Ducati Sporting Club UK > Model Specific - Discussion, Technical Problems and Help > 998 / 996 / 916 / 748 > clutch springs

[DC]

Hello chaps,

I have just fitted some new stainless clutch springs to the 748. After fitment I thought that the clutch felt lighter at the lever. Is this the case or am I getting that shiney is best syndrome?

The springs have the same spring curl and length as the original ones. To the touch of my fingers( compression) the springs feel about the same.
The end caps have been replaced with new ones. Again these are the same thickness as the original ones.

I do not know the ins and outs of the (Ducati?) clutch. What stops it slipping? Is it just good friction plates or does the spring strength pressing on the pressure plate have anything to do with it?

I feel like I am being a bit of a muppet with this one but any answers would be appreciated.

Thanks in advance,

DC.

[marksduc748]

stronger springs stop it slipping. are they genuine ducati ones? being hydraulic I dont think you would notice the difference unless youve fitted one of them aftermarket slave cylenders. could all be in your head pal. unless some one else thinks different.

[Shazaam!]

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.

[DC]

Hello mate,

I have fitted a uprated slave cylinder but I had used the clutch before I had changed the springs so I knew what it felt like before.
The springs are simply listed as stainless replacements for the 748/916. So I am assuming that they will be ok.

I think that you are right in that It is in my head Um, oh well. I think The clutch will soon let me know when I ride the bike and give it some jip

DC.

[DC]

Originally posted by Shazaam!
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.

As always, a wealth of information. Thanks very much Shazam.

Yep, I think I will get out on the bike and give it a test ride. I am sure it will be fine.

The thing is I have installed a radial clutch master cylinder and an uprated larger bore slave cylinder and this feels much lighter to what I was used to.
I stated earlier. It felt a bit lighter at the lever again after I had fitted the stainless springs and this made me think," I hope It doesnt start slipping?"

DC.

[Walenut]

I would hazard a guess and say that because the material used in the stainless steel spring is different to that used in the standard spring (different tensile, spring steel is specially made and heat treated) that they could well be different. The stainless one could be less efficient.

[rcgbob44]

Sounds like a head job to me!

[madmav]

I have replaced mine with the same as you and guess what ?

my clutch slipped !!!!!

[gasmanrus]

hey Shazaam! i was just about to say that

nice 1

[DC]

Originally posted by madmav
I have replaced mine with the same as you and guess what ?

my clutch slipped !!!!!

Oh bugger!

Never mind. If my clutch starts to slip then I will have to yank them out.
Would I be able to add a washer or two to add a bit of extra pressure on the plates or will this limit clutch travel?

Ta,

DC.