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Chockstone Forum - General Discussion

General Climbing Discussion

 Page 2 of 3. Messages 1 to 20 | 21 to 40 | 41 to 42
Author
Double, twin or single rope ...?

remdan
19/05/2004
5:32:25 PM
but shaggy is saying that if you clip one rope, then its half the force than two.
1x1/2=1/2

trent
19/05/2004
5:38:15 PM
there is enough movement to cause severe melting of the rope.

If you start out using the rope as a twin (clipping each rope to each piece via a single biner) you clip every piece in that manner. If you start out clipping only one rope per runner that is how you clip every runner. Think about it. You have just led a right trending rising crack where you clipped the first 5 pieces with the right (R) rope. The climb then heads up a blank face with bolts so you clip the both ropes into a bomber ring through the same biner. you move up and then suddenly peel off. Both ropes are running throught he same biner on the previous bolt but because the R rope is running through 5 pieces before that it has way more friction than the L rope. as a result the R rope will begin absorbing the fall before the L rope does. because of this it will start to come under tension (while the L isnt) and will indeed slide against the L rope. and every one knows that rope on rope friction is not cool.

There was a case in the American Alpine Journal of accidents where a guy on a free route did the above and as a result pretty well completely burnt throught the sheath of one of his ropes. It is a real threat. Yes lessening impact forces is the major reason to keep half ropes separated but cord melting is another.

saying all this my edelrid 8.3mm halfs were tested as singles and managed to take 5 UIAA single rope falls each. ive taken plenty of single cord dives on my halves and they keep coming back for more. just always keep them separate or at least running through separate draws on a bomber piece.

shaggy
19/05/2004
5:40:22 PM
This is true if we are talking about a static force, but we are not. We are talking about removing momentum from a falling object via a device elongating. If we take a person falling onto a single rope, it stretches, say 4 meters; on two ropes, the same fall would stretch the ropes about 2 meters; therefore the momentum of the falling person is absorbed in half the distance and thus doubling the force on the person.

try this experiment at home:
drive your car at 100km/h, brake and stop the car over 50meters, the from 100km/h again stop the car but over 25 meters, which situation causes more force on your body?

shaggy
19/05/2004
5:45:46 PM
If you climb on doubles, please do Not clip ropes together, physics and practice say that this is really bad.
There is a very big difference between double ropes and twin ropes!
If you don't understand it, dont do it until you do, and dont tell other people it is safe to do so!
M
19/05/2004
5:48:55 PM
The force is proportional to the elongation be that rope elongation or other dynamic factors. {force = mass X acceleration} so as more elongation results in a longer time to deaccelerate the force is lower. It is actually the time of acceleration/ deacceleration which is important not the distance/elongation. Two ropes clipped together will not elongate as far as a single rope under the same force. I doubt this is a linear relationship which means that althought two ropes clipped together will have a greater result in a greater force on the anchor than a single rope it wont be twice the force. If it was twice the force shared between two ropes then the impact on each rope would be the same (i think) as if the fall was on a single rope. i dont believe this is the case.
I do agree with Shaggys point which makes sense however a number of rope manufactures suggest that it is oK to clip together or alternatively.
Clipped together is better for your ropes but seperate is less likely to result in gear failure. Gear failure is more likely to result in an accident.
If the gear is bomber and significant elongation may result in hitting a ledge etc then clipping together with seperate biners to reduce friction may be a better option.

shaggy
19/05/2004
5:53:44 PM
MatD, You are correct, the equation is not linea, and it is directly related to the time rather than distance, but for the sake of simplicity it is a considerable more force than the other.
The ropes can take the extra force, but your gear might not, and your body probably rathers the lower force.

shmalec
19/05/2004
7:49:56 PM
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.

shaggy
19/05/2004
7:54:33 PM
or 3 - 4 years of engineering
M
20/05/2004
11:10:20 AM
On 19/05/2004 shmalec wrote:
>Hmmm….good to see such a humble group of contributors.
>
Im not sure quite what you mean or if you include yourself in this group.

Using physics to describe a climbing fall maybe interesting to some but it is not really very useful for most climbers. The dynamic factors, friction non linear elongation etc are difficult if not impossible to calculate. most physics describes ideal situations which climbing is not. The fall factors generated in testing are also not very relavent to actual climbing. Every bit of stretching and slipping reduces the peak force on the top anchor belayer and climber. If the force is higher there will be more streching and slipping thereby reducing the peak force.

