Despite other notable designs of the time like Lanchesters disc brake, drum brakes were standard fitment on cars for the best part of 60 years. Nowadays the effiency of the disc drake, has relegated them to a bit part player only found occasionally at the rear of your car.

 

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.

 

 Drum brake with drum in place

 Drum brake without drum in place

 

How a Drum Brake Works
The drum brake may look complicated, but the principle is fairly simple the shoes are pushed out against the drum surface to supply friction.

 

 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.

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.  As when 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.

How the Brake Adjuster Works

For the drum brakes to function correctly, the brake shoes must remain close to the drum without touching it. If they get too far away from the drum, as the shoes wear down for instance, the piston will require more fluid to travel that distance, and your brake pedal will sink closer to the floor when you apply the brakes. This is why most drum brakes have an automatic adjuster.

 

 Adjuster mechanism

As the pad wears down, more space will form between the shoe and the drum. Each time the car stops while in reverse, the shoe is pulled tight against the drum. When the gap gets big enough, the adjusting lever rocks enough to advance the adjuster gear by one tooth. The adjuster has threads on it, like a bolt, so that it unscrews a little bit when it turns, lengthening to fill in the gap. When the brake shoes wear a little more, the adjuster can advance again, so it always keeps the shoes close to the drum.

Some cars have an adjuster that is actuated when the Handbrake/parking brake is applied. This type of adjuster can come out of adjustment if the Handbrake/parking brake is not used for long periods of time.

How the handbrake Brake Works
The Handbrake brake on a car has to be actuated by a different power source than the primary braking system. The drum brake design allows for a simple cable actuation mechanism.


When the Handbrake brake is actuated, a cable pulls on the lever, which forces the two shoes apart.