Rope length vs. strength

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Rope length vs. strength

Postby Patrick Wilson » Jan 11, 2013 1:02 pm

I figure this is probably the best place to pose the question. Is there any relationship between the length of a rope and its strength? Say a 100 foot section versus a 300 foot section: when both of those are weighted under load, does one have an "advantage" over the other, so to speak? Does my question make any sense? :shrug:
Last edited by Patrick Wilson on Jan 11, 2013 2:31 pm, edited 1 time in total.
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Re: Rope length vs. strength

Postby Chads93GT » Jan 11, 2013 1:06 pm

The question makes sense but I don't see how it makes a difference. The breaking strength is the breaking strength. Wether it's 42' or 4200'
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Re: Rope length vs. strength

Postby Patrick Wilson » Jan 11, 2013 1:11 pm

Maybe it's not so much the strength I'm meaning to ask about, let me rephrase this: When using a longer rope, are you spreading out the load over more rope as opposed to a shorter rope? Or to put it another way, is a section of PMI Pit Rope loaded with 230 lbs of caver & gear in Golondrinas at the bottom of the drop under any more "strain" than the same scenario in say Neversink?
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Re: Rope length vs. strength

Postby Bob Thrun » Jan 11, 2013 1:30 pm

Patrick Wilson wrote: Or to put it another way, is a section of PMI Pit Rope loaded with 230 lbs of caver & gear in Golondrinas at the bottom of the drop under any more "strain" than the same scenario in say Neversink?

No, the entire rope will be loaded the same, except the top section will will have the additional weight of the rope below. If the caver is at the bottom of Golondrinas, the bottom will have a 230 lb load and the top will have a 290 lb load.
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Re: Rope length vs. strength

Postby GroundquestMSA » Jan 11, 2013 1:36 pm

Patrick Wilson wrote:Or to put it another way, is a section of PMI Pit Rope loaded with 230 lbs of caver & gear in Golondrinas at the bottom of the drop under any more "strain" than the same scenario in say Neversink?


Practically, I don't think so. Theoretically yes. The weight of the rope itself adds to the load. In Golondrinas the rope will add 80lbs. or so to the load. A free hanging piece of PMI Pit 900,000 feet long would be in danger of breaking under it's own wieght, so while it has the same strength as a short rope, it's using it all holding itself together.



*Bob beat me to it, and actually knows what he's talking about.
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Re: Rope length vs. strength

Postby Patrick Wilson » Jan 11, 2013 2:28 pm

Thanks for the replies. Honestly I was fishing for ways to explain this to some of my friends that go on-rope with me but have reservations about doing deeper pits. They posed that to me and I wasn't quite sure how to answer it, hopefully this will enable me to formulate a better answer to help them feel comfortable on longer drops.
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Re: Rope length vs. strength

Postby rjack » Jan 14, 2013 11:58 am

If you load the rope dynamically at all (which in small bits is inevitable) then longer pieces are better. A 1ft drop onto a 4ft piece of rope is a fall factor of 1/4th. A 1ft drop onto a 100ft rope is a fall factor of 1/100th. In an ideal world you'd load all ropes evenly and slowly of course so this wouldn't matter.
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Re: Rope length vs. strength

Postby knudeNoggin » Jan 15, 2013 1:38 am

GroundquestMSA wrote: A free hanging piece of PMI Pit 900,000 feet long would be in danger of breaking under it's own wieght, ... .


Just one of the reasons I prefer shorter lengths.
Btw, what would you hang it on?

:tonguecheek:
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Re: Rope length vs. strength

Postby Mike Rz » Jan 15, 2013 12:17 pm

it sounds like your friends are confusing the strength of the rope (which is constant) with elongation (bounce). Elongation is directly proportional to the length. for the same load, a rope which is 1300 feet long will stretch 10x more than a rope which is 130 feet long.
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Re: Rope length vs. strength

Postby GroundquestMSA » Jan 15, 2013 5:18 pm

knudeNoggin wrote:
GroundquestMSA wrote: A free hanging piece of PMI Pit 900,000 feet long would be in danger of breaking under it's own wieght, ... .


Just one of the reasons I prefer shorter lengths.
Btw, what would you hang it on?

:tonguecheek:


I see you aren't familiar with the deep caves of Ohio...
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Re: Rope length vs. strength

Postby wyandottecaver » Jan 15, 2013 8:17 pm

just to state a little more explicity what was already mentioned, even "static" ropes tend to have some stretch, and the longer the rope, the more stretch. That stretch does not impart greater strength to a rope holding a motionless load. It DOES impart some small resistance to rope failure and much much more importantly some capability to absorb rapid deceleration, under a moving load.

Most important of all, if your friends have reservations about doing deeper pits, assuring them of the rope strength is the least of your concerns! Increasing their competence, fitness, and confidence is. Some nervousness is natural, but coaxing someone with serious reservations down a deep hole is inviting trouble.
I'm not scared of the dark, it's the things IN the dark that make me nervous. :)
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Re: Rope length vs. strength

Postby eyecave » Jan 29, 2013 2:15 am

ok......as a rope breaks, the first thing that would happen would be the absolute stretch would come "out" and the tensile breaking strength would then be exceeded..... :banana_yay: ...so, i am going to go out on a limb here and suppose that a longer heavier more stretchierouttier rope would be under more strain and in danger of a fatal seperation of nylon occuring above me.......remember momentum and rope bounce here to....that only supports my radical suppositions.. :banana: ..

and as for those who fear exposure....ie "long drops"....or long ropes........your hips bones will shatter before the rope breaks..... :sadbanana: ..just repeat that mantra and remind them to focus on their attachment to the rope, rope dynamics, and their equipment procedure checklists......
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