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Old 05-22-2009, 06:03 PM   #1
StephenB OP
G(/)S ... what else!
 
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How-to: R100GS (paralever) to R100G/S (monolever) conversion (LONG)

I wanted to put this together for a while in a more comprehensive report and a fellow ADVrider finally "made me do it". This is NOT a step-by-step installation with exact measurements and coordinates, this is a conversion performed in a machine shop with the right tools at hand to make any necessary modification on the spot. Also, this isn't the only way how to approach that kind of modification.

For the benefit of those that have already studied the subject in detail and have decided to accept the monolever (dis)advantages over the paralever (dis)advantages, I am posting my experience with adapting a 1985-on monolever rear end to a paralever (GS) mainframe. The project went through more trial and error stages than described here and I will provide sources for the full story at the end:
  1. Modifying the rear shock bracket on the monolever swingarm
  2. Extending the swingarm
  3. Tapering the swingarm for tire clearance
  4. Modifying the rear wheel
  5. Modifying the exhaust can
  6. Fitting the rear end
  7. Rear brake cable bracket
What do you need to do the conversion:
  • 1985-on (or R80G/S or R80ST) monolever swingarm and driveshaft
  • 1985-on 4-lug final drive, choice of 37:11 (R80RT Mono), 32:10 (R100RT Mono) or 33:11 (R100RS Mono)
  • rear shock (with dimensions to fit a 1981-1986 R80G/S)
  • 2.50 x 18 R100R front rim (X-lace design)
  • ability to X-lace or outsource (e.g. Woody's Wheelworks)
  • access to a very good machinist
  • ability to improvise
Most images are linked to higher resolution pictures.

Part 1: Modifying the rear shock bracket on the monolever swingarm

I had a lower shock mount bracket welded to the rear swingarm, dimensionally equal to the G/S swingarm. I don't have any dimensions for that, my machinist took both parts and replicated the bracket. I fitted swingarm and a dummy (K100) shock and found that the swingarm is angled a lot more than stock. To the extend that the u-joint grinds in the swingarm! What's going on here? So, off on ADVRider I went to get dimensions as I sold my own 82 G/S a couple of weeks before starting this venture. Thanks to woodgrain and TEXASYETI, I was able to find the problem. A couple of sketches first.


I did not realize that the top shock mount location of the 88-95 GS was about 30mm (1.25in) lower than the 81-86 mounting point. What did that do to the lower shock mounting location: it needs to be moved by roughly 25mm! I used the good old papermodel to visualise:


Now, let's use some more sophisticated way of showing the difference:



The need for the swingarm extension was however not only based on the neccessity of moving the lower shock mount bracket as willbe discussed later. For one, it had to be moved back just shy of 1in further to the back to compensate for the lower upper shock mount of the GS frame.

Part 2: Extending the swingarm

Fitting a monolever swingarm to a 1990 GS frame is one thing, extending the swingarm at the same time another. I deemed the extension necessary, because the (GS) paralever swingarm is 35mm longer than the monolever of the G/S. That alone, with a stock GS front end, would lower the rear end. Therefore I decided to extend the swingarm moderately by 30mm. I calculated the force at the swingarm pivot point to increase by about 30%. Since the paralever GS uses the same frame, I am taking my chances and not reinforce that area on the frame. At the same time, the extension also requires a 17% higher spring rate of the rear shock, because of the change of the levers and dynamics. I found the formulas on the internet. Now that I knew what needed to be done I started work on extending the swingarm which turned out to be pretty easy: I had an adapter fabricated (rather than cutting the swingarm and welding a piece of tube inbetween):


I considered the lower shockmount pretty flimsy and had this mount modified with real metal bracket rather than 'folded' sheetmetal:


Note the distance of the lower shock mount bracket to the end of the driveshaft, moved back by just under 1in as described in Part 1. The position of the shock mount is determined by the distance to the studs of the final drive and the diameter of the shock eye.

Next up was the driveshaft, a much more difficult undertaking: my machinist had basically two ideas:

  1. Shrink-fit, pin and weld an 'outer' sleeve
  2. Shrink-fit, pin and weld a 'male-to-male' spacer

Unfortunately our first attempt, the outer sleeve was interfering with the diminution of the swingarm and could not be made to work.


