I'd like to get some more rear wheel travel out of my paralever suspension, so to get a better idea of the limits I took a few measurements of the shock length at differing swing arm positions. I took some measurements with just the swing arm and final drive bolted up to the bare frame, and some measurements with the fully assembled GS with just the rear shock removed.
Here are my measurements in millimeters:
swing arm topped out 520
drive shaft topped out 505
swing arm centered (zero u-joint angle) 450
drive shaft bottomed ou 402
tire bottomed out (Torance 130/80-17) 392
swing arm bottomed out 387
I found the limiting factor is the drive shaft in both top-out and bottom-out. I could hear the drive shaft rubbing, but because the bike was fully assembled I couldn't tell if the rubbing was from the yokes contacting, or from the drive shaft rubbing on the inside of the swing arm.
To get a ratio for shock stroke vs. wheel travel I measured the change in the distance between the sub frame and the center of the final drive when the swing arm topped out and the tire bottomed out and got 280mm.
travel/stroke = 280 / (520 - 392) = 2.18
So as just a rough limit, between the extremes of when the drive shaft starts rubbing the wheel travel will be:
(505 - 402) * 2.18 = 224mm = 8.8"
Some specs I found for the stock GS say it has 180mm (7.08") of wheel travel, so it seems reasonable to assume that there is no way to add more than 44mm (1 5/8").
I want to do some more study of the u-joint angles and to understand what is rubbing at the extremes. I also have to check if my shock body (Fox twin-clicker) can take a longer shaft, and what parts are available for it.