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  • Bridge construction details

    We constructed a bridge using the plans and materials as described in the "2012 Team Drawings" pdf file. (The only variation in spec is we used a 4x8 piece of HDPE plastic that we had left over from 2003 "Stack Attack", rather than purchase a piece of lexan.)

    The Field Tour video "Episode 5 - Bridges" (http://www.youtube.com/watch?v=-AMaqqmoLgQ) described "a test you can do to make sure your bridge has similar dynamics to the official bridge." Basically, placing 2 FRC batteries 28" from center of the bridge shouldn't cause it to tip, but at 30" it should tip.

    When we put even one battery at 28", the bridge tips! We're investigating hanging scuba weights from underneath (by cord, so that they're resting on the floor when balanced), so that the bridge behaves as the video shows.

    Has anyone else noticed this? Does anyone else think this is an issue? Or have we missed something critical in construction?
    "It's NOT just a robot thing!"

  • #2
    Re: Bridge construction details

    The HDPE plastic probably has a different weight than the lexan causing the bridge to react differently to the weight of the batteries.

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    • #3
      Re: Bridge construction details

      Maybe, but
      A) I doubt if there's much weight difference between Lexan and HDPE, and
      B) applying the weight evenly across the entire bridge surface shouldn't change the balance dynamics
      "It's NOT just a robot thing!"

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      • #4
        Re: Bridge construction details

        We completed the bridge construction on Saturday with the same "tippy" result (although we have no plastic on top at all).

        Our bridge is much easier to tip than the example given with the two batteries 28" from the center. I don't know if if one battery would have tipped our bridge, but my guess is that it would have been close.

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        • #5
          Re: Bridge construction details

          Originally posted by FRC811 View Post
          Maybe, but
          A) I doubt if there's much weight difference between Lexan and HDPE, and
          B) applying the weight evenly across the entire bridge surface shouldn't change the balance dynamics
          The bridge is not balanced on a point. It is balanced on a flat surface. Therefore there is a moment to overcome when the balance is changes.

          A suggestion: Add a 5LB weight underneath about halfway out from the center of the bridge on both sides. See if the reaction improves...

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          • #6
            Re: Bridge construction details

            I think this is a good opportunity for the physics students to get out a pencil and a piece of paper and figure out what is missing by intention from the team field drawings. It's not to hard to figure.

            Good luck.

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