[DanEC]

Car manufacturers are reporting torsional and bending rigidity for their frames/bodies all the time but I suspect most of it is derived from mega-buck super computers, finite element analysis and all that stuff. Small, custom, fabricators are not going to have access to that for the most part. Some assumptions and even rough calculations can be made looking at the structural properties of the main frame rails, number of crossmembers, etc. The structural properties for 4 inch round tube, 4 x 3 inch tube and 2 x 4 inch tube of specified thickness and steel grade are easy enough to come up with and I think if someone had the time they could make a ranking from that - but it would have some limitations.