Discussion Topic |
|
This thread has been locked |
tradmanclimbs
Ice climber
Pomfert VT
|
|
|
Apr 27, 2009 - 04:03pm PT
|
F*#K YOU OSMO! you are just twisting what I say arround. for the f*#king reckord from day F*#king one of this post I have said that you can use doubbles as twins when the gear is bomber but you should use them as doubbles when the gear is not bomber. I have also posted repetedly that a single half rope is NOt softer than a single single rope. what the f*#k ever....
Dont twist my sh#t arround MFCKER
|
|
rhyang
climber
SJC
|
|
|
Apr 27, 2009 - 04:17pm PT
|
Some people do prefer twins, and my PMI verglas 8.1's are rated as both halfs/twins. A partner in Cody, Wyoming brought some beal ice twins (7.7mm ?) that seemed to work ok, though they left fuzzy bits 'o sheath on my reversino on raps ..
|
|
dirtineye
Trad climber
the south
|
|
|
Apr 29, 2009 - 12:43am PT
|
Gee that's odd... I could SWEAR that the times I've been caught on a single strand of mammut 8.5 genesis were softer FEELING than the times I've been caught on a normal fat single.
Hell the doubles are like rubber bands in comparison, but that's just how it felt to me.
But any way, it's funny how nobody has even mentioned (that I noticed anyway) the main reason for climbing on two ropes as opposed to one-- which is, if you drop one, you still have a rope, LOL. And if you think that's a joke, just try dropping your single sometimes, on a multi-pitch route, hehe.
|
|
Osmo
Trad climber
Calgary, Alberta
|
|
|
Apr 29, 2009 - 02:06am PT
|
Keep your shirt on, Tradman; once again you seemed to contradict yourself, but OK, it seems you think that a thick rope (or 2 thin ones) gives a softer catch than one thin one, which (and I hope to prove it) everyone knows is B.S. Even Jim E. said so himself, 10 posts up re clipping both ropes on the first piece, and he's the guy who did those tests--maybe HE also feels they don't apply to climbing? I think we'll get to the bottom of this impact force stuff yet....
|
|
tradmanclimbs
Ice climber
Pomfert VT
|
|
|
Apr 29, 2009 - 07:09am PT
|
I ain't contradicting sh#t.
|
|
jstan
climber
|
|
|
Apr 29, 2009 - 06:10pm PT
|
At the risk of stating the obvious:
If you integrate the force displacement/stretch product for an 8mm and an 11mm rope and evaluate it at the same final value of stored energy you find the ratio of the stretches are inversely proportional to the rope diameters. So the stretch in an 8 mm rope is almost 40% greater than it is for an 11 mm if we assume all other properties of the two ropes are the same.
You travel an extra distance with the smaller diameter.
But our problem is we have not really defined “softness”.
If peak force defines “softness” we have to think some more. When a rope stretches more, the total amount of energy in foot pounds to be stored is increased by the product of the climbers weight times the length of extra stretch. To get an answer to this definition of softness we have to know the physical properties of the ropes and do the full calculation. To do this we need to assume how much rope is in the system and we need to include frictional losses over the biner. It may even be possible the relationship between the two ropes' peak forces will depend critically upon some of these assumptions.
So those with already prepared spreadsheets, assume away!
|
|
That Darn French Guy
Trad climber
Santa Clara, CA
|
|
|
Apr 29, 2009 - 08:01pm PT
|
BTW, does it matter if the ropes have different specs when climbing doubles?
Unlike twins it seems like you wouldn't care so much.
