[Kevin Sevcik]

I think your hands down favorite/best material to construct a robot out of is waterjetted and bent aluminum sheet metal, since you can design it to the exact requirements you need, given you an optimum strength to weight ratio.

Lacking the fabrication support necessary for the above, 57 has settled on the kit frame with a chassis of 1"x1"x1/16" wall square aluminum tube, mostly held together with more 1/16" sheet plates riveted with 3/16" aluminum structural rivets. It's pretty lightweight, plenty strong if you're not dropping your robot off a cliff, and we can get 20' sticks of the stuff for $6$20 from a local supplier, even on a Saturday. Which is harder to say about AM channel or 80/20 or another T-slot extruded aluminum.

Thick wall AM style channel is heavier and tougher than we need for any framing purposes. Thin wall channel would be flimsier than tube, if lighter. Either of the u-channel options means you have an entire side that can be a pain to attach things to.

80/20 style extruded aluminum is good for prototyping and quickly adjusting parts, etc. Only problem is that it's heavier than the 1/16" wall tubing that we use, for no more benefit than the quick propotyping.