Single-track vehicle

Cyclists on bicycles

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.

Wheeled

the bicycle and all its varieties, such as tandem, tall, and recumbent
the motorcycle and all its varieties, such as scooter and feet-forwards
the kick scooter and all its varieties
the unicycle and monowheel
the inline skate and roller ski when only one is in contact with the ground or when the second follows behind the first

Sliding

the snowboard and monoski
the skirider
the ice skate when only one is in contact with the ice or when the second follows behind the first

Intermittent contact

the pogo stick
the bipedal robot

Hydroplaning

the slalom water ski
the wakeboard

Approximately single-track

A vehicle is approximately single-track when the axle track is small enough with respect to the center of mass height to require leaning into a turn. Countersteering may or may not be required.