Ducati Sporting Club UK > Model Specific - Discussion, Technical Problems and Help > Supersports: SS / SuperSport / Supersport 950 > Better (less lever pressure) clutch unit

[UglyDuckling]

Wow !! Cornerspeed managed to get me a new clutch slave unit to my place of work within a day.

Thanks to all those involved.

Just hope I can get it fitted in a similar amount of time...."me - I hate bleeding Ducati clutches"...

[Iconic944ss]

You certainly might !!!

I remember a thread on here about fitting a bleed nipple to the end of the handlebar union (eg at the fluid high point) - might be a good idea while you are 'bleeding'

Good luck - Frank

[lemans]

If any one is interested I have a second hand one of these slave cylinders which I took off my 1995 super sport before selling it. It has only done about 2,500 miles and is in very good condition. I can't use it on my latest Duke (I've just ordered a new one so the old one's up for sale if anyone's interested. If not I'll stick it on Ebay.

[JonHsiung]

I can vouch and endorse the Evoluzione unit. It not only fixed the clutch slave problem by replacing it but also makes the clutch slightly easier to use.

[FiscusFish]

S'funny I compared my Evoluzione one with Ali's standard unit recently. It may have been my imagination but I couldn't feel any difference in the effort required to pull the lever in at all. Everything else appeared to be equal so explain that...?

Darren

[Shazaam!]

Reducing Clutch Pull

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 pushrod 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 pushrod, 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 so there's replacement radial master cylinders offered that reduce this movement, but at the sacrifice of higher clutch pull forces. For the street it's desirable, since the longer lever travel makes it easier to launch the bike from a stop by increasing the range of the "friction zone."

That said, the only practical way to reduce the clutch lever pull force is to change the diameter of the slave cylinder. The force-reduction aftermarket slave cylinder replacements offer around a 20% reduction but at the penalty of needing a longer clutch lever pull to get full disengagement.

Another solution would be to just reduce the distance between the clutch lever pivot point and the master cylinder in combination with decreasing the diameter of the master cylinder. This combination is not currently offered in the aftermarket, although Ducati did change the master cylinder’s bore from 13mm to 12mm in the 2001 model year in an effort to decrease the pull effort 15%. Yoyodyne sells the Brembo 12mm clutch master cylinder. $146. It’ll need a fabricated bracket for the reservoir.

Finally, a straightforward way to reduce pull effort is to reduce the force pushing the plates together. One way is to change the stock springs to ones having a lower stiffness. Another way is to reduce the number of springs from six to four. This gives a one-third reduction in lever pull as well as a one-third reduction in friction force in the clutch. Depending on the particular bike’s torque output, you might get slippage, but reports from owners say this approach works fine. The best way to test for clutch plate slippage is to apply full throttle power in top gear.

[JonHsiung]

...whoa.

good find/writeup...i guess i'll take a look at those springs too...

[Edited on 11-1-04 by JonHsiung]