Though many systems for slowing down a wheel on a vehicle have been designed and implemented over the course of more than a century, no design is more prevalent today than the disk brake. A disk brake system uses a rotor, usually made of composite materials such as iron, ceramic, carbon, and Kevlar®, that is fixed to a wheel and slowed using a caliper that forces brake pads to contact the disk from both sides simultaneously. Disk brake systems are used extensively on automobiles, motorcycles, bicycles, and other gas-powered and human-powered vehicles.
Disk brakes were developed as early as the late nineteenth century, but design flaws kept disk brake systems from entering mainstream use. The most difficult problem to overcome was heat transfer, or the disk's inability to distribute friction heat effectively. This problem was called brake fade and was quite prevalent on early models of disk brakes. Further, because roads around that time were primitive and unpaved, dirt and dust often came in contact with the disk brake system, reducing the power and effectiveness of the brake and often leading to premature wear of the individual components.
These problems were eventually solved by using composite materials that distributed heat more effectively and were less susceptible to quick wear. Other methods of solving the heat and dirt problems included drilling holes in the rotor itself, which allowed heat to disperse more effectively and dirt and grit to pass through without affecting the performance of the brake to a great degree.
In order to actuate the brake pads and make them press against the rotor with significant strength to slow the wheel, several methods have been developed. Cable actuated levers are common on bicycles, where massive amounts of force are not needed to slow the vehicle. On automobiles, motorcycles, and even many bicycles, hydraulic systems are employed to transfer power from the brake lever or pedal to the brake caliper. These hydraulic disk brake systems use a viscous liquid, usually an oil or other thick fluid, contained in lines rigid enough to contain the force of the moving liquid. When the brake lever or pedal is actuated, the oil or fluid is forced into the brake caliper, which in turn uses a series of pistons to thrust the pads against the rotor. Other systems of actuation include pneumatic systems and electromagnetic systems, both of which tend to be more complex but just as effective.