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General Climbing Discussion

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Double, twin or single rope ...? 19-May-2004 At 7:49:56 PM shmalec

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Hmmm….good to see such a humble group of contributors.

Regarding the impact force of two half ropes vs one…..as I understand it the area under the force vs elongation curve must equal the energy required to arrest the climber. In an ideal frictionless world this drives the peak tension on the rope. If the rope was an ideal spring the force would be proportional to elongation. This would mean that two ropes each taking half the energy of the falling climber would have a total force equal to 1.4 times the force of a single rope taking the same energy. This is the idealisation.

In the real world, some of the energy is absorbed in the knot on the harness and friction when the rope rubs on the biners. I would think that two figure of eight knots would absorb more energy than one, and two ropes would probably cause a bit more friction than one. So that’s a little less energy the rope has to absorb and a little lower impact force.

On the other hand, I doubt the rope acts as an ideal spring (ie force is not proportional to elongation) which is likely to increase the impact force a little. The more you reach the rope’s material limits the less like an ideal spring it will be.

Now, as far as high fall factors go on half ropes, all I know is that they are tested to something like 55kg at a fall factor of 1.7. If you weigh 90kg like Andrew and take a fall of 1.7 fall factor, you are now applying 160% of the UIAA loading. Now, I don’t plan on doing that unnecessarily based on a bunch of opinions from guys on a forum. If your first bit of gear is a good one, you can avoid this by clipping both ropes. Use separate biners if you like, although if both ropes are going from belayer to your first piece of gear and then to you I can’t see why you would need to (unless the ropes are different diameter).

Can I suggest a healthy bit of scepticism from anyone reading advice based on technical info like this posted on a climbing forum. Anyone can spout “expert” advice on a general forum like this having read 2 or 3 pages of a high school physics book.