I intended to align the Yamaha rear wheel in the Benelli frame as well as I could without resorting to my frame jig, which was in a storage unit across town. After all, for a static display it isn’t especially critical that I get it right. And according to Tony Foale’s experiments, a bike’s wheels have to be off the frame centerline by quite a bit before it has any detectable impact on handling, anyway. But after messing around, I decided to do it right. I retrieved my jig and set to work.
Using a plum bob, metal rulers, calipers and a laser, I confirmed that both the frame and swingarm are symmetrical only in the most approximate sense. Fortunately, as I’ve discussed previously, the only points that actually matter are the steering pivot and the swingarm pivot. The jig self-centers the steering tube, so I just had to get the swingarm pivot exactly perpendicular to the longitudinal jig rails.
Sadly, measurements also verified my suspicion that the right-hand passenger peg mount had been tweaked, probably in a crash. It is pushed in about a quarter-inch compared to the other side, and it hits the swingarm mid-travel. I’m not sure how to bend that outward. I might just cut the right passenger footpeg loop off completely. The left side would still be useful as an exhaust mounting point, as running the exhaust to the left side makes sense given the right-side chain run. But that’s a decision for another day.
Once I had the frame locked into place and the swingarm installed, I could more accurately locate the rear wheel.
And we’re good! With the wheel properly positioned, my truncated sprocket carrier allows the wheel to fit within the swingarm with just enough clearance for a thin little bearing spacer (which I’ll have to make up on the lathe). You can see that I was actually a little overzealous with my surgery; the bearing sits slightly proud of the carrier. I might make up a retaining ring from thin plate, which will slip under the sprocket bolts and cover the outside lip of the bearing. It actually doesn’t make any difference, because neither the bearing or carrier body has the ability to shift left or right as it is, but it looks sketchy.
Unfortunately, I didn’t have the same good luck with the sprocket; the teeth at the front hit the swingarm. I’m thinking I can just cut away as much as needed for clearance (yellow dotted line), then weld a flat section back in for strength. Even if somebody turns this into a running motorcycle someday, I don’t think that would cause an air-cooled ATC250R’s 23–26 horsepower to overwhelm the swingarm’s rigidity.
While I had the laser handy, I verified that the front wheel is within a millimeter or two of the steering axis, which is as good as it needs to be, and very likely better than the factory tolerances for a mid-70s Italian lightweight.
