80m rope stretch

How much will a 80m 9.5 stretch in general? What's the longest pitch you could rap off of safely?
Thanks

If you are counting on rope stretch to get you to a safe spot, you are gambling with your life. #Mistake

Have you even measured this rope to know its true length?

Well worst case scenario when you are counting on rope stretch to rap more than the static length of the rope (40m) is you go off rappel and lose the ends which magically snap back out of reach and if you are the real lucky type (which you sound like and not just being from Vegas) the rope starts whipping through the anchor (helped by wind mebbe) and blows around the corner as it falls through open air leaving you stranded.......

And this can get real tricky if your rope has no middle mark and no knots in the ends.

And seriously, read the tags, different ropes have different stretchy thingy numbers......

In general, who knows? But you can look up the static stretch in an Mammut Infinity 9.5 and it is 6.8% (for an 80 kg mass). So for a single rope rap of 40m (rope doubled), the rope will stretch half as much (springs in parallel) for a grand total of 0.5*0.068*40m = 1.36 m or about 4.5 feet. You can look up the static stretch for virtually any climbing rope.

Of course, your weight, the actual cut of the rope and the age of the rope are likely more significant so I wouldn't count on this for too much. I think Mammut, for instance, always cuts their ropes long to account for shrinkage due to aging and wetness.

No certified single is allowed to stretch more than 10% under 80 kg so you are looking at a maximum stretch of 2 m or about 6.5 feet (for a single (doubled) rope rap). This will, of course, scale with your weight. Personally, I wouldn't depend on rope stretch to get me to the next set of rap anchor.

Key word: safely

Thanks xam. This just happens to be the infinity. I thought I might get a decent amount of stretch with the small diameter but essentially the answer is not much. I'll stick with 40m until I get a feel for the rope.

Xam wrote: So for a single rope rap of 40m (rope doubled), the rope will stretch half as much

Not quite. Ropes have non-linear elasticity curves. When you put two in parallel, the modulus of elasticity of the system changes completely. Take the Beal Joker for example. It's rated as a half, twin and single. With 80kg as a single, it has a static elongation value of 8%. Add a second rope to make it a twin and it only drops to 7%.

The good news for the OP is is this means the rope would be longer than it would be if the elongation actually dropped by half.

20 kN wrote: Not quite. Ropes have non-linear elasticity curves. When you put two in parallel, the modulus of elasticity of the system changes completely. Take the Beal Joker for example. It's rated as a half, twin and single. With 80kg as a single, it has a static elongation value of 8%. Add a second rope to make it a twin and it only drops to 7%. The good news for the OP is is this means the rope would be longer than it would be if the elongation actually dropped by half.

If I rap from a rope doubled through an anchor, have I created two parallel springs or one extra long spring?

Oh and......

From the real world: WHAT you are rappelling matters too!!!!

I.e. free hanging vs. a high friction rap vs a rap with a kink or two vs. straight line.

Not to rain on 'nerd lab' but rapping IS outdoors........

be aware that as a rope gets older and more used they often shrink ...

ive had ropes which reached certain rap stations fine when new, but then barely reached them a few years latter ... its not the rock which has gotten bigger =P

if you are going to be doing raps without much margin .... make sure you practice reascending the rope off a rappel ... many folks cant do this in practice even if they know the theory ... its an essential skill

;)

Tx bear, good advice. My goal is to never carry 2 ropes again. Epics in the making

Seriously tho....

If avoiding two ropes is your motivation do keep in mind two things:

1. 80m is a Ton of rope to be lugging around, coiling, pulling etc., especially on shorter pitches, especially on shorter pitches on multipitch.

2. A lot of multi pitch raps are set up permanent anchor wise for two 60m ropes and 80m ain't getting you there.

3. Getting another 20m (i.e. HALF as long as 40m) makes multi pitch raps much faster and less epic.

mark felber wrote: If I rap from a rope doubled through an anchor, have I created two parallel springs or one extra long spring?

Two parallel. If the stress-strain curve of a rope was completely linear, we could say adding one more spring halves the elongation, and two more on top of that halves it again. But the stress-strain curves of dynamic ropes are not linear, which is why adding a second rope does not necessarily halve the elongation. Just look at the static and dynamic loading of a rope for an example. Let's use the Joker as an example again. It's rated as:

Impact force: 815 kgf (converted from kN to make things easier)
Dynamic elongation: 37%
Static: 8%

UIAA test is FF 1.78 with 2.3m of rope on the "climber side" and 0.3 on the belayer side.

In the case of the Joker, there would have been roughly 1798 kgf on the "climber side" of the rope during the test, and about 40% less, or 1078 lbf, on the belayer side.

Thus, during dynamic loading, very roughly, the rope would have seen 815 kgf along 2.5m of its length, and 489 along 0.3m of its length. We will just simplify that and say it was about 700 kgf on average across the length of the test sample. So 700 kgf under dynamic loading gets you to 37% elongation.

