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. The tandem bicycle or twin is a form of bicycle (occasionally, a tricycle) designed to be ridden by more than one person. The term tandem refers to the seating arrangement (fore to aft, not side by side), not the number of riders. A bike with two riders side by side is called a sociable. Patents related to tandem bicycles date from the late 19th century. In approximately 1898, Mikael Pedersen developed a two-rider tandem version of his Pedersen bicycle that weighed 24 pounds, and a four-rider, or "quad", that weighed 64 pounds. They were also used in the Second Anglo-Boer War. Tandem popularity began to decline after WWII until a revival started in the late sixties. In the UK The Tandem Club was founded in 1971, new tandems came on to the market from the French companies Lejeune and Gitane, and in the USA Bill McRea
founded Santana Cycles in 1976. Modern technology has improved component and frame designs, and many tandems are as well-built as mode Compared to a conventional bicycle, a tandem has double the pedalling power with only slightly more frictional loss in the drivetrain. It has about the same wind resistance as a conventional bicycle. High-performance tandems may weigh less than twice as much as a single bike, so the power-to-weight ratio may be slightly better than that of a single bike and rider. On flat terrain and downhill, most of the power produced by cyclists is used to overcome wind resistance, so tandems can reach higher speeds than the same riders on single bicycles. They are not necessarily slower on climbs, but are perceived as such, in part due to the need for a high level of coordination between the riders, especially if the physical abilities of the two riders are very different, requiring a compromise on cadence.