How the Clutch Works
The force required to pull the clutch lever is ultimately controlled by the engine's power output. A Ducati superbike with a maximum torque of 65 ft-lbs. being transmitted though it's clutch needs to have around 430 pounds of preload in the clutch spring(s) to prevent the plates from slipping.
This means that the hydraulic pressure on a typical 28mm slave cylinder needs to be about 425 psi to overcome the 430 lb. spring preload and disengage the clutch. The distance that the slave cylinder needs to move (the push-rod that in turn separates the plates) has to be at least the thickness of the 2mm dished plate in the clutch pack. Say 3mm tops.
About 94 lbs. of force is needed to be applied to a typical OEM 13mm diameter master cylinder piston to create 425 psi of pressure in the incompressible hydraulic fluid that, in turn, moves the slave cylinder. For every 1mm that the slave cylinder moves the push-rod, the OEM master cylinder has to move about 4.6mm. This requires the displacement of about 1.2cc of hydraulic fluid. But only in the ideal world.
In the real world, the master and slave cylinders are connected by a flexible clutch actuation line that expands a little under this 425 psi pressure. This undesirable expansion can be reduced in two ways. The rubber can be reinforced with Kevlar fiber or steel braid, or the line internal diameter can be reduced (by choosing a 2 or 2.5mm I.D. line rather than a regular 3mm) thereby increasing the line wall thickness (and it's strength.) The net result of having a real-world line is that it increases somewhat the volume of fluid that the master cylinder has to displace to get the same internal line pressure. This is often described as a "spongy" lever.
Continuing on. In order to apply the required 94 lbs. of force to the clutch master cylinder piston, the clutch control lever needs to be pulled, and here's where the lever's mechanical advantage comes into play.
The human hand can't repeatedly (without fatigue) apply 94 lbs. of force to the lever so the master cylinder lever is designed to provide a mechanical advantage - to amplify the hand's force. The OEM design for example provides between a 4:1 (two finger) to a 9:1 force reduction (end of lever.) This means that you have to normally apply between 23 and 11 lbs. respectively with your hand to release the clutch.
The lever's mechanical advantage has it's consequences, however. Instead of having to move the master cylinder only 9mm to disengage the clutch 2mm, the end of the clutch lever now has to move more than 3 inches. In a racing situation this is undesirable because longer level movement results in slower shifts. Consequently, there's after-market radial master cylinders offered that reduce this movement, but at the sacrifice of HIGHER clutch pull forces.
However, on the street it's desirable to have a longer lever movement, since the longer lever travel makes it easier to launch the bike from a stop by increasing the range of the "friction zone."
Reducing Clutch Pull
There are often complaints about the lever force needed for Ducati superbikes.
The most inexpensive solution to this is to remove two of the six clutch springs which reduces the lever pull force required by one-third. This also gives a one-third reduction in friction force developed in the clutch. Depending on the particular bike’s torque output, you might get slippage, but reports from owners say this approach works fine on unworn clutch packs. The usual way to test for clutch plate slippage is to apply full throttle power in top gear and see if it slips.
Another way is to replace the clutch springs with ones that have a lower stiffness or use three strong and three weaker springs.
The more common solution is to change the hydraulic ratio by replacing the OEM clutch slave cylinder with an after-market one that is larger in diameter. This diameter increase has its limits however because the penalty for force reduction is a longer lever movement necessary to fully disengage the clutch. The longer lever movement means that your take-up point will move farther away from the handlebar. This in turn requires that you change the 4-position clutch lever adjustment to get full disengagement.
Incomplete clutch plate separation results in difficulty in selecting neutral when the bike is idling at rest, and ultimately reduces friction plate life because of the plates dragging. About a 30% lever movement increase is the practical limit before there’s problems.
It's important to note that Ducati slave cylinders are not interchangeable between the pre-2001 and 2001+ superbikes because the push-rod length is different. When Ducati solved the slave cylinder leakage problem in 2001, they modified the slave cylinder on all bikes, except the SPS. That modification to the housing design required the use of a 10mm longer push-rod. Some of the cylinders on the market come supplied with a 10mm spacer so you can use it with both push-rods.
In model year 2000, Ducati itself decided to address the clutch pull complaints by changing the hydraulic ratio using a 1mm smaller diameter master cylinder. The result is a 15% easier-to-pull, 17% longer-to-pull lever. If you use an after-market slave cylinder with these later masters, you run into clutch engagement problems at smaller slave diameters than with the earlier bikes. (See Table)
Brembo Radial Clutch Master Cylinders
The main reason (other than for appearance and weight savings) that Ducati owners replace the factory set-up with a radial after-market unit is to obtain a different MC piston size (hydraulic ratio) and lever pivot point distance (mechanical ratio.) This allows you to obtain a shorter lever pull distance that theoretically promotes faster shifts, of particular interest for race bikes. However the lever pull force is higher than with stock OEM levers for both the 16x16 and 16x18 variants.
For the street, faster shifts at the sacrifice of an up to 36% harder-to-pull lever is a questionable trade-off. You really need to combine the radial units with after-market slaves to bring the clutch action back to the Ducati standard for lever pull force and distance. (See Table)
