UKC

Thinking out loud a bit here, but please bear with me.

From the 'climbing on double ropes of different diameters' thread:

>if you clip both [ropes] through the same anchor then you will rapidly find yourself in a world of very large impacts and potentially broken gear and bodies. That is why Twin ropes are much thinner and strechier than single or half ropes.

This seems obvious, when you think about it, however I am interested in the situation where you place two bits of gear, and clip a rope to each. Clearly the forces on each piece of gear will be lower than one piece of gear on a single rope, but what about the forces on yourself? If you fall is held by two ropes (on two bits of gear) are you dangerously multiplying forces on yourself?

I had a bit of a hunt around, and the mechanics of it is on this page:
http://newsgroups.derkeiler.com/Archive/Rec/rec.climbing/2006-05/msg00321.h...
which suggests that by taking a fall on two ropes, you are theoretically increasing forces on yourself by up to 40% (but in practice less, as ropes do not behave linearly)

I suppose I had always thought double ropes were stretchier than singles to allow for this? This worries me a bit, since I do this as fairly standard practice. If I'm coming up to, say, a crux overhang that I want to stitch up, I will stick in a bit of gear on each rope.

Does anyone else do this? I had thought it was fairly standard. Am I worrying needlessly? As it happens, my only trad fall to date WAS onto two bits of gear on a rope each - and it was a pretty big impact, compared to most sport falls I've taken.

In reply to plexiglass_nick:

I do the same, I am a wimp and place alot of gear, and don't want to fall so I back them up with each rope. I have taken two relatively small lobs and had no problem. I think it is standard practice, kind of half the point in using double ropes I think is that if one piece rips, you don't have quite as much of a fall before you hit the next piece of gear?

In reply to Reaver2k:

I think it's also to decrease the amount of fall if you take a fall while having a load of rope out to clip the next piece of gear. That and giving more choice of gear placements without needing to extend everything loads. And traverses to minimise rope drag.

In reply to plexiglass_nick:

A lot of this stuff is actually rather academic when you look at the forces involved. Basically you're not going to get into trouble if you're sensible (for a given definition of sensible), but basically you'll find in real life that people are falling onto two bits of gear at the same time all over the place (me included) and they're fine.

In reply to James Jackson:

Agreed - There is definitely a tendency (particularly of the American forums and groups) to really try to get inside the physics of it, which is useful to a point. I am more interest in the real world side of it.

Will wait for a few more replies, but it's looking like I will carry on happily clipping both ropes as usual.

Thinking about it a bit more, it's another reason FOR using doubles - you can trade off extension and fall force. So in practice, clipping both ropes would be good to limit extension (to avoid decking out, hitting a ledge, etc.) unless it's really big fall factor, (ie. more than 1) where it would probably be better to take the fall on one rope provided it's a safe fall.

In reply to Reaver2k:

Pretty much what I had thought... good stuff.

In reply to plexiglass_nick:

I'll often try to protect a crux with two separate bits of gear on two ropes. Not only is this standard practice, I always thought it was advisable. Its not so much about having a piece of back up gear if one fails but, I figured, being able to control the direction of the fall to reduce the hances of gear being ripped out.

In reply to plexiglass_nick:

A 1/2 rope on each piece will hit you harder than having just the one rope on both bits of gear but from experience it doesn't hurt any worse than any other fall. The upside is if you snap one (a worry on slate) then you don't fall way down to your next gear. You also fall less distance in theory, ideal if you're worried about the floor or ledges. I've done this occasionally with 2 full ropes where I was more worried about hitting the floor than the hit from the ropes, again, for short falls you hardly notice the difference.

Clipping both ropes into the same gear will have much the same effect but with the risk of some melting of the sheath if you're unlucky. Best use 2 q'draws if you want both ropes. You're right, it'll also load the gear more than a single would and more than hhalf of a pair of 1/2s would but it's a calculated risk, on a bomber nut it'll be fine, on a shallow alien I wouldn't even think of it.

jk

In reply to plexiglass_nick: I just had a read through of the post you quote. He is talking about a different situation, namely clipping a pair of singles (beal Joker) as a twin, i.e. into one anchor and not two, and also he goes on to talk aboiut the anchor force increasing, not the force on you.

