Torgue arm now fitted

[Gromit]

I can see what you're saying about the bike above, what about normal S's where the drive shaft is at an angle to the bevel box. That doesn't look right to me and surely straightening it out won't harm it?

If you read the jist of the Wiki link that Sandbar posted up it explains it really well.

This is the bit which probably says it best...

"A configuration known as a double Cardan joint drive shaft partially overcomes the problem of jerky rotation. This configuration uses two U-joints joined by an intermediate shaft, with the second U-joint phased in relation to the first U-joint to cancel the changing angular velocity. In this configuration, the angular velocity of the driven shaft will match that of the driving shaft, provided that both the driving shaft and the driven shaft are at equal angles with respect to the intermediate shaft"

[sandbar]

.......what about normal S's where the drive shaft is at an angle to the bevel box. That doesn't look right to me and surely straightening it out won't harm it?

You need to remember that BMW does not use constant velocity joints - it uses universal joints. If a universal joint is not absolutely straight, then the output shaft has slight fluctuations compared to the input shaft. The more the two parts are out of line, the greater the fluctuations.

The way to counter those fluctuations is to have a second universal joint mounted so that it is the exact opposite to the first one with its output shaft being parallel to the input shaft to the first universal joint. Then the fluctuations at the second joint are equal and opposite to the original fluctuation at the first joint. This is the way that propshafts work on rear wheel drive cars as well as drive shafts on BMW bikes.

If you just straighten out the second joint then you just transmit the fluctuations from the first U/J straight through to the bevel drive. That is the way that BMW used to do it - just the one universal joint at the front of the shaft! The /6 and /7 BMW had a reputation for heavy tyre wear. I would suggest that that was possibly due to the constant pulsing due to the fact that the one universal joint was always out of line. It had possibly not been an issue until the engines started to develop more power and also - dare I say it - also became not so smooth at 900cc and 1000cc.

Richard is, IMHO, right to say that the best way to increase ride height is to increase the length of the shock. It would then be good use of an adjustable torque arm to optimise the angle of the shafts.

sandbar

[SAS Tom]

I see so the bevel box being at an angle to the drive shaft is to cancel out the drive shaft being at an angle to the gearbox?

So why is the bevel box on a 'cup at the same angle as the driveshaft but not on a standard bike?

[sandbar]

I see so the bevel box being at an angle to the drive shaft is to cancel out the drive shaft being at an angle to the gearbox?

In a word - Yes!

So why is the bevel box on a 'cup at the same angle as the driveshaft but not on a standard bike?

I can think of many reasons, but they would all be 'impolite'!

Sometimes the efforts to keep the driveline in a straight line were fairly extreme. Check out the 'straight' drivelines on these two bikes.



sandbar

[lukedog]

Sorry that I have not replied, but I have been at the TT the last couple of weeks and arrived home early this morning.
Yes I agree with what you are all saying and did realize my error before I went to the TT, I adjusted the torque arm back to near the standard length.

[Mike B]

And, after reading all that high tech stuff.

I removed my shorter GS arm after about 50 miles because I simply didn't like it. LOL