MTtools lists static/dynamic elongations and impact forces:
http://www.mtntools.com/cat/rclimb/rope/BealFlyerDynamicRope.htm
Impact force (softness?) and elongation seem independent of rope diameter:
Twins- 07.4mm: 36% elong 7.4kN
Single 09.4mm: 37% elong 8.2kN
Single 10.2mm: 38% elong 7.4kN
Single 10.5mm: 37% elong 7.4kN
Clipping doubles together increases impact force and reduces elongation:
Joker twin-- 9.1: 29% elong 9.5kN
Joker single 9.1: 37% elong 8.2kN
|
|
rgold
Trad climber
Poughkeepsie, NY
|
|
|
Apr 29, 2009 - 08:53pm PT
|
I'm with Curly on this one (ok, ok, a bit of an in-joke---I'm speaking of Del Cross here). Nowadays engineers model viscoelastic materials with combinations of springs and dashpots in either series or parallel or both, so the pure spring analogy is tempered with varying types of viscous damping. And that's just the material part. Ropes have different internal constructions, different sheaths, and the nylon is heat-treated in varying proprietary ways. Nor is it clear that, even from a Young's modulus perspective, that one can compare cross-sectional areas just by knowing the respective diameters. The actual cross-sectional areas are the sums of the the cross-sections of the fibers, which do not occupy all of the main circular cross-section and may themselves be of different diameters, depending on the manufacturer and the rope diameter.
Tests by Attaway seem to verify that the entire system, during extension, does behave in an essentially linear way, so there is a system stiffness that is well-characterized by a Hooke's law constant. (This is in contradistinction to some of the pure rope load-elongation curves, obtained by slow pulling rather than dynamic loading, that suggest linearity falls off at either end.) The relation between the system stiffness and the rope stiffness with accompanying damping coefficients is not clear, as far as I know, (and I really don't know very much!) Perhaps the answer lies in Pavier's rope model paper, which I have only perused.
All of this just adds up to saying, as Del has already observed, that a simple Young's Modulus argument may not be decisive.
I don't think Jim Ewing's tests are the first to suggest that there may be little difference in the maximum impact forces for different diameter climbing ropes. I believe that the CAI section on Materials and Technique found similar results. I also think we should all be grateful to Jim for the testing he is able to do on his own time, and be very restrained about demanding more results ever, not to mention right away. His work is pro bono in the interest of the climbing community, and it is unseemly to suggest that he is in any way obligated to do more.
How does one explain the subjective climber's sense that half ropes provide softer catches? First of all, one cannot discount the psychological effect of myth. How many people take exactly the same fall on a strand of half rope and then on a full rope? Mostly, I suspect that people are saying something like, "wow, that was softer than I expected..." whereas in fact what they expected was a softer catch. So I am not persuaded that some of the things that all climbers "know" are actually true.
I am also not persuaded that some of things "known" by engineers are actually true. We have the Tacoma Narrows Bridge, the Ford Pinto, the space shuttle Challenger, and Kansas City Hyatt Regency Walkway, for example, to remind us of their sometimes catastrophic fallibility. On the other hand, people who place all their faith in experimental evidence typically understand neither experiments nor theory. One needs both.
The CAI hypothesized that half ropes slip more through the belay device, thereby reducing peak impacts, but that doesn't seem to correspond to the vast majority of practical catches when little or no slippage occurs. However, I suspect that most belayers are not conscious of what the CAI calls "inertial" slippage, which occurs when the brake hand is drawn towards the belay device without any rope running through the brake hand. Since this is the first thing that happens and typically just precedes the moment of maximum impact force, it is conceivable that this type of slippage does play a role in reducing peak impacts with a half rope without the belayer even realizing it.
It also seems possible that half ropes, used properly, make fewer bends through all the intermediate carabiners, so that the ability of distant sections to participate in Hooke's law energy absorbtion is greater than it would be for a single rope. (Remember that friction is exponential in the bend angle, so that small differences could be significant, especially cumulatively.) Since the climber experiences system stiffness, not rope stiffness, and the bends around biners contribute to system stiffness, it seems plausible to me that the perceived softer catches are an artifact of straighter rope paths.
