Clutch, Gearbox and Driveline
Clutch, Gearbox and Driveline
This domain tests the clutch's role, its adjustment, gearbox types and the driveline from gearbox to wheels. Expect questions on clutch slip, drag, free play, synchromesh and differential action.
Core rules
- Clutch: friction plate between flywheel and pressure plate; the release bearing and fork disengage it. A single-plate dry clutch is common in cars; multiplate wet clutches are used in motorcycles and automatics.
- Pedal free play is measured at the pedal pad before the release bearing touches the pressure plate. Too little free play causes slip and bearing wear; too much causes drag and gear crunch.
- Gearboxes: sliding-mesh (old), constant-mesh (gears always in mesh, dog clutches select), synchromesh (baulk rings equalise speed before engagement). Never force a gear; check the clutch and linkage first.
- Driveline: propeller shaft with universal joints, slip joint, differential, half shafts and CV joints. The differential lets wheels turn at different speeds in a turn while transmitting torque.
- FWD cars use a transaxle; RWD cars use a separate gearbox and final drive.
Worked example
The manual gives a clutch pedal free play of 20–30 mm. A technician measures 45 mm and the driver reports difficult engagement and a crunch into first. Reasoning: the extra free play uses up pedal travel, so the release bearing does not push the pressure plate far enough; the clutch drags. Adjust the cable or hydraulic pushrod to bring free play to about 25 mm, then recheck that the pedal still has reserve travel. If the play were 5 mm, the opposite fault - slip and release-bearing noise - would be the diagnosis.
Traps
- Adjusting free play at the wrong end or with the pedal depressed.
- Calling a dragging clutch 'slipping'; the symptoms differ.
- Ignoring a worn pilot bearing or faulty synchro when a gear crunches.
- Filling a manual gearbox with engine oil instead of the specified gear oil.