The Half Rope Puzzle

[freeclimb9]

Jim,

the "primary benefit" of double ropes is that you have two ropes. That can be handy for climbing up a wandering route, or getting down a big face (summary: avoiding excess ropedrag and descending efficiently). The skinny ropes are also handy for limiting the potential load during a fall.

 

[Jim]

Thanks. Shows my limted experience with two ropes on long routes. I've done two small ropes on steep water ice back east when I was a young buck, but not since. And I guess I never quite thought through the physics of the smaller diameter rope and shock load. It was just what all my friends were using at the time.

[mattp]

And there is more than that. With two ropes, you can reduce rope drag, you have less fear of the ropes being cut over an edge, you can rappel a full ropelength, you can belay two seconds, and you can also have a tight belay on the last piece you placed while you are reaching up and clipping the other rope over your head. This is very comforting if you are shakey at the stance from where you are trying to clip.

 

Personally, I find the redundancy and the flexibility of climbing with double ropes very helpful although the extra complexity is not always warranted and I have some partners (belayers) who just plain can't get the hang of watching two ropes.

[Jim]

Hmmm. I could become a convert here.

[fleblebleb]

Try it somewhere where you've had rope drag in the past, you'll be an instaconvert.

[Ross]

Look, here's the question, same as at the start:

 

- How can double ropes get away with being smaller than singles?

 

Here are the answers we've had so far:

 

- double rope technique is good for a variety of reasons.

(true. now back to the question)

 

- ropes rarely fail

(ditto)

 

- the second rope acts as backup for the first

(true, if cut. Yes, cutting is the primary reason for rope failure.

 

But if we're talking about rope strength for absorbing a fall, the second rope faces at least as much force: not good when it was enough to snap the first. (We're talking double-rope technique here, not twins clipped together, which have their own pros and cons.)

 

Also, if you clip alternately every 15 feet, the backup rope won't kick in until 60 feet plus rope stretch: 30 feet for the first one to fail, plus the 30 out on the second. Double ropes have low impact forces which means big rope stretch. On the first pitch, then, you need to be pretty high up -- or make your initial placements closer to each other-- before the redundancy applies.)

 

 

No question about the advantages of the technique. No question that rope failure -- like most gear failure -- usually comes from human error and not integral weakness.

 

But unlike twins, doubles take the force from a fall individually, and if the first breaks, the second faces a force at least as great. The only time this isn't true is for brutal falls exceeding fall factor 1, on later pitches, if the first piece is close enough to the anchor.

 

So either our singles are also way too fat and heavy (which I can totally believe) or double-rope technique should include closer placements on the start of the initial pitch and and an even-more-crucial close first piece on following pitches.

[freeclimb9]

Look, here's the question, same as at the start:

. . .

Actually, you first asked:

Why the smaller diameter?

 

[COL._Von_Spanker]

You are missing the point, which has been made a number of times in a number of different ways. A single half can take a fall, but as a precaution a second is added for all the mentioned reasons.The big ropes are overkill for the sake of safety;

 

You seem to be missing everyone is telling you.

 

 

 

 

 

 

[cracked]

In response to the original question: when you have a bomber piece near the belay, people often clip both half ropes to it. This will keep the force from a hard fall from coming onto only one skinny rope. Higher up, the fall factor will be less, so one rope is plenty. Also, the skinnier a rope, in general, the lower the impact force, so a sketchy piece is more likely to stay in.

[freeclimb9]

The big ropes are overkill for the sake of safety

Don't forget durability.

[COL._Von_Spanker]

Durability=safety

[freeclimb9]

Durability=not having to buy a new rope after climbing a big wall.

[Smoker]

Here is your answer to your question

 

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.

 

Comprende`?

 

Interpretation for the comprehension challenged to follow:

 

Half ropes are tested with less weight ergo they "can" be smaller and still pass the test. Most of us like our ropes light or lighter. The Beal "Stinger" is the lightest single on the market at 9.4mm (or used to be)

 

Comparing half ropes to singles are like comparing Apples and Oranges?

[COL._Von_Spanker]

Climbing a big wall=Are half ropes relavant?

 

(seriously, i've never climbed a big wall)

Edited January 6, 2003 by COL._Von_Spanker

[mattp]

Smoker -

I believe you are correct about the testing routines. However, I also think the underlying question is fair: if my 9mm stratos is strong enough to lead on, why don't I just use it as a single rope? Notwithstanding Ross' conclusion to the contrary, I actually think we HAVE addressed that question in this thread. I do use it as a single rope, but when I really want an unquestionnable rope (or as close to that as is generally thought practical), I use a rope that is made for that purpose.

[Smoker]

Single rope for big wall, with the haul line as emergency back-up Col. You decide if you use a static or Dynamic for the haul. If your a speed climber, does it matter?

 

I realize that the answer has been stated already. I am simply making sure that it is obvious enough for those posting and those simply reading.

[COL._Von_Spanker]

I was commenting on free climb 9's comment.

[Smoker]

Sometimes posts just sneak in there

 

My bad?

[Ross]

>obvious enough for those posting and those simply reading.

 

I'll take that, and I'll also give a possible answer to the question ignored.

 

The previous answers miss the mark -- maybe my fault for not asking clearly, maybe theirs for assuming without checking.

 

Here are the standard answers (Question: why can half ropes be smaller than singles?) and what's missing:

 

- half ropes are redundant, so you can get away with a thinner rope.

 

(In fact, they often aren't redundant:

 

If you fall putting a piece 60 feet up, putting them every 15 feet, and the first rope breaks, will the second catch you?

