Drum brakes, disc brakes, wheel cylinders, pistons, calipers, brake shoes??... If all these terms sound alien to you fear not. Modern braking systems found in today's vehicles are not as complicated as you may think, and servicing them is easily within the grasp of the home mechanic.
A Look At History
The predecessor to the modern drum brake was invented in the beginning of the 20th century and was used in virtually all vehicles until recent times. In the 1950s self adjusting drum brakes were invented, before then brakes had to be manually adjusted to compensate for brake wear. By the late 1960s and into the 1970s the front drum brakes on cars started to be replaced with disc brakes. Since then there has been a growing number of cars and trucks that have completely abandoned drum brakes in favor of disc brakes.
How Do Drum Brakes Work?
To understand how drum brakes operate, we first need to take a look at how they are constructed. As their name suggests, a drum brake consists of a hollow drum attached to the axle and wheel. The drum spins with the wheel. Inside the hollowed out drum are basically four parts: the brake shoes, wheel cylinder, self adjuster, and the parking brake pawl.
When you press the brake pedal, hydraulic brake fluid forces the wheel cylinder (colored green in the graphic) to extend two pistons out. These pistons are attached to the brake shoes (orange in the picture). The brake shoes are then squeezed against the inside of the drum (yellow in the picture). The friction caused by the shoes rubbing the drum slows the wheel down.
As the brakes are used, the shoe pads and the drum wear thinner. The self adjuster automatically compensates for this change in thickness. The parking brake pawl is basically a mechanical substitute for the wheel cylinder. When you activate your parking brake (sometimes called a hand brake or emergency brake), the pawl uses springs and mechanical devices to squeeze the brake shoes against the drum.
I Thought Drum Brakes Were Outdated, Why Do Some Cars Still Use Them?
Virtually all vehicles sold in the U.S. today use disc front brakes, however, some still use drum brakes on the rear wheels. Why is this? Well, for starters, the front wheels do about 70-80% of the braking, so the rear brakes do not get the same heat and friction abuse as the front brakes. Properly maintained drum brakes are more than adequate for rear wheel braking.
Another reason is cost, drum brakes are cheaper to make and install on vehicles, so it saves the car manufacturer some money.
A third reason is the simplicity of integrating the parking brake. There are no easy ways to make a disc brake operate like a parking brake, in fact, several rear disc brakes have built in drum brakes used solely for the parking brake.
Remember that rear disc brakes are not equal to front disc brakes. Virtually all rear brake rotors are not vented meaning they are usually less than half the thickness of front rotors. The brake calipers and pads on rear disc brakes (calipers are to disc brakes like wheel cylinders are to drum brakes) are smaller.
And finally, drum brakes may be making a slight come back. Some hybrid vehicles, like the Toyota Prius, use drum brakes on both the front and rear wheels.
Servicing Drum Brakes
Chances are, if you are reading about drum brakes, you are preparing to service the brakes on your vehicle. Here is How-To Matthew's Drum Brake Maintenance Article with step-by-step instructions and photos.
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