SL-Carry with you setup knot tests

[sum1sneaky]

Posted over at dz.com:
http://www.dropzone.com/cgi-bin/foru...177643#1177643

I just was wondering about the knots etc... so I did some testing.

I was doing a bit of testing with one I made out of 850lb dacron and found some interesting stuff about knots.

I've always tied my break cord in a circle using repeated square (surgeon's) knots - and this is what I've understood base689 to be using in the setup for the carry-on SL system.

This knot is pictured in the first image (sl-carry-knot1.jpg) and the second (test1_knot.jpg). In all the pics, I've substituted the the bridle with a carabiner for simplicity of loading etc. I then piled weight onto the setup, very slowly and carfully so that no acceleration except for gravity is applied to the system. In this first test, the breakcord broke just after I took the picture (test1_hang.jpg). Go ahead and make fun of my weights. Then I piled all this junk onto the scale = (test1_weight.jpg) and the weight was 105 lbs at break time! The carry-on worked flawlessly and the breakcord broke in the middle of the loop.

That seemed like a lot more than the 80 lbs it's supposed to be, so I decided to try another method, thus test2. In this test, the breakcord is tied to each end of the carry-on rig, and care would have to be taken to make sure there's not a lot of friction at the bridle attachment point (or else the force would be doubled!). The knots are detailed in (test2_knot.jpg). The weight is applied (test2_hang.jpg), and just after the last rock was added and that pic taken - it broke (test2_break.jpg). This time, the cord broke with 98 lbs. (test2_weight.jpg) - 7 lbs less than the previous method. This method would have a problem if the bridle bound the breakcord in any way - where as the circle/loop method used in test1 would always be free to break at the attachment to the carryon device.

I also though someone might try to lower the force required by tying one end of the breakcord around the carryon device and the other to the bridle - leaving just one strand of breakcord in between. (test3_knot.jpg) This would be a BAD thing however, as when the cord breaks - the loop will still be tied to both sides of the carryon rig (test3_break.jpg) and force will then be doubled or worse.

I'll be using it with the first method because it seems to have the lowest propensity for error. However, I would prefer it if it broke with less weight - any ideas folks?

I would post the pics here, but they're too big (59K each) and the limit for this board is 39K - why so tiny?

[BASE_689]

> I've always tied my break cord in a circle using repeated square (surgeon's) knots - and this is what I've understood base689 to be using in the setup for the carry-on SL system.
Yes indeed, the setup in your test 1 is EXACTLY what I am using, as well as your "carry-with-you-SL" is exactly the same as mine (except the length: mine has got shorter branches).
And the connection bewtween the "carry-with-you" part of the system to the bridle (=carabiner in your case) is EXACTLY the same I use.

> test 1: break at 105 lbs
> test 2: break at 98 lbs

Forget anything about test 3: is BLACK DEATH for the reasons you wrote.

You said that in test 1 the break cord broke exactly in the middle. Yes, of course it can happen, but the weakest point in a loop (of break cord) where there is a knot for forming (=closing) the loop, is the knot itself.
So, in a test where 20÷30 tests are performed, you should expect that in 80% (I am guessing, here) of tests breakage happens in proximity of knot.
That's why we bother so much closing the loop with a surgeon knot and NOT with "ANY" knot.
We know that surgeon knot simply distributes the load (transmitted through the cord) over the longest distance possible within a knot, and doing so, we should expect, with a surgeon knot, that the break load should be just slightly close (=slightly lower) than the break load onto a loop without knot (a loop formed through seam, for example; we cannot carry a sewing machine onto the exit, that's why we use surgeon knot).

