Hi nano0k, There's some very nice work there, nice fabrication! I will offer a bit of critique regarding swingarm. In your first and second pics on Pg 1 here, that swingarm is a very long tube; I believe too long. You'll be dealing with twisting forces along the whole length of that tube, caused by the leverage action of wheel's diameter twisting tube rotationally along its length from side thrust when cornering in either direction. That is, the wheel from top to bottom of its diameter acts like a long lever to twist the single swingarm tube along its length from cornering forces. Hard right hand turn would make the tube want to twist wheel top toward body; hard left would want to twist wheel under sidecar body [think I got that right]. Check out 70's Chrysler Corp. 'Torsion Bar Suspension' and you'll see that the twisting forces in a similar tube/shaft are even used to replace the entire front suspension and springs in a full sized vehicle! The longer the tube or shaft is the more it will have torsional twist. A shorter torsion tube has a lower twist rate, and a thicker-walled tube or a solid shaft has less twist along its length. Your 3/16"-.1875" wall tube isn't especially stout for resisting the twisting forces in this application of a single-sided swingarm tube withstanding the entire cornering force in a sidecar. In addition you have the leverage factor side to side [front to rear along its length] acting on that long single swingarm tube, making it want to bend near the forward mounting point and/or along its length due to cornering forces. Again, with leverage as with torsional twist, shorter is definitely better to control unwanted motion. Gussets and triangulation can help somewhat where possible and again a thicker-walled tube is obviously less prone to bending. See the single-sided swingarm in upper-right pics that Claude posted: it has a rectangular/square profile and much heavier dimensions overall, plus it's very much shorter. All of that will help resist torsional twist, and in his example, all but eliminate it. If it was 1/4" - 0.25" walled square/rectangle stock it would be even better, and solid stock is a possibility too. Increasing diameter or external dimensions also helps with torsional rigidity and resistance to twist or bend. The square profile would also help resist the bending load front to rear, as compared to round tube. Not saying yours won't work but I'd sure keep an eye on it during some rigorous testing and be sure and subject it to some maximum cornering forces with chair heavily loaded with ballast before trusting it on a long ride with passenger and kit. Have fun!