This looks like it could degenerate into one of those internet/forum discussions where it looks like there is an argument, when what is actually happening is we are talking about different things!!
Originally Posted by JCW
Do you think it was a matter of design and production compromises that prevented BMW from making the perfect parallelogram?
Yes I do - absolutely.
However I think that I could have worded that phrase a bit better. You obviously read it differently than I meant it. The word 'compromises' was only meant to apply to 'production'. Maybe the phrase should have read "design criteria and production compromises". My mistake - I am sorry.
You are right about the design's attempt to diminish the rise and fall of the rear of the bike due to the shaft drive - the so called 'jacking' effect.
However it does not alter the fact that if you want to eliminate any vibration (or the effects of it) from a shaft that has two universal joints, then the input and output shafts should be parallel. The further the shafts are away from parallel the greater the variations in angular velocity and therefore the increase in vibration and tyre wear.
I think we agree!!
Originally Posted by JCW
The monolever suspension of the 1980's had only ONE u-joint. That means that the angular ouput shaft of all these motorcycles suffered this catastrophic vibration you speak of. All those motorcycles, all those rear ends, teetering on the brink of annihalation.
Guess they survived OK for years, huh?
I do disagree with you about this. From the moment that BMW started to produce bikes with bigger engines and noticeably more power (compared to modern bikes that is a relative term) in the mid 70s with the R90S and then the 1000cc R100RS, the misalignment of the shaft became more and more of a problem. There was a continuous program of development on the drive train to reduce the vibrations and associated problems. There were vibration/shock dampers in the shaft ( late 70s) and a similar change to the gearbox in the early 80s. Alignments were changed, mountings were moved - all trying to address this issue.
There were no bevel drive pivots to wear and the shaft had its own seperate oil bath. The problems were not 'catastrophic' because the shafts and joints etc were over engineered. I don't think that they differ much in size from the parts used today, but even the most powerful BMW road bike of the 70s (R100RS)only developed about 70 hp IIRC.
In those days BMWs were also renowned for high tyre wear - again compared to modern times, that is a relative term.
Even before those days, this kind of realignment was necessary.
Crankshaft, gearbox output shaft and drive shaft as near as possible a straight line.
If anybody wants to find out if the vibration can be a problem, then just remove the bevel drive and re-connect the shaft so that the universal joints are out of phase.
Just think - the whole problem could be solved with the use of constant velocity joints