Rail/ladder chassis

[itstock]

I guess I understand your overall message, but comparing the torsional rigidity between the backdraft frame and that frame is completely irrelevant and meaningless. You have a jack, you couldn't do this with your example, but the backdraft does well.

This is toward the front right corner jacked up on the side frame rail, which jacks the right rear corner frame 2", and drops the front left frame 2" lower then the front right, basically using the left rear and front right as perfect pivot points with zero flex. This is not scientific by any means, but move it around at different balance points and check for deflection. The frame is certainly quite rigid and nowhere near your example.