[EricH]

For the drivetrain, unless you're actually using sheet, 1/8" wall tube can take everything thrown at it. I don't quite trust 1/16" in drive chassis applications. Even with the bumpers, the drivetrain takes a massive beating. From before bumpers were even allowed outside of the box, 1/8" has yet to fail on any robot I've known about. Welds have failed, but the structure has at best bent. And this is with full-speed, metal-on-metal, defense all over defense. Heck, falling from the 10' high bar in 2004 hurt my team's robot less than running our lift down when it was all the way down already!

Superstructure, yeah, I can see (and have used) 1/16" wall. But we aren't talking superstructure yet.

40 lb for a chassis is kind of light. A 50-60 lb chassis (with some of the superstructure power) is more like it, though I'd shoot for 50 when driveable. Heavy chassis=low CG. Low CG=less chance of tipping. As we all know, tipping == bad.

Designing for weight is not about making everything as light as possible. It's about tracking weight--where is it going, how much is going there, stuff like that. If you don't have one already, get a scale--you'll want both a platform type and a fish (or hook) type. Before a part goes onto the robot, weigh it and note the weight in the sheet. Try to get an estimate of how much the robot will weigh--most CAD programs have a way to do that. Then you know where you can remove a lot of weight quickly.