# Bridge question

The practice field ramp does not pass the battery calibration test with the dimensions given. Should the hinges be further apart?\$@#

I would leave the hinges where they are so that the bridge tips at the correct angles.

You may want to consider adding weight to the middle of the bridge (near the hinges) or the edges, depending on which way it is incorrectly calibrated.

He’s not saying that the bridge tips to one side, just that it requires less force than the real field should to tip in either direction. I would recommend adding weight to both sides or using a spring or other stretchy device.

How off caliber is your bridge? We constructed a low cost one ourselves and we were only able to move batteries 14" off center before tipping.

We are having the same problem, but we follewed the drawings very closely. I don’t understand how this could work.

it’s because they made the drawings to be cheap, not to be spot-on accurate.

the surfaces are also not spot on friction wise…

All bridges whether competition or low budget will need to be calibrated. If you build it well than the center of mass of your bridge is exactly in the middle. This means you imagine ALL the bridge weight as one side of your teeter totter which is I believe 3" on one side of the fulcrum. The 2 batteries with their bottom 28" from the fulcrum is the other side.

If you can only move the battery 14", then you need a heavier bridge, which means add 2 equal weights an even distance from the center. If you were so inclined, you could measure the actual distance from fulcrum to center, then do the math and figure out exactly how much weight to add.

If you want the dynamics as close to competition as possible, then pay attention to all measurements dealing with the hinges: their distance from the floor (and therefore also their distance from the deck), and their gap.

We are also haveing the same problem. On one side the batteries will go about 16 in and on the other side only 13 in before tipping. We tried adding weight to the center to see how much would be needed. It took 140 lbs to make the bridge balance with the batteries 28 in from the center. Has anyone come up with a solution that doesn’t involve putting that much weight on the bridge.

We made a cheap version using wood, and we had the same problem. however we fixed it by adding 40 pounds to each side.

It took 60 lbs on each side to keep the batteries at 28 inches on the better side (the one that will go to 16 in before tipping). With 60 lbs on each side it makes transporting it a real problem so it is not very practical to use this as a final solution. Has anyone come up with a way to use surgical tubing or something like it to fix these problems?

Just make the 60lbs on each side removable and transport as before.

Adding weight will not make the bridge behave differently.

You need to add a variable force, such as a spring or bungee cord, between the bottom of the bridge and the baseplate, and calibrate that until you get the right motion. Do this on both sides, of course!

We tried adding springs, and while it passes the battery tests, it still does not behave as the real ramp would as it falls down slowly. I think we will try adding weight next.

I think you are certainly wrong on this count.

Higher weight will make it harder to lift the CG of the bridge (which you certainly will do if you move edge of the bridge down).

It is pretty easy math. The pivot to the CG of the bridge (assuming symmetry) 3" (=barrier width/2 … actually from the this video it looks like the hinge point may be a bit wider than the barrier, but close enough). The pivot to then edge 41" (44"-3").

If it takes 15lbs at the edge, then the bridge weight must be ~200lbs (15*41/3).

Joe J.

You’re likely not going to get performance that is as good as the weight. Also, if you’re not tipping the same on each side, then you can fix it by not evenly spacing the 60 lb weights–if you move them around you can get both sides to balance properly.

Same problem.

I also noticed that our bridge is a bit flimsy. Lots of torsion when two people walk over it. I’ll be adding some 2x4 blocks between the 3/4" trapezoidal supports, and from these I’ll try hanging weights. I wish I had saved last year’s dead batteries!

And Joe Johnson, thank you for the explanation. My first instinct (as a physics teacher) was to claim that it should not matter how massive the bridge was. After checking the actual game pieces, and seeing the hinge points were at the same positions, I was trying to figure out how increasing the mass will matter.

The fact that the hinge points are off-center does indeed mean that the CG will be lifted when the bridge tips up.

I’m only upset that I didn’t see it myself

Man, I am having a hard time getting the bridge to conform to the tipping guidelines.

I made “pockets” out of 2x4 between the support trapezoids, 6 total, into which we can put 6 batteries. they need to be secured so they don’t slide out when the bridge tips, and they also need to be quickly installed and removed since those are 6 of our 10 batteries!

These things, being mostly lead, are the densest things we have, the pockets are as far as possible from the pivot, and they still do not get the bridge to go to the magic 28" tipping point (right now we get to 24").

Anyone else with more luck? I figure if we practice on this, we’ll only find it easier on the real field.

We have had the same experience. We had to add >110lbs to get the dynamics demonstrated in the video. Our engineer mentors (I’m not an engineer) tell us it does not matter where the weight is so long as it is evenly distributed relative to the center line. We proved that by testing with the weight all on the center of the bridge versus at the ends - same result in the 2-battery test.

As for the tipping point being different in one direction than the other, we found two problems with our bridge:

1. Note that the non-hinge side of the 2x4s on the bridge itself (not the base) is at the bridge center line, i.e. the 2x4 is off center. The first time we put the bridge on the base we had it backwards, which gave us a very out-of-balance bridge.

2. Pay close attention to the hinge mounting detail in the drawings. Notice that the 2x4 member on the bottom of the bridge itself is not flush with the edge of the hinge plate. We had our’s mounted flush. We moved it ~1/2" outboard and got a much closer match of 2-battery test tipping points on the two ends of the bridge.

Now we just need to figure out how to safely mount >100lbs on the bridge in a removable manner.

We haven’t finished calibrating our bridge yet, but we’ve come up with this to help with the calibration:

With (2) 14.2 lb 6.58" deep (round to 6.6) batteries, the bridge won’t tip at 28" from edge to centerline of the bridge, but it will at 30", per field tour video 5. So, that says the torque from the bridge weight is greater than the torque from the battery weight at 28", but less than when at 30". The bridge weight is 3" to one side of the hinge while the battery weight is the distance minus 3" plus half the length of the battery (3.3"), so the center of the battery is 0.3" further from the hinge than the battery edge is from the centerline of the bridge.

Given that, our chart is attached. If you know where your batteries cause the bridge to tip, you should be able to look up (or interpolate) your current deck weight and adjust it to get to the 28"-30" range.

Unless, of course, I messed this up again … :eek:

I just finished our bridge and here is what I came up with.

Looking at the game element drawings, the bridge pivots are 7" apart or 3.5" on each side of center. If you follow the low cost drawings, I think you will find your hinges end up 7" apart. Ours did.

Using the Youtube video the bridge is supposed to stay flat when 2 batteries are placed 28" from center and tilt when they are moved to 30". We weighed 2 batteries at 28 lbs. Working backwards from the 30" number you get:

``````           Weight of the competition bridge (Wcb)*3.5 = 28 * (30-3.5)
or Wcb = 212 lbs.
``````

Looking at our bridge, it began to tilt when the two batteries were at 18":

``````                                                       Wob *3.5 = 28 * (18-3.5)
Wob = 116 lbs.
``````

Ballast required to bring our bridge into spec:
Wcb-Wob = 212-116 = 96 lbs. (equally distributed about the CL of course!)

It’s very worthwhile to run through the excersise for the students. It’s a very accessable and practical problem.
If anyone has any corrections to this, I’d be happy to hear them (especially if I’ve missed something!) and if you need any help with the equations or how I arrived at them, I’d be glad to explain what I did.

Thank you and good luck!