About Drum Brakes
Drum brakes were standard on all cars until the 1950s, when disc brakes were first introduced. Drum Brakes were slowly phased out as disc brake technology became less and less expensive. Most cars sold today come with disc brakes standard, but many models still use drum brakes on the rear wheels.
Drum brakes are less expensive to manufacture than disc brakes, so using drum brakes on the rear wheels helps keep the cost of the vehicle down. Since the rear wheels usually only handle about 25% of the braking, it is perfectly safe to use drum brakes on the rear wheels. Drum brakes can provide just as much stopping power as the disc brakes that are used on rear wheels, but are more difficult to service than discs.
Drum brakes are more prone to brake fade than disc brakes. Brake fade happens when the high heat generated by slowing the car down heats the brake shoe to the point where it loses some of its stopping power. Brake fade can be dangerous, and should be a real concern, but it is a very rare occurrence in normal driving, even during stop and go traffic. Brake fade is usually only an issue on racecars that undergo heavy braking every few seconds. Be aware that older, more worn brakes are much more likely to experience brake fade than new brakes, so it is important that you replace your brake shoes sooner rather than later.
Most cars and trucks today use a drum brake for the emergency brake, even if all four of the regular brakes are discs. This is because the emergency brake needs to be operated by a separate system than the primary brake system. If the primary brake system should fail, the emergency brake needs to still be functional. A cable-operated drum emergency brake is simple and inexpensive to produce, and is able to safely and reliably keep a vehicle stationary. On vehicles that have drum rear brakes, the emergency brake will usually be built into the drum brake system. On vehicles with rear disc brakes, a separate emergency brake system is usually used.
How Drum Brake Shoes Work
Drum brake shoes sit inside the drum brake enclosure. When the brake pedal is pressed, the shoes are pushed out and into the drum, which is spinning along with the wheel of the car. When the brake shoes are pressed into a spinning drum, they are pulled slightly along with the drums. This digs the shoes into the drums harder. Because of this, drum brakes usually do not need to be power-assisted like disc brakes do.
Brake shoes are made of a semi-metallic compound that dissipates heat away from the area where the shoe contacts the drum. This heat transfer reduces brake fade and prolongs the life of the brake shoes and drums. Brake shoes wear down over time, and lose some of their ability to transfer heat as they do. As a result, new, thicker brake shoes are less likely to experience brake fade than older, worn brake shoes.
Why Replace Brake Shoes
Because the rear brakes only provide 25% of a vehicle's braking power, some people feel that replacing rear brake shoes in a timely manner is unimportant. This couldn't be farther from the truth. While your car may be able to stop effectively on a straight, dry road with just front brakes, if you try to slow down during a turn or in inclement weather, the rear brakes become crucial to maintaining stability while braking. Without rear brakes, a car could easily lose control while braking. Rear brakes are a very important safety feature on a car and should not be ignored.
Brake fade is more likely to occur on worn brake shoes than new ones because a thick, new brake shoe is much better at transferring heat away from the contact area than a worn brake shoe. Worn out brake shoes can wreck other, more expensive parts of the braking system if they aren't replaced. If the pad of the brake shoe wears out completely, the steel backing will scrape against the brake drum. This scraping can destroy the brake drum, and the heat generated can destroy other components of the braking system, which can get very expensive very quickly.
When to Replace Brake Shoes
Brake shoes are constructed the same way that brake pads are. A semi-metallic brake shoe compound is glued or riveted to a steel shoe that attaches to the drum brake system. Many drum brakes have a viewing hole located behind the braking enclosure to let you easily check the width of your brake shoes. On other drum brake systems, you will need to take off the wheel and brake drum to check your brake shoe's width. If you see that your brake shoes have less than 1/8th of an inch left of material, it is time to change them. Always remember that the most important safety feature your vehicle has are its brakes. Never skimp on the quality of brake components.