types of cam.
The conclusion reached is that, in order that any braking-system may operate efficiently, the linkage must be rigid and its geometry must be correct, so that neither the road spring-deflection nor the movement of the axle due to torsional reaction will affect the adjustment of the brakes.