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Are disc or drum brakes the more efficient and are
Page 207
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disc brakes ever likely to be used on heavy commercial vehicles?
AWhich type of brake is the more efficient
depends upon the application and also upon the weather. A simple disc is far less prone to expansion and contraction and will dissipate the heat generated during braking much quicker than the larger mass of a drum.
However, the efficiency of the disc brake is adversely affected in wet weather at low road speeds. This is because the disc does not revolve fast enough to spin water off its braking surface and there must be at least one revolution while the disc is being wiped dry when there is little or no braking effect.
When harsh braking from high speeds is frequently applied—a fire engine is one example— the high rate of wear to which the small friction pad of the disc brake is subject is of no consequence. For such vehicles discs are gradually being adopted as standard equipment.
Disc brakes have been in service on some heavy commercial vehicles since about 1961. One of the first companies to utilize this type was United Dairies. Through its associate company, Mickleover Transport, it equipped a number of Scammell semi-trailers with poweroperated disc brakes.