Thanks for the information guys. The round tube I have seems not to have any sign of a seam, so I assume that it's drawn. And I'm used to the devilish awkwardness of fabricating round stock. In any case, part of the spine is square tube, to copy Jurgen's elegant DIY rear suspension, for the two-wheel python; and so is part of the trike rear end. Obviously it's going to end up a cross-breed.
25 is right about us amateurs not understanding at first the extraordinary strength of steel: In the '80s I built a 70 foot steel proa, my first venture into steel as a boat-building material. When it was ready for roll-out, two of us used a makeshift ten-foot lever (again over-and-under round tubes close-welded together) to lift the boat off the concrete floor and onto a roller. I was really impressed by the strength of the lever, which seemed not to bend at all, visibly. But even more impressive was the boat itself. It balanced on a single midships roller, the extreme ends of the stems 35 feet away in either direction, yet we couldn't detect as much as 1/4 inch of sag overall. That was some box-beam! Bike framing should be fairly anxiety-free after that. Just don't want to dump my arse on the tarmac of Lon Fachwen, coming down its long, steep run, because my frame fails on a bump.
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