Disc vs. Drum Brakes: Why Most Cars Use Both

How does a disc brake stop the car?

A disc brake has a flat metal rotor that spins with the wheel. A caliper straddles the rotor, and when you press the pedal, hydraulic pressure pushes brake pads against both sides of it. The friction slows the rotor and the wheel with it.

Because the rotor is exposed to the air, it sheds heat well and recovers quickly after hard stops. The pads are also easy to inspect. A technician can often measure pad thickness and see the rotor surface without taking much apart.

How does a drum brake do it differently?

A drum brake is a hollow metal drum that spins with the wheel. Inside, two curved brake shoes sit on a backing plate. When you press the pedal, a wheel cylinder pushes the shoes outward against the inside surface of the drum.

The design is enclosed, which keeps the parts shielded from road dirt and water, and it works well as a parking brake. It holds heat inside the drum, though, which is one reason it is less suited to hard, repeated braking.

Why do the front brakes do most of the work?

When you brake, the car's weight shifts forward. The front tires are pressed harder into the road and the rears get lighter, so the fronts can use more braking force before they lose grip. As a result, the front brakes handle the majority of the stopping.

That is why many cars have larger or more robust brakes at the front and a simpler design at the rear. Disc brakes suit the heavier front workload, and a drum brake can be enough for the lighter rear job. Some vehicles, particularly smaller or lower-cost designs and some trucks, still use rear drums, while many others have discs at all four wheels. Your owner's manual or an inspection will tell you which your car has.

What are the drum-specific warning signs?

Many symptoms overlap with disc brakes, such as noise, a soft pedal or pulling. A few are more specific to drums:

• Weak or low parking brake: a lever or pedal that travels too far can mean the shoes are worn or the adjuster is not doing its job.
• Self-adjuster trouble: most drum brakes adjust themselves as the shoes wear, and a stuck adjuster can leave the pedal low or the rear brakes weak.
• Wheel cylinder leaks: fluid seeping from a wheel cylinder can soak the shoes and reduce their grip, and it may show as a low fluid level or dampness at the rear wheel.
• Scraping from the rear: a metallic scrape from the back wheels can mean the shoe lining is worn through.

A brake inspection covers all four corners, whichever design your car uses.