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'76 BMW R90/6 Brand New Clutch Slipping, Ideas?

B

banzaibob

New member
The bike: '76 R90/6, Siebenrock big bore kit, dual plug conversion, Alpha ignition, R100/7 final drive (more top end, lower rpms at highway speed). The bike runs GREAT! However. Last summer I notice some clutch slipping during hard acceleration so I decided to replace pressure plate, friction disc and pressure ring. It had the "heavy duty" diaphragm spring already installed from a couple of years ago.

Today while pulling a long hill against the wind at about 85mph....it started to slip. I lightened up on my throttle hand and it returned to normal. I put about 100 more miles on it today. No unnecessary hard acceleration. I also passed a couple of cars going 80-85mph, no more hint of slippage.

I am going to go back in there and replace the diaphragm spring, the one component I didn't change last summer. The old spring is about 10 years old.

However, I would like to ensure that whatever I do will solve the problem. I just read the Snobum article about placing "a .035"-.065" hard steel donut ring shim between the flywheel and the spring base." This would effectively "tighten" up the spring against the pressure plate. What is everyone's thoughts on this? Also let me re-phrase the article: The flywheel has an area that the diaphragm spring resides in...then comes the pressure plate. He's talking about taking hard shim stock and making a ring that the comes between the spring and the flywheel...am I envisioning this right? This would increase the clutch pull slightly but it's worth it not to have high speed slippage.

Your thoughts?
 
banzaibob said:
The bike: '76 R90/6, Siebenrock big bore kit, dual plug conversion, Alpha ignition, R100/7 final drive (more top end, lower rpms at highway speed). The bike runs GREAT! However. Last summer I notice some clutch slipping during hard acceleration so I decided to replace pressure plate, friction disc and pressure ring. It had the "heavy duty" diaphragm spring already installed from a couple of years ago.

Today while pulling a long hill against the wind at about 85mph....it started to slip. I lightened up on my throttle hand and it returned to normal. I put about 100 more miles on it today. No unnecessary hard acceleration. I also passed a couple of cars going 80-85mph, no more hint of slippage.

I am going to go back in there and replace the diaphragm spring, the one component I didn't change last summer. The old spring is about 10 years old.

However, I would like to ensure that whatever I do will solve the problem. I just read the Snobum article about placing "a .035"-.065" hard steel donut ring shim between the flywheel and the spring base." This would effectively "tighten" up the spring against the pressure plate. What is everyone's thoughts on this? Also let me re-phrase the article: The flywheel has an area that the diaphragm spring resides in...then comes the pressure plate. He's talking about taking hard shim stock and making a ring that the comes between the spring and the flywheel...am I envisioning this right? This would increase the clutch pull slightly but it's worth it not to have high speed slippage.

Your thoughts?
Click to expand...

Take it apart. Look for contaminants. Look for hot spotting. Then decide what to do. You ought not need a shim with a proper spring, but ...
 
20774 said:
Be sure the clutch cable is adjusted properly.
Click to expand...
Upon replacement of this last clutch, I adjusted the cable per the instruction of the actual factory manual cross referenced with the Snobum articles. Although I could not quote the detail chapter and verse, I'm pretty confident that I got it right (I could still be wrong). Nonetheless that is where I'm going to check first. There is about 5mm free play at the handlebar.
 
banzaibob said:
Upon replacement of this last clutch, I adjusted the cable per the instruction of the actual factory manual cross referenced with the Snobum articles. Although I could not quote the detail chapter and verse, I'm pretty confident that I got it right (I could still be wrong). Nonetheless that is where I'm going to check first. There is about 5mm free play at the handlebar.
Click to expand...

Too much spline lube on re-assembly?
 
banzaibob said:
Upon replacement of this last clutch, I adjusted the cable per the instruction of the actual factory manual cross referenced with the Snobum articles. Although I could not quote the detail chapter and verse, I'm pretty confident that I got it right (I could still be wrong). Nonetheless that is where I'm going to check first. There is about 5mm free play at the handlebar.
Click to expand...

I'm sure Snowbum has it right...probably got it from Tom Cutter. His method is in this post:

https://forums.bmwmoa.org/showthread.php?39773-No-adjustment-left-in-Clutch-rod&p=499945#post499945

It's more than just handlebar free play. It is free play at the back of the transmission as well as the position of the back-of-transmission clutch lever when you are halfway through the hand lever pull.
 
FELAW said:
Too much spline lube on re-assembly?
Click to expand...

