Sequential manual gearbox

A sequential manual transmission (or sequential manual gearbox) is a non-traditional type of manual transmission used on motorcycles and high-performance cars for auto racing, where gears are selected in order, and direct access to specific gears is not possible.

With traditional manual transmissions, the driver can move from gear to gear, by moving the shifter to the appropriate position. A clutch must be disengaged before the new gear is selected, to disengage the running engine from the transmission, thus stopping all torque transfer. This type of transmission is often referred to as a H-pattern because of the path that the shift lever takes as it selects the various gears. As the gear shifting process requires attention and dexterity that might be needed in managing sharp corners in car races, sequential transmission has been introduced into many competition rules as an alternative.

A true sequential transmission will very often use dog clutch engagement rather than the more usual synchromesh as fitted to a normal H-pattern road car gearbox. Engagement using dogs requires only a very brief interruption of engine torque to complete a shift into any adjacent gear. This allows shifting between gears without the use of the clutch. The clutch would normally be used only for standing starts.

Sequential manual transmissions work by providing the driver with the ability to select the gear directly before or after the gear currently engaged. Usually the shift lever is pulled back to select the adjacent higher gear and pushed forwards to select the adjacent lower gear. On a true sequential gearbox, the shift lever operates a ratchet mechanism that converts the fore & aft motion of the shift lever into a rotary motion. This rotary action turns a selector drum (sometimes called a barrel) which has three or four tracks machined around its circumference. Running in the tracks are the selector forks, either directly, or via selector rods. These tracks deviate around the circumference and as the drum rotates, the selector forks running in the tracks are moved to select the required gear. Only a true sequential transmission has a shift mechanism that operates in this way.

Sequential gearboxes are also used in nearly all modern motorcycles as it is too cumbersome to have a conventional H-pattern shifter and would take up too much space in the confines of a motorcycle frame. Having control over the gear shifter, typically with the rider's left foot, frees their hands to operate both the clutch and brake without letting go of the handlebars.

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