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Discussion Starter · #1 ·
'06 S1K Mono.... I was out riding today and was running at about 4500 RPM in fourth gear in traffic. I nailed it to pull out around a car and the clutch slipped then hooked up. Experimented a bit and found it does it whenever you power away in the mid RPM ranges. It slips then seems to get a grip. A fast get away from a stop sign doesn't seem to cause the slip, just powering away at higher revs. I will need to take another ride to be sure there is no slip on start up since by this time I was in an urban area where I thought it unacceptable to really power away from a standstill. Bike has 12,500 miles on it and the clutch stack is the original stock OEM parts. I don't make it a habit to slip the clutch and I don't bounce the engine off the rev limiter. Never had it on a track. The clutch has had an easy life I would say. When I got home I pulled the clutch apart and the driven plates are in spec at 2 mm thick. They are smooth and flat. The driving plates are just under the 3 mm spec...say 2.96 mm average. There is a little hammering wear on the basket slots but it is not bad. No oil or contamination noted on the clutch pack when I disassembled it. Neutral is easy to find. Shifts through the gears fine. The only non-OEM parts are a billet pressure plate (made by Hdesa USA), a SS spring set and a spider type spring plate instead of individual spring caps. Everything I have checked seems "right" to me. Any ideas what else I should be checking? Is the clutch stack just worn out at 12.5K miles since the friction plates are slightly under 3 mm? I will say that the friction material is not much thicker than the plate it is mounted on...is that "normal"? Should I be pulling the entire stack and measuring the total thickness? If so does anyone have a measurement for the entire stack height? Should I lightly sand the plain steel plates on a flat plate to roughen up the surfaces? Or should I go buy a Zero electric that doesn't have that pesky clutch lever?
TIA for your wisdom and ideas.
 

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The standard test for detecting worn-out clutch plate friction material is to test for slippage when accelerating with WOT in top gear.

You can often stop slippage by maintaining the force pushing the plates together. The idea here is to maintain the stack height as the friction material wears off and reduces the thickness of each plate. The higher the stack height, the more the springs are shortened/compressed, and the greater the force pushing the plates together. The clutch slips when there's not enough force supplied by the springs or the friction material is worn completely off.

The service limit on 3 mm friction plates is 2.8 mm. For 2.5 mm plates the service limit is 2.3 mm.

The plain metal plates come in stock 2.5 mm or 3 mm thicknesses. When the overall plate stack height wears 0.5 mm, replace one 2.5 mm flat plate with a 3 mm one. At 1 mm stack height wear, repeat for a second flat plate. At 1.4 mm - 1.6 mm wear, replace all the friction plates. Allowable friction plate wear is 0.2 mm (each).

The stack height is specified for the OEM springs, so if your springs are not OEM stiffness then the stack height needed to prevent slippage will vary. The stack height can only be increased a small amount over the OEM recommended height before the last plate hangs up on the basket.
 

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Sounds like the clutch pack is showing signs of normal wear-n-tear. I experienced the same issue and it slowly got worse but the bike was still ridable, just had to adjust my riding to the worn conditions. One thing I immediately noticed was how much quieter the clutch was after the change (as far as a dry clutch can be quiet :wink2:). As the clutch wears the noise it produces gets louder but it does so very gradually and thus you don't really notice the volume difference.

At 15,663 miles I replaced my original clutch pack with JC Pak :grin2: Here's my EVR 48t upgrade and I have some measurements there: Dry clutch change - AirCooledNut.com
 

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Not really going to help you out but I just recently purchased an air compressor and the first thing I did with it was blow out the clutch in my 06 SC. I was amazed at all of the clutch dust that came out of it.

If all of the plates are in pretty decent specs....what about trying new springs?
 

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Discussion Starter · #5 ·
Thanks for the advice guys. Moto I have an open clutch cover so there isn't much dust in the basket and the springs are only about a year old and supposedly 10% stronger than stockers. The service limits were very helpful Strega. I didn't see them in my service manual, only the original size. And as always a great pictorial write up Toby. You mentioned the increased noise and my clutch is pretty much as loud as my uncorked 2 into 1 Termi. And my basket is not nearly as hammered as yours was in the pictures. I guess it's time for a new clutch stack or to ride like an old man...which I am but I'm trying hard not to be. It still seems odd to me that I don't notice the slip when starting out from a stop but it's really unacceptable when you whack the throttle WFO in the mid RPM range. All that monstrous air cooled Duc torque I guess... :rolleyes: I'll get it apart in the next day or two and carefully measure it all up again with Strega's specs in mind and probably start looking for a new stack. Glad it's a dry clutch so it's easy.
 

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Discussion Starter · #7 · (Edited)
I finally found time to completely disassemble my slipping clutch. The total stack height is 35.2 mm (should be about 38.5 mm I think) and the friction plates are 2.95 mm on average. And I found the thickness spec for the drive plates in my shop manual...minimum 3.2mm. As they liked to say on Mythbusters...."Well there's your problem." Also the maximum gap between the the fingers and plates is supposed to be .6 mm and mine are 1.05 mm. No wonder it was so noisy. Also interesting was that the stack started with two of the 2 mm driven steel plates then the 1.5 mm wavy plate then the first friction plate. According to the book the first friction plate should be third in line between the two plain and the wavy plates. My clutch has never been apart to my knowledge. Factory error? Workshop manual error? Dunno... Anyway my clutch is toast at 12,500 miles and a new basket and clutch pack are going in and I will need to fabricate a clutch holding tool from the old plates. And buy a 32 mm socket. Thanks for the help.
 
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Discussion Starter · #8 ·
Follow up

I finally found time to install my new clutch pack and basket. A friend loaned me his Corse Dynamics billet clutch holder which made the job a piece of cake. Took the bike out for a ride today and no more slip so I'm a happy wrencher. The hardest part was torquing the hub center retaining nut to 190Nm/140 lb/ft. My 3/4 drive torque wrench only goes to 150 lb/ft so it was just about maxed out. I can't see how it would be possible to do the job with one of the handheld rather than bolt on clutch holder tools since it took me two hands on the torque wrench for to get it tight enough. Also my clutch is WAY quieter now.
 
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Discussion Starter · #10 ·
was it just missing the first friction plate?
Nope. I counted all the plates and all were accounted for. All of the friction plates were pretty much just worn down to the metal. If I had added another plate to thicken the clutch pack it might have lasted a few more miles, but it was just worn out. I was surprised it was gone at 12.5K miles since I don't abuse it but this is my first dry clutch bike and I guess that's to be expected unless you just cruise on the Interstate. Fortunately it's a very easy fix. If I hadn't changed the basket it would have been a ten or fifteen minute repair.
 
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