Brake Drums and Rear Brake Drums
A drum brake is a brake in which the friction is caused by a set of brake shoes or brake pads that press against a rotating drum-shaped machined surface when the brake pedal is engaged.
The term "drum brake" or “drum brakes” typically refers to a brake system in which brake shoes press on the inner surface of the brake drum to stop the rotation of the tires and consequently the forward or reverse the momentum of the vehicle. Drum brakes consist of a backing plate, brake shoes, brake drum, wheel cylinder, return springs, and an automatic or self-adjusting brake hardware system. When you apply the brakes, brake fluid is forced under pressure into the wheel cylinder, which in turn pushes the brake shoes into contact with the machined surface on the inside of the brake drum. When the pressure is released, return springs pull the brake shoes back to their rest position. As the brake linings wear, the shoes must travel a greater distance to reach the drum. When the distance reaches a certain point, a self-adjusting mechanism automatically reacts by adjusting the rest position of the shoes so that they are closer to the drum.
The brake shoes located in drum brakes are subject to wear and need to be inspected and replaced as required. Like conventional brake pads, brake shoe and brake drum wear are subject to driving conditions. In the 1960s and 1970s, brake drums on the front wheels of cars were gradually replaced with disc brakes and now most cars incorporate front disc brake systems. Drum brakes are still very often used for handbrakes as it has proven very difficult (and expensive) to design and incorporate a disc brake suitable for holding a car when it is not in use. Additionally, it is logical (and more affordable) to fit a drum handbrake inside a disc brake so that one unit serves as both a service brake and a handbrake.
Brake drums are typically frequently made of cast iron, although some vehicles have used aluminum drums, particularly for front-wheel brake drum applications. Aluminum conducts heat better than cast iron, which improves heat dissipation and reduces brake fade. Because aluminum wears more easily than iron, aluminum drums will frequently have an iron or steel liner on the inner surface of the drum, bonded or riveted to the aluminum outer brake drum shell.
While all vehicles produced for many years incorporate disc brakes on the front the primary reason for the continued usage of drum brakes on the rear (rear brake drums) is cost – plain and simple, they are cheaper to produce for the rear wheels. The main reason, as referenced above, is the parking brake system. On drum brakes, adding a parking brake is the simple addition of a lever, while disc brakes require a complete mechanism, in some cases, a complete mechanical drum brake assembly inside the disc brake rotor. Parking brakes must be a separate system that does not use hydraulics - it must be totally mechanical.