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I bought some Reynolds thinwall tubing to make up some side braces. I didn't want them to fit on the outside of the frame like the CC type product but rather they fitted inside the frame space. I was somewhat dissapointed then to find that the tubing would foul the Euro /5 tank or the airfilter box if they were to fit anywhere near the ideal place from the swinging arm area to the headstock. The only alternative I could find was to bend the braces and gusset ttem on these bends.
I've mocked one up using plastic tubing - what are your thoughts please on the rigidity - would they work? ( A metal version - not the plastic one!).

There is sidewards movement to consider as well. Your brace stiffens (somewhat) one axis only.

I would firstly find why there is a need for bracing.
Is it for track, where every little bit helps? Or for road use, where there is ideally no need to improve anything, except poorly set up/bent bikes.

road use, where there is ideally no need to improve anything

Sorry but I beg to differ with you on that point, this will be my 6th airhead and out of all of them the one with nothing more than the side frame braces and a welded up subframe felt the most "sure footed" of them all.
I wonder just how much the frame can flex and how much these braces that I have in mind to make would flex deflect compaired to straight ones?

I don't think that this bracing will have much effect. That's a pretty big "rectangle" formed by the frame tubes...I suspect all you'll do is transfer the loads into each end of the bracing and create problems for those local areas.

The motorcycle flutter report that was recently reported about (can't remember where) showed that the biggest problem with motorcycle stability was the front suspension with lateral instabilities. The external frame bracing did a good job of controlling instability especially when there was a tail load - pillion, bags, etc.

Your gussets probably won't do much to keep the "rectangle" of the frame loops from trying to close up...only the corner-to-corner bracing is going to do that.

Kurt in S.A.

OK, Reading up what I can find, comparisons are made with the Nortons Featherbed frame and that it uses the engine/gearbox assembly to brace the frame via its engine plates and headsteady.
I've seen pictures of the braces used to tie the front of the gearbox with the rear of the frames spine on the BMW, has anyone thought of also running a similar plate backwards from the front of the gearbox to the swinging arm pivots to stop the rear upright tubes from flexing back and forwards instead of the side braces and also using a headsteady - perhaps tying the two spine tubes to a welded lug on the timig chest?

Best of both worlds? (click on the picture and "view Image")

Except I presume no starter motor?

From Bamboo's picture, one can't tell if the head gusseting may have also gotten more reinforcing. I think that would be necessary for the best results from that bracing.

Hard to tell from the photos, but they did go with an extra lower triple tree: Lots more bracing on the highly modified rear suspension, too: More photos here: http://www.largiader.com/albums/83/index.htm (Sorry about the huge photos...)

This is an alloy Battery Tray cum brace on my friends Guzzi, as you can see it ties in the gearbox/engine to the swinging arm pivots.
This is what I'm thinking of doing to my BM instead of the side braces. Am I right in thinking I shoud be trying to eliminate fore and aft movement with this brace and leaving a proposed engine/gearbox to rear spine brace to take care of the up and down and sideways twisting forces.
I'd like to make it out of steel plate to keep them looking less clumbersome that say 10mm alloy.
Any suggestions on the guage of steel I should use if I were to form a 'lip' around its edges?
Again to replicate the Featherbeds headsteady I was toying with the idea of bringing down a box section from the spine tube to mount with the top of the timing chest - any thoughts?