Randy Illg:

"If tig is done correctly the heat input is minimal. The key to welding round sections is to do it 1/4 around then go to the opposite side and do 1/4 then 1/4 etc. stress relieving is only required when the section is thicker than .25". Local stresses can build up above this thickness. It really is impractical to heat treat motorcycle frames because you can't just put it in an oven . It must be held in a jig that will not move when heated. The whole science of heat treating is very complex. 4130 was originally formulated to improve the engine mounts on large radial aircraft engines prior to ww2. It was designed to be field repaired with oxy- acetylene equipment and put into immediate service. The outstanding thing about TIG (Heli-Arc) as you probably know, is that no atmosphere reaches the molten weld and therefore no hydrogen embrittlement and no O2 oxidation. The filler rod can be any good steel rod but for the best results we use rod specifically made for TIG-4130 welding."

Rear-most frame rails are welded in place, and those flimsy little metal plates that BMW thought sufficient to hold the head-stock steady are trashed and replaced with some good stout tubing:

The monoshock design as used by BMW positioned the shock on the right side of the frame. I have positioned the single shock in the center of the frame – centralizing both weight and forces imposed. The specific geometry of the cantilevered arrangement was 'borrowed' from the Ducati 900SS, then scaled to fit the BMW swing-arm & final-drive.

"There are 2 mounting points on the swing arm for the ride height adjuster to mount to so you can vary the ride height and the leverage put on the shock. The lower position will make the spring feel softer with a little more wheel movement. The upper position is based on the 900 duc. we have one here in the shop so I could compare it." – Randy Illg.

Shock in the pic (reversed for clearance) will be replaced by a custom Ohlins unit:

"I assembled the front end and wheels. The steering head angle is 26.5 degrees with the bike sitting on the bare wheels and the forks slid up in the triple trees 1 inch. With tires it should be about the same or slightly less (26 deg). The trail figures out to 3.96" at 27 deg and 3.69 inches at 26 deg so we are right in the ball park." – Randy Illg.

Resulting geometry:

One more tube, a transverse brace above the swing-arm pivot axis, two brackets to tie the main diagonal rails to the upper trans bolts, and another pair to connect them to the 'spine' ...

CRo: ALMOST COMPLETE !