A motorcycle transmission is a transmission created specifically for motorcycle applications.
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All manual transmission two-wheelers use a sequential gearbox. Most modern motorcycles (excepting scooters) change gears (of which they will increasingly have five or six) by foot lever. On a motorcycle either first or second gear can be selected from neutral, but higher gears may only be accessed in order - it is not possible to shift from second gear to fourth gear without shifting through third gear. A five-speed of this configuration would be known as "one down, four up" because of the placement of the gears with relation to neutral. Neutral is to be found "half a click" away from first and second gears, so shifting directly between the two gears can be made in a single movement.
Automatic transmissions are less common on motorcycles than manual. Types include include continuously variable transmission, semi-automatic transmission and dual clutch transmission.
The weight of the largest touring motorcycles (sometimes in excess of 360 kg or 800 lbs) is such that they cannot effectively be pushed backwards by a seated rider, and they are fitted with a reverse gear as standard. In some cases, including the Honda Gold Wing and BMW K1200LT, this is not really a reverse gear, but a feature of the starter motor which when reversed, performs the same function. In earlier times pre WWII, hand-operated gear changes were common, a lever was provided to the side the fuel tank. British and many other motorcycles after WWII used a lever on the right but today gear-changing is standardised on a foot-operated lever to the left.
Traditional scooters still have manual gear-changing by a twist grip on the left hand side of the handlebar, with a clutch on a lever also rotating. Increasingly they may be fitted with a continuously variable transmission or CVT instead, a kind of automatic transmission (as once used in the DAF car) that is stepless. Step-through motorcycles often have a three-speed foot change, but the clutch is automatic.
The clutch is typically an arrangement of plates stacked in alternating fashion, one geared on the inside to the engine and the next geared on the outside to the transmission input shaft. Whether wet (rotating in engine oil) or dry, the plates are squeezed together by a spring, causing friction build up between the plates until they rotate as a single unit, driving the transmission directly. A lever on the handlebar exploits mechanical advantage through a cable or hydraulic arrangement to release the clutch spring, allowing the engine to freewheel with respect to the transmission.
Pre-unit construction, also called separate construction, is a motorcycle engine architecture where the engine and gearbox are separate casings. In unit construction the engine and gearbox share a single housing.
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