Underbone in the context of Honda Super Cub

A scooter or motor scooter, is a motorcycle with an underbone or step-through frame, a seat, a transmission that shifts without the operator having to operate a clutch lever, a platform for their feet, and with a method of operation that emphasizes comfort and fuel economy. Elements of scooter design were present in some of the earliest motorcycles, and motor scooters have been made since at least 1914. More recently, scooters have evolved to include scooters exceeding 250cc classified as Maxi-scooters.

The global popularity of motor scooters dates from the post-World War II introductions of the Vespa and Lambretta models in Italy. These scooters were intended to provide economical personal transportation (engines from 50 to 150 cc or 3.1 to 9.2 cu in). The original layout is still widely used in this application. Maxi-scooters, with larger engines from 200 to 850 cc (12 to 52 cu in) have been developed for Western markets.

A centrifugal clutch is an automatic clutch that uses centrifugal force to operate. The output shaft is disengaged at lower rotational speed and engages as the output increases to a certain speed. It is often used in mopeds, underbones, lawn mowers, go-karts, chainsaws, mini bikes, and some paramotors and boats to keep the engine from stalling when the output shaft is slowed or stopped abruptly, and to remove load when starting and idling. It has been superseded for automobile applications by the fluid coupling, torque converter and automated manual transmissions.