[quote=ag_streak]Well, me and the welder didn't "click" I carefully explained what I was after, but he didn't seem to get it. I wanted a quick and dirty prototype, he wanted to maximize his price. He estimated $800 for the work, saying it was a lot of welds.
I see his point, so I designed another frame with fewer joints and welds. It uses a single tube main frame instead of twin flat-stock main rails. I thought the flat-stock idea was a bonus, but apparently, it causes more hand-work.
A large-diameter tube would have great stiffness and can be bent to the shape below in minutes on a tube bender (the diagram has angled bends only because they were easier to draw). Then a simple three-piece hinge part for the swingarm pivot and a single hole for the main pivot. Simple.
The swingarm is conceptually the same.
The hitch is five easy pieces. Steel rod instead of aluminum wouldn't weigh much more, and can be bent without kinking (I'd still prefer a hollow-tube hitch loop.) The plates that mate to the bike axle could be the ones posted by Ewok.
FWIW: I built something like your trailer during the boring winter of 2007 in Green Bay. Easiest pivot/best tolerances: use a universal joint yoke assembly (available at any farm supply store). The u-joint is way better than a bolt/bushing and it permits the appropriate up & down + side-to-side movement essential for a single-wheel trailer. Weld one end to your hitch assembly to attach to the bike & the other end to the trailer. Take pains to ensure the u-joint yoke is perpendicular to the ground (and parallel to the tire) before welding it on.
My design has a swingarm (on a bushing) plus a coil-over shock from a youth ATV. I use the trailer to transport a tandem bicycle or excess gear (grocery shopping when I'm dragooned into the task). Bullet-proof - I've run 90+ mph: no issues behind my R12GS. Doesn't change the beast's fuel economy one bit.