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Using Twin Ropes Singly?

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Postby brenta » Wed May 12, 2010 6:09 am

drpw wrote:Can you explain this one? How does falling on two ropes increase the force rather then falling on one? Say two 8.5mm doubles vs one 10mm single?

To avoid comparing apples to oranges, let's focus on two strands of one type vs. one strand of the same type. This is what happens if you use doubles that are not certified as twins as if they were.

The force on the top anchor depends on how fast the falling climber is arrested by the rope. Simplifying things quite a bit, a rope is a spring, and a stiffer spring produces higher forces. If you put two springs in parallel, you get an equivalent spring which is twice as stiff. Given the same amount of energy to absorb, a spring that is twice as stiff will produce a force which is higher by a factor of square root of two.

Real ropes are not exactly ideal springs, and when all is said and done the arrest force of two strands is actually less than 1.4 times the force applied by one strand. However, it is still higher, and, at least in theory, it may exceed the maximum allowed by the standard (12 kN).

On the other hand, there are several ropes nowadays that have multiple certifications. The Beal Joker is certified as single, half, and twin. The impact force as single (8.0-8.2 kN) is lower than impact force as twin (9.1-9.3 kN), but not by much. The PMI Verglas is an excellent example of light rope that is certified both as half and as twin.

Finally, when comparing impact forces of twins and doubles, remember that they are measured with different masses (80 kg vs. 55 kg). Hence, the values cannot be directly compared.
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Postby drpw » Wed May 12, 2010 9:46 am

Thank you for the explanation. That makes a lot of sense.
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Postby Diego Sahagún » Wed May 12, 2010 10:32 am

8 mm ropes are not made for single roped teams. That's all :!:
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Postby Autoxfil » Wed May 12, 2010 12:42 pm

http://www.mammut.ch/images/Seilfibel_E_030617.pdf

Page 15.

I think Will Gadd made a good argument for using doubles as twins as well.

It always concerned me, especially on ice screws, but I now do it with confidence with additional slack in one line - which is often easier to introduce than avoid.

rhyang wrote:
Autoxfil wrote:BUT, Mammut says all their doubles can be used as twins


Interesting -- where did you read this ? A half rope which is also certified for use as a twin will have both markings, and ratings for each test / usage. An example is the PMI Verglas 8.1 -

http://www.rei.com/product/767452

PMI Verglas 8.1mm x 60m Dry Half/Twin Rope Specs
Weight 42.6 grams per meter
Dimensions 8.1mm x 60m
Dry treatment Dry core and sheath
Impact force (half / twin) 5.2 / 8.9 kilonewtons
UIAA falls (half / twin) 8 / 18
Dynamic elongation (half / twin) 35 / 24 percent
Static elongation (half / twin) 8.9 / 7.2 percent


btw When you use such a rope, it can be clipped alternately (half technique) or together (twin technique), but not both during the same pitch. The reason usually given is that in the event of a lead fall, the difference in the ways the ropes run may cause sheath damage. The only exception in this scenario is clipping to the same piece of pro using different carabiners.

Confusing, huh ? :)
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Postby Autoxfil » Wed May 12, 2010 12:49 pm

Brenta, there's one other factor - slip at the belay device often limits the force to something below the impact force of the rope. Two strands of a double provide twice as much braking as just one strand, which is purely additive. This isn't a factor in light falls, but in the nasty falls that we are concerned about here, it's very much worth thinking about.
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Postby rhyang » Wed May 12, 2010 2:56 pm

Autoxfil wrote:http://www.mammut.ch/images/Seilfibel_E_030617.pdf

Page 15.

I think Will Gadd made a good argument for using doubles as twins as well.

It always concerned me, especially on ice screws, but I now do it with confidence with additional slack in one line - which is often easier to introduce than avoid.


thanks. Here's what the mammut brochure says -

But here you have the choice between twin rope technique, where both ropes run parallel through the protection and half rope technique, where the «left» and «right» ropes run separately through different protection points.


I'm not sure what this means exactly. Does it mean they have actually tested their half ropes as twins ? If so, where are the ratings and the test results ? Or are they simply trying to say "ehh, it will probably work .. buy our ropes instead of PMI or Lanex" :) I mean, sure it will work as long as you don't fall ..

I have Will Gadd's ice climbing book, but I'm not such a fan that I know everything he's ever said. What did he say on the subject exactly ?
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Postby brenta » Wed May 12, 2010 6:10 pm

Autoxfil wrote:slip at the belay device often limits the force to something below the impact force of the rope. Two strands of a double provide twice as much braking as just one strand, which is purely additive.

Good point about the slip, but friction is not additive.
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Postby Autoxfil » Wed May 12, 2010 6:28 pm

It wasn't Gadd - I found an article I read by him when I was researching this, but it's not the one that I was thinking of.

There is a bunch of reading on this topic, almost all of it junk. I'm going to contact several rope manf's and get their take.
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Postby Autoxfil » Tue May 18, 2010 2:56 pm

Here is the response from Sterling. Clif Notes: doubles as twins are OK, but they want to avoid any extra slack like I was adding to reduce impact force. Either use them as doubles, or twins (if the pro is good), but don't mix and match on a single pitch.

Sorry about the delay in getting back to you.  You can use half ropes (doubles) as twins but NOT twins as halfs.  If you use halfs as twins (placing both ropes through one piece of protection) you must continue to do that the entire climb.  You can’t switch between using them as halfs and then twins in the same route.  The concern is that if you don’t clip them through each piece of gear together, when you fall the rope will elongate at different rates and create the possibility for the ropes to rub together and weaken them.  The other concern is that if you use halfs as twins you will increase the impact force on the gear and need to ensure that it is bomber.  I would not allow the ropes to catch at different times as this creates the potential for the ropes to elongate at different rates.  If one rope crosses another and you fall, one rope could rub over the other and melt the fibers.
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Postby rhyang » Tue May 18, 2010 3:10 pm

Autoxfil wrote:Here is the response from Sterling. Clif Notes: doubles as twins are OK, but they want to avoid any extra slack like I was adding to reduce impact force. Either use them as doubles, or twins (if the pro is good), but don't mix and match on a single pitch.

Sorry about the delay in getting back to you.  You can use half ropes (doubles) as twins but NOT twins as halfs.  If you use halfs as twins (placing both ropes through one piece of protection) you must continue to do that the entire climb.  You can’t switch between using them as halfs and then twins in the same route.  The concern is that if you don’t clip them through each piece of gear together, when you fall the rope will elongate at different rates and create the possibility for the ropes to rub together and weaken them.  The other concern is that if you use halfs as twins you will increase the impact force on the gear and need to ensure that it is bomber.  I would not allow the ropes to catch at different times as this creates the potential for the ropes to elongate at different rates.  If one rope crosses another and you fall, one rope could rub over the other and melt the fibers.


Yep, this is my understanding. I would personally not use halfs as twins unless they are rated for it, or unless it's on a bolted route (or otherwise very solid gear).
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