SL question

[whatever 0]

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I do not see how 160 LB are needed to break the cord.

80lb break cord breaks is 'calibrated' and tested to break at 80lbs of force, that is a single strand of the cord

when you tie that single strand into a loop, you have distributed the force applied evenly to two strands of break cord, assuming it can slide a bit around the tie-in points to equalise the load

therefore, 2 x 80 lb = 160 lb

until about a minute ago I never checked for myself what the break cord I have tests at and as I could not find a scale nearby, I just rigged up something like my standard staticline setup, a girth-hitched (around the anchor point) loop of 5mm static cord with a rapide-link connecting to loop of break cord (which I knot with a surgeon's knot), I used another rapide-link and 5mm static cord loop, but doubled to be easy on my hands, on the bottom side of the break cord loop

I then pulled with all my might and managed to just lift myself of the ground before the break cord did it's job and broke.

I weigh 169lb.

It's a fun little experiment to try at home, just be selective when choosing an anchor point and don't get hit in the head by a rapide-link!

further to this discussion, be aware that there are several other factors that will affect exactly how much force it will take to break the break cord.


here are the main players:

-the size, shape and surface finish of the load points that contact the break cord

-the knot used to tie the break cord

-the amount of stretch in the anchor side of the system and the amount of stretch in the other side of the system

-the speed with which the load is applied


for the most part, if you keep it sensible (i.e. not using razor as contact points) these factors will not have a big influence on the force required to break the cord

caveat: all the numbers used here are approximate, do not be suprised by a variance of a few pounds either way

have fun!
sam

soon to be gone

[GaryP 0]

Hi 903 -You have posted 2 different setups/pic's.

Your 2nd pic shows equal force applied to both ends of the break-cord. I agree that there is no mech advantage with this setup.

Your 1st pic shows applied force travelling up the line and around the anchor point and back down to the load (break-cord). It does not travel up the line and then do a turn along the break-cord, it pulls from a point that has a mech advantage of 2.

I know it "looks" as if everything is static but it's the unseen "load" that is the determining factor.

Your 1st setup is similar to a climber being belayed with his foot caught in the rope as it's being pulled down by the belayer.

Anyway - to make absolutely sure of my comments I'm going out to get me some spring scales myself - maybe I'll be back tomorrow to eat some pie , just maybe.

g.

"Altitude is birthright to any individual who seeks it"

.

[pBASEtobe 0]

I agree. The first picture shown does have a 2:1 mechanical advantage, requireing 160 lbs. to break, whereas the second picture doesn't. This is because in the first picture only one free end is connected to the break cord. In the second picture both free ends are connected. This minor variation makes a BIG difference.

[base311 0]

Quote

Quote

I do not see how 160 LB are needed to break the cord.

80lb break cord breaks is 'calibrated' and tested to break at 80lbs of force, that is a single strand of the cord

when you tie that single strand into a loop, you have distributed the force applied evenly to two strands of break cord, assuming it can slide a bit around the tie-in points to equalise the load

therefore, 2 x 80 lb = 160 lb

(stuff snipped)

have fun!
sam

Man if I had a dollar for everytime I've seen this argument go down (btw, I'm with you)...

have fun!
Gardner

[tfelber 0]

From all the setups I've seen 80lb break cord is used to apply a force of 160lbs prior to breakage. Should we be applying 80lbs or 160lbs in a static line setup?

[sabre210 0]

This discussion is also current over at Blinc in the technical forum

http://www.basejumping.org/forum/showthread.php?t=23485

In a nutshell for those who don't venture over there, what tests point to is that a single strand of 80lb breakcord gripped and put under tension from both ends will break at 80lbs. However, for us to use this cord we have to introduce knots into the system which weaken it by an estimated 40%, as the knotted point creates a sharp edge which causes fibres in the breakcord to sever rather than snap (that's my understanding of it anyway) Therefore a single strand of 80lb breakcord will fail at approximately 48lb. By looping the breakcord we create 2 branches of 80lb = 160lb as stated but then factor in the weak point at the knot and you have a failure at approximately 96lbs which is closer to the optimum unknotted cord than the 48lb single strand. Taking into account that no jumper has as yet pulled an object down with them, but many have experienced premature breakcord failure putting them into a form of freefall assist, it makes sense to err towards the stronger setup than the weaker. As we only tend to static line lower objects, a breakcord failure at these altitudes can ruin your day.

Dexter, Base689, Sum1sneaky, Ray, Tom, riggersam and many others have added valuable and frankly life and time saving advice on this subject in many other theads here at dz.com and blinc.

As Gardner says, this little gem comes round about as often as the paperboy.

ian

[tfelber 0]

I went to Blinc shortly after posting my question, but no one is saying what kind of force we are trying to apply in doing a static line. There are several other factors involved in calculating the force that is actually being applied, for instance a steadly increasing force over a longer period of time produces less instantaneous force than a sudden jerk like a SL jump.

How is break cords strength tested? What's the minimum/maximum force that we should be applying?

I've seen some really nice CWY SL setups in the three current threads, but none of them are applying 80lbs of force to the bridle connected to the canopy. If this has been discussed repetitively I would think there was a general (not exact) consensus on the forces involved, correct?

