Disc Brakes, invented during World War II for aviation purposes (especially the demand of landing on carriers and other short strips), are the most popular and effective way of providing braking force for automobiles, aircraft, and even other wheeled vehicles like bicycles. Disc brake systems are reliable, easy to maintain, and light in weight, especially when compared to drum brake systems.
The parts of a disc brake system are the caliper, pads, and brake rotor|rotor (or disc). The caliper contains one or more pistons which are usually driven hydraulically, but may be actuated via a linkage as well. The piston pushes, which causes the pads to squeeze the rotor evenly, from both sides. This friction adds load to the wheel, resulting (hopefully) in the stopping or slowing of the vehicle due to the conversion of kinetic energy to heat energy.
The body of the caliper is generally made of either cast iron or aluminum. Aluminum handles heat nearly as well as cast iron and is considerably lighter, reducing the unsprung mass. Rotors are generally either steel or aluminum clad with steel, but in racing and extreme high-performance street applications carbon fiber brake rotors are now being used to further reduce unsprung mass. Some supercars (from, for example, Ferrari and Lamborghini) are fitted with Brembo carbon fiber brake rotors at the factory.
Brake pads are out of the scope of this writeup, but they are generally made out of some high-carbon compound which may contain steel, carbon fiber, or other high-strength and high-temperature materials. There are also a variety of ceramic brake pads now available which are supposed to outlast the car.
Caliper / \ / \ / \ / \| |/ ____ ____ _,--'| \_______ / __|_____|__ \ | | _ _ | \ \ \ | | | | |_______ | | |X | |=| | X,------.| | | |X | | | | X| ||==|X> <-- bleeder valve | |X | | | | X| ||__|_____________ | |X | | | | X`------' __ _____________ Studs |_|X | |=| | X| | brake line | | | | | |__________| | _,-'| | | | V _,-' | | | | | | |=| | (Xs near caliper and rotor XXX| | | | | are brake pads) /| Hub | | | | || or | | | | \| Hat | |=| | XXX| | | | | |_ | | | | <--- rotor/disc `-._ | | | | A `-.| |=| | | | | | | | | | | | Wheel | | | | attaches | |=| | here | | | | | |-| | |_| |_|
Shown in this diagram is a hydraulically-actuated disc brake system, featuring a ventilated disc. This means that the disc is actually two discs, separated from one another via spacers (shown as = in the diagram). This allows air to pass between the plates of the rotor for cooling. Brake fluid is forced through the brake lines by the master cylinder, which pushes the piston out of its cavity in the caliper. In modern cars it is common to have ventilated discs in the front and non-ventilated ones in the rear, since a ventilated disc weighs significantly more, and typically 60% or more of the braking is done by the front brakes.
Disc brakes can either be self-adjusting, or not. Because of the way the caliper is mounted, it can slide or "float" from side to side, which is what causes braking force to be applied evenly to both sides of the rotor. Older designs were fixed in place and had multiple pistons with opposing cylinders, which required adjustment, but fixed designs are no longer in use because they do not offer any advantages other than a slight improvement in the strength of the mounting hardware.
Most disc brake calipers feature a single-piston design for simplicity and low cost, but performance automobiles often have two or four piston calipers stock. Having more pistons spreads the braking force out evenly, and allows for greater forces to be applied to the disc for shorter stopping distance. In addition, each individual piston may be smaller, thus allowing for narrower caliper designs for special applications, such as motorcycle racing, and calipers for this purpose often have many pistons - eight or ten not being uncommon.
Most automobile brakes are hydraulically operated. A master cylinder mounted to the firewall (or to a brake booster, usually vacuum-driven, which is mounted to the firewall) is actuated by a lever attached to the brake pedal, which is mounted to or near the toe pan. This moves the piston in the cylinder, which causes fluid to be pumped down the lines to the brakes. What follows is a brief introduction to hydraulics, or the use of fluid to transmit power to do work, which will help explain how and why brakes actually work.
