Manual Transmission - Clutch

Clutch

In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to separate the engine and transmission when necessary. This is because an internal-combustion engine must continue to run when in use, although a few modern cars shut off the engine at a stoplight. The clutch accomplishes this in manual transmissions. Without it, the engine and tires would at all times be inextricably linked, and any time the vehicle stopped the engine would stall. Without the clutch, changing gears would be very difficult, even with the vehicle moving already: deselecting a gear while the transmission is under load requires considerable force (and risks significant damage). As well, selecting a gear requires the revolution speed of the engine to be held at a very precise value which depends on the vehicle speed and desired gear – the speeds inside the transmission have to match. In a car the clutch is usually operated by a pedal; on a motorcycle, a lever on the left handlebar serves the purpose.

• When the clutch pedal is fully depressed, the clutch is fully disengaged, and no torque is transferred from the engine to the transmission (and by extension to the drive wheels). In this uncoupled state it is possible to select gears or to stop the car without stopping the engine.
• When the clutch pedal is fully released, the clutch is fully engaged and all of the engine's torque is transferred. In this coupled state, the clutch does not slip, but rather acts as rigid coupling to transmit power to the gearbox.
• Between these extremes of engagement and disengagement the clutch slips to varying degrees. When slipping it still transmits torque despite the difference in speeds between the engine crankshaft and the transmission input. Because this torque is transmitted by means of friction rather than direct mechanical contact, considerable power is wasted as heat (which is dissipated by the clutch). Properly applied, slip allows the vehicle to be started from a standstill, and when it is already moving, allows the engine rotation to gradually adjust to a newly selected gear ratio.
• Learning to use the clutch efficiently requires the development of muscle memory and a level of coordination.
• A rider of a highly tuned motocross or off-road motorcycle may "hit" or "fan" the clutch when exiting corners to assist the engine in revving to the point where it delivers the most power.

The clutch is typically disengaged by a thrust bearing that makes contact with pressure petals on the clutch ring plate and pushes them inward to release the clutch pad friction. Normally the bearing remains retracted away from the petals and does not spin. However, the bearing can be "burned out" and damaged by using the clutch pedal as a foot rest, which causes the bearing to spin continuously from touching the clutch plates.