Our second attempt was the male-to-male spacer method and obviously that worked perfectly. So, as per the above drawing, a shrink-press-fit was used as means of interconnect with small bores for the trapped air to vent while the spacer is pressed in. The spacer was then pin'ed and welded. The whole assembly was smoothed out on the surface so that no sharp corner or edges from the weld beads exist. Looks like it was never touched. Unlike the HPN solution, the cush drive was maintained. Radial runout was measured before and after, the driveshaft runs true.

My Ohlins has a 95kg/mm spring installed. 70kg/mm is stock and for a 220lbs rider like me, a 85kg/mm is recommended. I added just over 10% to accommodate for the longer swingarm.

Part 3: Modifying the swingarm for tire clearance

Next, dry-fitting everything with the swingarm extension ... ouch, the tire rubes the swingarm! Hu? Upon closer inspection I realised I did not take into account the change of shape of the swingarm to accommodate the cush drive. So, the tire was rubbing there. Again, my machinist had the right idea: a piece of the swingarm was cut out and a steel plate welded back in. That fixed the problem.






Note the distance of the lower shock mount bracket to the end of the driveshaft. The position of the shock mount is determined by the distance to the studs of the final drive and the diameter of the shock eye.

Part 4: Modifying the rear wheel

Just for comparison, the 4-lug GS wheel and the 4-lug monolver cast wheel:


I have marked the area of concern with the red arrow. How does that show when you mount the GS rear hub to the monolever rear drive:



In my case, 21.5mm of the inner hub had to come off. I gave the job to my machinist, and he took excellent care of it. I won't bore you with any details, just because I don't have any:


Now, how does that fit all together?


That took care of just the hub fitting to the final drive. You need the 4 monolever wheel bolts which are shorter than the GS bolts. You also need to remove the final drive lower shock mount stud that is sticking out as can be seen above: it is screwed in so drilling a 1/4 hole and using a screwdriver to unscrew it will work. I had my machinist than cut it and machine a slot so that it can be used as a plug to be screwed in and out with a normal screw driver.

Now, the most important part is to make sure, that whatever tire size you're going to use is not rubbing at the swing arm. For the monolever conversion, I had the 18in rim laced approx. 1/4in off center to the left.





Part 5: Modifying the exhaust can

In order for the 18in rear wheel to pass the exhaust (despite the swingarm extension), the exhaust can had to be modified. My machinist just cut it flat off and welded a stainless piece in nicely:






Part 6: Fitting the rear end


Just perfect: the Kenda 270 that is currently mounted is a 4.50 x 18 with 121mm (4.76in) width and 27in diameter on the 2.50 x 18 rim @ 32PSI tire pressure. Clearances are as follows:
  • tire to modified exhaust collector: 13mm (1/2in)
  • tire to swingarm: 7mm (just over 1/4in)
  • tire to Ohlins: 4mm (just over 1/8in)



Part 7: Rear brake cable bracket

The rear brake was the only area where I needed 4 attempts (=designs) to get it right. Here are the three design that didn't work to my satisfaction:



This one is the last resort, it is as close as possible to the original rear brake cable routing on the GS.



That'll do.


I have done 8,000mls with this setup including a trip through the Colorado Rockies, New Mexico and Oklahoma on gravel and some other fairly intense stuff. The swingarm conversion held up nicely and the bike was rockstable at speed and on gravel. To be fair however this was all done with a 17in rear wheel with a 130/80-17 tire and a lot of offcenter lacing ... one of the not so successful design iterations (the Excel rim cracked) before I came closer to a G/S setup.

I have paid special attention to bike geometry when looking at all those mods and compensated the mods against the changes in geometry one-to-one so that in the end the bike's geometry is as close as possible to stock. Rake and Trail are as per the specification and the bike tracks superbly.

The above is a summary of the following webpages:

"The Making of the R100G/S": http://www.stephenbottcher.net/BMW/R100ST_2.htm
1990 R100G/S Logbook: http://www.stephenbottcher.net/BMW/R100ST.htm
Spoked wheels on a 1985-1995 Monolver rear drive: http://www.stephenbottcher.net/BMW/rearconv.htm






I don't think I forgot anything. I can extend in areas if things aren't clear.

