This year team 836 used a 3/16" thick aluminum plate, and boxed aluminum for the chassis and frame.
Ok here is the logic behind the plate. For one it is very hard to break, second you don't have to keep adding braces to make it sturdy and or add braces to mount stuff on, third if you need to lose weight drill some holes, but alot of holes. The most any other robot has done to the plate is a small nick, but to those who hit us at high speed i would kinda think twice, that plate isn't going to budge.