It's my understanding that RHS means rectangular hollow section (just as CHS means circular hollow section)
Therefore ERW in square form is just a type of RHS. Maybe ERW is made in a way that isn't suitable for the thicker sections?
So, when I look in the tube details appendix in Chris's book and see that RHS is specified for all the square tubes, I assume that means ERW RHS because that is what is available in the 16 gauge that is appropriate for this type of chassis.
I am using ERW, BTW. If I lived somewhere where ERW in 16gauge was not available, then I would use whatever was available (as long as it wasn't outrageously heavy) and not worry about it.
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