I will. The fork brace made a surprising difference. Passing lorries on the motorway gives no wobbles. Then again, i have replaced wheel bearings and adjusted the swingarm ones, making incremental changes also.
Looking at the frame, it's two rectangles only joined together at the bottom by the engine bolts. Tying the gearbox to the top tube looks the most secure, but I'm not messing the bike up for that. The back end where the subframe bolts also looks wibbly. These side braces should link the headstock to the swingarm and will triangulate the two big rectangles. Exactly how much impact that makes I really don't know. I do know that the oversized 'aerospace' pushbike racer I had with hand brazed joints was a lot stiffer than the Reynolds 531 "pipe-fitting" frame before it. The conventional frame bike would weave going downhill at 50MPH and cause the chain to hit the front derailleur going uphill, whereas the the hand made frame was a lot nicer to ride.
I will be putting some bends in these side braces and should be able to stick weld by cutting access holes to the long nuts through the tube. Fill the hole, grind it down.
The bike was 1 person power, so even a relatively meagre 60 or 70 horse power must be deflecting the simple BMW frame. I was thinking of getting the rear frame off to powder coat it and can see an ideal place to put a cross brace between battery and rear mudguard. The subframe is a simple hoop with no side to side support. Where does the twist from my weight and the push from the shocks go? The idea with both these braces is that I don't have to remove good things like the air filter or mess up simple battery maintenance or make it look unusual etc. My mate has a TIG welder, so I'm hoping if I get everything cut and tacked using my stick welder on the subframe, he can do a much better job than I will to finish it.
The R65 front forks are only 31mm, but the top and bottom clamps are thick aluminium. I could not believe the steel thing on some of the bigger bikes! Looks really wibbly. So no plans there.