The term disk wheel can refer to more than one type of wheel. It may refer to a type of wheel that accepts a disk brake, or it may refer to a wheel built as a disk which has no spokes or openings that air can pass through. A disk wheel, when referring to a wheel that accepts a disk brake, is common on cars, trucks, motorcycles, and bicycles. A disk wheel that refers to a wheel with no spokes or openings that air can pass through is common on certain race cars as well as on racing bicycles.
Motorcycles are a good example of a disk wheel that is meant to accept a disk braking system. The disk wheel differs from other types of wheels in that a round metal rotor can be mounted to the hub of the wheel. The caliper of the disk brake is mounted over the rotor, and when the brake lever is activated, the caliper's pistons push on either side of the rotor to slow the vehicle down. Most motorcycles feature this type of braking system, which is visible from the outside. Such brakes are also common on many cars, though they are not visible because they are hidden by the wheel and tire.
A disk wheel, when referring to a wheel with no spokes that air cannot pass through, is common on racing bicycles and on some types of racecars. On the bicycle, the disk wheel is commonly present only for certain types of races, and it is generally only mounted on the rear of the bicycle. The front wheel usually has spokes to allow air to pass through; otherwise, steering could be adversely affected in windy conditions. Common versions of rear disk wheels are made from carbon because it is a lightweight and rigid material. The carbon is molded into a seamless disk, with the tire mounted to the disk. Carbon disk wheels are designed to be extremely aerodynamic to promote high speeds during racing, but they are far more expensive than other types of wheels and not nearly as versatile.
Disk wheels on cars are of a similar design, but they are made from much stronger materials than carbon. Disk wheels on cars are made from steel or aluminum, and they feature a seamless design that air cannot pass through. On many cars, this is simply done for aesthetics and is rarely done for performance, though on certain types of racecars, the disk wheels can slightly improve aerodynamics.