Single and Half-Rope Impact Forces: Data!

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:

_________________________________________________________________________

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.

Politics (not directly climbing or flying related...)

It's no secret to regular readers of this blog (and thanks to those who have sent in comments on elbows, ropes and falling!) that I really don't like George Bush and his crew of Neocon nutters. I've always felt they were taking the US down some sort of rabbit hole into an alternate reality where it was OK to treat people like, well, not people. Where it was OK to invade a country (Iraq) not because that country had done something to the US but because, well, because. Where deficits didn't matter, corporate abuses didn't matter, workers didn't matter and the constitution was just a suggestion. I have frequently noted the Bush administration's invocation of "national security" to justify blatantly un-American actions such as Guantanamo Bay, the suspension of Habeas Corpus, torture and all the things we generally associate with third-world dictatorships but are now somehow OK for the US to sanction. In short, I've felt that Bush is the closest thing to a World War II fascist (Hitler, Mussolini) the United States has ever seen. I don't make that comparison lightly, or for shock effect. As soon as a country starts stripping individual freedom to preserve it something has gone seriously wrong.

This article in Slate does a much better job of explaining the Bush/fascism historical parallels than I can. End of political rant, gotta get one off every month or so...

Training: It's been cold enough to discourage much outdoor training here of late, but I've been hiking, running the dog (getting a dog has done more for my aerobic capacity over the last five years than anything else in life) and doing some yogacizing. Thanks for all the elbow suggestions from various people, I'm doing pretty much all of 'em, let's hope some healing results soon!

More on falling distances...

This discussion on falling distances with respect to clipping position is down there somewhere in the comments column, thought I'd post it as a new entry. I'm getting some email on this as well, it's been fun discussing it all, I hope the following helps people figure it out. Just for the record, it's generally safer to clip at waist level than over-head.

Anon wrote:

I've read your info regarding clipping and, while I didn't pull out the graph paper, I did use a string demo and am not seeing your logic.

Here's my scenario:
- assume a vertical face
bolt3 (30ft above ground - not clipped)
YOU (28ft above ground)
bolt2 (20ft above ground - clipped)
bolt1 (10ft above ground - clipped)
____ground_____________

If you overhead mis-clip 2ft from bolt3 (ie. 8ft above bolt2) then you'll have 8+2+2=12ft of slack rope and a 24ft fall from 28ft above ground. (You better hope you got less than 4ft rope stretch!!). If you mis-clip at your harness then you have 10ft of slack rope and a 20ft fall from 30ft above ground.

Seems to be safer to clip from the harness at bolt height. What am I missing!?

Will Gadd said...

Hi Anon, you're definitely right that clipping at waist-level is often safer than clipping overhead. That's the main point of all of this, so you're not missing anything in my mind in terms of where it's generally safer to clip. But your math is wrong in your scenario for total fall distance. I've written a few explanations below that I hope will help you understand...

In your scenario you don't fall twice the distance of the amount of rope you have above the last piece. That's the error I made as well when I wrote my book. Seems obvious that you would and that's how we are often taught to think of lead falls, but it doesn't quite work that way when part of the rope is "above" you. We actually end up the same distance below a piece as the amount of rope we had above the piece when we fell...

It's funny, I too refused to believe the graph paper for some time until I really counted the squares and thought it through. In your example you correctly have 12 total feet of rope in the system above the last piece at 20 feet. So you'll definitely end up with 12 total feet of rope below the 20 foot piece after the fall, right? What's 20 minus 12? 8. If your harness starts the fall at 28 feet and ends at 8 feet how far did you just fall? 20 feet... It's fun to run this scenario using a "long armed" climber who can clip 6 feet over his/her harness; Now there's 16 (10 to the missed clip, six from the missed clip to the harness) feet of rope above the last piece and the climber will end up 16 feet below the last piece--four feet off the ground. But the fall is still 20 feet, the "extra" six feet of rope going from the climber's harness to his attemped clip hand doesn't increase the fall.

So in your scenario, you actually fall 20 feet and end up 12 feet below the last piece--the two feet of rope going from your harness to the clipping point doesn't "double" or add to the fall distance. Count your squares on the graph paper, or with the string--the fall distance is exactly 20 feet (ignoring rope stretch, belayer feed, etc.) The KEY difference in the clip at waist and clip overhead scenarios is that the starting point for the fall is higher off the ground when clipping at waist level (safer). Belayers also tend to feed more slack than absolutely necessary, and climbers generally also pull more slack, which adds at least a few more feet of slack in the system when clipping overhead...

Just for fun and 'cause I'm a nerd, think of a climber who has a bomber piece at 100 feet above the ground. He climbs up another 20 feet, rattles in a sketchy piece, and starts climbing down to get back to his bomber piece. Unfortunately, just as he gets his harness level with the bomber piece he falls, the top piece blows, and he goes for a ride. How far is he going to fall? 20 feet? 40 feet? 80 feet? He has 40 feet of rope above the last piece now, our "classic logic" would tell us he's going to go 80 feet... Nope, he's going to end up 40 feet below the "bomber" piece because he had 40 feet of rope above it. Total fall 40 feet... This is just an exaggerated version of the clipping overhead scenario.

I've had a half-dozen discussions on this now. I expect a non-climbing math student would figure this out very fast, but as climbers we have a very strong, almost religious belief that fall distance equals twice the rope above the last piece. In the last two weeks I've had three major beliefs I hold about climbing seriously revised: fall distance, half-rope impact forces, and the use of Cordelettes...