Clutch Slip or Speedometer Error?
A 1.2-litre petrol hatchback with a five-speed manual gearbox (4th gear 1.00:1, final drive 4.10:1, tyre rolling radius 0.31 m) has done 68,000 km; the clutch was replaced 20,000 km ago. The owner complains that on a 12% gradient with four passengers the engine note rises sharply but the car does not accelerate, and there is a hot, burnt smell after the climb. On a level road in 4th gear at a steady 2500 rpm the speedometer shows 60 km/h, while the same car showed about 71 km/h when new. The clutch pedal has 4 mm of free play against a specification of 20-30 mm. The workshop owner wants to fit a 'heavy-duty' clutch kit and skip the pedal adjustment because 'the cable is self-adjusting anyway'.
1. Use the gearing to calculate the road speed the car should do at 2500 rpm in 4th, and work out the percentage slip. Does the data support clutch slip or a faulty speedometer?
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Step 1 - wheel speed. In 4th (1.00:1) the wheel turns at engine rpm divided by the final drive: 2500 / 4.10 = 609.8 rpm.
Step 2 - distance per wheel revolution. Circumference = 2 x pi x r = 2 x 3.142 x 0.31 = 1.948 m.
Step 3 - road speed. 609.8 x 1.948 = 1188 m per minute = 1.188 km/min x 60 = 71.3 km/h.
Step 4 - slip. Observed 60 km/h. Loss = (71.3 - 60)/71.3 = 15.8%, about 11.3 km/h of road speed.
Judgement: a speedometer error of 16% is possible but would not explain the engine note rising without acceleration on the gradient, nor the burnt smell. Those two symptoms point to the clutch slipping, with the speed loss confirming it: engine speed is rising faster than road speed. If the tachometer and speedometer are both suspect, verify the speed with a GPS before condemning the clutch - but here the physical smell settles it.
2. Explain the chain of events by which 4 mm of free play causes the slip and the burning smell.
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Free play is the gap that lets the release bearing sit clear of the pressure-plate fingers when the pedal is up. With only 4 mm instead of 20-30 mm, the bearing is preloaded against the fingers, so the pressure plate is partly lifted and clamp load on the disc falls.
Friction torque capacity is proportional to clamp load. On the 12% gradient with four passengers the torque demanded at the wheels rises, and once demand exceeds the reduced capacity the disc slips instead of transmitting drive - so engine rpm climbs while road speed lags, which is exactly the 15.8% loss measured.
Energy lost in slip becomes heat: heat rate is proportional to slip speed times torque, so a few seconds of slipping on a climb can take the lining surface well above its binder's safe temperature. The resin degrades and outgasses - the burnt smell - and the lining glazes, which lowers friction further and accelerates the slip. The flywheel can show blue hot spots and cracks.
Note also: on a genuinely self-adjusting cable, a 4 mm reading means the mechanism is at the end of its travel or a component (fork, bush, pedal box) is worn or seized. Either way, the cause must be found, not assumed.
3. List the checks and decisions you would make before removing the gearbox, and explain why a heavy-duty kit without correcting free play would fail again.
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Checks before removal:
- Cable routing, adjuster range and pedal-box bushes; confirm whether the system is truly self-adjusting and whether it is at end of travel.
- Release fork, pivot and bearing condition; a seized bearing or bent fork mimics wrong free play.
- Engine/gearbox mounts (movement can load the cable), and floor-mat or pedal-stop interference.
- Oil leaks at the rear main seal or gearbox input seal - oil on the lining causes slip and judder and must be fixed, not just cleaned.
- Verify road speed by GPS, and check tyre size, to rule out a speedometer error.
Decisions: if the lining is contaminated, glazed or rivets are close to the surface, replace disc, pressure plate and release bearing, resurface or replace a heat-cracked flywheel, renew the leaking seal, then set free play to 20-30 mm and road-test on a gradient with load.
Why a heavy-duty kit alone fails: it raises clamp load, but with the bearing still preloaded the clutch never fully engages. The extra clamp load just creates more heat and faster bearing wear, and the heavier pedal load stresses the cable and fork. A heavy-duty kit also makes a city car harsher and is not the fix for a maladjusted linkage. Correct the free play and the leak first, then choose the standard kit unless the vehicle genuinely tows or carries heavy loads.