Set up of your test 2 loads the break cord exactly with same forces as on test 1, with the negative (my opinion) that now in test 2 there are 2 knots.
Granted that the knot itself is the weakest point in the loop, granted that we should expect the break cord to break in proximity of knot (because of sharp bends the cord undergo along the knot), in this test 2 you introduce 2 weak points instead of 1.
Moreover, once onto exit point, with test 1 setup you have got to do only 1 surgeon knot, sometimes, fear (=always!) and unconfortability of exit point environment makes the job of doing the surgeon knot, a not so easy task.
With test 2 setup you have got to do 2 surgeon knots, very tight, yes, so the task would become more difficult.
Now I do my personal comment on the difference in break load between the 2 tests.
Break cord is rated 80 lb - 36 kg. Yes. This is by far NOT a perfect value. It is NOT that at 79 lb - 35.8 kg everything is perfect and at 81 lb - 36.7 kg everything breaks apart.
The 80 lb - 36 kg "nominal break load" should be read as follows: if you do a loop with the nominally rated 80 lb - 36 kg break cord, you should expect that your loop would break, in every single test you perform, under a load of 80lb-36kg ± Xlb-0.4536·Xkg. Only manufacturer knows what this "uncertainty" of X lb is.
80 lb - 36 kg should be meant as an "average" break load.
I justify the difference of 7 lb - 3 kg of your single trial for test 1 and for test 2 simply as the normal differences you get from one test to the successive one on same setup.
In a 500 yd - 457 m spool of break cord, you CANNOT expect that every single 10"-25cm piece along the whole spool has EXACTLY the same break load as the rest, i.e. 80 lb-36 kg. In the 500 yd-457 m spool of break cord, any single 10"-25cm piece will have a break load of 80lb-36kg ± Xlb-0.4536·Xkg. It's called "variance". Any single part of a physical system does NOT have EXACTLY the same characteristics as the rest, but there is a (minimal) difference from part to part.
To be able to do a real comparison between test 1 setup and test 2 setup, you should perform at least 20 tests on setup 1 and write down the values, calculate the average value, then perform 20 tests on setup 2 and write down the values, calculate the average value, and FINALLY do a real comparison between the average values.
But, not to be negative, apart from numerical values, I keep on using setup of test 1 and never going to use setup of test 2 for the reasons I wrote above.
Anyway, to comment the basic results, I never measured with a scale the break load of the break cord we use.
Yes, breaking at 100 lb - 45 kg instead of breaking at 80 lb - 36 kg actually means the actual break load is 25% higher than the nominal value of 80 lb-36 kg.
Actually, I do not know if the rated value of 80 lb - 36 kg is for a dynamic break or for a static break (which is the sort of test you made).
But, in the end, IMHO, I think that a break load around 100 lb - 45 kg is acceptable for our purposes.
Anybody else to make a comment on this "higher than expected" break load of break cord we all use?

[346]

I posted this reply to the thread on dropzone.com yesterday and thought it would be a good idea to post it here, too:

MIL-T-5661 Type I Plain Weave cord is designed to have a breaking strength of 80 lbs (minimum). That breaking strength is measured with one piece of material pulled in a straight line. Breaking strength will be reduced by any bends or knots used.

The knots you have in Test 1 and 2 use a "loop" of the material, that is, two pieces. In theory, that would take the breaking strength to 160 lbs. As you found in your tests, the material's strength is reduced (and is weakest) at the knot, so that is where it broke (your test2_break.jpg). A 97lb breaking strength indicates the knot reduced the strength by 63 lbs or about 39%. That's par... Poynter's Manual, Vol 1, Section 8.6 indicates a 40% reduction can be expected with a bowline knot.

I'd go with Method 1 as well - It keeps the whole sling from sliding around. And as you mentioned, I'd avoid method 3 like the plague...

Mark

[BASE_689]

In response to Nick D.G. post onto Dropzone - The BASE zone:
Also consider that the application of force on the breakcord is not a static load, like the test method shown. Your all up weight is accelerating away from S/L at the moment that weight is applied to the breakcord. I'm not a mathematician, but this increases the forces applied considerably.

I never seen, or heard of, a BASE jumper in tow (I guess we need a new phrase there) but the opposite, a premature parting of the breakcord, has occurred too many times.

In the same vein, if you compared how we packed for BASE in the old days, when free packing was the rule, and how it’s done now, it’s like night and day. We used to be afraid of any type of reefing that could possibly (even though it was imaginary) hang up the opening sequence. We’d pack thinking, open, open, open. When we should have been thinking, don’t open, don’t open, don’t open. We learned the hard way it’s easier to pack in a way that could blow your stuff up, and much harder to do it in a way where it wouldn't open at all.

One of the benefits of BASE specific gear, and BASE specific jumpers, is we don’t see much canopy damage anymore. I ripped the center cell rib completely out of a Cruiselite once time, and it scared me badly, but there were no BASE canopies available at that time, so I sewed it back together and kept on trucking . . .

