Perhaps the most important part of any vehicle is its brakes. Acceleration and maneuverability would be of little value if we could not slow down or stop the vehicle. Anybody that has been in a near-miss accident situation can attest to the value of their car's brakes.
When you operate the brakes on your car, you are using hydraulic pressure to actuate each wheel's brakes. By pressing on the brake pedal, you force a piston in the master cylinder (mounted on the firewall in front of the driver) to apply compression to the brake fluid (actually, modern cars use two pistons, one for the front brakes and one for the rear. This is to allow partial braking ability in the event of a failure.). Since fluids do not compress well, the pressure is forced through the brake lines to each wheel cylinder. At each wheel is another cylinder, properly named the wheel cylinder. This cylinder is mechanically connected to the brake mechanism, and will operate the mechanism with a force that is proportionate to the pressure in the brake line. How this mechanical action takes place at each wheel is dependent on whether disc brakes or drum brakes are used.
Disk brakes use a hardened steel disk called the rotor that is attached just behind each wheel. This rotor rotates with the wheel, and provides a surface upon which the brake pads can operate. When driving, the pads lightly rub against the rotor, ready to apply pressure when the brake pedal is depressed. This constant rubbing also serves to wipe water off the rotor, since water would act as a lubricant and would reduce the friction between the rotors and the pads. The wheel cylinder is attached to the caliper, which is the mounting structure for the pads. The caliper is mounted firmly to the suspension such that it does not rotate with the disk. When pressure is applied to the brake pedal, the wheel cylinder in turn directly applies pressure on the brake pads, pressing them against the rotor. This squeezing pressure creates friction between the pads and the rotors, converting the kinetic energy of the moving vehicle to heat, thus slowing the car down. Cooling fins are often designed into the rotor to dissipate excess heat.
Drum brakes use a hardened drum attached behind the wheels to provide a surface upon which friction can be applied. Drum brakes use shoes, rather than pads to provide braking. Though the name is different, the function is similar. In drum brakes, the shoes are held away from the inside of the drum by a series of springs. These springs are part of the mechanism that allows the drum brakes to operate properly. When the brake pedal is depressed, the pistons in the wheel cylinder are forced outward, pushing the top part of both shoes against the interior surface of the drum. The adjuster that connects between the bottoms of both shoes forces the shoes to rotate slightly, thus introducing the rest of the braking surface to the inside surface of the drum. Once again, the friction between the drum's inner surface and the shoes converts kinetic energy to heat as it stops the car.
Drum brakes are the older of the two technologies, though they are still in use today. The disk design has several advantages over drum brakes. The constant contact between the rotor and the pad keeps water out. Drum brakes don't have this feature, though they are designed so that water does not easily find its way to the braking surface anyway. Disk brakes are much simpler, relying on a large piston rather than mechanical linkages to directly apply braking force. They are easier to repair and can often be disassembled for pad replacement by removing one bolt. Drum brakes, on the other hand, require that a series of springs be removed to replace the shoes. They also require adjustment to keep the shoes at the proper distance from the drums, although this is done automatically through the adjusters when backing. Overall, the disk design is far superior to that of drum brakes.