Proper adjustment of a bungee system (or "Hooray! Another bungee system thread")

[Chuck F.]

We are installing overhead bungee systems. I understand the general principals behind how they work, but I wonder about how to adjust them so that they work best.

Is there any kind of "formula" where I can plug in the height of the ceiling and length of the room and find out how long the cables and bungee should be?

I just want to avoid causing damage by having too much tension, or a sloppy set-up with too little tension. Ideally, I would also like the sockets to be out of the way when the strips are not in use.

Advice?

(PS - Also, do you think that the grid of a suspended ceiling is strong enough to act as an anchor for the cables near the scoring box? The actual pulleys will be anchored to the walls, but I can't think of a better way to secure the machine end of the cables without going to a whole lot of trouble . . .)

[SJCFU#2]

I'm not aware of any design formulas however there are a few things you might want to keep in mind:

As far as height goes, you probably want to keep everything high enough that fencers aren't likely to hit anything (at least not without specifically reaching for it). Nine or ten feet above the striip is usually enough, although you might be able to get away with the bungee cord being a little lower so long as it's far enough off to one side to avoid getting in the way (just be sure to keep it high enough to not have people running into it).

One of the most common problems I've heard are the cables getting twisted. There are two things you can do to address this. First, try to get any twist out of the bungee cord before installing it - that will help to minimize the torque applied to the system and causing the cables to twist. Second, try locating the central anchor point (where the cables are anchored prior to dropping down to the scoring machine) a few feet higher than the end anchors (where the blocks for the bungee cord attach to the wall) so as to maximize the angle between the cable as it enters and exits the block (the "<"), This will help to pull the cables apart, keeping them from twisting. If you can't get this seperation in the vertical plane than perhaps you can get some in the horizontal plane by placing the central anchor over where the scoring machine will sit and the end anchors somewhere between there and the edge of the strip.

The bungee should probably be just long enough to stretch across the space with no one hooked up (sometimes the weight of the bungee cord will provide enough tension to keep everything in place). You might want to cut the bungee cord a little longer than you need at first, then tie loops in it near the middle to shorten it until you find the ideal tension.

The cables should be obviously be long enough to reach from the blocks at the ends of the stip to the central anchors and down to the scoring machine. I also like to tie a knot in the cables a few feet back from the fencer end sockets so that when no one is hooked up the fencer end socket will dangle down at a convenient height. You probably want a little slack between the central and end anchors (if only to allow a little more length for fencers to run off the end of the strip without pulling your cable apart) but not too much (you probalby want to be sure there will be at least a little tension in there whenever both fencers are hooked up, even when they are both back near their respective two meter lines).

As far as the suspended ceiling is concerned - I would think it would be possible to use the support grid for the central anchor point (I string a length of clothes line across our gym, from one side to the other, with two bungee systems anchored at the middle). If the grid isn't strong enough then you can always add a little extra bracking to the grid (just move a couple of tiles out of the way and add a few wires between the grid and anchor points overhead set at diagonal angles so as to take the additional load).

One other tip - mark each end of the cable so that as you plug the overhead into the machine you can keep track of which plug goes in which side (the other option would be to provide two central anchor points so that you can keep the cables seperated from one another).

edit: Also, be careful about locating the ends near anything breakable - I once saw someone take out a window when they released the end socket while standing near mid strip and it snapped back.

[Mergs]

I'm pretty sure that no one has worked out the math for figuring it out, although it shouldn't be too much of a problem. You could do a rough estimate of the cords by taking the height of the box off the ground, square it, add 64 (half the strip length +1m for runoff, squared) and take the square root of the sum. As for the bungee, depending on how high off the ground you want the ends of the wires to hang, say 4' and the box is at 7' off the floor, then 14m + 1m + 1m = 16m (because 1m roughly 3.3' (39.97" to be exact, or 3.3308')). Tension will depend on the size of the bungee. 1" diameter bad. 1/4" diameter, much better. Remember, you are just trying to keep enough tension on the moving pulley to keep the wire from drooping onto the floor, not so much that when one of your youth fencers gets to the warning line on the other end of the strip, it slingshots him back into the wall!

As for using the suspended ceiling for support, it would depend on what the structure is. Pics would help.

That's about as good an answer I can give this early in the morning with only 1/2 my daily caffeine dosage consumed. If you can give me more details about the dimensions involved,

[Chuck F.]

OK, the bungee systems are up.

I decided to try a few different things to see what I liked best and, in the end, have 2 different types of systems.

In one of the set-ups, there is small diameter bungee cord (1/4", I think), and the pulleys hang a little lower than I would like when the system is not in use. I don't want to tighten/shorten the bungee too much, and then regret it later because something got damaged. But, if I can safely increase the tension and raise the pulleys a bit, I would be happier.

How much can a bungee cord stretch safely before it will break?

I could perform my own experiments, but I would bet that others have already been down this road.

[SJCFU#2]

How much can a bungee cord stretch safely before it will break?

IIRC most bungee cord can stretch to more than twice its original length. However if you want actual numbers then I suggest looking for technical data on manufacturer's sites - the stretch and breaking strength should both be listed somewhere in there (I would also take those numbers with a grain of salt - they're probably based on brand new cords and I would expect performance to decay over time).

BTW, one way to increase the tension without permanently shortening the cord is to tie a loop into the cord somewhere near the middle (where is isn't likely to ever reach either of the blocks near the end). That way if you decide that you need the extra length back all you need to do is untie the loop.