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I have a roller I built for rolling 1 inch square tube and I was just wondering if the frame would be stronger if it was rolled at the d1 and d2 area instead of making the bottom rails out of three pieces. a2 and f2 area would remain straight but the d2 area would be curved.
I hope I am not confusing everyone. Just thinking outloud

The Series 4 Lotuses used bent sheet metal for several crossmembers.

Depends on how big the radius of the bend. If it's a big long bend like the trendy-exo-that-shall-not-be-named tubes, you'll need to go bigger* than 1" .063 because you're changing straight tubes into big long compression springs. And I think you'd need to match that curve in the N1 and N2 tubes so the upper frame would match the lower frame or it'll be tough to panel the sides. But I like the concept, and bought myself a set of 1" square tube tools for my bender some years back so I could try building a frame that way in my copious free time. I've forgotten the radius, but it's smallish...I'm guessing four inches; just enough to make the upper and lower frame rails out of one piece of tube (each), by bending those tubes at the A1/D1/F1 junctions and the N1/J1 junction instead of welding them out of parts A1/D1/F1/N1/J1.

*a lot bigger, and that's only an estimate.

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Locost builder and adventurer, and founder (but no longer owner) of Kinetic Vehicles

I was thinking one continuous roll from a1/d1 to d1/f1 leaving a1 and f1 straight and just curving d1 to mate the two. I am not sure what you would have to do to the top frame to make it look right though. Probably more trouble than its worth.

At the 'element' level the tube doesn't know that it is bent - think free-body diagram. But the bent tubes will be carrying moments that straight ones don't experience. So as Jack points out, the design becomes buckling-constrained rather than seeing just the simple tension/compression loads of a conventional space frame. How to assess buckling risk is the challenge.

Going to bigger diameter tubes offers reduced risk of buckling, but defining critical loads may still be a challenge. As an alternative to increased tube diameters you could reduce span, but you might end up with a birdcage .... Bigger tubes are probably the more practical approach, as long as you can calculate critical loads, and of course, make a rational estimate of what the design loads are (frontal collision maybe?), I wonder how may bent-tube exo-cars have been through that process?

But I do like the looks.

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Warren
Isuzu Pickup/SR20DE, +401 COLD frame
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