"....I found some engineering formulae to calculate the bending moduli of square and round profiles....." Siwmae 25hz, The quote above from your conversation with Henry prompts a question:Which is stronger? Obviously for tubing of identical width and wall-thickness, the round will have less steel than the square, and therefore be lighter. But what about relative strengths?
I too do 'seat-of-the-pants' building, designing everything in the drawing office of the mind's eye for a long 'brooding' period before ever I pick up saw or angle grinder; more than by any other method, for everything that I make. Time consuming, and leading to a good deal of trial, perceived error, and re-shaping once you do actually start the physical making. Often I feel embarrassingly amateur and ultra-slow, when I read how fast some of the professional engineers on the list can turn out rideable bikes, from concept to riding down the road. And showroom quality finish too, sometimes. But - well - the artist's way sketched above is just amazingly pleasing in the final results, both aesthetically and for engineering soundness. I do all the furniture that I make by this method now, and I've even built boats thus, up to 22 metres. Hairy at these sizes. There can be panics along the way: "Oh Jesus, I've blown it! There's no way out of this jam..." (There always is with steel, though; violent, murderous, filthy - but wonderful - material!) But still, I'd never revert to more orthodox techniques. Did you ever hear that quote from Michelangelo Buonarotti, that he saw himself as 'releasing' his sculptings from the raw block of stone, and never knew exactly what he'd get to in the end. The Galeria in Firenze, where they now keep his original 'David', also contains a number of his never-finished pieces, where you can actually see the rough-surfaced figures emerging from the blocks. At least with HPVs the amount of time, materials and money invested is less frightening. And you can always re-cannibalise.
I'm building one of my current bike projects using a spine made of two round steel tubes welded in contact with stitch-welds at regular intervals, over and under, like a Winchester rifle. I've used this idea before on a much bigger scale, and it makes a beautifully strong, rigid beam. Just curious to know if anyone has any properly-competent-engineer's knowledge of the relative strengths of this versus rectangular tubing? All professional information humbly welcome.
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