[MechEng83]

Has anyone built the "cheap" version of the bridge? Ours is pretty much built to spec but doesn't have dynamic properties coming close to the competition bridge in the FIRST video on their site.

Using 2 batteries, our bridge tips at about 15 inches out, rather than the 28" claimed. I did some math (explained below) and came up with a weight for our bridge around 70 pounds, whereas the competition bridge should be more like 170 pounds. Anyone else notice this, and what are your recommendations? I've thought about just adding 100 lbs of weights to the underside of the bridge, but didn't know if other people had more elegant solutions.


Math:
The hinges are 7" apart, meaning the pivots are 3.5" on each side of center.
Since the batteries weigh approximately 14 pounds each, and the center of gravity is approximately 3 inches inboard of that, we can calculate the balanced gravitational moment of the bridge and battery. For our home-built bridge, the battery side moment is (15"-3"-3.5") X 28 lbs. The balance bridge moment must be 3.5" X W[bridge]. Solving for W we get ~70 lbs.
Now, take the competition bridge in the FIRST video -- replace our 15" with 28" and you get 170 for W[bridge]. No wonder they said the field was particularly heavy this year!