[DexterBase 0]

Here's a post I made in the other static line thread. It details the system I have been using very successfully.

[GaryP 0]

This I like ! NOW I'm happy

...and what's best - it's simple.

In Eastern Oz our most jumped S has an area about 15metres long at the exit pt along the railing with MULTIPLE layers of electrical tape on it. I was once pushing for ppl to at least use WHITE tape (white painted railing) to minimise the visual impact but found myself to be the worst culprit in the end. The council wokers clean it off every 3 months or so but it is still a real eyesore.

This system would really minimise this mess.

Onya' DexterBase mate
g.
AB86

"Altitude is birthright to any individual who seeks it"

.

[baseclimb 0]

Yo Dude,

As we had this discussion before, I might just add my mix to the ramble.

I use single static line setup ( as opposed to loop system) , doubled with the same setup, about 10 - 15 cm further up the bridle, to provide a backup, with a 46 or 48" PC.

Quote

However, for us to use this cord we have to introduce knots into the system which weaken it by an estimated 40%, as the knotted point creates a sharp edge which causes fibres in the breakcord to sever rather than snap (that's my understanding of it anyway) Therefore a single strand of 80lb breakcord will fail at approximately 48lb. By looping the breakcord we create 2 branches of 80lb = 160lb as stated but then factor in the weak point at the knot and you have a failure at approximately 96lbs which is closer to the optimum unknotted cord than the 48lb single strand.

Now, 40% loss in strength? What knots are you using? As I have done my test previously, and just done them again, twice, I found that with the knot I use, the break cord always (always: relative, isn't it?) breaks above 34kgs ( or 74.957 lb's).
Test 1. Lifted 34kgs. then with same break cord, lifted 35kgs or 77.162 lbs, and broke.
Test 2. Lifted 35 with fresh break cord for a while, put it down, then lift it again, not broken.

Now, 74.9 and 77.1 does not give me a loss of 40%. To me, this resulted in a 93-96% strength.

Also, one of the first things I did when I bought my rig, was to test the pin tention. Yes I like mine 'soft', and it not even close to 8lbs'. Lets say I go with the about 20-30 lbs of force, it leaves me with 45lbs plus, plus the second system of another +73lbs, plus the 46" - 48" pc. If something goes wrong here, I think it my time then.

Knots used on both ends, is like a figure of 8, with an additional full loop, before completing the knot.

Previous results, was with standard figure of 8 knot.

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Taking into account that no jumper has as yet pulled an object down with them, but many have experienced premature breakcord failure putting them into a form of freefall assist, it makes sense to err towards the stronger setup than the weaker. As we only tend to static line lower objects, a breakcord failure at these altitudes can ruin your day.

If you think/know/tested/don't know your setup, and think/know/don't know that it will break @ 40%, then I would propably do the same.

Other random thoughts:
We are happy with a human PCA, but how reliable will 'this setup' be in terms of strength.

I don't think any of my PCA buddies will be able to hold a 34kg snatch force with one bicep, one hand, and a bridle routed in-out @ the same end of the hand, and at the same time, provide me with a full duplicate system. Yet, we are happy with so much more force (subject to set-up)

Don't we also continually do these PCA's from the same objects /heigts as this over-kill SL?

A thought I had, was to set-up a single SL system on a PCA, and see if the break cord will break, before you PCA buddy let go. What will this test? Maybe not much, but the so called 'Other forces' might be identified, or their ghosts might get kicked.

As W has mentioned, what is the long term stress you put with an overloaded system on your bridle, attachment points, etc. Over engineering the SL system is not the solution for doing it wrong in the first place (thinkig of SL failures of the past, that makes people so paranoid.) , for now we introduce new stresses. I am convinced that all the SL setups that failed, weren't done 'right', for even now, we can't seem to agree on what is right.

Also, why do we go to such great lengths in getting the PC size for a FF right, when we are happy with such great forces in a SL?

Happy jumps in this weather Dude, and C-yah in France! Maybe we can give the Ski thing a go as well.

[TomAiello 25]

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Also, one of the first things I did when I bought my rig, was to test the pin tention.

What body position were you in when you tested it? You were wearing it, right?

Testing it on a table yields radically different numbers from testing it when worn, and especially when worn and curled into a tight tuck.

If you got less than 8 pounds with it all tightened down, I'm curious what the tension was like on a table.

-- Tom Aiello

[email protected]
SnakeRiverBASE.com

[baseclimb 0]

Yes, it was very low, but as I have stated above, lets say I opted for the 20-30lbs standard, their is still a lot to play with.

(For interesting sake, I think I need to do that pin test again, which, off-course would not affect my argument above, unless it is above 30lbs. I bloody don't hope is more that that. Also, being into areals, I don't plan in doing a tuck on my SL jumps )

Was also thinking last night, that the way people make their knots with break cord that is actually a flat webbing, as you would have got in climbing, is that thye don't feed the webbing 'flat' around a knot, as we did in climbing (Yes, No?) will result in loading the break cord on the one side more arount this corner, and hence, weakening it dramatically.

I pay a lot attention in doing the knot. Should actually to a few tests on badly woven knots as well.

C