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Discussion in 'Airheads' started by spartanman, Sep 21, 2010.
The stock unit seems marginable for my 77RS. Anybody make a stouter clutch for these bikes?
I think marginal is accurate. BMW makes the best high performance clutch for them. It's a '81 on clutch. It is the best solution.
Yeah, but you have to change the input shaft. And I like the heavy flywheel. I really want a drop-in replacement that uses a better diaphragm spring and clutch material.
Unless your engine is a fire-breathing monster, a stock clutch should be up to the task. Your spring and pressure plates may need freshening. I think Snowbum had some tips on shimming to increase pressure when necessary.
Stock engine. New spring, pressure plate, friction disk and flywheel have about 2K miles. New rear main seal and oil pump o-ring. No sign of oil leakage. Didn't overdo the spline lube. Clutch is adjusted to spec. I've noticed some slipping at high loads in high gear. I figure the diaphragm spring isn't up to the task.
I don't think worn bits is the answer. I remember when these bikes were brand new. The clutch is marginal.
Siebenrock and the other place sell early clutch stuff. It makes for a STIFF clutch pull and still doesn't work that well in my opinion. A late model clutch IS the best option. I can understand wanting some flywheel on a CR500. Maybe even a 250 but why on an airhead? IMO, they are WAY better without it. And the clutch is WAY better to boot!
a defective pressure plate maybe?
If you want to throw your $$$ away, Motoren Israel can probably help. They manufacture good stuff for racers and the high performance crowd but the prices are insane. Look em up via Google and rest easy in the knowledge that they take credit cards.
You can easily shim your spring to get more pressure and then add an 'easy clutch' system but you'd better keep an eye of the ancillary parts.
The late model clutch is the way to go.
Educate me: Why is the later clutch better than the pre-81 version? I owned an '82 RT. My recollection was that it revved faster and the clutch was easier to pull, both good things. But the engine vibrated more than my '77 RS or old '78 S, which is what I'd expect with a lighter flywheel. The RT also took more revving to get underway from a stop, again due to the lighter flywheel.
Why is the later clutch better? Because they don't slip with a lighter pull at the same time.
I suspect leaving from a stop is the best place to appreciate the earlier flywheel. I guess I just don't worry about that one small parameter. Hellfar, I read that some want a lower first gear so they don't have to slip the clutch so much with their 2.91:1 FD. I have a super tall close ratio first gear that is close to taking off in second with a regular tranny and no flywheel AND I live in San Francisco! 99k miles on the original late model clutch at it STILL doesn't slip. I do wheelies and ride over the ton ALL the time. Same story with my other late model and it has more miles than that.
Vibration? I don't think it was the flywheel.
There's a couple of other ways to get the spring closer to the plate too, but that's probably the easiest and cheapest way to increase the pressure using the older parts.
Assume you have the same engine. Install a heavy, then a light flywheel. The heavy flywheel setup will run smoother because there is more rotating mass (inertia) to maintain constant engine speed between firings and absorb combustion pulses. It's all a moot point right now because I fixed the problem.
I had adjusted the clutch cable by the book with the bike cold. it turns out the clutch adjustment changes slightly when the bike warms. No suprise, really. I rode it for a few miles, then added a tad more free play at the lever. No more slip.
Thanks everyone for the help. I appreciate it.
Motobins do a heavy duty clutch spring.
http://www.motobins.co.uk/bmw-parts.php?model=R Series 2 valve Twin
Thank you. Good to know.
Update: The clutch slipped again this morning under high loads in top gear. I have to remove the trans to fix a notchy shift cam roller and replace the pawl spring (pre-emptive maintenance).
Just ordered one of these from Motobins:
CLUTCH DIAPHRAGM SPRING-HEAVY DUTY TWINS 1955-80 *RECOMMENDED FOR ALL 1000cc*
(NOT R45/65 1978-80)
Part No. 32000
I'll let you all know how it works.
I have found that new pawl springs are just as likely to break as old ones. I don't change them unless someone insists and have never had any problems for it. I have seen a couple of rollers split but I don't remember ever seeing one get a flat spot. Personally, I don't like the feel of a metal roller. It makes my bike feel like a K bike. At least to my toes. IMO, even that is not good.
I have always adjusted clutches by the book and never had a problem for it. Normally, cable free play isn't reduced by the bike getting hot, it is reduced by revs. Centrifugal force somehow forces the spring out just a tad. Lightly take out the cable play with your fingers at the lever and rev the engine. You can feel the spring moving back and forth.
I forgot to add that shift quality can be effected by the one shifter cam coming loose from the cam's gear. Check and make sure there is no slop there. The setup works loose and makes for notchy shifting. I lot of people forget to check that.
Thanks for the tip on the shifter cam. Disagree on thermal effects. Consider how idle speed increases when the engine warms up if there is insufficient cable slack. Parts of different materials thermally grow at different rates. Agreed, a new spring can fail too, though statistically, the one with the more stress cycles will fail first, all other things equal. And blasmephous as it sounds, I'm replacing the plastic roller with a k-bike steel equivalent. I find the k-bike shifter action to be less vague than the airheads.
Just so you know that is no different than the BMW HD spring available at any US dealer for the same amount of money $58.87 U.S.
The spring was introduced on all big twins after the R100 debuted in late 1976.
(The /5 spring is still available and has a lighter clutch feel. part # 21211250288 same price.)
Part number for the R100 spring you just bought in ENGLAND is probably 21211234035, $58.87, same as here in the US.
Check in your home country first, it'll save some aggravation and shipping $$$