In the late 20th century the word motorcycle is used to describe a variety of two- or three-wheeled motorized vehicles, including minibikes, mopeds, and motor scooters. Some of these are smaller than bicycles. The scooter, which has been extremely popular in Italy, has wheels from 8 to 14 inches (20 to 36 centimeters) in diameter. Among the larger motorcycles are the three-wheeled vehicles used by many police departments. They carry equipment in a trunk mounted between the two rear wheels.

The motors are usually air-cooled internal- combustion engines ranging from 25 to 4,500 cubic centimeters (1 1/2 to 275 cubic inches) in displacement-- the volume within a cylinder being displaced by the range of motion of a piston (see Internal-Combustion Engine). The larger the displacement, the greater the power. In Europe engines are rated by volumetric displacement, while in the United States they are rated by horsepower. A displacement of 100 cubic centimeters (6 cubic inches) equals approximately 5 to 10 horsepower. The smaller engines are one cylinder, while the larger motorcycles have multiple-cylinder engines. The fuel in some motorcycles is a mixture of oil and gasoline, while others use only gasoline.

The transmission is normally through chain or gearing between the engine and the two- to six-speed gearbox and then by chain, shaft, or belt to the rear-wheel sprocket. Controls on handlebar grips govern the throttle action and often the front-brake system. Rear-wheel brakes are normally operated by a foot pedal.

Motorcycles such as those manufactured by Harley-Davidson in the United States and by Honda in various countries have much of the same standard equipment as automobiles--gas tank, battery, spark plugs, muffler, generator, shock absorbers, oil pump, and turn signals, among others. The much smaller mopeds are far more compact and carry little more equipment than a motor and a small fuel tank.

Motorcycle Racing
Motorcycle racing began in 1897 on a track in Richmond, Surrey, England. Although many early events were informal road races or city-to-city, most racing today is closed circuit--it takes place on confined tracks.

Flat-track racing is done on oval dirt tracks from 1/2 to 1 mile (0.8 to 1.6 kilometers) in length. Motorcycles used in such racing have an engine displacement of up to 750 cubic centimeters (45 3/4 cubic inches). Short-track racing is done on ovals of less than 1/2 mile (0.8 kilometer) in length, and the motorcycles have a maximum displacement of 360 cubic centimeters (22 cubic inches). Some short track racing is done in indoor arenas.