ASK THE MXPERTS: THE SMARTEST CLUTCH QUESTION WE’VE EVER BEEN ASKED

THE SMARTEST CLUTCH QUESTION WE’VE EVER BEEN ASKED

Dear MXA,
I know this is going to sound like a stupid question, but since I don’t know the answer, it is the smartest question I can ask. How does the clutch on my CRF450 work? What actually happens inside the engine when I pull the clutch lever?

A motorcycle clutch modulates the transfer of torque from the engine to the transmission through a mechanism that is basically a dead man’s switch. The lever on the left side of the handlebar throws the switch that connects the internal combustion part of the engine to the rear wheel or, more accurately, disconnects the engine from the wheel.

All modern motocross bikes employ a wet clutch design where multiple undriven friction plates are stacked in an alternating pattern with steel drive plates to create the clutch pack. The clutch pack is sandwiched between the clutch basket and the inner hub. When the plates in the clutch pack, undriven and drive, are in contact with each other, the complete unit transfers power via the inner hub to the drivetrain. When the clutch pack is pulled apart, by pulling in the clutch lever, the connection with the inner hub is broken by spreading the undriven fiber plates away from their fellow steel drive plates. Springs are used to hold the plates in the clutch pack together to transmit power to the drivetrain. And when the rider pulls in the clutch lever, he feels the spring pressure in his fingers. It is the pressure of the springs that he must overcome to separate the undriven fiber plates from the driven steel plates. Pulling the clutch lever disengages the clutch by lifting the pressure plate and separating the clutch pack, allowing the two sets of clutch plates to spin independently.

Clutch springs are used to hold the plates in the clutch pack together to transmit power to the drivetrain. They come in soft and stiff versions. We’ve never used a softer clutch spring.

When the clutch lever is released, your bike moves because the plates connected to the rear wheel are grabbed by the plates connected to the engine. A rider can modulate how quickly the rear wheel is engaged by how fast he releases the clutch lever. If he pops the clutch by releasing the lever quickly, the fiber and steel plates bite into each other instantly and produce rapid acceleration—or an unwanted wheelie. If the rider releases the clutch lever slowly, the soft-fiber undriven plates engage slowly with the hard steel drive plates by slipping against each other. The slippage controls how rapidly the engine’s power is transmitted to the rear wheel.

This slippage is both a rider’s friend and enemy. By slipping the clutch, a rider can modulate the power in infinite ways. He can aid the engine in getting up on the pipe. He can use the slippage to get the engine to pull a taller gear. He can use the clutch as a secondary throttle by engaging and disengaging the power train without shutting the throttle off; however, too much slippage will cause the fiber clutch plates to overheat and eventually lose their ability to hook up with the steel plates. When a clutch slips its plates without the lever being pulled, your race is over. Once a clutch starts slipping, you are on a death march to overheating, worn plates and contaminated oil. The only solution is new clutch plates.

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