What I'm saying is that the major stresses that subframe connectors endure are not the folding of the car like it was a taco, hood to trunk. It is more from twist like a candy cane and bending from left to right like a brat laying on its side. (Gotta love food similes). Round subframe connectors will deal better with the bending and the twisting. Square may deal better with folding, but that's not really a problem.
Bicycles are all round tubes and they are straight so shaping can't be the reason why the use round tube. Nor is is aerodynamics.
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351W 89 Mustang GT Convertible