Rear drum brakes work in tandem with front disc brakes to help a car stop. Most cars had drum brakes on both the front and rear wheels until the 1960s when disc brakes were developed. Today, many cars still employ drum brakes in the rear wheels.
A drum brake stops a wheel by using components contained in a drum that rotates within the wheel. When a driver presses down on the brake pedal, a foot inside the drum presses against the wheel to slow it down. This creates heat from the fiction. Drum brakes are much less efficient at dispersing the heat than disc brakes, which consist of brake pads on each side of the rotor that are exposed to the air outside. Too much heat from friction that is not dispersed decreases the effectiveness of drum brakes. However, because between 60 and 90 percent car's stopping power comes from the front wheels, many cars can be built safely today using drum-brake technology, which is less costly, on the rear wheels.
On the whole, disc brakes are more effective because of their more advanced heat displacement. The two main physical principles involved in stopping a car from moving are friction and heat. Applying resistance to a moving wheel causes friction, which in turn causes heat that has to be dispersed so the system continues to work effectively.