Transmission

Transmission

The transmission is controlled by a clutch lever under the left hand in standard configurations, a twistgrip throttle on the right handlebar and a gear lever at the left foot.
The gear lever operates by shifting gears when it is pressed or lifted. A normal street motorcycle is put in first gear by pressing the gear lever, while second and all further gears are reached by lifting it. Downshifting is done by pressing the gear lever. Neutral sits between first gear and second, so a small lift out of first causes the gearbox to change into neutral, but a large movement causes the gearbox to change into second gear. In contrast, racing motorcycles have all gears arranged "below" the first gear, thus pressing the gear lever always shifts up, while lifting it shifts down.
Modern motorcycles normally have five or six forward gears. Only the largest touring motorcycles (most prominently, the Honda Goldwing) and a few models that are routinely used with a sidecar are fitted with a reverse gear.
The clutch is typically an arrangement of plates stacked in alternating fashion, one geared on the inside to the engine, and 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 buildup between the plates until they rotate as a single unit, driving the transmission directly. A lever on the handlebar, through a cable or hydraulic arrangement, uses mechanical advantage to release the clutch spring, allowing the engine to freewheel with respect to the transmission.
The most commonly used transmission is a sequential gearbox. From neutral, you may select either first or second gear, but higher gears may only be accessed in order - you may not shift from second gear to fourth gear, without shifting through third gear. Internally, a rotating cam on the shift lever operates cogs on two counter-rotating shafts carrying a variety of gears. One shaft is geared to the final drive mechanism, and the other to the clutch. Operating the shift lever slides individual gears on one shaft, to intersect with a matching gear on the other. The small mass of the whole arrangement allows for extremely quick gear changes. Also, gear synchronizers typically found in passenger cars with manual transmissions are not necessary. The two shafts are always geared together (except in neutral), always spinning at a speed nearly approximating the next higher or lower gear ratio. Aided by beveled edges on the gears, shifting gears is simple for novices - no double clutching or grinding of gears. Advanced drivers can perform "full-throttle upshifts" on racing mounts, but this risks both the warranty and mechanical integrity.
Final drive from the gearbox to the rear wheel is typically accomplished with a chain, which requires both lubrication and adjustment for elongation (stretch) through wear. The lubricant is subject to being thrown off the fast-moving chain and results in grime and dirt buildup. Chains do deteriorate, and excessive wear on the front and rear sprockets can be dangerous. Many motorcyclists replace the chain and both sprockets as a set to maintain efficiency and safety. Many manufacturers offer cruiser models with final drive options of a belt, or a shaft. A belt drive is still subject to stretch, but operates very quietly, cleanly, and efficiently. However, belt drives are limited in the amount of power they can transmit. The belt is frequently toothed. A shaft drive is completely enclosed, the visual cue is a tube extending from the rear of the transmission to a bell-housing on the rear wheel. Inside the bell housing a bevelled gear on the shaft mates with another on the wheel mount. This arrangement is superior in terms of noise, cleanliness, and is virtually maintenance free. However, the additional gearsets are a source of power loss and add to bike weight.