Angle of the dangle on Paralever bevel drive/swing arm

[Roger Chatterton]

After 2 fairly recent g'box re-builds, my 'box has now refused to allow me to select 1st gear. Consequently the 'box is with Steve Scriminger, to havethe selector internals inspected (100 GSPD paralever).
The esteemed Mr S suggested that fitting an adjustable torque arm to the paralevers, when adjusted correctly under rider and luggage load, should help to flatten the sharp angle suffered by the UJ at the bevel box end of the shaft.
So, bowing to superior knowledge and experience, I have tried to hunt down an adjustable torque arm - tried all the usual suspects, HPN, Moren Israeli, BMWBoxer Parts, BoxerParts etc., to no avail.
Does anyone have any experience of such an arm, or where, if anywhere, it is possible to buy one ?

Much though I love my GS, there is no doubt in my mind that the shaft and gearbox are the weakest link on the paralevers. Clearly the shaft was the earliest iteration of the paralever design,and as such, over reasonable mileages, proves to be over stressed due to the angles involved, and from the sliding/hammering from the splines/UJ's. Apparently the 1100's and 1150's had the splines further up the shaft, and as such took some of the stresses away from the bottom UJ.

Discuss.......

PS Before anyone mentions it, yes I have an Ohlins, but yes, I have had it shortened to same length as standard BMW GS shock length. This obviously reverts that angle back to standard.

 

[Paul Rochdale]

Here's one although it looks over-engineered to me. I would have a go at making my own, modifying your own one and using stainless steel studding.

http://www.advrider.com/forums/showthread.php?p=19762234

 

[Roger Chatterton]

Paul,
if I had the slightest clue how to make/modify the existing arm I would give it a go ....

The picture you found in the link above illustrates exactly the effect I am looking for, ie to straighten angle between FD and UJ. Thanks for that -

I wonder if Migsel might do something by way of adjustable arm? I have a Migsel sidestand, and the way that adjusts would be the way I would approach things ! Maybe that's why I'm not an engineer !!!

 

[Rob Farmer]

Roger,

I thought you'd had your Ohlins rear shock shortened to reduce the angle?

 

[Roger Chatterton]

Roger,

I thought you'd had your Ohlins rear shock shortened to reduce the angle?

Hi Rob
Keep up at the back there! See my PS in my post!!!

Having Ohlins shortened only restores bike
Shaft angle back to original BMW spec. That angle is still pretty
steep and is what can stress the rear UJ, in conjunction
with the sliding splines. Surely this is why they
modified the design for 1100 and subsequent
Oilhead paras?

 

[ChasF]

Either Motorworks or Bins used to sell an adjustable paralever arm but that was a few years ago.

I thought the standard way to reduce the angle was to fit the R100R shock.

I'm sure if the splines were 'bottoming out' both shafts and gearboxes wouldn't last 5 minutes let alone 40k miles!

 

[Roger Chatterton]

No, splines are not bottoming out of course- that
would be ridiculous!
Maybe I don't make myself clear....it's the
angle between final drive and the UJ.
This angle has to transfer all the engine power
to the rear wheel via the shaft, whilst also allowing the
shaft to move in and out via the splines, to accommodate
changes in road surface and subsequent suspension
height. Too many forces placed on the UJ and too
steep an angle.

 

[Redboots]

Too many forces placed on the UJ and too
steep an angle.

There must be a far bigger angle range at the gearbox end. UJ's are very robust things. Usually crap lubrication screws them up.

 

[Paul Rochdale]

Roger
An engineering company would cut across your existing torque arm. Drill and tap threads in each side then let in a length of SS studding. Fixed at one end (with a c-pin) the other end could be twisted and locked with a lock nut. Not too difficult with anybody with the right facilities.

 

[Beemerboff]

If the angle is steeper the UJs are under more stress, and will get hotter.
This can lead to them loosing their grease, and then rapid failure.

Applying an angled load, together with insufficient or incorrect lube on the splines can also lead to them galling under load, which locks them in place.
This also locks the suspension, and overloads the UJs and the gearbox bearings.

These were common problems a long time ago on IRS sports cars like the Triumph TR6 and the Datsun Z series, but the knowledge gained while solving the problem there doesn't seem to have transferred over to our bikes.

