Motorcycle transmission

Underbone and miniature motorcycles often have a three to five-speed foot-shift lever, but the clutch is automatic (usually a centrifugal clutch). This type of clutchless (no manual clutch) transmission still must have the gears shifted manually by the rider, and the system is commonly known as a semi-automatic transmission.

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Automatic and semi-automatic transmissions 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 modern