But as I told you kids: Don't try this at home!

I assume no liability or responsibility if you try those mods on your own bike with a different result. Some of these mods are not for the faint of heart and should only be performed by a professional.




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'72 R75/5 The Blues

www.stephenbottcher.net

StephenB screwed with this post 05-24-2009 at 06:15 PM
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Old 09-11-2009, 05:37 PM   #2
SOLO LOBO
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Awsome info Stephen!

Just caught site of this and thought this might be a good place to put it:



1 Pivot Extension Kit G / S


Monolever vibrating extension 50mm for BMW R 80 G / S. By extension, the resonant behavior of the BMW R 80 G / S is significantly improved. ) In conjunction with a longer front fork (eg, USD fork or comparable is also achieved more ground clearance for the BMW R 80 G / S. We use the Extension Kit 50mm vibrating primarily in connection with USD forks of a length to 860mm. With this modification, the suspension geometry remains largely intact, and the behavior improved significantly in this light conversion.
Delivery Kit 1:
1) resonant Extension 50mm
2) 50mm extended Kardanglocke
In exchange against your Kardanglocke or send us your swingarm with shaft drive and we'll build it for free at
3) 4 threaded bolts (with hexagon socket for easy mounting)
4) Extended brake rod made of stainless steel (including wing bolt and clevis)
We are available to you from the phone and installing the rocker available. Even the selection of the fork, we advise you gladly.


http://www.cti-fahrzeugtechnik.de/ep...s/1014-001-002

379 Euro (inc 19% VAT)
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Quote:
Originally Posted by Stagehand
your bike is suitably dirty. Well done.
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Old 09-17-2009, 08:34 PM   #3
igormortis
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Quote:
Originally Posted by SOLO LOBO
Nice. What I don't understand however, when you look at the other kit where they refurb your swingarm, there's no additional clearance for the tyre. What gives?
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Old 09-20-2009, 08:58 PM   #4
igormortis
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X-lacing + Excel

Stephen - I know you've probably covered this before, but just to clarify:

In your (and perhaps Woody's) opinion, was the problem with the Excel rear inherent to mating it to a x-lace hub, or did it only show it's limits because you pushed the offset too far? i.e would you use an Excel rear again with the minimal offset you have now?
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Old 09-20-2009, 09:48 PM   #5
StephenB OP
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Yeah, basically. X-lace hubs and center-lace with maximum offset didn't work out in this case, the exiting angles of the hub in conjunction with an 1/2in offset center-lace might have been too extreme. But I am sure there are some other gurus in either Europe or OZ/NZ that use a different technique and may be able to make it work. I think you have to do some drilling of the hub bores to correct exiting angle as stress relief and also account for offset when drilling the rim. With gong back to a 21/18 G/S-type configuration I kinda said good bye to the idea of a wide rear tire and just added some 1/4in offset to give it some space so even a 120 tire running slightly wider than normal won't give me trouble. Tires are cheaper too and there is a decent selection of street, gravel and dirt tires in 120-18.

Woody's/Zach was very good about the whole thing and got me something that I like in the end. He warranted his work and provided the labor for the rework, I provided the 18in X-lace hub.

Would I do it again? If it came up again, I probably would, if stress-releaving the hub bores and drilling rim specifically for decent offset and use of a x-lace hub would be included. Don't know if that guy is out there though.

Would I use another Excel rim with minimum offset? Don't think so. I convinced myself that the BMW x-lace design seems stronger and more resilient to side forces than an center-lace rim. To even underline the subjectivity of my statement: it appears beefier.

QED.
__________________

Some of the above is fact, some is fiction, some is my personal imagination and some is just simple truth. [me]

... i'm not touring around the world, but neither are most of the guys i see running overloaded spam cans ... [bmwblake]

Stephen Bottcher
Ontario, Canada

'72 R75/5 The Blues

www.stephenbottcher.net

StephenB screwed with this post 09-21-2009 at 06:21 AM
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Old 09-21-2009, 04:40 AM   #6
igormortis
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Thanks Stephen. All the work you've put into documenting all this is appreciated.
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