Brake Drum
A heavy metal drum attached to the wheel hub that rotates with the wheel and is friction-clamped from the inside by brake shoes.
The brake drum is the key rotating component of a drum brake system. It is a heavy, bowl-shaped cast iron drum bolted to the wheel hub that rotates along with the wheel and tire. When you press the brake pedal, hydraulic wheel cylinders push brake shoes outward against the inner surface of the drum, creating the friction necessary to slow or stop the vehicle.
While disc brakes are standard on the front wheels of modern passenger vehicles due to superior heat dissipation, drum brakes are still commonly used on the rear wheels of compact cars, trucks, and heavy-duty commercial trailers because they are inexpensive to manufacture and double effectively as parking brakes.
Like rotors, brake drums wear down from friction. Over time, the inner drum surface can develop grooves, scoring, or become out-of-round (warped). Symptoms of drum brake issues include a pulsating brake pedal, squeaking or grinding noises during braking, a weak parking brake, or a dragging sensation.
During a brake service, mechanics inspect the drum's inner diameter and compare it to the maximum allowable limit stamped on the drum. Drums can sometimes be resurfaced on a brake lathe, but must be replaced if they are too thin or heavily damaged.