Ducati Clutch Tuning Voodoo
by Larry "Shazaam!" Kelly
Clutch engagement problems are quite common, but solving them often requires a fair amount of trial-and error work.
Usually, an uneven take-up or excessive slipping signals signals the need for a clutch pack replacement, often just caused by normal wear loss of the friction material on the plates. The mileage between replacements has a very wide range, anywhere between 3,000 and 30,000 miles, depending upon the amount of city riding and the rider’s launch style. If the clutch type is a slipper, it’s normal for the friction material to wear-out sooner since it’s function is to allow the plates to slip past each other under high engine-braking loads during downshifts. Some friction materials do better than others regarding wear and engagement smoothness, just like brake pads.
Sometimes engagement problems begin after you’ve just replaced your worn-out stock clutch components, upgraded to an aftermarket clutch or changed to a slipper clutch.
The key to smooth engagement is controlling the frictional force developed between the smooth plates (that are driven by the engine clutch hub) and the plates that contain friction material (that drive the rear wheel through the clutch basket.)
The amount of force developed between these plates is controlled by the stiffness of the clutch springs, specifically by the amount of preload on these springs. When the clutch is fully engaged, the friction force developed between the plates needs to be greater than the engine’s applied torque to prevent slip. About 430 lbs is needed on a stock superbike.
When you pull-in the clutch lever, the hydraulic pressure applied to the slave cylinder overcomes the spring’s preload and progressively reduces the force pushing the plates together until they begin to slip. During this time the dished plate(s) in the stack act to provide a progressive reduction in the inter-plate force as the plates separate a few millimeters and you get full disengagement.
When you engage the clutch the opposite occurs. The reduced hydraulic pressure on the slave cylinder allows the push-rod to move the spring-loaded pressure plate toward the plate stack (a millimeter or so) until the plates begin to touch. Keep in mind that when you move the lever you are changing the POSITION of the pressure plate. You have only indirect control over the forces between plates.
The forces between plates is controlled during this transition (between disengaged and fully engaged) by a dished plate that is included in the stack to smooth this transition. This plate acts as a spring (pushes back with a force) when it gets flattened between adjacent plates by the movement of the pressure plate.
So, the force pushing the plates together first come from the smaller force produced by flattening the dished spring plate, and later, a much greater force produced by the preloaded clutch springs.
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.
Here’s some examples:
Haynes Manual: 748, 916, 996
7 friction plates - one curved plate with two friction surfaces
Minimum friction plate thickness = 2.8mm
2.0mm plain plate #1 2.0mm plain plate #2 3.0mm friction plate #1 1.5mm dished plain plate (convex side facing toward you) 3.0mm friction plate #2 2.0mm plain plate #3 3.0mm friction plate #3 2.0mm plain plate #4 3.0mm friction plate #4 2.0mm plain plate #5 3.0mm friction plate #5 2.0mm plain plate #6 3.0mm friction plate #6 2.0mm plain plate #7 3.0mm friction plate #7 2.0mm plain plate #8 pressure plate (line up marks)
38.5mm plate stack height
Haynes Manual: 916SP, 916SPS, 996S, 996SPS
8 (thinner) friction plates - two curved plates with one friction surface each
Minimum friction plate thickness = 2.3mm
2.0mm plain plate #1 1.5mm dished plain plate #1 (convex side facing toward you) 2.5mm friction plate #1 1.5mm plain plate #1 2.5mm friction plate #2 2.0mm plain plate #2 2.5mm friction plate #3 2.0mm plain plate #3 2.5mm friction plate #4 2.0mm plain plate #4 2.5mm friction plate #5 2.0mm plain plate #5 2.5mm friction plate #6 2.0mm plain plate #6 2.5mm friction plate #7 1.5mm plain plate #2 2.5mm friction plate #8 1.5mm dished plain plate #2 (convex side facing away from you) pressure plate (line up marks)
38.0mm plate stack height
Ducati Service Manual: 998
6 friction plates - one curved plate with two friction surfaces
Minimum friction plate thickness = 2.8mm
2.0mm plain plate #1 1.5mm dished plain plate (convex side facing toward you) 2.5mm plain plate #1 1.5mm plain plate #1 3.0mm friction plate #1 2.0mm plain plate #2 3.0mm friction plate #2 2.0mm plain plate #3 3.0mm friction plate #3 2.0mm plain plate #4 3.0mm friction plate #4 2.0mm plain plate #5 3.0mm friction plate #5 2.0mm plain plate #6 3.0mm friction plate #6 1.5mm plain plate #2 2.5mm plain plate #2 1.5mm dished plain plate (convex side facing toward you) pressure plate (line up marks)
38.0 mm plate stack height
A too-high overall thickness of the plate stack will cause drag between plates when the clutch is disengaged making it difficult to select neutral when the bike is stopped.
The last 2mm steel plate can be interchanged with a 1.5mm curved plate facing away from you if you want to adjust the stack to get the 38mm stack thickness, or if you want a more progressive, soft clutch engagement. A commonly-used substitution that adds an additional spring plate to the stack.
Any of the steel 2mm plain plates can be exchanged with 1.5mm plain plates to reduce stack thickness or increase stack thickness as needed to achieve your desired overall height. You will see the need for this as friction plates wear, so keep your steel plates from pervious clutch replacements as spares to be used as height adjustments later. The 2mm and 1.5mm steel plates only need replacing if they’re scored or warped.