When discussing the braking hydraulic system it is important to consider that the master cylinder piston area is less than the sum of the piston areas of the slave cylinders. What this means is that, like a lever, we are trading distance for force. Let's look just at the front brakes; If we have a 3/4" piston (a typical size) then we've got a .44in2 of area. If we have single-piston disc brakes, we might have two 2" pistons to deal with, for a total of about 6.3in2 of area. Thus, for every 6.3" we moved the master cylinder we'd have only 1" of total movement - half an inch on each side.
In the real world though, the brakes don't have to move anywhere near this far. The pads are in fact less than an eighth of an inch away from the rotor at all times; usually more like one sixteenth! So we only need one-eighth inch of total movement to bring the pad in contact with the rotor, which means we need to move the piston a bit less than one inch. Generally speaking, vehicles today have a "dual master" system, which has two pistons in the same master cylinder body so that they are actuated at the same time. The cylinder has separate ports, and usually one goes to the front and one to the back. Some manufacturers, notably Subaru, split the system differently, so that one cylinder drives the front left and rear right wheels, and the other handles the front right and rear left. Since the Subarus use rotors of nearly identical sizes at all four corners, this provides balanced braking force if you lose either half of the braking system.
Remember also that I said that this tradeoff is like a lever; you trade force for distance (or vice versa). We have to move the cylinder six point three times as far because the caliper piston area is about that many times greater than the master cylinder piston area. However, at the same time the force applied is multiplied by the same factor. Thus, if we pressed down with 200 pounds of force (remember there's a lever, providing leverage, and possibly a brake booster adding force here) we would have 454 PSI of pressure at the master cylinder, but since we would also have that same 454 PSI applied across the entire system (Boyle's Law states that pressure is equivalent throughout a closed system) then we would multiply it by the 6.3in2 of caliper piston area and wind up with a total of 2860 pounds of pressure, or 1430 pounds of clamping force at each front wheel. The ability to stop a vehicle is based on a combination of the clamping force, and the total friction.
Typically speaking, modern automobiles have ventilated disc brakes in front, and either a non-ventilated disc in the rear, or a drum brake, though some vehicles do have rear ventilated discs. While disc brakes do sometimes suffer some slight loss of functionality in wet weather, modern brake pads generally reduce or eliminate the seriousness of this drawback. Additionally, during periods of heavy braking the discs generally heat up to the point where any water will evaporate from them quite rapidly. The only time this is not true is when the brakes are partially or completely submerged, in which case drum brakes will be at least as ineffectual as discs.
Bicycle disc brakes are typically mounted to the hub, and depends on special mounting hardware on the forks or frame (depending on whether we're talking about front or rear brakes, respectively.) They usually provide a single, cross-drilled rotor of less than five inches in diameter. Both cable- and hydraulically-driven models are available; the cables are less durable, but they weigh less. Cable-operated brakes require careful adjustment, while most hydraulic brakes are self-adjusting. The weight difference is 60-120 grams additional for hydraulic, but as a disc brake system weighs a minimum of 630 grams this is relatively negligible. For comparison, a full water bottle weighs over 800 grams.
The only advantage of pad brakes is weight - again, you will save something like 600 grams on average. However, the soft pads necessary to not damage the soft rims found on most bicycles mean that your braking force is limited. This is not usually a problem as a good pad brake will definitely let you "lock up" a wheel when on dirt or gravel, and if you can do this, you don't need more braking force. However, disc brakes also let you better modulate your braking force than pads, because the amount you must press the brake lever with pads varies widely with speed. Disc brakes, having hard pads, do not have this problem. Also, they perform much better in wet or muddy situations because rather than braking around the edge of the wheel, they brake towards the middle, meaning that the braking surface stays cleaner in any kind of weather. Also, when a wheel is warped it is still ridable but pad brakes may not work correctly or may even have to be removed. Since disc brakes are attached to the hub, they still work when the rim is bent.