Basejumper.com - archive

We played with 80 pound breakcord and a tensile tester yesterday. Our test wasn't very scientific (we want to do more soon and get some graphs), but we found that when looping the breakcord, the break happened at an average of 156 pounds. This is in line with what base689 explains in this post.

At one point I believed that since the loop breaks at one point only, the pull force would be only 80 pounds. Our tests and base689's explanation made me think otherwise.

I've used this setup in two static-line jumps, and it worked great. I actually used a longer secondary backup loop, but I imagine that the snatch force for the backup is already enough that it's not going to increase the required load. I do believe it's important to make the backup loop significantly longer, to make sure you don't actually create a 320 pound loop.

Does anybody else agree that 160 pounds seems a lot? Has there been much testing as to what is actually necessary? I used to believe it was the pin-pulling or velcro-peeling that was important, but I had somebody explain to me that it was the sudden lift of a heavy canopy from the packtray that caused the biggest snatch in the deployment sequence. This made sense to me, but nonetheless 160 pounds sounds like a lot to me.

Does anybody use a single strand of breakcord with a loop on both ends to attach it?

Does anybody know what kind of strength the bridle-attachment point at the canopy is supposed to withstand? If anything, that would seem like the weakest point in the link to me, is that true?

Thanks,

Jaap

I know a jumper who had a pilot chute snag on a static line jump. He got down thinking everything went fine. When he went to pack the next day he noticed his bridle and pilot chute missing and a rectangular hole where his bridle attachment used to be. He called up the manufacturer (Asylum) and found out that the bridle attachment point functioned exactly the way it was designed. It is supposed to break away at 400 pounds of force.

I've done similar tests as you and come up with similar results. My tests were even less scientific because I just used my home weight set and kept adding weights until I could get a piece of break cord to break. The tests were too unscientifc to draw any real conclusions, but a single strand of break cord will break at 75-80 pounds. If there is a knot in it, it won't break at the knot but somewhere near the middle of the piece. Looping the break cord doubles its strength. I've actually had 160 pounds hang from a static line with a looped piece of break cord tying the two ends together without breaking it. I could only assume that the static line itself was taking some of the load.

Would it be possible to put a more reaistic static line set up in your tensile tester (instead of a looped piece of break cord)? Something like a piece of dacron with each end tied together by a piece of break cord? It would be interesting to see if the dacron takes some of the load and actually increases the strength required to break the break cord. It would also be interesting to see the difference between a looped piece of break cord tying the two ends together versus a single strand.

I want a tensile tester! It would significantly reduce my risk of dropping a 50 pound weight on my foot

I have asked a few questions regarding this in the recent past and have not been completely happy with the response. One of my questions was how much force should be applied?

From my background, I am a test engineer for the aerospace industry, and reading stuff here and on Blinc, I have conceeded to the following:

1. There is indeed a pulley effect which doubles the effective force required to break the loop.

2. The knot in the breakcord, due to the angular components of the force, reduces the strength of the cord by as much as 54%.

3. The dynamic loading of a static line jump creates an instantaneous force much greater than 80 lbs when using an 80 lb weight.

4. Gradually adding weight to a piece of breakcord yields results consistent with Item 2 above.


While I would still like to know how much force should be applied, I quit asking because what's being done works. (I have two static line jumps and they both opened) However, I think using a loop of breakcord between the bridle and a static line attached to an object, requires 160 lbs of force to break but due to the dynamics of the force applied and the decrease in stength due to knots, we achieve the expected results which is about 80 lbs of force.

Disclaimer: None of this has been tested to any measurable degree except that it has opened my parachute and many others.

Edited to add: If you really want to quantify the results of your tests with real world application you need to look at application of force over time. Then if you decrease the amount of time the change in force is applied over you can get an estimation of instantaneous force. Also, different types of knots would yield useful information.

That's very comforting to know. I have to study this attachment point a little better. Somehow the thought of having 400 pounds attached to something so easily tearable as F111 fabric doesn't make sense. I guess it works by spreading the load over a large area of the canopy.


We actually had the breakcord tied between two improvized short bridles made from standard webbing. I'd like to try it with round lines (instead of flat webbing) too, to find out if that has an effect. I doubt it though.


We'll be trying that for sure, although I think we're just going to see 160 versus 80.


No kidding! I had never seen one before, but those things are fun to play with! Then again, anything that allows you to break shit is fun to play with. Maybe that's why BASE is fun too.

I posted a couple things in a thread last summer that might be of interest here:

http://www.dropzone.com/...ost=1177752;#1177752

Mark

tfelber :
" While I would still like to know how much force should be applied, I quit asking because what's being done works."
................................................

That statement pretty much sums it all up right there.
If you are showed the proper way to Tie yourself off with Break-Cord
It will work all the time / every time./ You need to just, Go With It.
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