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Old 02-11-21, 12:03 PM
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It's very hard to cold-bend tubes without them just kinking without the proper bending die the right size. You can heat them up, push one end into a block of wood drilled the right size, and bend it down by hand. I've done this on tubes to unbend a bend a bit and it worked OK but idk whether the heat cycle is doing the tube any favours, and I didn't end up using those tubes as they weren't symmetrical enough. This is the other thing about bending chainstays: you have to get them exactly the same. That's what I like about the design of this double-barrelled bender: (I don't have one but it looks cool). You often don't care exactly what the bend is so long as it's the same.

As for making a square on the end I guess you might have to do it in steps with a series of mandrels and dies. Don't know. If you just bash something square in there at the final size it's probably going to split the tube.

You could just push the end into a bit of square tube the right size and braze it up at both ends, filling in as much of the gaps as possible and leaving a good sized fillet on the long end. Then sand it into a nice smooth transition. A bit of 16 gauge 3/4" square tube would have pretty much exactly the right ID. It wouldn't need to be 4130 or anything fancy since it's thick enough and anyway the round tube is still there on the inside.
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