I did some hunting around the Internet and took a few trips to local MC dealers with a tape measure in my pocket to collect some info on rear suspensions. I'm not sure of how wheelbase is measured, but I put down what I measured with the bike supported and the suspension topped out, plus an adjustment for the longer swingarm. Here's the tally; wheelbase, travel front and rear, swingarm length, final drive ratio:
bike wbase front rear salen final
R80G/S 1465 200 170 410 3.36
CRF250R 1478 315 315 ? -
R100GS 1514 225 180 455 3.09
R1200GS 1520 190 200 533 -
F800GS 1577 230 215 622 -
HP2 1610 270 250 575 -
HPN-Adv 1620 295 240 510 -
ORGS 1622 280 280 535 2.91
It seems 100mm is a common monolever swingarm extension. I wanted to get some more travel and figured I'd try 125mm, so 80mm more than the max of the R100GS paralever swingarm.
I marked the cut with a Sharpie pen, put the arm in the fixture, then went at it with a hacksaw
. I put the cut where the swingarm tube runs exactly parallel to the centerline of the bike so that both swingarm tube and the extension tube ends would need 90 degree cuts.
Here's a view of the section.
The 2"x1/16" tube I mentioned in the previous post was intended for the cross brace. I bolted the back end of the arm at the 125mm index of the fixture then measured the angles the cross brace would need. I set the brace to give the maximum support and have just enough clearance to fit an 18" wheel.
I used this hole saw and a vertical mill to cut the tube ends. I set the head at the needed angle then made the cut.
The hole saw wasn't long enough to cut straight through the tube, so I needed to cut until the saw bottomed, remove the tube from the vise and cut off the scrap, and then remount the tube and continue with the cut.
I drilled this small hole in the bottom of the brace to vent the tube while welding.
Here's the arm and additions cleaned up and ready for welding.
I used masking tape to hold the extension in place for tack welding. I cut out small sections of tape and made the tack welds.
I used this big C-clamp to hold the brace in position for tacking.
There wasn't much room between the bottom of the arm and the fixture so I needed to use this short tungsten cap to get in there.
Welding in progress.
And the finished arm out of the fixture.
I measured the arm before and after welding with this height gage to check the alignment.
Here's the arm installed with a fender and shock mock-up.
I really like the look of the triangle the arm and big cross brace make just in front of the wheel.
I'll need to look into and decide on a shock before setting up the shock mounts.