BRAKE "FADING"

Rotorua Morning Post, Volume 2, Issue 586, 18 July 1933, Page 2

BRAKE "FADING"

MAiSrt iMPiORTANT REASONS FOR LOSS OF POWERL DA'NGER INViOLViED. The loss of bralcing power in the course of a long descent is a fault wihich, at the least, is discohcefting and at the worst may hecome positdvely dangerous. Nowadays (states the Motor), when the average car is fitted with only one s.et of brakes operated either by the peclal or the hand lever it is particularly iinportant that the linings and the cooling facilities should meet all requirements. Otherwise, if the hrakes "fade," there is no reserve set to bring into action. A few figures will sh'ow the vast amount of energy which the brakes are called upon to dissipate when a car is descending a hilj. Suppose that for a mile, the road slopes continuously at a rate of 1 in 10; this is quite a moderate gradient, but neverthe less a car will descend vertically through 528 feet in this distance. A medium-sized vehicle, weighing about 32cwt., will lose potential energy (by descending dn this way) to the extent of 2^00,000 foot-pounds. Were it not for the action of the hrakes this would be converted into kinetic energy and would suffiee to. add about 130 m.p.h. to the speed of the car. It is clear that when the driver uses the brakes to maintadn a steady speed, the whole of th'e energy lost in the descent must be dissipated in the form of heat at the brake drums; At a speed of 30 m.p.h. the brake will work at a rate of 30 h.p. and if the cooling facilities are not sufficient to cope with the rate at which the heat units are produced, the temperature of the brake drums and linings may rise to an extent prejudicial to efficient braking. This is the main cause of "faddng" phenomena. Modern linings of good quality have a very high resistance to the effects , of heat, maintaining th'eir co-efficient of friction to a surprising extent, even at temperatures so high as 500 de- ■ grees F. Consequently, unless very poor linings have been fitted as a mistaken form of economy, brakes which fade on long descents are usually troublesome owing to the expansion of the drums. 0 On the many high-grade cars ribhed ; drums of considerable size are employed, the ribs serving not only to stiffen the periphery, but also acting as fins to dissipate th'e heat. The plain steel drum cannot get rid of the heat so readily, and if it is of unduly small diameter its temperature may rise to a very considerable extent on a long hill. High leverages are used in the brake-operating gear, which means that an expansion of the drum amounting to only one-hun-dredth of an inch may result in serious pedal movement. This point should empahise the fact that it is esesntial to keep the brakes in proper adjustment when using a car in a hilly distriet. Otherwise, a pedal which comes near to the floorboards when appliied at the commencement of a long descent may meet the boards hefore the bottom of the hill is reacbed, s0 greatly reducing the braking effort which can be applied. Furthermore, the various brake levers are usually arranged. so as to work most efficiently when the adjustment is correot. As the pedal gets nearer to th'e floorboard the lever angles change, and there is usually a distinct loss in the power which can be applied by the foot. Some Effective Precautions. Apart from the provision of stiff, well-cooled drums, the car manufacturer should make the brakes of adequate siize. This will reduce the rise of temperature and will keep the pressure exerted per square inch of lining with'in reasonable limits. The ownerdriver on his side should see that material of good quality is used when the brakes require relining and should keep the breaking system in proper adjustment. In the case of a car which ds prone to this fading trouble the engine should be used to give additional braking on long descents, employing second or third speed, as required, to enh'ance the effect by increasing the revolution speed of the crankshaft. The same plan can ihe used in emergencies. It may be of interest to mention, in ;onclusion, that the expansion of the irakp. drums on a long descent has

provided a difficult problem for inventors of automatic brake adjusters. Such devices operate by shortening the linkage between the pedal and th'e cams when the shoes movie outward beyond a predetermined limit, so com. pensating for lining wear. Expansion of the drums will cause the adjustment tb be made, and then, when cooling occurs, the brakie shoes will rub because the drums have contracted.