The Half Rope Puzzle

[Ross]

Why the smaller diameter?

 

Totally puzzled. I've climbed on halfs/doubles a few times and see the rope-management advantages, but I just can't make sense of the skinny width.

 

Take the classic half-rope scenario: stemming between two cracks, alternating pieces in each every 15 feet or so. A fall while placing a piece means a 30 footer (plus stretch), just as it does for a single -- except it's on an anemic little strand.

 

If it blows, the "backup" rope sees a situation just as bad and worse -- 30 feet, plus all the remaining energy that wasn't absorbed by blowing the first.

 

Here's a common response:

 

"the work done on the first rope will have used up most of your KE, so the second rope will have an easy catch. "

 

but he's suffering from a head injury. You've got out 30 feet on that second rope, too, and in the very best case -- that the first piece absorbed all of the KE from the first fall but blew just as you were coming to halt -- you face a fall of the same factor as before. In reality, it's going to be harder than the first one : more KE, and a fall of the same distance plus a few more feet to span the distance between the cracks.

 

So why are half-ropes any different from singles? Don't they need to take the same kind of punishment?

 

Ross

[mattp]

Good question, Ross. I don't know the answer, but I believe it to once have been reported (like, maybe 20 years ago however) that there had never been a case when both ropes in a double or twin rope system failed. I don't know how many times a single rope has actually broken, but I have heard of plenty of times when one was cut over an edge.

[forrest_m]

ross - all ropes, whether single, double or twin - are capable of holding several falls all by themselves. the thinner they are, the fewer they can catch, but that's ok because it's a redundant system. your single rope has to be "overkill" because if it fails, that's it. most people choose to retire their ropes long before it has sustained the damage of 10 or 11 UIAA test falls.

 

people choose the two-rope systems because they judge that the other benefits (greater total diameter, redundancy which reduced the chance of being cut by edges and rockfall, full length rappels, two followers, reduced rope drag, etc.) will outweigh the added danger of greater elongation.

 

having taken numerous leader falls on double ropes, i can't say that i've really noticed a huge difference in fall distance because the shorter slings and more numerous pieces seem to even out, i.e. if you take a long fall, you're likely to have tension in both strands when you are caught.

[forrest_m]

oh, and if you are worried about a long section of runout, and you are worried about getting a lot of stretch, you can always clip both ropes to the same piece (using separate draws).

[Smoker]

What Forrest said.

 

 

 

Copy from somewhere listing the tech stats:

 

 

 

Rope Testing Standards

 

The International Union of Alpine Associations (UIAA) is the governing body that develops testing standards to evaluate climbing ropes. It is important to understand these standards and testing procedures, and how they compare to actual use of climbing ropes.

 

Falls Held

 

The UIAA test that determines a ropeís strength is called the drop test. The test uses a 176-pound (80-kilogram) weight for single ropes and 121 pounds (50 kilograms) for half ropes. In the test, the weight is tied onto the rope above an anchor. The weight is then dropped 16.4 feet on a 9.2-foot section of rope. This creates a fall factor of 1.8. The fall factor is determined by dividing the amount of rope out of the anchor by the vertical distance fallen. The most serious theoretical fall is one with a fall factor of 2. The higher the fall factor, the more limited the rope is in absorbing the energy of the fall. During the test, the weight is dropped repeatedly until the rope breaks, and that is the number of falls held.

 

The UIAA fall test puts a great deal more force than on a rope than real climbing situations. If you have a rope that held 7 falls in the fall test doesnít mean you need to retire it after 7 falls while sport climbing. But if your rope has sustained long, hard falls close to the belay, then you should retire it.

 

Impact Force

 

Impact force is determined during the same fall test. A climbing rope is manufactured to stretch during a fall to absorb the energy generated by the fall. The UIAA standards for impact force states that no more than 2640 pounds are to be transferred to the climber during the first fall of the test.

 

on

 

 

[snoboy]

most people choose to retire their ropes long before it has sustained the damage of 10 or 11 UIAA test falls.

 

 

I think I might consider retiring myself if I had taken 10 or 11 UIAA falls!

 

[snip] there had never been a case when both ropes in a double or twin rope system failed. I don't know how many times a single rope has actually broken, but I have heard of plenty of times when one was cut over an edge.

 

I've heard this one too. With the addition that a single rope has never broken in real life.

[Ross]

Ok, gotta be clear.

 

Yes, there are advantages to using double ropes, and that would make a fine additional thread. I'm with you on the benefits of the technique.

