This build diary shows how we handle basic problems. It is intended for people new to racing and low budget guys and ladies like oursevles. Nothing fancy here.
The Triumph swingarm was extended 3 inches. It was cut and a 3-inch lengths of rectangular tube were spliced in using butt welds. The butt welds are probably good enough but I would be in real bad trouble if there was a fracture at or near the weld. We added four gusset plates across the butt welds, one on each side of the swingarm, for safety's sake. Our welder calls them "fish plates."
The modified arms are longer and the swingarm assembly will be more flexible unless strength is added. I looked at two options. One was to build a "C" shaped truss under or over the swingarm. The other was to add a "C" shaped box section. I drew up free-body diagrams showing the forces, reactions, and moments in the swingarm. This is complicated so I looked at brake load, power load, weight load, and torsional twisting load separately. The box added the most strength in the right places with the least amount of added metal. The Triumph has a McCandliss style twin shock swingarm with 2-inch deep rectangular tubing. A McCandliss swingarm made from smaller diameter round tube or a single shock cantilever arm are different animals. The added box might not be the best option for them.
The added box was discussed with the welder. He made some suggestions and gave me some sheet steel that is compatible with the Triumph metal. We are a low budget operation so I cut and bent the pieces. I work for free so this saved money. The welder welded everything up. A big problem that none of us anticipated was contraction during welding. The arms pulled toward each other. The welder spent a lot of time dealing with this issue and getting everything right. I did not ask what he did and I do not want to know. I do know that a lot of heat, force, cussing, and cigars were used.
I seldom get everything right the first time around so I do not invest much money in paint. I use a paint system that is inexpensive and easily patched after future changes. It is to clean the metal as good as I can, spray on red primer, follow with gray primer, and topcoat with satin black. I use Rustoleum rattle can paint. This was how my parents did it. It can look good if the blemishes are sanded smooth between coats.