[ThaneS]

I've just finished our bridge and I note the same thing. Ours tips with the batteries at around 13". However, we haven't put a sheet of polycarbonate over the top yet. I think this should make the bridge less prone to tipping, as there is more weight keeping it in the center position (if my brain is thinking about physics correctly). Getting a 4x8 sheet of .25" polycarb isn't the easiest thing for us, but another sheet of .5" plywood weighs about the same (50 lbs), so that's probably what we're going to try first. The only other discrepancy between our bridge and the plans is that the plans seem to dimension the nominal thickness of the plywood, and not the actual, easily available, thickness that we used (15/32" instead of 1/2").

On a completely different note, I haven't seen much discussion of robots falling off the bridge. There's no lip on the sides, and if a wide robot tries to go over it, there's only 5" of clearance on either side. I expect to see a lot of tipped robots.