Logbooks of '72 R75/5, '79 R100T, '82 R80G/S Paris-Dakar, '93 R100GS PD, '71 CB750 and others, a.k.a.
Paralever -> Monolever Conversion
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:
- Modifying the rear shock bracket on the monolever swingarm
- Extending the swingarm
- Tapering the swingarm for tire clearance
- Modifying the rear wheel
- Modifying the exhaust can
- Fitting the rear end
- 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
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:
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.
- Shrink-fit, pin and weld an 'outer' sleeve
- 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.
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:
Part 7: Rear brake cable bracket
- tire to modified exhaust collector: 13mm (1/2in)
- tire to swingarm: 7mm (just over 1/4in)
- tire to Ohlins: 4mm (just over 1/8in)
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.
The final "product":
Experience so far:
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.