How Drum Brakes Work?

Drum brakes work on the same principle as disc brakes: Shoes press against a spinning surface. In this system, that surface is called a drum.

Location of drum brakes

Many cars have drum brakes on the rear wheels and disc brakes on the front. Drum brakes have more parts than disc brakes and are harder to service, but they are less expensive to manufacture, and they easily incorporate an emergency brake mechanism.

we will learn exactly how a drum brake system works, examine the emergency brake setup and find out what kind of servicing drum brakes need.

The Drum Brake

The drum brake may look complicated, and it can be pretty intimidating when you open one up. Let's break it down and explain what each piece does.

Parts of a drum brake

Like the disc brake, the drum brake has two brake shoes and a piston. But the drum brake also has an adjuster mechanism, an emergency brake mechanism and lots of springs.

First, the basics: Figure below shows only the parts that provide stopping power.

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Drum brake in operation

When you hit the brake pedal, the piston pushes the brake shoes against the drum. That's pretty straightforward, but why do we need all of those springs?

This is where it gets a little more complicated. Many drum brakes are self-actuating. Figure 5 shows that as the brake shoes contact the drum, there is a kind of wedging action, which has the effect of pressing the shoes into the drum with more force.

The extra braking force provided by the wedging action allows drum brakes to use a smaller piston than disc brakes. But, because of the wedging action, the shoes must be pulled away from the drum when the brakes are released. This is the reason for some of the springs. Other springs help hold the brake shoes in place and return the adjuster arm after it actuates.

Drum Brake Assembly

drum brake assembly

A drum brake assembly is used to bring the rear wheels of most vehicles to a stop. Fluid pressure from the master cylinder causes the wheel cylinder to push the brake shoes against the brake drums which are attached to the vehicle's rear wheels. The friction between the stationary shoes and the revolving drums causes the drums to slow and stop the rear wheels.

Worn drums and shoes, however, can cause unreliable stopping, excessive pedal effort, or brake pedal pulsation.

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