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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.[a]
With traditional manual transmissions, the driver can move from gear to gear, by moving the gear lever 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 enable the driver to select only 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 and aft motion of the shift lever into 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. Only a true sequential transmission has a shift mechanism that operates in this way.
Nearly all modern motorcycles use a sequential gearbox. (A conventional H-pattern shifter would be cumbersome and take up too much space in the confines of a motorcycle frame). Controlling the gear shifter with the rider's foot allows their hands to remain on the handlebars.
Some automatic transmissions provide a degree of user shifting input. One commercial example is the Tiptronic transmission. User shifting through buttons or lever does not necessarily mean that the transmission is a manual transmission.
Beyond the ease of use from a driver's standpoint, an additional benefit of sequential manual gearboxes is that use of the clutch via foot pedal or hand control can be minimized or completely obviated, with the clutch used only for starting from a complete stop. Formula One cars of the 1990s made the most high-profile debut of this technology in motor sports, and enhanced variations on this theme are still in use in many forms of road racing and drag racing today. The simple push-pull action of the shift mechanism also lends itself to semi-automatic control using either hydraulic or pneumatic actuators—a system often referred to as paddle-shift. Instead of a manual gear lever, the driver is provided with (usually) a pair of flipper paddles on the steering wheel. Rally cars often utilize just a double-acting single paddle: pulling on the right-hand paddle makes an up-shift and pulling on the left-hand paddle makes a down-shift. The paddle-shift system will use a sophisticated electronic control unit to provide the necessary logic to operate the shift mechanism which includes the auto blipping of the throttle on downshifts to rev match, and a momentary torque cut for upshifts. This type of paddle-shift system fitted to race and rally cars should not be confused with most of the current crop of paddle-shift systems fitted to some high-end road cars. These systems often use a dual-clutch transmission or a conventional torque converter automatic transmission which allows driver input to select the gears.
Although a sequential manual transmission can (depending on the model) offer faster shift speeds and thus faster race times, many road-car-driving enthusiasts prefer a standard manual transmission, which—with a clutch pedal and the ability to skip gears—allows more driver input and a more traditional driving experience.
- Cameron, Kevin (1998). Sportbike Performance Handbook. Motorbooks. p. 82. ISBN 9780760302293.
- Cameron, Kevin (2009), Top Dead Center 2, Motorbooks, p. 58, ISBN 9780760336083
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