I've made a start on the frame at last. Having mentally sorted through various options, I've decided on welded in bracing with a drop out lower rail for the purposes of engine removal, and more importantly, oil filter replacement. The problem with that is that the engine is only attached to the frame with two studs, and those would have to be loosened to remove the frame rail, which would leave the engine hanging on two studs through the left hand rail. I rather suspect that wouldn't do the studs much good. To remedy that I thought I'd utilise an idea I saw somewhere else and add a mounting plate from the top two gearbox mounting studs to the frame bracing rails.
As I've mentioned before, I want the motorcycle to look like a BMW that's off to the races, and not like a racing motorcycle that happens to have a BMW engine in it. With that in mind, I bent up a couple of tubes that would run under the fuel tank and then drop down to meet up with the swinging arm pivot once they were clear of the carburettors and their air filters. Bending the tube didn't present much of a problem as my principle tube bender is a serious industrial piece of kit intended for series production work
I used one and a quarter inch 14 gauge CFS 3 tube for the braces because the front down tubes are that diameter. However, the brace tubes also meet up with the tubes that run from the swinging arm pivot up to the frame's top tube, and those tubes are one and an eighth of an inch in diameter. Not only that, but they're ovalised where they meet the swinging arm bosses, narrowing their width even further. This was all going to make the joint with the brace tube quite interesting, even without the three quarters of an inch tube that runs just above the swinging arm bosses getting in on the act. Which is why, although I have a serious tube bender, I don't have a tube notcher. It took me about half an hour to cut the mitres by hand using a hacksaw and a selection of files. My theory is that if you cut tube joints by hand you tend to develop a "feel" for the job and even oddball examples like this one become much less trouble, and cutting simple ninety degree mitres takes so little time that setting up a tube notcher would probably take longer.
With the brace tubes shaped, I cut and mitred a cross brace that will eventually support the top mountings for the torque plate from some more one and a quarter inch tube, and after rummaging around a little I found some offcuts of one and an eighth inch tube to tie the tops of the brace tubes to the rear of the engine cradle.
The idea is that there'll be another inch and a quarter cross tube between the brace tubes at (or close to) the points where they're tied to the rear of the engine cradle. From either end of that cross tube another pair of tube will run up to meet the top tube, at (or very near) the point where the brace tube from the front of the frame meets it.
Aside from that, I want a pair of gussets tying each of the down tubes to the top tube where they cross, a cross tube under the frame where the original footrests mounted, box in the front of the factory headstock gussets, and a gusset arranged so that there's something other than the swinging arm boss tying the top and bottom of the frame together.
While I haven't perhaps cut as much tube as I would have liked, I have gone from a vague idea of what I was after to a fairly concrete plan, albeit one that exists only in my head. It should all be fine, as long as I can remember it all....
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