Hi Jonathan,

Ah sorry, missed what you meant by floating.

Ali vs steel parallel question. I've always thought that items like ali profile or aluminium extruded RHS etc. are pretty true due to the extrusion process used. Steel box can be resistance welded along one edge, which I would suspect is not so good. If going for steel sections on critical parts which held the rails then I'd be considering having the faces ground. But I'm not an expert to my suggestion is that you post this up for a better response!

Twisting calculations were from Roark's formulas for stress and strain, an old University text book I have about 1" thick full of tables and equations. Excellent book, but heavy going in places. If you search the internet for 'twisting constant K', which is a bit like I values for bending, you might get somewhere.