we used (correct me if i'm wrong) 80/20 1" steel box tubing this year to build our frame. for those of you that were in the practice area behind the pits at SLR, you know just how strong the frame was. our robot ran full speed into a solid concrete post during an autonomous mode practice run and came out unscathed, we did more damage to the post!
and i must admit, making modifications during the build season was a snap, we just used either a hand saw or a plasma cutter to hack off the parts that we didn't need
