I spent some time working on the rear wheel this morning. I got the grody old tire off, dropped the engine into the frame and lined the rim up with the centerline of the bike as carefully as I could. At that point, I discovered something truly amazing.
According to my measurements, the front and rear sprocket offsets are almost exactly spot-on — as in, the difference is less than the width of my laser. Wow. My original (very imprecise) rough calculations had the two sprocket planes drastically off. The Bultaco uses 520 chain and the Suzuki 530, so the existing rear wheel sprocket is thicker. Knowing that, I used the inside edges of the sprockets, which will remain consistent. (I will, of course, need to have a 520 version of the GS sprocket custom made at some point.) This seems way too unexpected and coincidental to actually be true, so I’ll need to measure this again after I get the rear engine mounts welded onto the frame. But I can’t worry about those steps until after I get the rear rim mounted, the steering stops fabbed, and the rear shock fitted.
One decision I do need to make now is whether or not I stick with this current GS450T rear rim. I already have two matching wheels that I got very, very cheaply. But the 17″ diameter may end up looking a bit too cruiser-ish, and combined with the narrow WM3 width leaves me with very few decent tire options. Furthermore, the rim is in pretty bad shape — even worse condition than I initially thought. Not only is there an incredible amount of rust and corrosion in the hub (which I knew), to the point where the bearings hardly turn, but when I pulled the Cheng Shin tube-type tire off, I discovered a lot of pitting and corrosion under there as well. In addition, somebody must have changed the tire with a screwdriver at some point; the inside of the bead lip is full of burrs and gouges on one side. I tried to clean it up and dress it with a scotchbrite pad, but it still looks pretty nasty. I am wondering if it might not even be completely airtight with a tubless tire. (That may be the reason for the tube-type tire. Or, perhaps it was just a cheaper/more expedient choice.) There is also one small but noticeable nick (maybe a millimeter or two deep) in the outside edge of the lip I hadn’t noticed previously.
I could look around for an 18″ rear rim from a GS400/450 standard or sport. Considering that it would be otherwise identical to this one in all the good ways, it might be worth pursuing. I could also consider going with wire spoke GS wheels, which are plentiful, but that would require a new front wheel as well, and salvage spoked wheels tend to be rusty and out of true. I really want to have cast wheels on this bike if possible; they’re an important part of making it look like a genuine road bike, distancing it visually from its dirt bike roots.
My short-term goal is to get this project to the point where it can put some new tires on it and roll it around on its own two wheels. At that point, it might actually be something like a motorcycle, rather than just “creatively placing random parts in close proximity.”