In the interests of keeping it simple and accurate
two ropes clipped together will not stretch as far as a single rope. This will result in a higher force on the top anchor. How much higher will depend on a host of factors such as slippage through the belay device.
I am not going to comment on the friction between the two ropes melting the ropes except to say its worth bearing in mind.

Not taking advice from a climbing forum is an excellent piece of advice, except that its advice on a climbing forum so you should treat it with sketicism.

Seriously though, anything posted on a forum or that you are taught by another climber for that matter should be treated with some sketicism. Get you information from lots of sources, make sure you understand what you are learning and make up your own mind about what advice you are going to take. I also believe a general understanding of how dynamic forces are involved in arresting falls is useful knowledge for any climber.

adski
20/05/2004
4:03:57 PM
-- MatD wrote:
"Not taking advice from a climbing forum is an excellent piece of advice, except that its advice on a climbing forum so you should treat it with sketicism."

Thanks for your advice, MatD, i'll give it the consideration it deserves.. ;-)


I usually clip both doubles in to the first bit of gear, but never really thought about it. Thanks for all the info so far, now i'm reasssessing clipping both:

Cons:
- possible heat damage to rope
- less optimal loading on biner
- more impact on gear, possibly leading to failure

Pros:
- less extension leading to possible further fall distance (groundfall?)
- less possibility of belayer getting burnt hands or worse - losing control

I think i'll continue clipping both.
joemor
20/05/2004
4:25:24 PM
or just put two crabs on the first piece?
kieranl
20/05/2004
8:21:49 PM
Have to agree with Rich. Can't imagine how clipping both ropes into the first piece of gear can increase the impact force.

shaggy
21/05/2004
12:44:25 AM
I was about to type in the formulas but decided that would be too geeky, so, just try the experiment suggested above (driving car), and all will be revieled
M
21/05/2004
10:11:07 AM
an easy experiment to demonstrate this effect is to get two elastic bands. place one over a finger and your thumb and open you hand quickly so that the elastic band resists. now use two elastic bands. its more difficult and you can feel more force on your finger and thumb. the elastic band is a bit like a dynamic rope and the thumb and finger are the climber and top anchor. Two elastic bands like two ropes will exert more force than one.
If you want to be technical this experiment is far from perfect as with you hand you will just increase the force to compensate, opening the rubber bands just as far. A falling climber doesnt get any heavier or fall faster on two ropes so the result is that the ropes wont stretch as far. because the ropes dont stretch as far the climber will stop quicker. the climber stops more quickly (negative acceleration) because there is more force exerted on them over a shorter period of time. more force on the climber means more force on the top anchor.

my understanding of the rope burn issue is that this only occurs if the ropes have been seperated and returned together. I also feel that serious rope burn would be pretty rare even if the ropes were seperated and returned together but I dont know for sure.
kieranl
21/05/2004
9:11:45 PM
Are you talking about impact force on the body or shock-load on the piece?
I don't really care if the ropes are damaged while arresting a severe fall.

"A falling climber doesnt get any heavier or fall faster on two ropes so the result is that the ropes wont stretch as far. because the ropes dont stretch as far the climber will stop quicker"

This is just nonsense. Two ropes don't stretch as much as one because the load is distributed over a wider area. Two ropes stretching slowly can take the same (or less) time than one rope stretching quickly.

Don't condescend to us, if this is a real effect please quote us the science (journal articles etc).
Nico
21/05/2004
10:23:34 PM
There are some practical considerations as well as the theoretical physics. I have to assume that my rope will hold a fall, and (mostly) I assume that the pro that I place will, too. I climb mostly moderate grade trad routes, often with a fair amount of zigging and zagging. In this situation, having a right hand and a left hand rope reduces rope drag - the route zigzags but the rope(s) go straight on up, on either side. It also reduces the chance of gear being pulled out by unexpected sideways or upwards forces which can occur with a single (zigzag) rope. If you do come off, it's essentially falling on a single rope - whichever was the last one to be clipped - with the pieces below as backup.

If the route straightens out, then I can't see any problem with clipping both ropes to the same piece of pro - BUT through two krabs at slightly different lengths, or different length quickdraws. If the krabs are side-by-side there are all sorts of varieties of cinch possible. The cinch is the knot or buckle used to hold a saddle on a horse, where the girth goes through two steel rings. This can bring you to an embarrassing halt.
M
22/05/2004
8:54:46 AM
Kieran,

If I came across as condecending I am very sorry. I was just trying to reduce the concept into very simple terms. I do not know how much knowledge of physics anyone reading these posts has but I suspect that there is a wide range. As a scientist (not a physicist) I often have to explain scientific concepts to the general public and students and it is often useful to reduce ideas to a very simple level. I am always aware of the risk of being condecending and feel sorry if anyone feels condecended to.

