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I'm looking into buying a rope for rock climbing, and I know that I need a dynamic rope for lead climbing. However, trying to find information on the elongation percentages of ropes is proving tricky.

Some sources say that an elongation of 6.5% is typical for a dynamic rope, but others put this number at around 30% (with a maximum allowed of 40%). The rope I'm looking at is a 6.5% elongation, and the salesman says it's dynamic. However, some websites reference static ropes at "under 10% elongation" while others say "1-5%".

Can someone help clarify?

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This should not have been migrated. It's about sports equipment, not physics theory. –  roviuser Jan 29 '13 at 14:53
    
Agreed, this is a question about gear for an outdoor activity, which this Stack Exchange answers all the time. The elongation is one of the basic metrics of a climbing rope, like the temperature rating of a sleeping bag. –  DavidR Jan 29 '13 at 16:25
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migrated from physics.stackexchange.com Jan 29 '13 at 15:37

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3 Answers

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You are apparently confusing static and dynamic elongation. You must use rope that is UIAA-101 / EN-892 certified for the task at hand. You will be buying a Single type rope unless you specifically know otherwise.

Quoting Beal Ropes:

Dynamic elongation:
This is the stretch of the rope during the first UIAA test fall. It must be less than 40%.

Static elongation:
Measured under a load of 80kg it must not exceed 10% for single rope, 12% for double rope, and 10% for two strands of twin rope together.

So you see the static elongation must be no more than 10% for a Single climbing rope, yet to meet the impact force requirements the dynamic elongation is going to have to be much higher than that (though it may not exceed 40%).

Impact force

Values required by the Standard:

  • Single rope: Impact force lower than 12kN holding the first factor 1.77 fall with a mass of 80kg.
  • Double rope: Impact force lower than 8kn holding the first factor 1.77 fall with a mass of 55kg.
  • Twin rope: Impact force lower than 12kN holding the first factor 1.77 fall with a mass of 80kg, on two strands.

The impact force printed in the technical notice must not be lower than the poorest result found by the certifying laboratory.

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First of all, DO NOT buy a rope or ANYTHING for that matter that you will trust your life to, purely based on what a single salesperson says, or what I say. Find a few experienced climbers (at least 5+ years of experience each), preferably certified guides and ask them their opinions. Now on to your question:

There is ambiguity in this because apparently ropes are labeled by their percent elongation for 'static' and 'dynamic' testing/use, in addition to themselves being called static and dynamic ropes. That is, you can have a rope that is static (meant for top roping, hauling, etc NOT for lead climbing) or dynamic (designed for lead climbing) and any rope from either category can be testing for 'static elongation' and 'dynamic elongation'. i.e. static ropes will have crappy dynamic elongation by definition.

See the petzl nomad for example:

http://www.petzl.com/us/outdoor/verticality/dynamic-ropes/98-nomad

Static and dynamic elongation is precisely defined here:

http://www.rei.com/learn/expert-advice/ropes.html

In other words you should ask the salesman if he is talking about static or dynamic elongation. 10% dynamic elongation is certainly not enough for lead climbing considering the Nomad has 30% dynamic elongation and that is certainly a lead climbing rope.

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I don’t think you can go wrong with any UIAA-certified climbing rope, can you? –  zoul Jan 31 '13 at 7:40
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You have a couple of perfectly good answers here from DJBunk and Mr.Wizard. However, it appears from your question that you need a better understanding of why or how you would buy a rope with certain specs in terms of elongation. The elasticity of a dynamic rope is a compromise. If it's too elastic, then when you take a fall with a high fall factor, you could end up hitting the ground. If it's not elastic enough, then it will stop a fall so abruptly that you'll be get hurt by the force of your harness on your hips.

For example, say you're just starting the second pitch of a multi-pitch climb, and your belayer's station is an anchor on the side of a vertical cliff. You climb some distance above your belayer, and then just as you're about to clip into your first protection, you slip and fall. In this situation, your fall factor is 2 (the highest possible), there is nothing for you to hit, and you're going to fall past your belayer. If you had a choice in this situation, you would choose the most elastic possible rope, since it would decelerate you as gently as possible.

On the other hand, suppose you start climbing from ground level, clip into your first protection at the first opportunity, 3 meters off the ground, and then after another 2 meters of climbing you're getting ready to clip into your second opportunity for protection. It was unwise to get yourself into this situation, because if you fall while trying to make this second clip, the rope will only come up taut while you're 1 meter above the ground, and it will probably not stop you before you hit the ground. In this situation, you're wishing your rope was a little less elastic.

In reality, you only bring one rope for a day of climbing, and on any given day there may be some situations where a higher elasticity would have been better and others where you might have preferred a stiffer rope. You use a dynamic rope that's designed for this general type of lead climbing, and you carry out your climb in a style that is basically a matter of habit and tradition. These habits and traditions are based on the typical characteristics of a standard dynamic rope.

Suppose hypothetically that dynamic ropes weren't standardized, and you bought one that was more or less elastic than the typical rope. This would be bad and potentially dangerous for your climbing partner, who has built up habits and expectations based on experience with rope that has the standard specs. If you're talking about lead climbing in the gym, then the bolts on the wall are set up at heights so that with a standard rope, you're likely to be OK if you fall while trying to reach the second clip.

I don't really know of a lot of reasons to own rope that is more or less elastic than a normal dynamic climbing rope. The only example I know of is that if you're really into canyoneering, you might want to bring a static rope for a day of canyoneering, because you don't want a bouncy rappel, and the rope is never going to be loaded with more than your static body weight.

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Good commentary. +1 One point: elasticity and peak impact force are not directly correlated (inversely); you can find ropes that have moderate stretch yet have a "soft catch." I presume this is done by having a flatter force curve. Also, you might consider adding something about a dynamic belay as it is very relevant. –  Mr.Wizard Nov 2 '13 at 4:54
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