There are a number of attributes that differentiate between mediocre and a good brake drum. The following highlights these and what they mean to the end user.
HEAT ABSORPTION DEVICE (heat sink)
The braking system in a vehicle converts the vehicle energy of motion into heat energy by means of friction between the brake lining and the brake surface of the brake drum when the vehicle’s brakes are applied. The surrounding parts, such as brake lining, rim, tire, hub, bearings, and drum absorb heat by conduction and radiation. The surrounding air is heated by convection. Radiated heat to surrounding parts is small and heat dissipated by convection to the surrounding air is slow relative to the actual time of braking.
For these reasons nearly all the braking energy must be stored in the form of heat in the brake drum. This energy is stored by heating the drum material to a higher temperature than the drum temperature prior to the brake application. A heavier drum will store more energy than a light one will. A heavier drum is stronger and more fade resistant.
CYLINDRICAL BRAKE LINING INTERFACE
The brake drum moves past a stationary brake lining. When (if) the brake surface does not move past the lining uniformly, the brake drum has run out. This will overstress the brake drum, linings, and attaching hardware, causing variable heating of the brake drum braking surface which results in heat checking, excessive ware of both the drum and lining, pulsing of the brake, and shortened life of both the drum and lining. Also to keep the drum dimensionally stable under these conditions, it must be a relatively heavy brake drum.
A heavy precise brake drum does not reduce the need for linings to fit the drum correctly. If the linings are not contoured to fit properly, the brake will not produce proper breaking torque. Poorly contoured linings cause additional mechanical stress in the drum, shoes and lining, and brake hardware. This additional stress hastens failure and greater wear.