Bridge question

[Chi Meson]

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

[Chi Meson]

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.

[jm2713]

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.

[DELurker]

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 ...

[sandiegodan]

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!

[jdunston94]

surgical tubing. plain simple light. that allowed ours to behave quiet nicely.

[Chi Meson]

We got two 50-lb flat weights. At 14" diameter, they fit nicely between the two center support trapezoids. We put a 2x4 block across with just enough room to fit the weights without slopping around.

A carriage bolt that goes down from the top of the bridge, through the hole in the weights, and through the 2x4, holds them in place. With a wing-nut, the bolts are easily removed when we have to store the bridge.

Thanks to our Pit Captain, Tulsa, for working this problem through.

[Jim Wilks]

Originally Posted by jdunston94

surgical tubing. plain simple light. that allowed ours to behave quiet nicely.

Installed where, how?

[TerryS]

Originally Posted by jdunston94

surgical tubing. plain simple light. that allowed ours to behave quiet nicely.

If you use surgical tubing or another kind of spring to balance your bridge keep in mind that the resulting spring force increases as it stretches. So, you will be increasing the equivalent weight of your bridge as you tilt it. If you build a tilting mechanism that can tilt your bridge then it should have no problem with the competition bridge, but you may also end up over designing it. This could be a problem if your robot needs to go on a diet.

[Dad1279]

We had to add about 160 lbs, which behaves differently than any bungees or surgical tubing we tried.

[Taylor]

Read this. It's not so much how much weight you add to the bridge; it's where you place that weight. If the weight is as close to the center of the bridge as possible, it will be more effective. Saying how much weight was added to the bridge is meaningless unless it has some location dimensions with it.

[Ernst]

Originally Posted by jgw

Installed where, how?

You can attach eye bolts to the trapezoidal braces on the moving part of the bridge and on the fixed, central base, then run surgical tubing between them. However, you should exercise caution when dealing with enough surgical tubing under tension to simulate 100+ lbs of weights.