Single and Half-Rope Impact Forces: Data!

November 30, 2006
Will Gadd

Jim Ewing is a bud of mine and the head rope guru over at Sterling. We’ve been emailing back and forth about single-rope impact forces vs. half-rope impact forces (as well as the discussion on this blog) for the last few months. Single ropes are tested with a nasty (1.77 fall factor) fall with an 80kg weight, half-ropes are tested with a nasty fall with 55kg. This weight difference has always struck me as odd–I weigh the same (about 85K all dressed up for winter climbing) whether I’m climbing on a single or half ropes, so why is there a different test? Half-rope technique is to generally clip the strands individually, so the impact will normally be on one strand… Furthermore, many people assume that because the “published” numbers for half rope tests show lower impact forces then using a half-rope will result in lower impact forces on a piece of protection (never mind the test weight is different…). Fortunately, Mr. Ewing has access to a drop-test tower and the knowledge to use it. He completed the following tests over the last 24 hours (he also reportedly did some training in the in-house Sterling cave…), here’s the data from Jim on “certified” half ropes tested as single ropes:

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Here’s the total picture.

Rope A. 80kg-7.35kN, 55kg-5.39kN, published with 55kg-4.85kN

Rope B. 80kg-8.15kN, 55kg-6.23kN, published with 55kg-6.3kN

Rope C. 80kg-8.23kN, 55kg-6.25kN, published with 55kg-6.5kN

Rope D. 80kg-9.22kN, 55kg-5.88kN, published with 55kg-6.1kN

These drops were conducted without the regulation conditioning but complied with all other requirements and procedures. Relative humidity was 42%, temperature was 20ºC for 48 hours.
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Jim also noted that his four test ropes were all new and from different manufacturers, so his data should offer a pretty good spectrum of what’s out there for half ropes tested as single ropes. This is the first solid data I’ve ever seen on half ropes tested as singles, thanks Jim!

Now the fun part: comparing single rope impact forces to half rope impact forces when tested as “single” ropes. Jim’s tests show half rope impact forces with an 80kg weight testing from 7.35kN to 9.22kN. Here are some numbers (taken directly from the BD and Sterling’s web sites):

BD “Joker” 9.1mm: 8.2kn
BD “Booster III” 9.7mm: 7.3Kn
Bd “Apollo II” 11mm: 7.7kN
Sterling “Nitro” 9.8mm: 9.0kN
Sterling “Pro”10.1mm: 8.6kN
Sterling “Mega” 11.2mm: 8.7kN

This range is from 7.7kN to 9.0kN; not a lot of difference from the Half rope range of 7.35kN to 9.22kN…

I draw five main conclusions from Jim’s data:

1. Half ropes likely do not offer significantly lower impact forces than single ropes in high fall-factor falls where one strand is clipped as is common.
2. Rope diameter alone is NOT a good indicator of impact force (some of the “fat” 11mm ropes offer lower impact force than the “skinny” single or half ropes).
3. The “published” impact numbers may not mean much (there’s a wide range between the published and actual in Jim’s data).
4. Terrain is more important for rope selection than impact force. If I’m heading up on a route with sketchy gear I may just use my standard single rope, simpler. A single rope with low-impact force may actually be better. But, for routes where the gear is all over the place then half ropes are likely better for less drag (and possibly less chance of both ropes getting cut…).
5. I’ve got a lot more questions than answers about rope stretch (elongation) with different fall loads–these fall tests are with a very harsh (1.77) fall factor. What happens with low fall-factor loads in terms of elongation and impact forces?

Thanks very much to Jim for working on this. I think this data is the kind we need more of in the climbing world–it challenges our assumptions about equipment in a good way. I don’t think there are many “absolutes” in climbing; the systems we use are surprisingly complex and sometimes very non-intuitive. The best we can do is to try and understand our gear as best we can, and then use what’s appropriate for the situation at hand. Even then we’re likely to get it “wrong” at least some of the time, so having a good margin for error is perhaps the most important part of the climbing process. In climbing we’re always trying to balance multiple different factors; speed vs. safety, speed to get to safety, going light to go fast, bringing enough gear to stay alive if the fast idea doesn’t work, not taking so much gear that progress stops in a dangerous place, backing up gear in case we fall vs. placing so much gear that we will fall, etc. Perhaps those of us involved in the “climbing education” business are placing too much emphasis on the “right way” and not enough on “think it through.”

WG

PS–There’s also a discussion on this going on at Rockclimbing.com.