Clutch, Gearbox, Drive Chain, Belt and Final Drive
What the exam tests
Power flow from crankshaft to rear wheel: clutch operation and adjustment, gearbox layout, and chain, belt or shaft final drive condition.
Clutch
- Motorcycles use a wet multiplate clutch: alternating friction and steel plates in a basket, pressed by springs, released by a pushrod and lifter.
- Lever free play is typically 10-20 mm. Too little causes slip and burnt plates; too much causes drag and difficult neutral selection.
- Scooters use a centrifugal clutch with shoes and a bell; engagement rpm is set by spring tension, and the variator changes ratio continuously with rollers on a ramp plate.
Gearbox
Constant-mesh design: gear pairs stay in mesh and dogs select them, moved by forks on a shift drum. Note the shift pattern and find neutral before stripping. Shims set mainshaft and countershaft end play; a worn fork gives jumping out of gear under load.
Final drive
- Chain: slack measured at mid-span is typically 25-30 mm. Adjust at the rear axle with the wheel aligned to the same mark on both sides, check for a tight spot, and lubricate with proper chain lube rather than used engine oil.
- Belt (scooter CVT): check width against the service limit, glazing and cracks. Worn rollers make acceleration flat and noisy. Variator nut torque and thread locking matter.
- Shaft drive: check bevel-box oil level for leaks and contamination.
Worked example
A scooter loses top speed while the engine revs high. The belt measures 20.0 mm against a 21.5 mm service limit, and the rollers are flat-spotted. Reasoning: a narrow belt sits low in the front pulley so the ratio cannot reach overdrive, and flat rollers prevent full ratio change. Replace belt and rollers as a set, then re-check top speed.
Traps
Over-tightening a chain and ruining the countershaft bearing, setting slack with a rider aboard, reusing a split-link pin, and adjusting clutch play at the engine instead of the lever.