[Veselin Kolev]

If you can actually find a 2.5x2.5 with a .090 wall, awesome

anyway... I think that what you are trying to do is quite possible, and be able to take pretty large hits. The thing you want to avoid is having long beams that are unsupported, where they can easily crush inwards. I think that if you place a small welded aluminum "peg" every 8" or so, just like a square hole in the 2.5x2.5 with another box beam welded in there, maybe a 1x1x1/16 wall, perpendicular to the 2.5x2.5, you would have a light and very strong frame. Kindof like ghetto internal trussing.

The one thing that worries me is that the 2.5x2.5 is really wide. If someone rams you really hard, it could crush upon itself, even if it has a crossbeam right behind it. If you internally brace it somehow, like the ghetto trussing mentioned above, your idea of using really thin walled stuff would work very reliably.

Oh, and I like your idea of the wheels in the beam. Very hot.