Drum Brakes

Drum brakes are a friction-braking design in which curved friction shoes are forced outward against the inner surface of a rotating drum that turns with the wheel. They predate the disc brake and were once fitted at all four corners of most cars, but they have since been largely superseded by discs at the front and survive mainly on the rear axles of less expensive vehicles. Their continued use reflects a balance of cost, mechanical simplicity and a particular suitability for the parking-brake function rather than any advantage in outright performance.

Inside the drum sit two brake shoes lined with friction material, held against return springs. When the driver presses the pedal, hydraulic pressure acts on a wheel cylinder that pushes the shoes apart so they press against the inside of the spinning drum. Friction between lining and drum slows the wheel. Many designs are self-energising, or self-servoing: the rotation of the drum tends to drag the leading shoe harder into contact, multiplying the applied force. This effect lets a drum brake generate considerable stopping force from relatively modest pedal effort, which was valuable in the era before brake servos became universal.

The layout brings genuine economic and packaging benefits. Drum brakes are cheaper to manufacture, and their enclosed construction protects the friction surfaces from road grime and water, which suits them well to the rear of a vehicle. That same enclosure makes them convenient for incorporating a mechanical parking brake, since a simple cable and lever can spread the shoes directly; this is why drums, or a small drum-in-hat arrangement, often persist even where rear discs are fitted.

The defining weakness is heat. Because the friction occurs inside a closed drum with little airflow, the heat generated under hard braking has nowhere to go. The components soak it up until the friction material loses its grip, producing brake fade, a dangerous reduction in stopping power during prolonged or repeated heavy braking, such as a long mountain descent. Trapped heat can also expand the drum away from the shoes, lengthening pedal travel. Water and brake dust collecting inside the drum can further upset performance, and the mechanism is more awkward to inspect and adjust than an exposed disc.

For these reasons drum brakes have given way to disc brakes wherever braking loads are high, particularly at the front, where most of a car's weight transfers under deceleration. On the rear of small, light and budget cars, however, where the braking demand is lower and cost matters, drums remain a practical and durable choice. They are best understood in contrast with disc brakes and through the phenomenon of brake fade that exposes their principal limitation.