Brake Drums

By Norman Nock


This article covers the basic workings of the brake drums and shoes. The pressure of the brake shoes against the drum starts with the force applied to the brake pedal. The shoes are pushed out against the brake drum; one end bears against an anchor point, or heel, the other end is pushed out by the wheel cylinder. When the drum is rotating in the direction shown in Figure 1, the friction force between the brake drum and the lining tries to turn the shoe around the anchor point. The shoe is then forced more strongly against the drum, like a wedge. This is known as self-energization end bears against an anchor point, or heel, the other end is pushed out by the wheel cylinder. When the drum is rotating in the direction shown in Figure 1, the friction force between the brake drum and the lining tries to turn the shoe around the anchor point. The shoe is then forced more strongly against the drum, like a wedge. This is known as self-energization.



The wedging action starts at the toe, or the wheel cylinder end. and keeps increasing as it nears the anchor point. If the drum is revolved backwards (reversing) there is no self-energization. Therefore, you have less braking when going backwards.


The rear brakes have one leading shoe and one trailing shoe, as shown in Figure 2, unlike the front brakes that have two leading shoes. When the brakes are applied the wedging action is only on one of the shoes, depending on the direction of rotation of the drum. The reason for this type of brake is so you will have stopping power by always having a leading shoe. and when using the hand brake you will always have a leading shoe which ever way you park on a hill.


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