[MrForbes]

Actually a 4" tall frame has some interesting advantages....but following a strict weight budget (for the rest of the robot) is required.

Here's a look at how the frame can be layed out, the I beam is a neat shape for the sides, wheel mounting is pretty simple with just a 3/8" bolt for each axle going thru holes drilled in the fiberglass frame.

The 4" high 1/4" thick wall around the outside of the robot is incredibly tough, no bumpers needed, but bumpers can be attached easily. The drivetrain area is open so it's easy to work on, yet the chains are protected from damage from colliding with other robots.

One downside to this material is the limited availability...think truck freight and a bit of a lead time, and a quantity purchase. I think it might be easier to just find some aluminum C channel, if you have a design that could get by with it. Spacers between sections of channel would make a reasonably strong structure, with good accessibility compared to rectangular tubing.