There are a few good photos out there of German bikes which are actually raced successfully off road and on, and almost all have gone to the trouble of making the drive line as straight as possible, usually including realigning the motor.

 

[Rob Farmer]

I'd be wary of dropping the rear height lower than standard without dropping the front an equal amount. The steering gets very strange if you lower the backend.

Did scriminger explain why the uj would cause an issue with first gear selection? When my output shaft bearing collapsed last year the shaft was flapping outbound so much it cut a groove in the side of the gearbox case. It still changed gear without a hitch

 

[Roger Chatterton]

Hi Rob
I'll know more once he has had chance to
open up the g'box and inspect.
I don't think that the first gear issue is necessarily
related to shaft angle etc. Steve was explaining
some general g'box problems/ solutions.
As I was leaving Steve's place, someone else
was also delivering him an airhead g'box for repair!!!

 

[GoSlowFallOver]

Hi Rob
Keep up at the back there! See my PS in my post!!!

Having Ohlins shortened only restores bike
Shaft angle back to original BMW spec. That angle is still pretty
steep and is what can stress the rear UJ, in conjunction
with the sliding splines. Surely this is why they
modified the design for 1100 and subsequent
Oilhead paras?

Re-awakening an oldish thread … I've had stock shock for 130,000kms (with one joint replaced) and have recently treated myself to a Wilbers shock (OK, it was second hand but better than the stocker). Like Roger, I had it shortened to the stock length of 485 mm to preserve the joints. But I wondered if that is only part of the equation …

When riding, as opposed to being on the centre stand, we want the gearbox - shaft - bevel to all be in line, or as close as practicable. I think this means setting preload for given rider and load such that ride height (i.e. drive angles) are optimal.

I measured the ride height with the stock shock and rider only, and then set the preload on Wilbers to get exactly the same ride height, again for rider only. So in theory I still have 'stock' geometry.

Now I'm wondering if this arrangement is the best possible …

There must be a particular angle of the shaft (relative to the gearbox/frame) at which everything is in alignment? If I knew what the corresponding eye-to-eye shock length was, I could set ride height accordingly. Problem is that when I'm sat on the bike, I can't see the alignment of shaft - bevel.

I could take the shock off (again!), use a jack to raise the rear wheel until everything is in line, then measure shock mounts centre to centre. But I'm a lazy sod and wondered if anyone has done this before? If so, do we have *the* critical dimension? If not, I'll take one for the team and report back.

Does the logic stacks up?

 

[Redboots]

There must be a particular angle of the shaft (relative to the gearbox/frame) at which everything is in alignment?
Does the logic stacks up?

I don't seel the lenght of the paralever arm affecting anything except the angle the bevel box is in relation to the swinging arm.
My paralever is a custom lenght and it jacks the back up a bit by pulling the bevel box down.

If you want to have a straight'ish line between the shaft and the gearbox, your shock and subsequent travel would be really short.
A variation on this would do it: Engine Riser, but only raising the front.

No

 

[GoSlowFallOver]

I don't seel the lenght of the paralever arm affecting anything except the angle the bevel box is in relation to the swinging arm.
My paralever is a custom lenght and it jacks the back up a bit by pulling the bevel box down.

If you want to have a straight'ish line between the shaft and the gearbox, your shock and subsequent travel would be really short.
A variation on this would do it: Engine Riser, but only raising the front.

No

With calibrated 85 kg load (i.e. the chap from next door in his best off-road thongs), scientific instrumentation (i.e. masking tape, broken bamboo blind slat and a keen eye with glasses), I reckon this is about the right angle for swing arm to be in line with the bevel. The shock centre to centre distance was measured at 469 mm (OK, 470 mm), compared with free length (extended) of 495 mm.

For my own reference and easier measurement, this equates to a distance from bottom edge of rear mudguard to floor of 420 mm.

Yet to ride with this amount of pre-load ….

[Ed] Oops - image insertion didn't work …. Mods - can I insert link instead? https://www.dropbox.com/sc/99wrhwyjbth7w4f/z-fiEY9gXB

 

[(RIP) Arkwright]

 

[Roger Chatterton]

Precisely Mr Arkwright!


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[Roger Chatterton]

Here's a pic of my Adjustable Torque Arm that arrived from Steve Scriminger's this morning. Made by Arkwright as above .....

Looks good and this should do the business I hope