Normally in situations like this I would reasonably question everything. However I'm pretty confident that I got the right amount...applied a VERY small amount with a toothbrush, also like a thin coat of paint.
 
How worn were your old clutch components? I’m thinking a new diaphragm spring will take care of your problem. Check the difference in height between the new and old. I always brake clean the surfaces that the clutch comes in contact with before installing the clutch assembly.
 
As others have stated, you need to pull it apart and look for a smoking gun (oil or grease contamination), or incorrect diaphragm spring free length.

FWIW, when I had my /5, I pulled the tranny to do a spline lube and decided to replace the input shaft seal. After buttoning it up, the clutch started slipping.

I had installed the seal incorrectly and of course, gear oil had contaminated the friction disc.
 
Just a note on spline lubrication: Apply the lube to the shaft, not to the clutch hub. Then, any excess will be pushed back along the shaft and when flung will be behind the clutch surfaces. If applied to the clutch hub any excess will be pushed forward by the shaft, and when flung will line up with the front clutch disk surface.
 
If the spring is the problem, then I have to think that adding a shim might only delay the inevitable later failure from continuing wear. Where did that "heavy duty" spring come from?

I've seen a few diaphragm springs that were no longer "flat" when placed on a piece of glass - including a "brand new" one (I wonder...) that a shop once tried to sell me (they are no longer in business). A warped spring may cause slipping, besides causing uneven wear on the adjacent parts.
 
STEVENRANKIN said:
Not to be a jerk but are you sure you used a correct spline lube? The wrong stuff can be a problem. St.
Click to expand...

No prob, the stuff is down in the shop right now and I simply cannot remember the brand name. However it was specifically formulated for spline lube....maybe Enduralast, Starburaugs (sp.)?
 
Next big question: If I go the route illustrated in the Snobum article, using a shim, can I use stainless steel as the shim material? There are no local sources for hardened shim stock in the 7" x 7" in size other than stainless.
 
Major update: Transmission and clutch are both off and inspected. First off, the old spring is fully .052" (1.3mm) shorter than a brand new heavy duty spring. This is probably a big deal. Not only that, but I spent a couple of hours at a buddy's machine shop making a "top hat" shim that fit inside the small end (see photos). We kind of figured that there was no way to cut hardened stainless to make a donut to bed in the flywheel. The shim is not going to move so we made it out of leaded steel to improve lubricating properties. The shim "lip" measures right at .040".

However.....the breather valve on top of the engine has either a failed gasket or a crack in the tube. Oil mist is escaping, dripping down and accumulating in the bell housing/clutch area. There is wetness....but NOT around the trans seal and definitely NOT coming from behind the clutch and accumulating on that "shelf". That may have also contributed.

Yet there's more!!! One of the 2 ears that holds the clutch lever/arm on the back of the trans is starting to get seriously hogged out. It hasn't failed but the arm/lever is seriously wobbly and a small bushing will have to be inserted to keep the aluminum for eventually breaking loose.

The pressure plates have some light burnishing, kinda in between glazing and normal. see pics.UsC8xaJSTeKiWAVCgcIpJw.jpgoGT3dchKSVG8AE4iNyUsIg.jpgR5ktZJI%RI+yOEYZNrAGWg.jpgfullsizeoutput_440.jpgfullsizeoutput_43f.jpg
 
You need a new diaphragm spring.

The correct height of your diaphragm spring is listed in the service manual (section 21 - clutch) with minimum tolerance of 19.0 mm +/- 0.5 mm. You measured 17.7 if my calculations are correct from your note. You need a new diaphragm spring. Really.
 
Reiterating the history: The entire clutch was replaced about 12 years ago. I opted for the "heavy duty" spring and the Sachs friction disc, etc. It gave great service. This last summer the clutch started slipping. Pressure plate, disc, etc showed wear, glazing, etc. I replaced all of the components...except the spring (AHA, you say!). This past week I noticed light slippage under hard conditions. Looking at those "new" parts that I replaced I noticed that they were salvageable. However...the spring measured a full millimeter beyond service limits! (AHA, you say!)

The lesson here is that it is operator/mechanic error. I consider myself to be a good motorcycle mechanic with a decent local reputation.....but I made a mistake by not checking the spring.

I have decided to not use the "top hat shim". There is simply no examples of it out there, although I'm sure it would work well for racing applications. The setup that I had worked well for 12 years, that's a decent service life for a clutch in a high performance engine that sees heavy service. I see no reason why that same setup would not give another 12 years. Right now I'm in the process of buttoning it all up. I'll keep you posted.
 
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