Another issue, by the way, is that it takes force to bend a rope over the top biner, and that too will detract from the full rope length's elastic capability. Smaller diameter ropes will need less bending force.
Finally, one might wonder if the subjective sense of softness for half ropes, even if it is not based on prior expectation, might be possible even though the impact forces are the same for half and full ropes. John has rightly asked what that subjective sense of softness actually is.
One claim I'd like to see documentation for is that the UIAA testing drives the rope into the plastic deformation range, making the results less relevant to smaller falls that are held elastically. Note in this regard that one has to distinguish between plastic deformation and the effects of damping, which might make the rope recover its initial length rather slowly, without however having undergone plastic deformation.
|
|
tradmanclimbs
Ice climber
Pomfert VT
|
|
|
Apr 29, 2009 - 11:04pm PT
|
RG, I would argue that precieved softness is just that, precieved. There is no way to prove or disprove the level of precieved softness. I do know from several decades of experience that most falls are softer than we expect them to be. Often we stress over how bad it would be to come off right here and then we do fall and its no big deal. WOW! that was soft and easy. Guess I will go up and send now,. This holds true regardless of the rope system be it Single , half or twin. This is a big part of Arno's Warrior way theory. My problem with the Just let go theory is that when I get cocky and lose my concerns about falling everything is just fine untill something goes wrong. I have learned over the years that while most falls are softer and safer than expected a few falls are much, much worse than expected but that is for a diferent thread.
Allmost Without ecxception the falls that felt hard to catch or really yanked me were either sport falls on the first or 2nd bolt or short trad falls close to the ground. 8ft fall on 20ft of rope kind of thing.
I did have one hard longer trad fall when gear ripped that felt pretty hard. 25ft fall on 50 ft of 10.6mm Sterling marathon rope, ripped 3 pieces. I felt that with doubbles I may have been able to keep the direction of pull correct so that the 3rd piece may have held. The first 2 pieces were gonners regardless of what I did short of an intrevention from god..
The point of all this is that I nor anyone else can prove that one fall FELT softer than the other unless there is a drastic diference in the type of fall. We can tell the difference between the feel of a factor 1 fall and a factor .15 fall but not the difference in in those same falls experienced with diferent rope systems. Especially if the events happen weeks, months or years apart and are not the exact same falls.
My last fall on doubbles was caught on a single strand of 8.8mm sterling marathon half rope. I had close to 100ft of rope out nd fell 10 or 12 ft. man that was one soft fall. My last real fall on a single rope was a bout 10 or 12 ft on 60 ft of 10.2mm Beal rope. That one felt really soft as well? which was softer? I honestly couldn't tell you.
|
|
Dr.Sprock
Boulder climber
Sprocketville
|
|
|
Apr 30, 2009 - 01:45am PT
|
OK, see if this summary makes sense:
Long Fall 80 feet - 100 kg drop test:
8 mm static will stretch the least = 5.75 meters and 4.5 kN
then a 10 mm static = 8.25 meters and 4.8 kN
then a 11 mm dynamic = 8.75 meters and 4.10 kn
then a 8.8 dynamic = 11.25 meters and 3.54 kN
Short Fall 6 meters - 100 kg drop test
8 mm static 3.85 meters and 6.6 kN
10 mm static 4.2 meters and 7.3 kN
8.8mm dynamic 5 meters and 5.5 kN
11.0mm dynamic 5 meters and 5 kN
|
|
Dr.Sprock
Boulder climber
Sprocketville
|
|
|
Apr 30, 2009 - 11:42am PT
|
i think the small static line is kind of like clothes line, you know how when you pull on clothes line, and it stretches like crazy for a short time, then gets tighter than heck, right before it snaps.
i am afraid that is what we are seeing with the droop figures.
are the kN figures different at each end of the rope?
the harness has webbing, and the body absorbs energy, but the top anchor is solid, so there are some non linear things happening along that rope during stretching?
see, i was smart enuff to the grunts do all the work, all i have to do is mouse around. leaves more climbing time.
this took me 5 minutes to search:
(pdf is easiest on a 600 by 800)
http://209.85.173.132/search?q=cache:7pMKITus-3wJ:www.mra.org/services/grants/documents/Using_low_stretch_ropesFinal.pdf+a+comparison+of+stretch+forces&cd=1&hl=en&ct=clnk&gl=us
|
|
Osmo
Trad climber
Calgary, Alberta
|
|
Wow, you guys!--what a lot of great discussion!