On the other hand, the static elongation, which is 80 kgf, gets us to 8% elongation. If we look at this from a percentage prospective, going from 8% elongation to 37% elongation requires very roughly 10.2x the load for only 4.625x the elongation. In other words, not linear.

I'm not here to argue that climbing ropes are linear, and obviously most rappelling is done outside, but it may be worth observing that:

• 1798 is the translation of 8 kN in lbf, not kgf. Bottom line: the 18.75 ratio is off by a factor of 2, roughly.
• The static elongation numbers reported by manufacturers are upper bounds. I would not use them when I need a lower bound, as in "how much stretch can I count on?" For instance, Tendon reports the exact same static elongation for the Master 8.9 regardless of whether one or two strands are used. Does it mean that changing the load mass from 40 to 80 kg does not change the stretch of one strand?
• Static elongation is not affected by damping, but dynamic elongation is. Even a linear system, if it has any damping, will "look stiffer" when stress is applied rapidly.

brenta wrote:I'm not here to argue that climbing ropes are linear, and obviously most rappelling is done outside, but it may be worth observing that: * 1798 is the translation of 8 kN in lbf, not kgf.

Oops, you're right. Corrected.

I have an 80m infinity and a pair of photons from Sterling. If rapping is your motivation for the 80, save your money. Rarely has it saved me a rap, most of the time I break it out to cut down on pitches climbed if I know i can skip bolts on a near vertical line, etc. (i.e.Sunset Streaks or Leonids on ECM in 2 pitches from the mid 2nd pitch bolts, but its still 3 raps from the top) I have been told you can rap the route in two with two 60s. I have yet to bring two ropes to a place set up for a single 60 (ECM, Corte Madera), but I am able to climb more routes in less pitches with the 80. And if I am climbing at or near my limit I am spent at the end of a 60m pitch, no chance of hanging on for another 20...

80 has also been nice in places like Josh where some rap stations are long with a 70 (Hemingway), alpine butterflying a third to the party in the middle of the rope, or for TR super pitches. I got mine on sale figuring I could chop it a few times making it an economical decision.

Maybe one day I will set it up on something for TR solo and get 80m super strong, but probably not.

20 kN wrote: Not quite. Ropes have non-linear elasticity curves. When you put two in parallel, the modulus of elasticity of the system changes completely. Take the Beal Joker for example. It's rated as a half, twin and single. With 80kg as a single, it has a static elongation value of 8%. Add a second rope to make it a twin and it only drops to 7%. The good news for the OP is is this means the rope would be longer than it would be if the elongation actually dropped by half.

Ok...that's reasonable. Basically, we are talking about a factor of 2 in the numbers I put up above. Still, I wouldn't count on it to get me to the next rap anchor if I am cutting it close.

20 kN wrote:But the stress-strain curves of dynamic ropes are not linear, which is why adding a second rope does not necessarily halve the elongation. Just look at the static and dynamic loading of a rope for an example. Let's use the Joker as an example again. It's rated as: Impact force: 815 kgf (converted from kN to make things easier) Dynamic elongation: 37% Static: 8% UIAA test is FF 1.78 with 2.3m of rope on the "climber side" and 0.3 on the belayer side. In the case of the Joker, there would have been roughly 1798 kgf on the "climber side" of the rope during the test, and about 40% less, or 1078 lbf, on the belayer side. Thus, during dynamic loading, very roughly, the rope would have seen 815 kgf along 2.5m of its length, and 489 along 0.3m of its length. We will just simplify that and say it was about 700 kgf on average across the length of the test sample. So 700 kgf under dynamic loading gets you to 37% elongation. On the other hand, the static elongation, which is 80 kgf, gets us to 8% elongation. If we look at this from a percentage prospective, going from 8% elongation to 37% elongation requires very roughly 10.2x the load for only 4.625x the elongation. In other words, not linear.

I don't know about this one...you are comparing the response of a system at (a) very different strain rates and (b) very different strains. For (a), as you know, the high strain rate response and the quasi-static response of a system are very different and generally not directly comparable and you would not expect the same type of response. For (b), all material systems are non-linear when observed over their entire stress-strain curve, so to compare them at radically different points in that curve seems a bit disingenuous.

Still, since both regimes are within the intended use of a rope, it is pretty compelling that these systems can't be considered linear when looked at across the practical range of use.

Xam wrote: I don't know about this one...you are comparing the response of a system at (a) very different strain rates and (b) very different strains. For (a), as you know, the high strain rate response and the quasi-static response of a system are very different and generally not directly comparable and you would not expect the same type of response. For (b), all material systems are non-linear when observed over their entire stress-strain curve, so to compare them at radically different points in that curve seems a bit disingenuous. Still, since both regimes are within the intended use of a rope, it is pretty compelling that these systems can't be considered linear when looked at across the practical range of use.

You are correct, dynamic loading affects the stress-strain curve differently than static loading. I was only trying to illustrate a rough example given the very limited information provided by the manufacturer to demonstrate that doubling the quantity of ropes does not simply halve the elongation.

You can get a few extra feet by simul rapping. Just FYI.