Sequential manual transmission

A sequential manual transmission, also known as a sequential gearbox, or a sequential transmission, is a type of non-synchronous manual transmission used mostly for motorcycles and racing cars. It produces faster shift times than traditional synchronized manual transmissions and restricts the driver to selecting either the next or previous gear, in a successive order.

A sequential manual transmission is unsynchronized and allows the driver to select either the next gear (e.g. shifting from first gear to second gear) or the previous gear (e.g., shifting from third gear to second gear), operated either via electronic paddle-shifters mounted behind the steering wheel or with a sequential shifter. This restriction avoids accidentally selecting the wrong gear; however, it also prevents the driver from deliberately "skipping" gears. The use of dog clutches (rather than synchromesh) results in faster shift speeds than a conventional manual transmission.

On a sequential manual transmission, the shift lever operates a ratchet mechanism that converts the fore-and-aft motion of the shift lever into the rotation of a selector drum (sometimes called a barrel) which has three or four tracks machined around its circumference. Selector forks are guided by the tracks, either directly or via selector rods. The tracks deviate around the circumference and as the drum rotates, the selector forks are moved to select the required gear.

A sequential manual transmission is not to be confused with a "sequential" shifting function sometimes fitted to the hydraulic automatic transmission, marketed with terms such as "Tiptronic" or "SportShift". This function allows the driver to select the previous or next gear through the use of buttons or a lever (usually near the gear shifter or steering wheel); however, the mechanicals of the transmission are unrelated to a true sequential manual transmission.

Most motorcycles use a sequential manual transmission. The rider controls the gear shifter with their foot, allowing their hands to remain on the handlebars, and gear selection uses a layout of 1 - N - 2 - 3 - 4 - 5 - 6 (for a typical 6-speed gearbox, said as "one down, five up"). However, most modern motor scooters do not use a sequential manual transmission; instead using either a hydraulic automatic transmission or a belt-driven or chain-driven continuously variable transmission. Underbones, however, often use a semi-automatic transmission with an automatic centrifugal clutch, but will still retain the conventional foot-operated gearshift lever, such as the Honda Super Cub.