physics of Half rope method


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Whitehorse Jeff

Trad climber
Fairfield, CT
May 5, 2009 - 06:58pm PT
In answer to the oft repeated question:" Why do they test half ropes singly with only 55kg", I was the American delegate (of the AAC) on the UIAA Safety Commission when the Half Rope Standard was put in place. Tests done at the time by Pit Schubert of the DAV (German Alpine Club) showed that any half rope that would withstand 5 UIAA test falls with a 55 kg test mass would also withstand one UIAA test fall with the 80kg mass. The argument presented to the members of the Commission at the time was that it would be impossible to get the lighter ropes which climbers (and guides ) wanted, to pass a harsher test, and that the real goal was to insure that a half rope would sustain at least one of the worst imaginable falls in the field, that being the fall represented by the UIAA single rope test. The repeated falls caught with the 55kg mass were accepted as the minimum repeatable test standard to insure this desired result. Over the approx 20 years that we've had the half rope standard , the Standard has proved to be high enough. (before the UIAA Standard was approved, people just climbed on half ropes without UIAA approval, assuming that both ropes would never be cut at the same time in the same fall, which has proved to be true in practice).
I accept that this reason may seem less than scientific to many. It was based on many tests carried out by the different certified UIAA test labs at the time, and as I remember was passed in near, if not total, unanimity by the voting members of the Commission, National delegates and Rope Manufacturers alike. There is one thing that encourages me at present as to the wisdom of this decision at the time-- rope manufacturing has made huge progress over the past 20 years, and this test still seems to hold up. Modern half ropes are much better than the ones I first used in France and Great Britain in the 70's.

May 6, 2009 - 09:51am PT
Very interesting.
That Darn French Guy

Trad climber
Santa Clara, CA
May 6, 2009 - 07:33pm PT
"any half rope that would withstand 5 UIAA falls with 55 kg would also withstand one UIAA fall with 80kg"

So a double rope *is* meant to catch a fall on a single strand.

The problem of that spec is everything is reported at 55kg: An 80kg person doesn't know what increase in force and elongation can be generated and IF IT IS STILL SAFE for his back.

Since we have ropes that are rated twin/double/single at 9.2mm, they shed some light there:

55kg - elong 32% - force 6.0kN (double)
80kg - elong 37% - force 8.2kN (double)
+46% - elong 16% - force 37%

So can I expect to read a double's spec and say if it's rated 8kN at 55kg, then my max impact force for my 80kg ass will be a hefty 11kN?
del cross

May 6, 2009 - 08:25pm PT
80kg? Did you gain weight, Dan?
Whitehorse Jeff

Trad climber
Fairfield, CT
May 8, 2009 - 01:01pm PT
"The problem of that spec is everything is reported at 55kg: An 80kg person doesn't know what increase in force and elongation can be generated and IF IT IS STILL SAFE for his back."
The main concern is that the rope will hold one Factor 1.78 (or so) fall with an 80kg mass. My main point in leading is to avoid such severe falls, especially since even without a pack or a rack of gear I always exceed the 80kg mass weight. I accept that falling off is likely to be DANGEROUS for my back or other parts. I use a rope so as to still remain attached to the part of Mother Earth I am attempting to climb.I also accept that climbing is not really Safe and that falling off is often quite Unsafe. I don't mean to be flippant-- our activity (addiction?) is a "fine sort of madness" isn't it.

Ice climber
Pomfert VT
May 8, 2009 - 04:37pm PT
Jeff we all know that climbing is dangerous but it seems to me that the origional thiought process was a bit skewed. Man these ropes break way to fast when we use the 80KG weight so we better make it lighter. That may not have been the intention but that is certainly how it looks.

Trad climber
Calgary, Alberta
May 24, 2009 - 01:23am PT
Things have been quiet on this thread for quite a while--everyone's cooled off. But quite a few guys have questioned the 55kg tests for half-ropes over time, and I'm not sure that they know yet, that there's a very good and valid reason for the 55kg. A comment like this last one from Tradman is a cute anecdote, but may only confuse them even more, as it's not true at all.

But "Francis" got the explanation straight from the UIAA about a month ago (Apr. 20, 2009), to the effect that the UIAA standards require a 'single' rope to hold at least 5 test drops before breaking; and they require a HALF rope to hold at least ONE drop in the same test, because it typically holds an entire fall on its own in double-rope climbing, when the second-last piece is some distance below the last one. Now here's the point: one is an awkward number because if the rope survives one drop but breaks on the second one, it could be that it BARELY held ONE, or it could be that it NEARLY held TWO, or anything in between. It would be nice to know a little more, since conditions, age, or manufacturing irregularities could push the result over the line at either end, particularly below the one-drop end. So the UIAA looked for a drop-weight that would abuse the rope the same amount in 5 drops as the 80kg would abuse it in ONE. That turned out to be about 55kg.

Now the testers can drop away, and have more data to compare between different ropes or different chunks of the same rope: if a particular rope never breaks on the 4th drop, and sometimes makes it to 7 or 8, they can say it's definitely a solid half-rope. But if it sometimes breaks on the 3rd or 4th drop, they may eye it suspiciously, and re-rate it as a twin or send it back to the lab for beefing up.
Chicken Skinner

Trad climber
May 24, 2009 - 01:29am PT
Smartass answer. Shorter pitches.

old toad

Trad climber
yosemite, Ca.
May 24, 2009 - 11:58am PT

I climb on a single 9.2mm rope now....