 

Nope. The ground will. See my first post.

 

Fortunately, the first one won't break from a fall alone. See below.)

 

 

- bigger ropes are more durable. thinner ropes save weight

 

(Yes, Rope Knowledge 101. The question was NOT "why does anyone climb on an 11 mm?" but rather "what is different about double rope technique that makes it possible to get away with smaller ropes?" It ain't the redundancy. See above.)

 

 

- The UIAA tests them at a lighter weight, dummy, so they can be smaller.

 

(Um, WHY does the UIAA test them at a lighter weight? Not because they both hold the fall -- they aren't twins and don't share the weight. I think I know the answer now -- see below.)

 

 

- All ropes can hold you, stupid. The bigger singles are just for safety.

 

(same as above. What, if anything, makes double ropes appropriate for a different standard?)

 

 

- If a fall breaks the first rope, the second will hold you since the first will have taken some of the force.

 

(Sorry: if you place your pieces the same distance apart, the second rope will face at least as much force as the first. There's one exception -- see below.)

 

 

- A second rope is a good backup in case the first is cut.

 

(Totally true, but the UIAA tests in question are about breaking strength, not the edge-cutting test (which should be more popular, yes).

Still the same question: what about half rope technique makes it possible to accept a lower standard in pure drop strength, compared to singles?)

 

 

----

I think this is why:

 

* The second rope will never see a fall factor of much above 1.

 

It's not because of redundancy. There is NO redundancy on the first pitch until you are higher than 4x(distance btwn pieces). Above that, the falls put less strain on the rope anyway.

 

It's for the second, third, and fourth pitches, at the start of the climb.

 

The UIAA single rope tests cover a fall factor of 1.78 for an 80kg climber -- a fall that could only happen near the anchor on a second+ pitch. The half rope test is for a 55kg climber. That's odd -- why 55Kg? The ropes DO NOT SHARE THE WEIGHT, unless clipped as twins, which is a different technique, with different pros and cons.

 

Because they're trying to simulate a lower fall factor. A fall factor of 1.78 at 55Kg is the same as an 80Kg fall at factor 1.2. In fact, "factor 1.2" is sometimes cited as a standard in other discussions about half ropes, even though the UIAA uses the strange 55kgx1.78 fall factor test.

 

They could, of course, change the drop distance and use an 80Kg weight, but I bet it's easier to change the weight than to move the equipment set at a given distance.

 

Why does 1.2 make sense?

 

Because unlike a single, half ropes can never face a factor 2 fall, not alone. Unlike singles, double ropes always face fall factors of less than 1.3.

 

A fall right off the anchor on pitch two is a factor 2 fall, but it's shared by both halfs.

 

A fall higher up can exceed 1.2 for the first rope but not the second. Say the first piece is 4 feet up and the climber falls putting in another 12 more feet up: 24 foot fall on 16 feet of rope= fall factor 1.5. It might break.

The second rope, though, now faces a fall of 8 more feet feet on 16 feet of rope=.5, much smaller.

 

5 feet and a big 25 feet to the next? Factor 1.6 on the first, but only .16 on the second. You just can't generate a huge fall factor on the second rope alone.

 

 

Overanalysis? Ask the folks 60 feet up who think the second rope will hold them. It won't, but it doesn't need to.

 

The second rope is redundant for only two cases: when one is cut, and at the start of the second-and-higher pitches -- because THAT is when you need the backup.

 

Except for those cases, you ARE climbing on a skinny single.

 

And it'll hold you just fine then.

 

Ross

[mattp]

Sorry Ross, but we've been going round and round about this all day. If you lead on double ropes, and place a piece of gear every fifteen feet, alternating clips, without regard for where the crux climbing lies, you are not properly using double rope technique in my opinion.

[Greg_W]

Explain your rationale please, Matt. I'd like to go to doubles and I'd like as much info as possible.

[COL._Von_Spanker]

Except for those cases, you ARE climbing on a skinny single.

And it'll hold you just fine then.

Ross

 

That's what people have been saying, just not in idiot terms.

 

BTW: Why did you ask a question and than argue with every person that gave you and answer, finishing off with a full matmatical disertation? You clearly either think you know what you are talking about and are looking to sound smart and be and argumentative prick, or are just and argumentative prick.

Edited January 7, 2003 by COL._Von_Spanker

[mattp]

Simple, Greg. When I lead a rock pitch, I place more pro near the crux's and less pro when I do not feel that a fall is likely. If I want redundancy -- like when I am facing a scary move with crater potential below -- I place more gear. And I prefer to have both ropes connected to an anchor that is high enough on the pitch to prevent a crater when I believe a crater is a reasonable likelihood. This is easily possible over 95% of the time (either by clipping both ropes to the highest secure piece or by placing two pieces of pro and using runners to reduce drag where neessary) so there is rarely a need for me to expose myself to that situation which has repeatedly been described here as the reason that double ropes do not add redundancy.

[freeclimb9]

Climbing a big wall=Are half ropes relavant?

Jugging on a thin rope wouldn't be my preference (they'd wear out quickly, and be more susceptible to edge cut due to the smaller area they present to an edge).

[mattp]

"Jugging on a thin rope wouldn't be my preference"

 

No kidding! For fixed ropes on expedition climbs, I have seen 3/8" nylon boat rope and I've even used it upon occasion. But I don't see the advantage of the thin ropes when you're on a big wall. Hanging on, swinging on, and jugging on those things just doesn't instill confidence.