As a climber you are far more experienced than myself and I do not wish to comment on the technicalities of lead climbing as there are many on this forum, including yourself who are far more qualified to do so.

My main interest was discuss the forces on protection, climber and belayer when two ropes are compared to one. I firmly believe that two ropes will result in a larger force on the anchors belayer and climber than a single rope of the same diameter. I think that others who have posted above and seem to have engineering experience agree on this point.

In terms of quoting scientific journals this is probably quite difficult. I suspect data such as this (refering specifically to climbing ropes) is more likely to be in honours or PhD thesis than a journal. If it is in a journal, it is likely to be in a journal which I have only limited access to without significant effort.
If you are genuinely interested in this please let me know and I will endevour to find some specific scientific evidence. Alternatively I could write to a rope manufacturer and enquire as to weather they have any data they could supply.

Either way please accept my apologies.

Mathew
M
22/05/2004
1:43:55 PM
I have found some info on this topic from PMI. Although this provides evidence to the above statement perhaps the most relavent part to climbing is the last paragraph.

It was at the following webpage

http://mtncommunity.org/dcforum/DCForumID1/1142.html

the basic text is as follows

PMI's response:

Hello Bill,

I have tested and can say for certain that the impact forces on PMI's 8.1 and 8.6mm ropes are low enough to safely be used as twin ropes. We also purposely had the certifying lab test our 8.1mm as BOTH half and twin to the UIAA/CE tests so that we could legally dual-label them as twin and half.

I have NOT tested all brands of "half" (a.k.a "double") ropes. What I can say is that any other brand half rope that has a higher impact force rating than shown below will of course also have higher impact forces than shown below when two are used together as twins. Any such combo may still test out below the UIAA limit of 12 kN, but you'd have to get that info from the rope manufacturer or someone able to do such tests.

Here are some actual test results:

A) PMI Fusion 8.6mm half rope - UIAA "1/2" rope test - single strand of rope - 55kg test mass - 5.9 kN impact force - 17 falls held

B) PMI Fusion 8.6mm half rope - UIAA "1" rope test - single strand of rope - 80 kg test mass - 7.3kN impact force - 3 falls held - this is a FAILING result and why we can't/don't sell the 8.6mm rope as a "1" rope

C) PMI Fusion 8.6mm half rope - UIAA twin rope test - two strands of rope - 80 kg test mass - 8.7 kN impact force - stopped test after 19 falls held - perfectly safe result

For further comparison, the PMI Verglas 8.1mm rope tested as a twin (w/ 80kg mass) has impact force of 8.4 kN. This is a little less than our 8.6mm tested in the same manner, but equal to that of the PMI Cirque 10.6mm "1" rope. Hope all this helps you make safe climbing decisions.

Regards,

Chuck Weber
Quality Mgr. / ISO Coordinator
PMI
P.O. Box 803
LaFayette, GA 30728
kieranl
22/05/2004
10:31:10 PM
This is good information. I'll have a look at this over the next few days. If there is stuff that I don't understand (almost a given) I will ask for help from the more technically gifted of the forum.
Kieran

shmalec
24/05/2004
2:22:50 PM
Good find Mat. Interesting stuff.
Can I tentatively venture a simple explination…..no promises though. :-)

Forget everything else for a minute and just think in terms of stiffness. Intuatively, we know that a bungy cord is not very stiff (its stretchy) and has a very gradual, low impact force when you fall on it. On the other extreme, a steel cable is very stiff (its hard to stretch) and would arrest your fall with a sharp jolt and high impact force.

The difference between one rope and two is like that but not as extreme. One rope is less stiff (more stretchy) and will give a slightly lower impact force. Two ropes are more stiff (less stretchy) and will give a slightly higher impact force.

By impact force I’m talking about the “pull” of the rope on the climber. But the principles apply to the protection as well.
For any given fall, the stiffness drives the impact force, rope stretch and time to arrest fall. They are all related.

I think the recommendation for clipping both to the first bit of pro came from John Long's "How to Rock Climb" series of instruction books (for what its worth).

Don’t know if this helps anyone. My personal opinion is that its pretty academic in the end. Small differences in the way you place your protection, whether or not you wear a helmet, grease your hands up with sunscreen or get bitten by a bull ant mid pitch have more impact on your safety. But its an interesting problem anyway.

 Page 2 of 3. Messages 1 to 20 | 21 to 40 | 41 to 42
There are 42 messages in this topic.

 

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