[MrForbes]

We managed to avoid that problem by welding up a nice 27 pound steel frame for the bottom of the robot. And we have about 10 lbs of pneumatics, too.

One rookie Tucson team we've been following has a really light robot like that, they came up with a neat design for their sheet metal sponsor to make, and their robot is somewhere around 85 lbs.

Since the bridge causes short wide robots some issues, especially when there are balls under the end of the bridge, you might see if there is some heavy thing you could add to the bottom that would make bridge climbing easier, and flipping less likely. We're looking into this for our robot.