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5mm cord, Is it safe for anchor setups?
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By Eric Krantz
From Black Hills
Oct 28, 2012
smoke break, pitch 5 or 6 (or 7??) of Dark Shadows

H.. wrote:
pretty sure just because there is a single loop on the leg doesnt make the tension on the rope any different (unless you made each side independent with a clove hitch or something). little experiment you may want to try with fishing line: 10 lbs test + 15 lbs weight. Tie the line directly to the weight and slowly lift (SNAP). Make a single loop around the weight and lift (SNAP). Im no expert but...pretty sure a single loop doesnt increase the strength at all.


You should get together with 20kN and discuss how 1+2=4.


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By H..
From Washingtonville NY
Oct 28, 2012

Eric Krantz wrote:
You should get together with 20kN and discuss how 1+2=4.


I am not a physics expert, and my logic may be (probably) flawed, but my reasoning follows as such (with meaningless made up numbers);

100 lbs on a single strand = 100 lbs of tension on the rope.

100 lbs on a loop = 50 lbs pulling from each side = 100 lbs of tension on the rope.

After reading the paper with data comparing single strand and loops, I would appreciate someone explaining the flaw in my logic. It still doesnt follow the 'two sides to the loop = twice the strength' logic anyway, since it went from 3k-->5k for the 7mm nylon, with that being the largest increase in overall strength with the loop for what they tested.


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By ARowland
Oct 28, 2012

Tension does not add that way. Imagine hanging a 100 pound weight from a single strand. The weight exerts 100 pounds force downward on the cord. But in order to hold it up, the anchor has to exert a 100 pound force upwards. But, practically be definition, the cord only has 100 pound tension on it. Forces are considered individually, and are not added to the reactions they create.


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By Eric Krantz
From Black Hills
Oct 28, 2012
smoke break, pitch 5 or 6 (or 7??) of Dark Shadows

H.. wrote:
50 lbs pulling from each side = 100 lbs of tension on the rope. After reading the paper with data comparing single strand and loops, I would appreciate someone explaining the flaw in my logic.


For a single loop, 50 lbs pulling on each side makes one hundred pounds force on the object. Don't confuse that with tension in the rope. The tension in the rope is still 50 lbs.


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By H..
From Washingtonville NY
Oct 28, 2012

Eric Krantz wrote:
For a single loop, 50 lbs pulling on each side makes one hundred pounds force on the object. Don't confuse that with tension in the rope. The tension in the rope is still 50 lbs.


Tug of war: 50 lbs pulling left, 50 lbs pulling right... not 100 lbs of tension in the rope? Is that where I am being dumb?

Thanks for the explanations btw, its been bothering me for some time.


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By ARowland
Oct 28, 2012

H.. wrote:
Tug of war: 50 lbs pulling left, 50 lbs pulling right... not 100 lbs of tension in the rope? Is that where I am being dumb? Thanks for the explanations btw, its been bothering me for some time.


Yes, this is where. It might be easier to think of the situation where you pull with 50 lbf and the other end is tied off to a rigid object. It's still 50 lbf.


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By michaeltarne
Oct 28, 2012

Another easy way to think about it- a single loop (in this sense) is the same as having two strands. One 50lb weight on a string means 50lbs of force. If you add a second strand (or make it a loop) each strand only has to hold up half the weight, or 25lbs.


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By Ed Rhine
Oct 28, 2012

I would not personally use it for anchors or a cordulette, but I have totally used 5mm cord before for raps stations on alpine climbs and feel there is absolutely nothing wrong with that.


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By James Arnold
From Chattanooga
Oct 28, 2012
Chew toyed

1+2 does = 4, for greater values of 1 and 2

To answer the question, I would avoid it, but, as Ed alludes to, it will hold "bodyweight"...


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By Locker
From Yucca Valley, CA
Oct 28, 2012
...

"5mm cord, Is it safe for anchor setups?"




...
...


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By John Husky
Oct 28, 2012

I think that there is a lot more to it than this. Physics was 20 years ago, but:

If the rope was 2 knotted ropes at the biners instead of a clipped loop, I think the forces would be different.

The force where the biner contacts the sling is eventually on a single strand. There is one loop of material making two strands between the biners, but we are still talking about a single strand.

I recall from John Long's book that there is a difference in the way a nylon loop acts versus spectra in this situation. I don't know the physics behind it, but I think some imput from some one who does would be helpful here.

They sell 8mm perlon right at the gear store you know.


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By Eric Krantz
From Black Hills
Oct 28, 2012
smoke break, pitch 5 or 6 (or 7??) of Dark Shadows

John Husky wrote:
I think that there is a lot more to it than this. Physics was 20 years ago, but: If the rope was 2 knotted ropes at the biners instead of a clipped loop, I think the forces would be different. The force where the biner contacts the sling is eventually on a single strand. There is one loop of material making two strands between the biners, but we are still talking about a single strand. I recall from John Long's book that there is a difference in the way a nylon loop acts versus spectra in this situation. I don't know the physics behind it, but I think some imput from some one who does would be helpful here. They sell 8mm perlon right at the gear store you know.


Over a carabiner (or anything causing an angle or curve in the rope), the outside of the loop radius has relatively higher tension than the inside radius. The outside gets stretched, and the inside gets scrunched together. Because of this inequality (a portion of the rope supporting more than it's fair share of load), the overall failure strength goes down. That is also why ropes fail at the knot.

Edit: I don't know about spectra vs nylon, but I would not be surprised to find that the spectra core cuts through the nylon sheath at high loads and sharp bends. It would super cool to slow-mo film a rope failure, kinda like those ones with bullets hitting stuff.


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By Jim Titt
From Germany
Oct 29, 2012

John Husky wrote:
I think that there is a lot more to it than this. Physics was 20 years ago, but: If the rope was 2 knotted ropes at the biners instead of a clipped loop, I think the forces would be different. The force where the biner contacts the sling is eventually on a single strand. There is one loop of material making two strands between the biners, but we are still talking about a single strand. I recall from John Long's book that there is a difference in the way a nylon loop acts versus spectra in this situation. I don't know the physics behind it, but I think some imput from some one who does would be helpful here. They sell 8mm perlon right at the gear store you know.


The theory is simple really but suprisingly hard to describe.
If you hung a length of cord (or tape) which had no friction and no bending resistance over an object and then weighted both sides with X the cord would be loaded by the weights combined or 2X.
However cord or tape over a karabiner are neither frictionless nor free from bending resistance (usually lumped together as the resistance factor, in fall force theories this is called the karabiner factor) and so the load on the cord at the top of the karabiner is X divided by the resistance factor on either side. More simply not all the load on either side gets to the tape at the top.

In practice with tape there is something else to consider. Tape is tested over large drums to obtain the true strength of the material and this is strength is reduced over the smaller diameter of a karabiner. If we used 15kN rated nylon tape over a karabiner we reduce this rating by for example 15% so now it is a 12.75kN tape. If it was all frictionless thatīs all combined force the two sides would hold. However using a resistance factor of 1.7 it would hold a total of 21.67kN and if you look at a sewn nylon sling with three tracer threads (the marking for 15kN) you will find this is near its rating.
(Contrary to what one would think nylon tape has a worse karabiner efficiency than rope despite it feeling more flexible).

Dyneema etc are more difficult as they are slipperier (particularly internally in the material) and you find the loop strength as a proportion of the single strand strength is comparatively lower.

And if I used a cordalette (which I donīt) it would be 8mm cord!


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