Machining 21.5mm off the center of the rear wheel hub moved the hub that much closer to the swingarm. To fit a wide tire like the Gripster or TKC-80 I would need to offset the rim to the left to get the needed tire clearance.
With a stock wheel and Metzler ME 880 140/80 tire mounted I measured the gap between the swingarm and tire to be about 10.5mm. I also measured the Gripster to be about 4mm wider than the ME 880, so as a rough figure I thought I'd need about 14mm of offset to have a 3mm tire to swingarm gap. I didn't think I could move the rim over that much and have proper spoke/nipple engagement with stock length spokes. I did have a set of stock length spokes I had bought to use with the paralever rear end, so I decided to do a temporary wheel build with them to get the length difference needed to have proper engagement.
The spoke holes in the hub and wheel are drilled so they are close to being aligned when the wheel is assembled. With a rim offset of 14mm the holes would no longer be aligned. I decided to re-angle the holes a little to reduce the bend of the spokes. Due to the geometry of the wheel some spokes needed a little more rework than others. At first I used a hand drill with a twist drill to do the rim, but then switched to a small end mill chucked in the hand drill which gave me better control.
I used an appropriately sized twist drill to do the hub.
During my first attempt at building the wheel I found the spokes on the right hand side started to protrude into the nipple so much that the allen key would no longer engage. To continue I ground 3mm off the threaded end of half the spokes. I used this caliper as a length gage while grinding.
The rear wheel lacks the bearings of the front wheel, so poses a challenge on how to support it for truing. I used the final drive and swingarm held up in a vise. I found that with no oil and the drive shaft splines disengaged it wasn't too bad. I needed to take the wheel off the drive to work on the spokes that had nipples inside the hub though. I didn't need super precision for the run-out, so I just used this setup with a sheet metal pointer held with a c-clamp.
Once I got the wheel built I checked the tire clearance with several different tires mounted, and it seemed the 14mm offset would be OK. I used this 3mm screw and nuts as a depth gage and found I needed the left hand side spokes 6mm longer. The spokes had enough of the length threaded so that I would be able to use the shortened ones on the right hand side.
Here's the wheel with a TKC-80 tire mounted that shows the offset rim and a little of the tire clearance.
And a view of the left to compare.
The spokes on the left side of the rim have just a few millimeters of the tread engaged. It is enough to hold the bike up, but I don't think will be strong enough to ride with. I need to get some longer ones to do the final build.