| Dished Clutch Plates
During acceleration the clutch is prevented from slipping by the frictional force between plates. The higher the torque output of the engine, the more frictional force that is required. In order to get more frictional force, you need more force to push the plates together. The force holding the plates together is determined by the compression of the clutch springs, which in turn, is controlled by the stiffness of the springs and the amount that they are compressed, i.e. the height of the clutch plate stack.
There are two values of frictional force that can exist between the clutch plates: static and sliding. The coefficient of static friction is always higher than the coefficient of sliding friction.
Smooth progressive clutch engagement requires that the force transition between sliding and static occur gradually, and this is why a curved dished plate is placed in the clutch plate stack.
When the clutch lever is released, the push-rod moves the pressure plate, which in turn moves the slipping plates together, transmitting increasingly higher torque to the rear wheel. At some point, the force between plates is sufficient to cause a transition between static and sliding and the difference in the two coefficient of friction results in an abrupt change in torque output. The clutch grabs suddenly.
To minimize this suddenness, a dished plate is added to the plate stack. A dished plate is essentially a spring that exerts a force between the plates when it becomes flattened so as to give a more progressive force between plates than can be achieved using the stiffer clutch springs.  |