At the root of it, the clutch serves to engage and disengage the engine from the rear wheel. Without a clutch to couple and decouple the crankshaft and the transmission, you’d have a really hard time starting, stopping, and shifting your bike, so let’s have a look at how it works.

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The clutch may appear pretty complicated, but it's a fairly simple assembly that consists of an outer basket, inner hub, pressure plate, a bunch of clutch plates, and clutch springs and bolts.

The basket is geared to the crankshaft by the primary gear, and the inner hub is splined onto the end of the transmission’s input shaft. Nested between the basket and the hub are the clutch plates, with alternating friction plates, and steel plates. The friction plates have tabs that mesh with grooves on the outer basket, while the steel plates have tabs that sit in grooves in the inner hub. So each type of plate is locked into either the hub or the basket.

The pressure plate sits on top of this stack of plates and forces them all together with springs. When the clutch lever is out the clutch springs keep everything sandwiched together and the basket—geared to the crank—and the hub—attached to the transmission—are locked together so that the engine can drive the rear wheel. Pull the clutch lever and the pressure plate is forced away from the clutch pack, which permits the plates to slip past each other and the outer basket and inner hub to turn independently.

As you’re releasing the clutch lever and opening the throttle, the clutch plates are gradually pressing together and gripping each other, and as they do torque is fed from the engine to the rear wheel.

You may be wondering, why are there so many plates? It all comes down to the clutch’s capacity to transmit engine torque. We won’t get into the mathematics of it, but the torque capacity of a clutch is a function of the frictional force and the radius of the clutch plates. So, you can either have a larger diameter clutch with fewer plates, or a smaller diameter clutch pack with more plates. You could also up the spring pressure, but that would make for a really heavy clutch-lever pull.

Some bikes, like older BMWs and Moto Guzzis with longitudinal crankshafts that run front-to-back, use a single-plate clutch that's as big as a dinner plate, just like in a car. That works with those bike's layouts because they have space for a big clutch, but on most bikes the crank runs across the frame and design limitations mean a smaller-diameter clutch is desired. In that case, you gotta have more plates to handle the torque.

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