A single-track vehicle is a vehicle that leaves a single ground track as it moves forward. Single-track vehicles usually have little or no lateral stability when stationary but develop it when moving forward or controlled. In the case of wheeled vehicles, the track of the front and rear wheel usually follow slightly different paths when turning or when out of alignment.
Single-track vehicles have unique dynamics that, in the case of wheeled vehicles, are discussed at length in bicycle and motorcycle dynamics, that usually require leaning into a turn, and that usually include countersteering. Single-track vehicles can roll on wheels, slide, float, or hydroplane.
A bicycle, often called a bike (and sometimes referred to as a "pushbike", "pedal bike", "pedal cycle", or "cycle"), is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist.
Bicycles were introduced in the 19th century in Europe and now number more than a billion worldwide, twice as many as automobiles. They are the principal means of transportation in many regions. They also provide a popular form of recreation, and have been adapted for such uses as children's toys, general fitness, military and police applications, courier services and bicycle racing.
The basic shape and configuration of a typical upright, or safety bicycle, has changed little since the first chain-driven model was developed around 1885. However, many details have been improved, especially since the advent of modern materials and computer-aided design. These have allowed for a proliferation of specialized designs for diverse types of cycling.
The invention of the bicycle has had an enormous effect on society, both in terms of culture and of advancing modern industrial methods. Several components that eventually played a key role in the development of the automobile were invented for the bicycle, including ball bearings, pneumatic tires, chain-driven sprockets, and tension-spoked wheels.