The clutch slips (geared bike)

This is the first cause to check, since adjusting it is free. Free play is the small amount of slack at the lever before the clutch starts to engage; it guarantees that, with the lever released, the clutch fully clamps the plates. If there's no free play (adjustment too tight, cable poorly set, or a hydraulic system that doesn't fully release), the clutch stays slightly disengaged all the time: it doesn't clamp fully, slips, and heats up. The tell: slipping combined with, on testing, zero free play (slack) at the lever — the clutch engages the moment you touch the lever. The fix principle: restore correct free play (often around 2-3 mm at the lever tip — check your manual) via the adjuster/cable tensioner, or by checking the hydraulic system. This is an easy DIY job, to try first before suspecting the oil or the plates. This cause overlaps with the clutch adjustment for the hard-to-shift symptom (shared solution) and the clutch slipping under power-loss symptom (Batch 1). Important: this is for cable/manual adjustment — not a scooter's centrifugal clutch. If, with free play correctly set, the slipping persists, look toward the oil and then the plates. A simple adjustment is often enough to fix early-stage slipping.

On a motorcycle with a wet clutch (the vast majority), the clutch sits in the engine oil: the nature of that oil is therefore decisive. A modern car oil, or an oil labeled "energy conserving," contains friction-modifier additives designed to reduce friction — exactly the opposite of what a wet clutch needs, since its plates are meant to grip each other. Result: such an oil makes an otherwise perfectly healthy clutch slip. This is a common and easy-to-fix cause, typically appearing right after an oil change done with unsuitable oil. The tell: slipping that appeared after an oil change (possibly with a non-motorcycle oil or one not rated JASO MA2), with otherwise correct lever free play. The fix principle: do an oil change with a MOTORCYCLE-rated oil meeting JASO MA or MA2 (and a fresh filter) — an easy DIY job. The slipping generally disappears after a few miles with the right oil. This cause overlaps with the engine oil cause for the hard-to-shift symptom (shared solution). Important: never reuse a car "eco" oil on a wet clutch, and follow the manufacturer's grade and spec. If slipping persists with compliant oil and correct free play, the plates are probably worn.

If the free play is correct and the oil is compliant but the clutch still slips, the clutch plates are worn. The multi-plate clutch stacks friction plates and steel plates, clamped by springs; with wear (mileage, starts with the clutch slipping, overheating), the friction material thins and glazes (a smooth, shiny surface that slips), and the springs lose their tension: clamping pressure becomes insufficient and the clutch slips under load. The tell: confirmed slipping that persists despite correct free play and compliant oil, often on a high-mileage bike or one that's pulled a lot of load (cargo, hill starts). The fix principle: open the clutch cover, measure the friction plate thickness and the free length of the springs against the manufacturer's specs, and replace whatever's out of spec (ideally the full kit) — an intermediate job, doable by an equipped home mechanic. Budget-wise: a clutch kit runs roughly 60 to 200 euros depending on the model. This cause overlaps with the clutch-slipping-under-power-loss symptom (Batch 1). Important: this is for manual clutches, not a scooter's variator/centrifugal clutch (safety lock). Only replace the plates after ruling out free play and oil, which are far cheaper.