Figure 7-14.Self-energizing and servo action.
problem that occurs during heavy braking is brake
fade. During panic stops or repeated harsh stops, the
brake linings and drum develop large amounts of heat
that reduces the amount of friction between the brake
shoe and drum. This reduction in friction greatly
decreases the stopping ability of the vehicle, and, in
most cases, additional pressure directed on the brake
pedal would not increase the stopping performance of
The enclosed design of the brake drum assembly
does not allow for cooling air to enter the assembly and
therefore heat developed during braking must be
dissipated through the brake drum and backing plate.
As the brakes heat up due to repeated application,
cooling air flowing past the drums and backing plates
is limited. This condition causes the radius of the drum
to increase more than the radius of the brake shoe. As a
result, a change in pressure distribution between the
linings and the drum occurs, which reduces the braking
ability of a vehicle by up to 20 percent.
The enclosed design also does not allow for water
to be expelled rapidly should the brake cavity become
wet due to adverse weather conditions. The water
reduces the frictional properties of the brake system
and must be removed to restore braking ability. This is
a very dangerous situation and drastically reduces the
stopping ability of the vehicle until the system is dry.