 

Yes, ropes aren't likely to fail. Most accidents are human error. I'm with you there, too.

 

The question is:

 

- Why are half ropes made in smaller diameter than single ropes?

 

In use, they'll face the same falls as single ropes -- see above -- so why a lower standard in strength? The UIAA force tests even subject them to a lighter test than singles.

 

The "backup" idea is pretty questionable. In my example above, you'd have to be maybe 75 feet up for the second one to keep you from cratering -- the low impact force of double ropes means a lot of stretch. That leaves alotta time for you to climb essentially on one thin rope...

 

And if it's just a lower standard we're accepting, why isn't that also the case for singles with limited use? I'd gladly take a rope on my next alpine climb if it could hold only three factor 1.5 falls -- instead of 11 at factor 2 -- but weighed half as much.

 

Still baffled: half rope technique makes sense -- but don't the ropes face the same abuse as singles?

 

Ross

[jesse_mason]

- Why are half ropes made in smaller diameter than single ropes?

 

Forrest writes "all ropes, whether single, double or twin - are capable of holding several falls all by themselves. the thinner they are, the fewer they can catch, but that's ok because it's a redundant system. your single rope has to be "overkill" because if it fails, that's it."

 

i think this pretty much answers your qeustion

 

Cheers Jesse

[snoboy]

jesse_mason: I think if you re-read his second post, where he clarifies, and perhaps changes his question, you'll see that Forrest didn't actually answer his question(s).

 

The "backup" idea is pretty questionable. In my example above, you'd have to be maybe 75 feet up for the second one to keep you from cratering -- the low impact force of double ropes means a lot of stretch. That leaves alotta time for you to climb essentially on one thin rope...

 

And if it's just a lower standard we're accepting, why isn't that also the case for singles with limited use? I'd gladly take a rope on my next alpine climb if it could hold only three factor 1.5 falls -- instead of 11 at factor 2 -- but weighed half as much.

 

 

[fleblebleb]

A related question is why do we have single ropes ranging from 9.low mm to 10.5 or 11 mm? I always thought it's a durability and cut resistance vs. weight tradeoff.

 

And, what's the difference between a 9 mm half rope and a thin single rope? What if the half rope has monofilament and the single doesn't?

 

How thin do half ropes get, anyway?

[jesse_mason]

Forrest writes "all ropes, whether single, double or twin - are capable of holding several falls all by themselves. "

 

So 75' or a whole pitch it shouldnt matter, yes you will have more stretch.

 

And if it's just a lower standard we're accepting, why isn't that also the case for singles with limited use? I'd gladly take a rope on my next alpine climb if it could hold only three factor 1.5 falls -- instead of 11 at factor 2 -- but weighed half as much.

 

To make a rope lighter is to make it smaller, barring technical advances in lighter materials, I routinely take one half rope into the hills, mostly on easier climbs, but iam well aware of its diameter and its vulnerabilty to being chopped.

 

Cheers Jesse

 

 

[Ross]

No offense to Forrest, but that's not right.

 

The assumption that double ropes are "redundant" overlooks two problems:

 

- even the backup rope must take the full force of the original fall and more (see the first example above).

 

- when placing gear every 15 feet, the second rope will catch you without cratering only when you're above 75 feet or so. (again, see above). Until then, that first rope had better hold.

 

I'm not saying double ropes don't work, but I am asking how they can get away with a smaller diameter. If they really are adequate, maybe our singles are fatter than they need to be -- since doubles each need to take the full force applied to a single.

 

Ross

 

[freeclimb9]

Why the smaller diameter?

In a word, weight.

 

[mattp]

Ross, I don't know what you are talking about with this 75' minimum for the second rope to prevent a groundfall. Yes, if you have anywhere near enough rope out to hit the ground, you are likely to find that in the event of a fall there is sufficient rope stretch that you will crater. And yes, if you fall through space you will gain speed until you reach terminal velocity so that a partial stop after 30 feet would theoretically allow some carry-over momentum and a second catch at 60 feet would be intense indeed. But when I go climbing, that is never how it works.

 

Two ropes are two ropes. That is called redundancy. And most people I know look around for some kind of pro as close to (but before) they make a hard move as possible. In fact, I often clip both ropes just before a hard move and wherever possible this means that I am looking at a maximum of a twenty foot fall onto either rope. Also, most natural rock climb takes place on terrain that is less than vertical so there is friction between climber and rock during a long fall.

