# Help Improving Bridge Acuracy

My team got finished building some cheaper versions of the bridges, but just like everyone else that has done this, we are having problems making the bridges accurate. I’ve heard you can put 50 lbs. on either end to make it work, but i’m not sure where on the bridge that would go. Does anyone know what we need to do?

Thanks

Assuming your bridge won’t see heavy testing, the natural suggestion would be to mount weight wherever the other end of the bridge will pass the two-battery test.

However, the one bridge I’ve seen in person was built to the drawings and passed the test until excessive testing pushed the double-hinge out of spec, throwing the balance of. Are you working off of FIRST’s team drawings, or are you estimating it yourself?

Alright we’ll try that.

Any ideas on what to use as weight and how to mount it? Also about where have you noticed the weight goes (just so we don’t have to spend so much time finding the general area ).

At the risk of sounding unhelpful, the answer is “wherever it means your bridge passes the two-battery test.” Without knowing your bridge (even in the face of FIRST’s drawings for teams), it’s not really possible to answer much better.

It is the center of mass of the weight that matters. To have an appropriate effect, it should be centered between and above the hinges.

The problem of getting your team field bridge to behave like the ones you’re going to see in competitions is a combination of statics AND dynamics. just like life If you position some weights on the other side of the fulcrums so that the battery balance test is passed, you will be dealing with the static difference of your bridge. You should be able to figure out some things about how to deal with bridges, like how hard to push on one edge to get it low enough to drive onto, for instance.

If you want to practice balancing on your bridge, the rotational inertia about the hinge points will become important. So the distribution of the weights you applied to achieve the right static balance will be factors in how the bridge reacts to your fast or slow driving back and forth across it. The reason tight-rope walkers carry a pole is to increase their rotational inertia and decrease the effect of small errors in their walking balance.

You can make the bridge behave statically correct by weights of different amounts at the appropriate distance for each. The chance that it will behave dynamically the same as the metal one is yet to be determined. And we haven’t even stopped to consider the hinge friction characteristics under varying loads. :eek:

The way you can test your bridge is through the two battery test. If I remember correctly FIRST had a youtube video demonstrating the test. They also told you how far apart the batterys should be placed. I would put the link up, but I am currently in my Econ class and youtube is blocked in our school.