Drum brakes are a brake system that slows a wheel’s rotation by pressing curved brake shoes against the inside of an attached metal drum.

Where disc brakes have a metal disc attached to the hub to which the wheel mounts, drum brakes use a metal drum that rotates along with the wheel. Brake shoes are basically the same as the brake pads used with disc brakes, but the two shoes are curved to match the inner surface of the rotating drum.

Virtually every vehicle manufactured today employs disc brakes on the front wheels. This is because disc brakes are, by most standards, superior to drum brakes, and the front wheels are more important when it comes to stopping a vehicle. They do most of the work — unless you do a majority of your driving in reverse. Some vehicles still use drum brakes for the rear wheels, but four-wheel drum brakes are rare.

How They Work
Automotive brakes use friction to convert the vehicle’s momentum into heat. When you step on the brake pedal, a piston in the vehicle’s master cylinder pushes hydraulic fluid through tubes to the braking system at each wheel. With drum brakes, a hydraulic cylinder/piston complement between two brake shoes then pushes each shoe outward and into contact with the inner surface of the rotating drum.

The brake shoe lining material creates friction against the drum, which slows and ultimately stops the wheel. Brake shoe material is designed to prevent noise and withstand excessive heat.

Pros and Cons
The drum brake’s principal advantage is that it requires less hydraulic force than do disc brakes, which made drums more feasible when power-assisted brakes were a luxury rather than commonplace. (Manual and power brakes use the same principle, employing pistons and hydraulic fluid, but power brakes add a vacuum- or hydraulic-powered booster that helps you push on the master cylinder with greater force than you could unaided.) Drums require less force partly because their design is self-multiplying: Brake shoes typically are hinged to each other and/or the stationary backing plate in such a way that, when the shoes contact the drum upon brake application, the drum’s rotational force pushes them even harder onto its surface. The resulting friction is greater than what would be produced by the cylinder’s force alone.

This design also marks one of the drum brake’s principal disadvantages — less linearity and a tendency to lock up. The use of antilock brakes mitigates this shortcoming, but the enclosed nature of drums tends to trap water and air, so they remain more apt than discs to lose effectiveness. Water interferes with the goal of friction, and airflow is necessary to cool brakes, which fade if they overheat. The drum design may exacerbate brake fade beyond the loss of friction from the linings: If the drum and brake shoes overheat and expand considerably, their size and shape relative to each other may change enough to decrease the contact area.

Another advantage of drum brakes is that they can serve as a parking brake with the addition of a few simple components. On the flip side, in order for drum brakes to operate without a mechanic’s intervention as the shoes wear, they must include mechanical adjusting provisions that are less reliable than the inherently self-adjusting disc-brake design.

Information for this was taken from the Cars.com’s glossary, written by Joe Wiesenfelder.

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Answered by Joe Bruzek on October 14, 2008 in Glossary | Permalink

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