ANALYSIS of dynamometer and field tests leads the author to suggest the following criterion for determining when to use bimetallic brake drums: When cooling air is available, the bimetallic drum will outperform the standard all-cast-iron drum. When cooling air is not available, then its use, except to eliminate squeal, gives only a dubious advantage.
Cooling air is of little help to the standard cast-iron drum, he explains, because this type acts merely as a heat reservoir. The bimetallic drum, on the other hand, functions as a heat exchanger, for the heat is rapidly conducted from the friction surfaces to the aluminum fins, where it is dissipated by the cooling air. Thus, the brakes can be used steadily without their getting so hot as to become ineffective.