The Motorcycle Clutch Basics

Everyone who learns to operate a motorcycle with a manual clutch has to acquire the knack of coordinating clutch engagement with opening of the throttle. Most of us had to get it wrong a few times – stalling the engine, advancing in a series of lurches, or revving the poor thing to an embarrassing degree as we got the coordination a bit wrong.

It is a peculiarity of spark-ignition engines that they produce little useful torque until they are spinning somewhere in their range or good breathing, so it's almost impossible to ride away at idle – revs are essential. And it takes the beginner a few tries to find the place in lever travel where the clutch bites, and to be able to waste little time in getting to it. Once you get it right, engines revs are coming up just as the clutch gets an initial grip, allowing the rider to feed throttle as required and move smoothly off.

Racers naturally use a somewhat more extreme technique. They simply pin the throttle and control the engine with the clutch level, simplifying the coordination task. If the engine bogs, pressure one the clutch lets revs rise, increase torque to sustain the drive. If the front wheel starts to come up, again, pressure on the clutch lever brings it down. This method requires a certain ruthlessness that must be acquired, but assures fast departures. Curiously, some racers never learn this. Some of the best starts are of 250cc riders, who are able to hold the engine at constant revs while the machine accelerates to the point of clutch lock-up.
Let's look at what happens as a clutch starts is made. As the clutch bites but the motorcycle has not yet moved, all the power the engine is making goes into driving clutch slip. This sends clutch plate temperature steeply upward. As engine torque, applied through clutch and gearbox to the rear wheel, starts to move the motorcycle, the clutch becomes a power divider, sending to the motorcycle what it can use, dumping the excess into clutch heat. As an example of why this is so, imagine a motorcycle at the instant it passes through 3 meter per second of forward velocity. Assuming for the moment that it is accelerating at one 'G' (Just under 10 meters per second) and all-up weight is 272 kilogram, instantaneous horsepower being used for acceleration is about 10.9 horsepower. If the rider is making a spirited start and uses engine revs and throttle roughly 89 horsepower of that is going into heating the clutch.

Moments later the motorcycle is at 48 km/h and can accept more power as 48 horsepower. Less power is heating the clutch and this decrease continues until clutch lock-up. This explains why my early small engine sized motorcycles always needed servicing of the clutch, all that engine power going into it as starting slippage heat raised its lightweight stack of clutch plates to a temperature high enough to warp or even crack a plate or two. So if you increase the power of your motorcycle engine you should also look into improving your clutch to match the power.
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