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A motorcycle transmission is a transmission created specifically for motorcycle applications. They may also be found in use on other light vehicles such as motor tricycles and quadbikes, go-karts offroad buggies, auto rickshaws, mowers and other utility vehicles, microcars, and even some superlight racing cars.
Most manual transmission two-wheelers use a sequential gearbox. Most modern motorcycles (except scooters) change gears (of which they increasingly have five or six) by foot lever. On a typical motorcycle either first or second gear can be directly 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, and are mostly found only on scooters and some custom cruisers and exotic sports bikes. Types 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. To avoid accidental operation, reverse is often engaged using an entirely separate control switch - e.g. a pull-toggle at the head of the fuel tank - when the main gearshift is in neutral.
In earlier times (pre-WWII), hand-operated gear changes were common, with a lever provided to the side of the fuel tank (above the rider's leg). British and many other motorcycles after World War II used a lever on the right (with brake on the left), but today gear-changing is standardised on a foot-operated lever to the left.
Scooters, underbones and miniatures
Traditional scooters (such as the Vespa) still have manual gear-changing by a twist grip on the left hand side of the handlebar, with a co-rotated clutch lever. Modern scooters were often fitted with a throttle-controlled continuously variable transmission, thus earning the term twist-and-go.
The clutch in a manual-shift motorcycle transmission 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(s), allowing the engine to freewheel with respect to the transmission.
Automatic and semi-automatics typically use a centrifugal clutch which operates in a different fashion. At idle, the engine is disconnected from the gearbox input shaft, allowing both it and the bike to freewheel (unlike torque converter automatics, there is no "idle creep" with a properly adjusted centrifugal clutch). As the throttle is opened and engine speed rises, counterweights attached to movable inner friction surfaces (connected to the engine shaft) within the clutch assembly are thrown gradually further outwards, until they start to make contact with the inside of the outer housing (connected to the gearbox shaft) and transmit an increasing amount of engine power. The effective "bite point" is found automatically by equilibrium where the power being transmitted through the (still-slipping) clutch is equal to what the engine can provide. This allows relatively fast full-throttle takeoffs (with the clutch adjusted so the engine will be turning near its maximum-torque rpm) without the engine slowing or bogging down, as well as more relaxed starts and low-speed maneuvers at lower throttle settings and rpms.
Above a certain engine speed - when the bike is properly in motion, so the gearbox input shaft is also rotating quickly and so allowing the engine to accelerate further by way of clutch slip - the outward pressure of the weighted friction plates is sufficient that the clutch will enter full lock-up, the same as a conventional plate-clutch with a fully released lever or pedal. After this, there is no clutch slip, and the engine is locked to and providing all of its available power to the transmission; engine rpm is now dependent on the road speed and the current gear ratio (under either user control in a semi-auto, or reliant on road speed (and sometimes load/throttle position) in a CVT setup). In a typical CVT, the gear ratio will be chosen so the engine can reach and maintain its maximum-power speed as soon as possible (or at least, when at full throttle, in a partially load-dependent system), but in a semi-auto the rider is responsible for this choice, and they can ride around all day in top gear (or first) if they so prefer. Also, when the engine is turning fast enough to lock the clutch, it will stay fully engaged until the RPMs fall below that critical point again, even if the throttle is fully released. Below the lock-up point, partially or fully releasing the throttle can lead to the RPM falling off rapidly, thanks to the feedback loop of lower engine speed meaning less friction pressure. This toggle-like mode of operation can lead to certain characteristic centrifugal-clutch-automatic behaviour, such as being able to freewheel rapidly downhill from a standstill, with engine braking only being triggered by turning the throttle briefly (and not then cancellable without braking to quite a slow, gear-dependent pace), and lockup triggering at a lower speed with full versus minimal throttle.
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.
In many modern designs, the engine sits in front of the gearbox. From a sprocket on one side of the crankshaft, a chain or sprocket directly mounted to the clutch will drive the clutch, which can often be found behind a large circular cover on one side of the gearbox. The clutch is connected to the gearbox input shaft. For motorcycles with chain drive, the gearbox output shaft is typically connected to the sprocket which drives the final drive chain.
Most manual motorcycle gearboxes have "constant mesh" gears which are always mated but may rotate freely on a shaft until locked by a toothed sliding collar or "dog clutch". Since the gears are always rotating and can only be accessed sequentially, synchromesh is not generally needed. To save space, both shafts may contain a mixture of fixed and free-spinning gears, with some gears built into the sliding parts.