[EricH]

Originally Posted by 75vs1885

Thanks!
I'd tend to think that the bent sheet metal would cause flexing within the frame, causing and uneven chassis. Then all the problems evolve.
Could you use 3x1 box? it would have more strength

That would depend on the bends of the sheet metal. There's a reason a lot of teams like to use sheet metal; if it's handled right, it can be as strong as channel and box. And, it's a lot lighter than box extrusion.

Some things to keep in mind for sheet metal: bends and gussets increase strength, if properly applied. This design appears to be proper application, and there's space in the outer rail for bumper supports/backing which will also increase strength.