Gearbox

How it works

The gearbox lets the engine speed match the bike's road speed. On almost all motorcycles it is a sequential gearbox: you change gears one at a time, in order, with the foot lever (often 1st down, neutral, then 2nd to 6th up). Inside, two shafts carry gears that are always in mesh; dogs (side teeth) lock the chosen gear to its shaft. Selector forks, moved by a drum linked to the gear lever, slide these gears across. On most bikes the gearbox shares the engine oil, hence the importance of suitable oil. Hard or noisy shifts usually come from a misadjusted clutch, worn-out oil or a misadjusted gear lever. A gear that jumps out under load, however, signals internal wear of the dogs or a bent fork: that is a workshop repair, often requiring the crankcases to be split.

Maintenance tips

• ✓Before suspecting the gearbox, check clutch free play, the oil and the gear lever.
• ✓Shift firmly with the clutch fully disengaged, without forcing the lever.
• ✓Use the specified oil rating (JASO MA/MA2 if the gearbox shares engine oil).
• ✓A gear that jumps out under acceleration should be diagnosed quickly: riding on makes dog wear worse.
• ✓Internal gearbox repair means splitting the cases: it is workshop work.