| The higher the height of the clutch pack, the greater the clutch spring preload. So, as the clutch pack friction material wears-out, the pack height gets shorter, until the force between plates is insufficient and the clutch slips, at first under high torque conditions such as at launch, and later even when the lever is not pulled at speed. Time for a new clutch.
This is also the cause of the annoying phenomena of clutch “judder” such that when you release the lever, and while the spring plate is being compressed, the clutch springs can’t develop a sufficient force to prevent slipping so the plates slip, grab for an instant (causing vibration-induced slip,) grab, slip and so on, until the slipping stops as the engine torque is reduced when the bike acceleration eventually drops-off.
Here’s the Physics behind it. The friction force developed between two surfaces depends on whether the surfaces are sliding past each other or not. For a given amount of force pushing any two surfaces together, less frictional force will develop between sliding surfaces, than between surfaces that don’t. Once surfaces are together it takes more force to get them to slide than to keep them sliding. One they begin to side, however, they’ll keep sliding unless you push them together harder.
It’s these alternating higher-lower friction forces that cause the vibration that you perceive as judder. Sometimes adding a second spring plate to the pack will help to damp-out certain vibrations, so Ducati specifies a different number of flat plates, dished plates, plate thickness and stacking sequence for different models. The common spec however is stack height. Ducati clutch packs are stacked to 38mm ± 2mm.*  | 
05-Sep-2005, 23:11 
Shazaam!
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