Both slow a wheel by pressing friction material against a rotating metal surface, but they do it in opposite ways. Use the buttons to look at them side by side: a disc brake clamps pads onto a disc, while a drum brake pushes shoes against the inside of a drum.
Disc and drum brakes share the same principle: turn the car's motion into heat through friction. Where they differ is which surface they squeeze. A disc brake uses a flat rotor that turns with the wheel and a caliper that clamps friction pads onto it from the outside. A drum brake uses a round drum that turns with the wheel and curved shoes that press outward against its inner wall.
That single difference explains most of their behaviour. The disc's friction surfaces are open to cooling air, so it stays cooler and its performance holds up better under repeated hard braking. The drum encloses its friction surfaces, so it keeps heat — and, because a shoe gets dragged around by the drum, it also produces strong braking with relatively little pedal force.
Most modern cars use disc brakes at the front, where braking loads are highest, and often drum brakes at the rear, where the loads are lower and their low cost and enclosed parking-brake action still appeal. Seeing them side by side makes the trade-offs between heat, cost and force obvious.
In a disc brake the rotor turns with the wheel, and a caliper straddles its edge. When hydraulic pressure reaches the caliper, its piston pushes a friction pad against each face of the disc, so the disc is gripped from both sides at once.
Because the disc and pads sit out in the moving air, the heat they generate is carried away quickly, and many discs are further vented with internal fins. That is why disc brakes hold their performance through hard, repeated stops and resist fade.
In a drum brake the friction surfaces are inside. A wheel cylinder at the top pushes the two curved shoes outward until they meet the spinning inner wall of the drum, and the friction between shoe and drum slows the wheel.
As the drum turns it drags each shoe around with it. That drag pulls the leading shoe into the drum, so the shoe's wedge action adds to the force the cylinder applied. The result is a lot of braking from a modest pedal effort, which is the drum's main strength.
The catch is the enclosure. With its friction surfaces hidden inside, a drum cannot shed heat as readily, so during sustained heavy braking its temperature climbs and its friction drops. That loss of grip as things get hot is called fade, and drums are more prone to it than discs.
When the brake is released the hydraulic pressure falls and a return spring pulls the shoes back off the drum wall. That spring is what stops the shoes from dragging and lets the wheel spin freely again.
Front brakes do most of the work under braking, so they are usually discs. The rear can often manage with drums, whose lower cost, compact shape and easy parking-brake action suit a lighter duty. That is why a car may have discs at the front and drums at the back.
A disc brake squeezes friction pads against the outside of a spinning disc, while a drum brake pushes curved shoes outward against the inside of a rotating drum. One clamps from outside, the other presses from within.
A disc brake. Its rotor is open to the air and can be vented, so heat escapes quickly. A drum encloses its friction surface, so heat builds up inside, which makes drum brakes more prone to fading under hard, repeated use.
Because of self-energising. As a leading shoe is dragged by the rotating drum, it is pulled into the drum and adds its force to the applied force, so a drum can produce a lot of braking with relatively little input.
A disc brake. Its friction surfaces are open, so water and dust are thrown clear and drying happens quickly. A drum can trap water inside, which can reduce braking until it dries out.
They are cheap, compact and produce strong braking at low cost, and their enclosed design suits the mechanical parking brake. That is why they often survive as the rear brake on smaller or cost-sensitive cars.
A disc brake. The pads and rotor are open and easy to reach and inspect, whereas a drum brake hides its shoes and springs inside the drum, which takes more work to open up and reassemble.