[Jim]

I think the point being made is that the system is not redundant unless both ropes are always clipped through all protection points. And if you're taking a leader fall on one small rope, and that's ok, why are larger single ropes necessary? I've used double ropes and understand the need in zig-zag routes, but this discussion brings up an interesting point.

[Greg_W]

Not true, Jim. It is redundant, in that the second rope acts as a backup for the first. Say you're climbing on a double-rope setup and are 60' or 75' up with ropes clipped alternately to 5 or six pieces or pro. Due to rockfall from above, a baseball sized rock hits one of your ropes and chops through to the core. Your other rope is intact and you can finish the pitch knowing that you have one solid rope left. If you were in this same situation with a single it would be really bad, with a redundant, double rope system you can escape in relative safety.

[COL._Von_Spanker]

It seems like it could be bad if you clip half ropes together in the same piece when you clip them together its like having a big fat 14-16mm rope thus adding more shock to the gear.

 

Is one of the advantages of the half rope that due to the small diameter it stretches more thus decreasing the amount of shock to the system?

 

 

[iain]

but then if you had fatter ropes you wouldn't need to clip both lines to the piece...ah the classic cc.com overanalysis. which knot is the BEST one?

[freeclimb9]

It seems like it could be bad if you clip half ropes together in the same piece when you clip them together its like having a big fat 14-16mm rope thus adding more shock to the gear.

Do the math. Two 9mm ropes would have a similar cross section area to a 12.5 mm rope.

Is one of the advantages of the half rope that due to the small diameter it stretches more thus decreasing the amount of shock to the system?

Yes. The stretch intiates at a lower energy than would a larger diameter rope.

[mattp]

Jim -

I repeat: two ropes is two ropes. That IS redundancy because, by definition, if one rope should fail you are counting on the other. As I noted above, I have never heard a story of a rope breaking from force, but I have heard plenty of stories of their breaking over an edge. And also as I noted above, I have never heard of two ropes being cut at the same time.

 

What is this "unless you clip both ropes at every anchor" thing? When using double rope technique (as opposed to twin) I never clip both ropes to every protection point but I do sometimes clip both to the same piece right before a crux move or I place two pieces of gear shortly below a crux move and clip both ropes. (And don't tell anybody, but I don't always use two separate biners when I clip the ropes to a single piece, as is recommended by the rope manufacturers and the alpine club. And I have taken plenty of falls this way and I ihaven't shredded a rope yet.)

 

I know nobody who faces a crux move and doesn't look for an opportunity for anchors for both ropes unless they are worried about rope drag, and in this case they often secure both ropes but put a couple of long runners on their gear.

 

I believe Forrest is right: fat ropes are WAY stronger than they need to be. I climb on a single 9mm Edelweiss quite often, and it is rated to hold a fall over an edge with a single strand. I believe it is plenty safe. But when I'm on something more serious, I want two ropes or a fat one. It is kind of like doubling up on pro: one bomber piece just before a hard move is by definition sufficient. But most of us want two.

 

And I agree with Ian - overanalysis is an issue here. The main thing is to climb at some minimum standard of safety and beyond that, do what you are comfortable with. Every pitch need not be set up as might be a rescue rig, but some poeple feel the need to have cordellettes' equalized anchors with directionals at every belay on a long alpine rock climb.

Matt

[COL._Von_Spanker]

Just did the math, yer right. I should have done that first.

 

How is the elasticity of a rope measured? Are ropes assigned a spring constant?

[mattp]

Watch out for that math. If what you did was to determine that the cross sections add up to the same surface area, you have only determined just that: cross section add up to the same surface area. I would venture a guess that this does not correspond directly with stretch or with the dynamic characteristics of the system.

 

Consider, also, that if nothing else there will always be some difference in the tension placed on the two ropes, even if both ropes are clipped to every piece using twin rope technique, because one strand or the other will almost certainly have a slightly greater bit of slack or an extra twist in it.

[COL._Von_Spanker]

Yeah, that's why I asked about the elasticity.

Dem skinny ropes seem really boingy.

[mattp]

"Dem skinny ropes seem really boingy. "

 

They are, and I think they stretch more than a single fat one though I could be wrong. The stretch is often a disadvantage when you are hanging on the ropes, and you gotta be careful not to let go of the rope at the bottom of a rappel some times.

[Jim]

Ok. I'm a little slow on the pickup today, so humor me. So the primary benefit (aside from tricky rope mgmt) is for the possibility of a rope being cut - nothing more?