Q What is the explanation of fade when the brakes
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have been applied over an extended period? Are disc brakes less likely to fade than drum brakes and if so why? And is mosture detrimental to brake efficiency?
AIn the case of drum brakes, fade is
frequently caused by the drum expanding more than the shoe when excessive heat is generated by continuous or severe application. A greater amount of heat is dissipated through the drum than the brake shoe and when the drum expands differentially the area of drum/shoe contact is reduced. And this tends to cause fade.
Differential expansion of the disc and pad mechanism of a disc brake does not reduce the pad contact area and fade is less likely to result from severe application. The coefficient Of friction of a pad or lining material may deteriorate at elevated temperatures and consequently a cool running brake of either type is less liable to fade than one which tends to overheat whether or not there is differential expansion. In typical applications a disc brake operates at a lower temperature than a drum brake of comparable capacity in which the same amount of heat is produced. The pad is open to the air and if the disc is not shrouded the flow of cooling air over the disc is generally greater than the flow over a drum at a given speed. The fitting of a shroud over a disc has the advantage that it reduces the amount of abrasive material in the friction area in dusty operating conditions. But it reduces heat dissipation from the area.
While special linings and pads are available having a. better anti-fade characteristic than conventional materials, they are normally more costly or have a shorter wearing life.
The effect of moisture on brake efficiency depends on the amount of moisture on the linings. A small amount may increase the friction property of the lining and the first application of the brakes after the vehicle has been standing overnight and moisture has condensed in the drums may produce a higherthan-normal rate of retardation. If the linings are soaked in water their friction coefficient is reduced and full brake efficiency is not restored until the brakes have been dried out by continuous or repeated application.