I'm not 100% on this so I'll hypothcise a little... If you fall onto two anchors each sharing half the impact force, each rope disapates half the energy of the fall. Elongation of the rope reduces the energy transmitted to the climber - effectively you are on 2 springs in parallel. A spring requires a certain force per distance elongated (hence the other post talking about linear behaviour) - hence you can say that the energy disappated is linear as well. As the force is same force is spilt bewteen two ropes there is less energy for each "spring" to absorb. Furthermore because the springs are in parallel the energy when recombined at the knot will not increase much (as an example take two bulbs - rigged in parallel they use the same voltage as one bulb but burn more energy) so in theory the energy is less. Its a very different situation... Thats most likely clear as mud?

In reply to plexiglass_nick:

The maximum impact force allowed for a double rope is 8kN I think (but the most common ropes Beal and Mammut are around 5/6kN).

For a single rope it is 12kN.

The impact force is the peak tension in the rope, so it is the same as the force transmitted to you.

From the article that suggests you'd increase the force on yourself by 40%, well, 8kN +40% is roughly 12kN. Basically you're pretty much within the bounds of single rope territory anyway, and you'd never worry about that.

In reply to mike kann:

Not sure I really want to get too drawn into the mechanics of it, as ultimately we're arguing about the finer points of a model which is not very applicable in a real world situation... (modelling the ropes as behaving elastically, which they don't) but here we go.

Not sure I am convinced it makes sense that the mechanics are different. After all, it doesn't matter from the point of view of the rope what the 'anchors' are attached to - whether the 'anchors' are a single anchor (ie. attached by being clipped into the same QD) or attached by ultimately being placed in the same bit of rock. The only place where this makes a difference is the force on the anchors, as you have two which share the load, rather than just the one. No?

Also if energy is force times distance, if you reduce stopping distance (as extension is less with 2 ropes in parallel) then surely the force must increase if the total amount of energy is the same?

In reply to katonka:

A-ha. So double ropes are sufficiently more stretchy to cope with there being two of them. I guess I'm just wanting to be sure they are designed for that, and it looks like they are.

In reply to jkarran:

I am glad I am not the only one who does it... definitely options to bear in mind if there are worries about decking out.

In reply to plexiglass_nick:


Agreed. The one/two anchors scenario makes no difference to the force on YOU. The force at the anchors is the same too, but in one case it's shared between 2.


Force will increase over having just one rope. F=ma, you will stop quicker (greater deceleration), so there must be a greater force.

In reply to plexiglass_nick : I think I would look at it like this; if I could guarantee not hurting myself whilst falling (no ledges etc), and was way above the ground, then I could "afford" to have a really elastic rope to make stopping the fall as soft as possible. I think that's what a bungee jump does.

Of course in the real world there's stuff to hit on the way down and the ground's usually closer than we might like so rope extension must be limited. The consequence is a harder fall, which of course also means more load on the anchors.

I'm pretty sure that whether you fall on one rope or two, you'd have to be in a pretty unusual situation for it to make a critical difference to how soft/hard the fall was.

I can remember seeing similar debates about the merits of various tying on knots due to their different strengths (generally between 60 & 70% of the rope strength). I always say "any knot that doesn't come undone will do" because if you're up to that kind of force you are really in the sh*t and lots of other things will be breaking. The merits of various knots lie in their other properties (ease of tying/untying, shape, etc) not in their absolute strength.

In reply to plexiglass_nick:

People seem to be doing the classic thing of going down the analysis route and getting it rather wrong. There's this lovely thing called 'impact':

f * delta_{t} = delta_{P} (1)

which can clearly be seen from the integral of the formula (sort of) shown above:

f = m*(dv / dt) (2)

So, with two ropes, delta{t} is clearly less than with one, therefore the instantaneous force (on gear and climber - the two must be the same!) is greater.

So, the post above which states that the force is the same is wrong. The momentum change is the same, the instantaneous force is different (but the integral force is the same). Context implies that instantaneous force is the subject under discussion (as it's this that will generally do the damage).

Anyway, as I initially said, these discussions are usually completely pointless as generally you'll be alright (given sensible, solid gear placements). No point in clipping two ropes though - you get a smoother ride (if there's nothing to hit) by clipping one.

Glad to see not much has changed on UKC in the past while I haven't posted

In reply to plexiglass_nick:

you hear things like this all the time though- tieing a figure of 8 knot in a rope reduces its strength by 20%.

does this mean that by having 5 on a single rope it should theoretically disappear....?

i suppose have 2 ropes of different diameter clipped to the same runner would give more force than two thinner ropes- but still, unless its completely marginal!....