Werner, you don't need a rope!!!

May 24, 2009 - 01:22pm PT
And here I thought this was going away. This isn't about physics. It is about
whether you will need protection at the level of the UIAA drop test.

What seems to be happening here for simple technical rock is the UIAA is
counting on the other half rope as a backup for those cases where a half rope
fails a standard 80Kg. single drop test.

If so then anyone who actually needs protection at that level also needs to think
carefully as to whether the half rope setup is right for them. For now you have
to start worrying about placement positions and/or clipping as a twin.

For simple technical rock where you really are accepting these kinds of falls use
two singles. In alpine climbing where rope cutting and weight are issues and
there is not much protection to start with the logic might tend to weigh in on
the twin or half rope side where being able to rappel long distances is also a
big plus.

Bottom line, a purely technical rock climber whose eyes are not crossed can
make some use of this alpine technology.

If your eyes are crossed, think twice.

Seems to me.

Boulder climber
san diego
May 25, 2009 - 05:24pm PT
sounds like wisdom is .4mm

Trad climber
Chicago, IL
Sep 10, 2010 - 09:54am PT
Being completely new to the sport and also an engineer....a certain calm comes over me when I explore the different ropes and techniques. I can't seem to find the answer to one question though. How do I tie a half rope system into my harness? I'm assuming two figure 8's would put a torque on the harness and could cause a dangerous situation. Please let me know what knots and how you tie in using half ropes and twin ropes if it differs. Thanks in advance for the insight.

I would also appreciate any names of reference books/websites that detail out the half rope system.

Sep 10, 2010 - 10:54am PT
You just tie in each rope (whether half or twin) to your tie in points the same way you tie in with a single. Though with half ropes you want to pay attention to left vs. right ropes being tied in the same way for both leader & follower. Not sure about reference books .. I learned from more experienced partners.

Trad climber
Chicago, IL
Sep 13, 2010 - 01:27pm PT
Top roping question: When taking my wife and kids out Iíd like to top rope with two ropes.
1. Should I set up two separate anchor systems?
2. I weigh 230lbs so the rope is being punished when Iím in the air but does it really matter if I use a single(double), half(double) or twin(double) for top roping?


3. When leading, why canít I just use two singles while using half rope technique? If I stager the protection shouldnít the load mainly be on one rope anyways?

Trad climber
Jan 14, 2014 - 01:41pm PT

For the physics minded, the journal article about double ropes has been written:
B. Ernst & W. Vogel, "Determination of the redistribution shock load in climbing double rope system"
If you don't want to pay the big bucks for the article, see if you can get a free copy through your local library.

Regarding the question about the use of multiple "single" ropes, the question becomes one of finding a sweet spot where everything works OK. For ropes to work, they must simultaneously not stretch too much, must keep the force on the anchor & the deceleration of the arresting climber down to a dull roar, and not sever during fall arrest. The use of multiple "single" ropes will increase the force on and the deceleration of the arresting climber. If handled intelligently dual "single" ropes will PROBABLY work out without surprises for a top-rope situation where the anchors are reliable. BUT, it is possible that such a system will push things outside the sweet spot where everything works OK and challenge the anchor and/or the climber with unexpectedly high forces. Hence, no "sound practice" can recommend dual "single" ropes.

Trad climber
Poughkeepsie, NY
Jan 14, 2014 - 02:30pm PT
It is worth noting that the article referenced appears, from the list of figures, to deal with one strand of a twin rope system failing. It does not (for good reason) consider the multitude of scenarios that might arise in half rope technique.
Big Mike

Trad climber
Jan 14, 2014 - 09:12pm PT
First of all, thank you for the very interesting discussion. Lots of interesting info here.

I've been climbing for more than fifty years and I think I need rope stacking lessons. When I flip one of those piles, I get the mother of all Gordian knots half the time.

I got a little rope trick for anyone who has ever had this issue. I simply stack both ropes over my personal anchor,starting with really long loops, which each loop progressively shorter than the next. This way, the next loop you stack won't cross under the previous loop which is what causes snags.

So start with six or seven foot loops (or longer if you can) and work your way down to six inch loops by the time your partner reaches the belay. If you do this correctly the rope will flip perfect every single time. The most important part is paying attention to the length of the loops as you belay.

This trick works real nice for single ropes too.

Also never stack the ropes seperately. Treat them as one.
Big Mike

Trad climber
Jan 15, 2014 - 11:15am PT
Doug from Hobart

Trad climber
Mar 5, 2014 - 01:33am PT
I also use your "rope trick" Big Mike. Works well as long as you are careful with the stacking, otherwise you can end with a monumental mess.

On another note, I'm looking at getting a new set of half ropes. Currently using 8.0 mm Mammut Pheonixes which I've had for a few years and been very happy with, but they are about ready for replacement.
Pluses: at 42 gms/m they are the lightest ropes I've found; they've been pretty durable despite their thinness; less bulky in the pack.
Minuses: They do seem to tangle more easily than previous ropes I've used; fall rating is lower than other half ropes.
Anyone out there got any comments/suggestions? Anything new on the market I should look at?
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