What is a drum brake?
A drum brake, as the name suggests, is a type of braking device that generates stopping power by pushing a friction material into contact with a rotating drum.
How does a drum brake work?
Unlike a disc brake assembly, which features a caliper pressing brake pads onto a disc, a drum brake relies on a pair of brake shoes being moved into contact with a drum.
In a simple drum brake, a pair of moveable brake shoes are located on a backing plate with retaining pins and clips. This assembly is then bolted to the axle housing, suspension upright or trailing arm. A double-acting hydraulic cylinder is mounted between the shoes and, when brake pressure is applied, the brake cylinder pushes the shoes apart.
This action presses the friction material on the shoes against the interior of the drum, which is clamped to the wheel hub. This causes the drum to slow, providing the required stopping power at the wheel. When the brake pedal is released, springs pull the brakes shoes back into their original position so that they don't drag on the drum and are ready to be applied again.
A drum brake is often hydraulically operated, as described above, but older set-ups may rely on cables or rods to transmit force from the brake pedal to the actuating mechanism of the drum brake.
When being employed as a parking brake, however, the handbrake is usually connected to the drum brake by a simple cable. This pulls on a lever in the drum brake assembly that forces the shoes into contact with the drum and prevents it from rotating.
Do new cars still use drum brakes?
Drum brakes are predominantly used by older cars - and are very common among classic cars. That said, some less expensive modern cars feature hydraulically operated drum brakes on the rear axle for reasons including cost and simplicity.
Many modern and aftermarket set-ups also feature a small internal drum brake assembly inside the rear disc brake; when the parking brake is applied, the internal brake shoes are pushed outward. This prevents the disc from rotating, locks the wheel and secures the car.
Besides appearing visually neater, this configuration also prevents the disc brake's pads being clamped to a still-hot disc, which can cause an uneven build-up of pad material on the surface of the disc - resulting in an unpleasant vibration when braking.
Applications of drum brakes
Front brake assembly: Front drum brakes are only found on older classic cars. In many cases, besides being larger than the rear drums, these sometimes feature finned drums - which are better at dissipating heat, improving braking performance during heavy use.
Rear brake assembly: Providing stopping power for the rear axle is the most common application of drum brakes. A drum brake also easily functions as a parking brake, helping further keep costs and complexity down.
Parking brake: Rear drums, besides providing stopping power, are also employed for parking brake duties. Pull the handbrake and the brake shoes will be pushed into contact with the drums, locking the wheels on that axle and preventing the car from rolling away. Many cars, alternatively, feature small integral drum brakes inside the rear disc brakes.
Transmission brake: These parking brake assemblies are often found on older or industrial four-wheel-drive vehicles. In this configuration, the drum brake is mounted somewhere in the drivetrain - such as on the output of a transfer case - and often provides stopping power to all of the wheels.
Why did manufacturers change to disc brakes?
Firstly, disc brakes can provide better stopping power and aren't as prone to fade as drum brakes. Repeated hard stops can quickly overheat a drum brake, as the enclosed set-up cannot dissipate heat as quickly as a disc brake - because the entire assembly of a disc brake is exposed to the air passing by it, and the disc itself is often being ventilated to further improve cooling.
Secondly, disc brakes require little maintenance and are self-adjusting. Drum brakes, while often featuring a self-adjustment mechanism, require more regular attention and can be difficult to set up properly.
Similarly, servicing a brake disc assembly - changing the pads, swapping out the disc - is far easier than tackling the myriad springs, retaining clips and pins found in a drum brake.
A properly maintained and configured drum brake is capable of providing good stopping power in normal conditions, however, and often well suited for the rear axle of a car. That said, the visual appearance of a disc brake is considered by many to be worth the additional outlay alone.