The rope is being dissected and it's obvious that there are a heck of a lot of angles to that simple four-letter word. On the other hand, I'm pretty sure that only very few of the characteristics and differences are significant. But in any case, as I said at least a couple of times before, when comparing impact forces in ropes of different sizes, they really have to be the same TYPE of rope: same polymer, same heat-treatment, same manufacturer, etc., so when we see a list of rope sizes and impact force/elongation combinations, we can only hope that the tester really cared to be fair by using only ropes of the same materials, as the numeric results quickly become impossible to correlate with each unrelated difference that's thrown into the mix.
As for effective cross-sectional areas, it's true that different ropes have different densities, and I think it may have been from Beal that I most recently saw a comment that a more accurate way to rate rope diameter is by weight per unit length, which I've considered for a long time, since seeing how some ropes were tightly woven while others were quite loose. Another practice used to address the same problem is to tension a rope somewhat while measuring--apparently you should know the approximate diameter of the rope and apply a tension proportionate to the X-section area.
I don't think Jim Ewing's tests need to be defended against perceived criticism, from me or anyone else: clearly he does the tests prescribed by the UIAA, which definitely have a good purpose, and I'm sure it's clear that I applauded his extending the 80kg tests to half ropes, plus who knows what other tests he does? And I don't think my comments to him can fairly be taken as any kind of a demand. A suggestion is another thing, and surely reasonable in view of the apparent confusion among climbers, about impact forces. And besides, it may be easy enough to rearrange a drop tower to investigate impact forces resulting from realistic fall factors--if so, what could be the harm in a suggestion like that? I think Jim E. can take care of himself. However, I was serious about doing some of my own testing, and will look into it as soon as the ice is out of the creek.
A lot of good comments also passed on the subject of 'softness' of catches. (One thing I noticed is that little if any mention was made of the two different views of a fall: that of the climber and that of the belayer, but I think most climbers realize that the major impact is on the protection, then on the climber, and finally on the belayer, unless there is no protection.)
In any case, it seems to me that there are two main components to the softness or hardness of a fall experienced by any component in that chain of protection: one is the peak force, and the other is the impulse, which is basically the product of the force and its duration.
It could be that a peak force seems softer if it takes longer to build up and then release, as in the case of a long fall with the same fall factor as a short fall. A couple of people have mentioned that falls that seem hard are commonly short ones--think of the difference between someone jabbing you with a finger, and then putting the same finger against you and pushing hard: the jab will likely get your attention more than the push, even at the same force. On the other hand, if there's any difference between the effects of the two impacts on protection, if a piece is marginal in respect of the impact force, the long fall/long impact duration is more likely to tear it out than is the brief impact.
The Psychological effect of myth is an interesting point, too, and worth considering, but then, just how does a myth get started? Is there a difference between BEING happy, and just THINKING that you're happy? Is there a difference between a soft fall-stop and just THINKING that it's soft? Although it may be true that very few people fall often enough to be sure of a fair comparison of impacts from different sizes of ropes in similar fall situations, I have no doubt that if you took a fall on a thin rope and expected a soft stop, but instead got a bone-jarring yank and hurt your back, you would realize that it was a hard stop, no matter what the rope looked like--so I just don't buy the "expected soft catch" concept very much.
And I assure you that whenever I say that "I" or "all climbers" "know" something, I'm always pulling someone's tail to some extent, because I'm well aware that it sometimes happens that things well-"known" turn out to be a bit less than completely true.