Nick
BASE 194


I answered:
> ...consider that the application of force on the breakcord is not a static load... ...your all up weight is accelerating away from S/L at the moment that weight is applied to the breakcord...
> ...but this increases the forces applied considerably
Well, let's see.
Our body in freefall WOULD exert a high force.
Why "WOULD"? Simply because before break there no force applied anywhere (= not to a fixed point), gravity force is pulling you down and you accelerate, after break again gravity continues to pull you down.
Your body does exert a force ON A STATIC/FIXED POINT only in the few split milliseconds you load the break cord and it breaks. Before putting tension on break cord and after break, there only gravity with no other force that opposes to it, simply it accelerates you.
The bottom line is the above: even if your accelerating body would be capable of exerting a high force onto a fixed ppoint, the break cord cannot hold yourself more than its proper break load.
Rather, I think an interesting argument is the following.
Granted that 100 lb - 45 lb applied statically to break cord loop breaks it, what would happen to our break cord loop if a device capable of applying a variable force (=your body in freefall) actually start exerting an increasing force onto the break cord loop?
When would the break cord loop break if such a variable force is applied dynamically/instantaneously to it?
Would the break cord loop dynamically break BELOW 100 lb - 45 lb?
Would the break cord loop dynamically break ABOVE 100 lb - 45 lb?
I think the dynamic test can only be performed in a laboratory, putting break cord strings between the clamps of a dynamometer and setting it to pull at a fixed speed (perhaps few test at different speeds) and having it to record the load applied between the clamps, up to break of loop. And having a look at the results, especially reading the load AT break.
If you take a 100 lb - 45 lb and apply it gradually to a break cord loop, at the very end of releasing it breaks the break cord loop.
Given for granted that if you step "very fast" over a brick on the pavement, you exert onto brick twice your weight (=dynamic load) just for a split millisecond and then you exert just your weight, if you step "very slowly" over a brick, at the end of slowly process of releasing/loading weight you end up with your weight onto brick.
So, if you take a 100 lb - 45 lb load and release it istantaneously onto a break cord loop, it WOULD exert for a split millisecond twice its weight, but simply our break cord loop cannot hold such a weight and it breaks exactly AT its (dynamic) break load.
It would be really interesting to know if and how much the dynamic break load differs from the static break load of our 80 lb - 36 kg break cord.
If you don't believe to the fact that instantaneously applying a weight onto a surface, the surface itself bear for a split millisecond bear a load twice the weight of the object, try the following.
Take the scale you got at home.
First test: put only one foot onto the scale and very slowly transfer your weight from the foot on pavement to the foot onto the scale: you will notice that the indicator on the scale increases up to your weight, without never exceeding your own weight.
Second test: put only one foot onto the scale and transfer IMMEDIATELY, as fast as you can, your weight from the foot on pavement to the foot onto the scale: you will notice that the scale indicator will exceed your weight (you should read for a while twice your weight, but system itself could be not so fast in "chasing" the actual (maximum) weight value) and then, oscillating it will stabilize onto your weight.

[BASE_689]

And to your post, Mark, I replied:
> MIL-T-5661 Type I Plain Weave cord is designed to have a breaking strength of 80 lbs (minimum).
> That breaking strength is measured with one piece of material pulled in a straight line.
> Breaking strength will be reduced by any bends or knots used.

WOW!!!!!! Really breaking load is with ONE SINGLE PIECE OF MATERIAL CONNECTING TWO PULLING HEADS?!?!?!?!?!?
Until I read your post, I ALWAYS thought that, just because ALL THE BASE MANUFACTURERS were talking about 80 lb break cord and because our use in BASE (for SL jumps) is to tie such a break cord in a loop, the 80 lb breaking load was referred to the break cord ALREADY SET IN A LOOP!!!!!!

So, finally, I discover that what we get is NOT the 80 lb break cord being little more robust than the "nominal" value, but actually is the opposite way around.
What we actually rig for SL jumps is a 160 lb - 73 kg break cord but because the (surgeon) knot deteriorate (=sharp bending radius) 40% (I didn't imagine the deterioration was so HIGH) of the characteristics of the cord itself, we get something like:
0.6 · (160 lb - 73 kg) = 96 lb - 44 kg
That's how comes the 100ish lb value of the tests of sum1sneaky!!!!!
I justified the results coming from the tests starting from the wrong break load value!!!!!!!
It's true: you never stop learning!!!!!
But, honestly, I think that the BASE manufacturers SHOULD inform buyers of break cord about the real value of break load (so when you do a loop you simply get twice the break load) and about the high deterioration (40%) of doing the (surgeon) knot.
I mean: it SHOULD be way more evident and way more clear in our world what we are EXACTLY using!!!!
Thanks anyway, Mark!!!!