Minibike

A minibike, sometimes called a minimoto or pocketbike, is a miniature motorcycle. Most minibikes use two-stroke engines and chain drive.

Like go-karts, the first minibikes were made by enthusiasts from spare parts found in their garages. They were first popularly used as "pit bikes", for drag racers to travel in the pits during races in the late 1950s. They were very useful for this purpose, as they could maneuver very well in the tight pit roads, fit in about the same space as a small bicycle in a trailer or pickup. As racers brought them home and used them around their neighborhoods, many children liked the idea of having a mini motorcycle and started building their own. The June 1967 Popular Mechanics magazine had an article with plans. All you needed was an engine from a lawnmower and a neighbor with a welding torch.

A market for minibikes developed, and from the early 1960s many cottage and major industries developed to meet the demand. Most lawnmower shops began carrying a line of mini-bikes. Minibike companies include Arctic-Cat, Rupp, Taco, Heath, Gilson, and Fox, many of which also made other power toys such as go-karts, trikes and choppers. Traditional motorcycle manufacturers also began coming out with models like the Honda Z50A, inspired by aspects of minibikes. In America the peak of the minibike/go-kart era was from the late 1960s to the early 1970s. Many of the brands, foremost Rupp, have gained a cult-following of enthusiasts and owners.

These early minibikes usually had a power train with a small four-stroke, horizontal crankshaft, flathead engine. The transmission usually was of a crank-mounted centrifugal clutch and chain drive to a rear sprocket. As the minibike and the mini-powersports field grew, Comet introduced a continuously variable transmission, much like a snowmobile's, called the Torque-a-Verter, which automatically adjusts gear ratios, resulting in better top speed and acceleration.

...
Wikipedia