And that even applies of course to things "known" by engineers, though the Tacoma Narrows Bridge is possibly the only fair example mentioned, of engineering 'in discovery'--that's the thing about engineering, like ALL science: it's always developing and improving, and I doubt you could find an engineer anywhere, who claims to know everything.
The Ford Pinto was largely an issue of esthetics and quality control, and yet, I know a couple of guys who even now, many years and several cars later, still nurse fond memories of their old Pintos as the all-time favorite, so maybe the designers and even the Engineers, got something right on it; the space shuttle Challenger was a disaster, not of engineering, but of political interference, and the Hyatt Regency Walkway failure resulted from on-site exchange of materials by the contractor, rather than bad engineering.
True enough, experimental evidence and engineering are not separate fields at all, though some people who don't realize it, love to claim that they're unrelated. After reading some erudite comments on ropes in the last several posts, I suspect that I'm not the only engineer contributing to this forum anyway, though no one else is owning up to it.
And finally, someone mentioned Pavier's rope model, but unfortunately, I actually went climbing, and so haven't had much chance to look it up, and haven't found it yet--very interested to see what it is. I only hope it's very simple, as a simple approximation would almost certainly be adequate (and so, BEST) to describe the behavior of ropes as climbers see them. Unfortunately, such models tend to be overblown because they naturally become vulnerable to criticism if they neglect a number of characteristics, even if those are negligible in the overall performance of the device--the climbing rope in this case.
|
|
dirtineye
Trad climber
the south
|
|
Someone please throw in Impulse as a consideration in the GREAT Softness Percetion Debate, LOL.
And, don't you just think even a little that spreading the force out over a longer time would make it less JERKY?
THe way I used to understand it, Jerk was a term for third derivative, and had to do with, well the actuall jerk that you get when you yank on somethign hard and fast as opposed to long and slow. Seems like that would have application here as well.
|
|
tradmanclimbs
Ice climber
Pomfert VT
|
|
seems pretty clear that what we thought to be true is not. Clipping a single 1/2 rope to a manky piece is not a get out of jail free card. You do however have better tools with the 1/2 rope system for dealing with mank. easier to equalized with the 2 ropes and easier to have correct direction of pull. just don't kid yourself that the forces are automaticly lighter just because you are useing doubbles. You can MAKE the forces lighter by useing the system correctly.
Furthermore My pet peeve and the motovating factor for me throwing a bone in this debate is DON'T effin tell me that I am gonna die because I ocasionaly use doubbles as twins when the gear is bomber.
|
|
Whitehorse Jeff
Trad climber
Fairfield, CT
|
|
In answer to the oft repeated question:" Why do they test half ropes singly with only 55kg", I was the American delegate (of the AAC) on the UIAA Safety Commission when the Half Rope Standard was put in place. Tests done at the time by Pit Schubert of the DAV (German Alpine Club) showed that any half rope that would withstand 5 UIAA test falls with a 55 kg test mass would also withstand one UIAA test fall with the 80kg mass. The argument presented to the members of the Commission at the time was that it would be impossible to get the lighter ropes which climbers (and guides ) wanted, to pass a harsher test, and that the real goal was to insure that a half rope would sustain at least one of the worst imaginable falls in the field, that being the fall represented by the UIAA single rope test. The repeated falls caught with the 55kg mass were accepted as the minimum repeatable test standard to insure this desired result. Over the approx 20 years that we've had the half rope standard , the Standard has proved to be high enough. (before the UIAA Standard was approved, people just climbed on half ropes without UIAA approval, assuming that both ropes would never be cut at the same time in the same fall, which has proved to be true in practice).
