Operation

• Both disc and drum brakes work on the same principle: applied friction between two surfaces reduces the motion of one hard surface while reducing the softer surface's thickness. In the case of brakes, a disc or a drum spins with the wheel, and pads are applied with hydraulic pressure to the disc or drum surface. The friction between the two surfaces generates heat and also nibbles away a small portion of the stationary pad. The motion of the disc or drum, however, slows and comes to a stop. Eventually, the soft pad or shoe will require replacement.
  
  

Disc

• Disc brakes are flat discs attached to the spinning wheel hub. Attached to the stationary axle is a caliper surrounding a portion of the disc. Two pads, installed in the caliper, are on each side of the disc. When the brake pedal is pushed, hydraulic fluid is pumped into the caliper, and the pads squeeze the rotating disc. The squeezed disc loses momentum and will come to a stop with enough applied pressure.
  
  

Drum

• Drum brakes have a hollow drum attached to the wheel. Inside the drum is an actuator that instead of squeezing pads together, as in a disc brake, pushes two "shoe" pads out to contact the inside surface of the spinning drum. The friction between the shoes and the drum reduces the drum's spin, and the wheel will slow and stop.
  
  

Anti-lock

• Both disc and drum brakes have a tendency to "lock" when actuated. Locking brakes work by the pads locking onto the drum or disc surface and abruptly stopping the wheel. If the car is at a moderate speed, this will cause the wheel to skid, possibly making the car uncontrollable. Anti-lock brakes (ABS) is a computer system that rapidly engages and disengages the pads faster than a human operator can. This slows the wheel while the quick alternating release and pressure cycle prevent lockup.
  
  

Disc and Drum Brakes Together

• Many car models have both disc and drum brakes with discs on the front and drums in the rear. Drum brakes are mechanically simpler than the disc and caliper design and thus less expensive. These cars, however, will still have front disc brakes because disc brakes are more responsive to driver input. This response is important on the front turning wheels whereas the stationary back wheels do not always require this kind of performance. These two brake styles reduce the cost for the car while providing safe and responsive handling.