In reply to Twisty:

Nah, it's one of Zeno's Paradoxes stated in easily understandable terms for the climbing fraternity. Just a geometric series, shurly? (P.S. Stop calling me Shirly!)

In reply to Twisty:

no. you would have a rope which had 5 weak points. (each independent weak point being '20%' less strong than the rest of the rope.)



still think too many people worry far too much about the fizziks of climbing.

In reply to satori:

Exactly, my darling climbing also-Physics-educated lovely. How are the operations going these days?

In reply to James Jackson:

Oh, I see you've come out as a not-lady now No more operations then. I did follow them with rabid interest. I just remembered a word that few will know:

Penderosa

In reply to James Jackson:

ha ha. haven't seen you around here for ages. i've had to resort to taunting merkins ffs.

so they let you out of the young offenders place then?


operations are going fine. only another 7 operations to go (inch at a time) and i'm officially a woman.

In reply to James Jackson:


mighty big steaks.




and too many hedgehogs.

In reply to satori:

Yeah, they let me out of the first one and I quickly got committed again for requesting to do one of those things that makes you a doctor-in-all-but-medial-knowledge. Perhaps I can offer cheap ops to you in the future?

In reply to satori:
>

And the grumpy barman who liked head. And the huuuuge pizzas that a certain someone cooked!

In reply to James Jackson:

Make that Medical!

In reply to James Jackson:

lol. only just spotted your profile pic (despite it being nearly 3 years old).

i bet your feet were still on the ground when that was taken.

In reply to satori:

F*ck off, they were at least three feet higher than ground level!

In reply to katonka:

Katonka is right.
In additin if you look at the stats for most modern half ropes, you'll see that they are very similar to that of twin ropes anyway.

Also, as long as you are not talking about greater than factor 1 falls, two single ropes clipped like this will not cause too much problem anyway. Essentially this is because one rope will always catch slightly before the other and this one will absorb most of the force.

In reply to James Jackson:


Surely not ? the force on the climber comes only from their rope, the force on a running belay is the resolution of forces between the climber's rope and the rope going to the belayer, which is always higher, isn't it ?

In reply to GrahamD:

Which will be equal, otherwise your rope has broken.

In reply to James Jackson:

In an ideal model, which has been pointed out, doesn't actually apply in real life. Also, like I've said twice now, no point in overanalysing it for reality's sake. And I'm bored now.

In reply to James Jackson:

P.P.S. I did make various assumptions, but there you go.

In reply to katonka:


This is true. However, the UIAA drop test is performed with a 55kg mass for a half rope, whereas for a single rope an 80kg mass is used. The peak impact force quoted by the rope manufacturers will be that established in the UIAA drop test http://www.theuiaa.org/upload_area/cert_files/UIAA101_DynamicRopes.pdf ie using the specified mass for that class of rope.


Using the classic impact force equation, a half rope that just meets the UIAA spec will produce an impact force of 9.8kN with an 80kg mass/factor 1.78 fall. Using two together effectively doubles the elastic modulus and the impact force would become 13.5kN, which exceeds the single rope spec.

If the peak impact force of the half rope is more like 6kN then the impact force for an 80kg mass will be 7.4kN, and with two together it will be 10kN, so within the UIAA single rope spec (and thus on that criterion they could also be classed as twin ropes, although the UIAA spec does include other criteria which the half ropes may fail to meet).

Usual caveats apply re applicability of classical impact force equation and advisability of avoiding high factor falls in the first place.

In reply to James Jackson:

Am I missing something here ? maybe we are talking different scenarios ?

Simple example - assuming runner is frictionless. Climber hanging from running belay, rope runs from them, through the runner and back down to the belayer. Force in rope = X so sorce experienced by climber is X, force experienced by belayer is X and force experienced by runner is 2X.

In reply to GrahamD: You are certainly correct in regard to the situation you describe. It's called the pulley effect and all climbers should really be aware of it. It is a significant factor, for example, in this BMC article about the failure of a small wire: http://www.thebmc.co.uk/Feature.aspx?id=1477

It's difficult to tell precisely what situation James Jackson is trying to expound about, but if it's the same one as yours then he's wrong.