How Disc and Drum Brakes Work
The two types of brakes used on modern cars are disc breaks and drum breaks. All new cars have disc brakes on the front wheels, while the rear wheels may use either disc or drum brakes.
Disc brakes, sometimes spelled as “disk” brakes, use a flat, disk-shaped metal rotor that spins with the wheel. When the brakes are applied, a caliper squeezes the brake pads against the disc just as you would stop a spinning disc by squeezing it between your fingers, and slows the wheel.
Drum brakes use a wide cylinder that is open at the back, similar in appearance to a drum. When the driver steps on the brake pedal, curved shoes located inside the drum are pushed outwards, rubbing against the inside of the drum and slowing the wheel.
The Difference Between Disc and Drum Brakes
Disc brakes are generally considered superior to drum brakes for several reasons. First, disc breaks do a better job dissipating heat. Under severe use, such as repeated hard stops or riding the brakes down a long incline, disc brakes take longer than drum brakes to lose effectiveness, which is a condition known as “brake fade.” Disc brakes also perform better in wet weather, because centrifugal force tends to fling water off the brake disc and keep it dry, whereas drum brakes will collect some water on the inside surface where the brake shoes contact the drums.
Why Many Cars Use Rear Drum Brakes
All cars sold in the United States use disc brakes for the front wheels, but many cars still use drum brakes in the rear. Braking causes the car’s weight to shift forward; as a result, about 70% of the work is done by the front brakes. That’s why your front brakes tend to wear out faster. Drum brakes are less expensive to make than disc brakes, largely because they can also double as a parking brake, whereas disc brakes require a separate parking brake mechanism. By fitting disc brakes to the front wheels and drum brakes to the rear wheels, manufacturers can provide most of the benefits of disc brakes while lowering costs.
Even so, a car with disc brakes on both front and rear axles will provide superior braking performance in wet weather and on long downgrades. Incidentally, you should never ride your brakes when driving down a long incline. Instead, downshift and let the engine control the car’s speed.
How to Tell If Your Car Has Disc or Drum Brakes
If your car was built in the last thirty years, it most likely has disc brakes on the front wheels, but it may have drums in the rear. If the car has wheels with big openings, you may be able to see some or all of the brake assembly. Seen through the wheels, disc brakes have a flat rotor set back from the inside surface of the wheel and a wider piece (the caliper) at the front or rear of the disc. Drum brakes have a cylindrical drum that sits flush against the inside surface of the wheel.