It’s common to find a combination of disc and drum brakes in new cars today. The typical setup consists of front disc brakes and either disc or drum brakes in the rear connected by a system of tubes and hoses that link the brake at each wheel to the master cylinder. So what’s the difference between drum and disc brakes?

Drum brakes

Early automotive brake systems used a drum design that was invented in 1902 by Louis Renault. Drum brakes consist of a backing plate, brake shoes, brake drum, wheel cylinder, return springs and an automatic or self-adjusting system.

The backing plate holds everything together and is attached to the axle. It forms a solid surface for the wheel cylinder, brake shoes and assorted hardware.

When you apply the brakes, brake fluid is forced under pressure into the wheel cylinder, which in turn pushes the brake shoes into contact with the inside of the drum. When the pressure is released, return springs pull the shoes back to their rest position.

Brake shoes consist of a steel shoe with the friction lining riveted or bonded to it. As the brake linings wear, the shoes must travel a greater distance to reach the drum. When the distance reaches a certain point, a self-adjusting mechanism will automatically adjust the resting position of the shoes closer to the drum.

The wheel cylinder consists of a cylinder that has two pistons, one on each side. Each piston has a rubber seal and a shaft that connects the piston with a brake shoe. When brake pressure is applied, the pistons are forced out pushing the shoes into contact with the drum. 

Return springs pull the brake shoes back to their rest position after the pressure is released from the wheel cylinder.

Disc brakes

Disc brakes have now become the more common form in most passenger vehicles, although many still use drum brakes on the rear wheels to keep costs and weight down as well as to simplify things on the engineering front to accommodate a parking brake.

The first-ever car disc brakes were patented by Frederick William Lanchester in 1902, though it took another half-century for his innovation to be widely adopted.

The disc brake is a lot like the brakes on a bicycle. As you apply brake pressure, fluid from the master cylinder is forced into a caliper where it presses against a piston.

The piston, in turn, squeezes two brake pads. The two brake pads on each caliper are constructed of a metal “shoe” with the lining riveted or bonded to it.  The pads are mounted in the caliper, one on each side of the rotor. The disc rotor is made of iron with highly machined surfaces where the brake pads make contact with it.

The rotor used in disc brakes is fully exposed to outside air, which constantly cools the rotor, greatly reducing its tendency to overheat or cause fading.