[Peck]

Originally Posted by Camren

Preferably aluminum but i'm unsure how it will handle the abuse so possibly steel.

if aluminum can't take the abuse of being a robot cart, you should take another look at your design. Steel will, in all likelihood, hurt more then it will help.

For example (over engineering it): If your robot weighs 200 lbs and you have 4 support beams holding that weight, then each beam will receive (ideally) 50 lbs. Basic mechanical physics tells use that the beams will be receiving a compression from the weight. Go to your school (or local) lifting room and put a 50 lbs weight (or 50 lbs equivalency) on the top of some aluminum of about the dimensions you want to have for your support beam. If it notices the weight at all then beef it up.