Disc brakes are a brake system that slows a wheel’s rotation by squeezing an attached metal disc in a viselike hydraulic caliper.

Disc brakes use the same principle as bicycle handbrakes, but on a bike the brake pads press against the wheel itself. On a car, the disc is part of the hub to which the wheel is mounted. The disc, technically called a rotor, is clearly visible through spoked wheels.

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.

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 disc brakes, one or more pistons in each brake caliper then push the brake pads in contact with the rotor.

The pads create friction against the rotor, which slows and ultimately stops the wheel. The brake pad lining material is designed to prevent noise and withstand the excessive heat created by the friction.

Pros and Cons
Disc brakes are favored over drum brakes for several reasons. First, they’re more linear in their operation and less prone to locking up. Second, the open design allows more efficient cooling than enclosed drum brakes. Although heat is an unavoidable byproduct of friction, if brakes get too hot, they become less effective — a condition called brake fade. Disc brakes are also more resistant to water because the rotors sling it away and the pads serve as a squeegee. 

Finally, disc brakes are inherently self-adjusting, which means they can remain fully functional without a mechanic’s intervention as the pads wear. Before the 1960s, drum brakes did not self adjust; most do now, but the self-adjusting relies on mechanical provisions that are less reliable compared to the disc system.

Disc brakes require more hydraulic pressure to operate than drum brakes do, which is why power brakes are now the norm, even in entry-level vehicles.

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