Transmission, Clutch and Final Drive
What this domain tests
This domain covers primary reduction, wet multiplate clutches, constant-mesh gearboxes, shift drums and forks, and chain, belt or shaft final drive. Questions ask you to calculate ratios and diagnose slip, drag, jumping out of gear and chain wear.
Core rules
- Overall reduction = primary ratio × gearbox ratio × final drive ratio. Higher numerical reduction gives more torque and lower speed.
- A wet multiplate clutch uses spring pressure, friction plates and steel plates running in engine oil. Correct oil matters because friction modifiers can cause slip.
- Clutch lever free play is typically small, around 10–20 mm at the lever; too little free play causes slip and release-bearing wear.
- Constant-mesh gearboxes keep gears engaged; the shift drum and forks slide dogs into engagement.
- Chain slack is measured at the tightest point with the motorcycle unladen or as specified, commonly about 20–30 mm. Over-tight chain loads countershaft and wheel bearings.
- Sprocket wear shows as hooked teeth; replace chain and sprockets as a set when badly worn.
Worked example
Engine speed 7000 rpm, primary reduction 3.0:1, fifth gear 1.0:1 and final drive 3.0:1. Wheel speed = 7000 ÷ 3.0 ÷ 1.0 ÷ 3.0 = 777.8 rpm. With a 0.60 m wheel circumference, road speed = 777.8 × 0.60 = 466.7 m/min ≈ 28 km/h. The calculation shows why lowering final gearing increases acceleration but reduces top speed.
Traps that cost marks
Setting chain slack with the rider seated unless specified; using car engine oil with friction modifiers in a wet clutch; adjusting clutch cable to zero free play; replacing only the chain on hooked sprockets.
Key points
- Overall ratio multiplies primary, gear and final ratios.
- Wet clutches need suitable oil and correct free play.
- Constant-mesh gears engage by sliding dogs.
- Chain slack is set at the tightest point.
- Replace chain and sprockets together when worn.