I'm going to review the swingarm modification so I'll have it all in one post...
The stock DR swingarm:
The swingarm is made from three pieces. There is the cast front section that houses the bearings and there are two forged arms. They are connected with four square-drive bolts per side. In addition to the bolts, the arms are also epoxied to the front piece. The epoxy looks just like JBweld. After thoroughly destroying my first swingarm I discovered the joints that hold the three pieces together are incredibly strong and don't need to be reinforced. Even welding on the swingarm did not seem to effect the epoxy.
Here is the main problem:
The arms are C-sections where most swingarms are fully boxed. This allows a lot of flex.
Another aspect of the swingarm that might be causing flex is this E-section on the outside:
After the whole thing is boxed, the swingarm will be strong enough that much of this E-section will be unnecessary. Since I don't want to add much weight, first I drilled a decent sized lightening hole and then I ground out the center fin of the E-section.
I don't know what alloy the swingarm is made out of so this leaves a lot of guess work. I have chosen 5052 Alu for boxing the rear swingarm. 5052 provides the best as-welded (no temper) strength. ESAB recommends 5356 filler rod for the least chance of cracking when welding 5052, so that's what I'm going with.
I made the shape in cardboard first, then aluminum
And then I welded it all up.
It was difficult welding the section around the brake bracket due to the difference in thickness of the pieces. I later had to grind down the welds around the two chain-stay hole to get it to mount.
I weighted the swingarm before and after. These modifications added 13oz.
Boxing in the swingarm has significantly stiffened the chassis. The bike now handles like you would expect a dirt bike to.
After I started this project I found out that replacing the DR swingarm with one from an RMX was not difficult and is reported to give the same result. If I wanted to stiffen another bike, I would try the RMX swingarm first.