I accept that this reason may seem less than scientific to many. It was based on many tests carried out by the different certified UIAA test labs at the time, and as I remember was passed in near, if not total, unanimity by the voting members of the Commission, National delegates and Rope Manufacturers alike. There is one thing that encourages me at present as to the wisdom of this decision at the time-- rope manufacturing has made huge progress over the past 20 years, and this test still seems to hold up. Modern half ropes are much better than the ones I first used in France and Great Britain in the 70's.
|
|
rhyang
climber
SJC
|
|
Very interesting.
|
|
That Darn French Guy
Trad climber
Santa Clara, CA
|
|
"any half rope that would withstand 5 UIAA falls with 55 kg would also withstand one UIAA fall with 80kg"
A-ha!
So a double rope *is* meant to catch a fall on a single strand.
The problem of that spec is everything is reported at 55kg: An 80kg person doesn't know what increase in force and elongation can be generated and IF IT IS STILL SAFE for his back.
Since we have ropes that are rated twin/double/single at 9.2mm, they shed some light there:
Joker:
55kg - elong 32% - force 6.0kN (double)
80kg - elong 37% - force 8.2kN (double)
+46% - elong 16% - force 37%
So can I expect to read a double's spec and say if it's rated 8kN at 55kg, then my max impact force for my 80kg ass will be a hefty 11kN?
|
|
Whitehorse Jeff
Trad climber
Fairfield, CT
|
|
"The problem of that spec is everything is reported at 55kg: An 80kg person doesn't know what increase in force and elongation can be generated and IF IT IS STILL SAFE for his back."
The main concern is that the rope will hold one Factor 1.78 (or so) fall with an 80kg mass. My main point in leading is to avoid such severe falls, especially since even without a pack or a rack of gear I always exceed the 80kg mass weight. I accept that falling off is likely to be DANGEROUS for my back or other parts. I use a rope so as to still remain attached to the part of Mother Earth I am attempting to climb.I also accept that climbing is not really Safe and that falling off is often quite Unsafe. I don't mean to be flippant-- our activity (addiction?) is a "fine sort of madness" isn't it.
|
|
tradmanclimbs
Ice climber
Pomfert VT
|
|
Jeff we all know that climbing is dangerous but it seems to me that the origional thiought process was a bit skewed. Man these ropes break way to fast when we use the 80KG weight so we better make it lighter. That may not have been the intention but that is certainly how it looks.
|
|
Osmo
Trad climber
Calgary, Alberta
|
|
|
May 24, 2009 - 01:23am PT
|
Things have been quiet on this thread for quite a while--everyone's cooled off. But quite a few guys have questioned the 55kg tests for half-ropes over time, and I'm not sure that they know yet, that there's a very good and valid reason for the 55kg. A comment like this last one from Tradman is a cute anecdote, but may only confuse them even more, as it's not true at all.
But "Francis" got the explanation straight from the UIAA about a month ago (Apr. 20, 2009), to the effect that the UIAA standards require a 'single' rope to hold at least 5 test drops before breaking; and they require a HALF rope to hold at least ONE drop in the same test, because it typically holds an entire fall on its own in double-rope climbing, when the second-last piece is some distance below the last one. Now here's the point: one is an awkward number because if the rope survives one drop but breaks on the second one, it could be that it BARELY held ONE, or it could be that it NEARLY held TWO, or anything in between. It would be nice to know a little more, since conditions, age, or manufacturing irregularities could push the result over the line at either end, particularly below the one-drop end. So the UIAA looked for a drop-weight that would abuse the rope the same amount in 5 drops as the 80kg would abuse it in ONE. That turned out to be about 55kg.
Now the testers can drop away, and have more data to compare between different ropes or different chunks of the same rope: if a particular rope never breaks on the 4th drop, and sometimes makes it to 7 or 8, they can say it's definitely a solid half-rope. But if it sometimes breaks on the 3rd or 4th drop, they may eye it suspiciously, and re-rate it as a twin or send it back to the lab for beefing up.
|
|
|
SuperTopo on the Web
|
Let us know!