The clutch slips (geared bike)

Engine speed rises faster than road speed, especially when accelerating hard or climbing a hill in a high gear: the clutch is slipping instead of transmitting full power to the wheel. The symptom is very distinctive and easy to recognize — when you open the throttle under load, the tachometer shoots up (the engine "races in the void") without the speed following proportionally, sometimes with a smell of hot friction material. The approach is to confirm the slipping, then narrow down the cause, from simplest to most involved. Three causes, in the order you check them. First, poorly adjusted clutch free play (no slack at the lever): the clutch stays slightly disengaged all the time and slips — a free adjustment, to do first. Next, unsuitable oil: on a motorcycle, a wet clutch sits in the engine oil, and a car oil with "energy conserving" friction-modifier additives makes an otherwise healthy clutch slip — you need oil rated JASO MA/MA2. Finally, worn clutch plates (friction material too thin/glazed, tired springs) that no longer clamp hard enough: kit replacement, an intermediate job. You tell them apart with the slip test, whether there's free play at the lever, and the context (did the problem appear right after an oil change?). Urgency threshold: a slipping clutch wears and heats up; it will eventually fail. It's not immediately dangerous, but should be addressed, starting with the free causes (free play, oil) before opening the cover.