A compressive source of tension?

General Discussion
1

I have been hung up for some time with the preconception that a cord breaks because of the tension it is under. A weak fibre somewhere snaps under the tension and dumps the load it was holding into its neighbouring strands. One of these exceeds its breaking point and it likewise snaps, dumping its load into the remaining fibres, then suddenly, many fibres find themselves subjected to a load in excess of their breaking point and they all let go together and bang, the whole cord fails.

By thinking this way – that tension snaps fibres – I had created a mental image of knots failing because in tight radius bends, the outside strands were heavily strained, so any weak fibres would be quickly overloaded and snap, again the cascade of shedding force into fewer and fewer fibres would result in a break of the cord.

But is this perception correct? For some time now there has been the contradiction of the many claims that cords break ‘just outside the knot’ – how can this be? Did the cord really break inside the knot on the tight radius, but because of shape changes as the pressure was released, it just looked like it broke outside of the knot? Certainly in my own trials using a polyester braid, there was absolutely no question where the cord broke – on the first load shedding tight radius within the knot, and all my samples showed this quite clearly. Yet when I tested a cotton cord, the knot shrank to a characterless ‘stone’ and there was every indication that the cord had broken just outside the knot, there was a clear tuft sticking out of the rock solid knot. It did not fit my perception, I could not understand it, so I sort of ignored the contradiction and put it down to the fact that it must be something weird to do with the cotton.

BUT, have I been missing something. Is compression an important part of cord/fibre failure?

Considering for a moment cooked spaghetti. Get hold of a piece and pinch it between your fingers - no surprise - it breaks, compression made it break. Now take a strand, hold it gently but firmly in each hand and pull it to try and break it. Chances are that most times the break will come in your hands, as you are holding it tight to stop it slipping and the pressure of gripping it makes it break, again, compression made it break, not tension. Could it be that compression might be a more important mechanism of breaking than tension – it certainly seems to be so in mono-filament spaghetti.

So how could compression cause a mono-filament to break? Obviously if the compression comes from opposite shear forces, the cord is effectively scissored through, but in knots we are not looking at any scissor/shear style structures. In knots we generally have rather soft compression surfaces - the internal radius of a tight bend or occasionally the constrictive compression of a number of round turns. Cold this extreme, but wide spread compression be a cause of breaking threads?

Consider for a moment a packet of Bassetts Liquorice Allsorts, or more precisely, one of the round squidgy ones covered with little blue balls (OK, yes they are my favourites). Hold it across its diameter between your finger and thumb and then squeeze. As you squeeze, the diameter between finger and thumb gets narrower, but the diameter at right angles to this gets bigger. A compressive force in one direction is making an expansive force at right angles to it. Consider then, this liquorice allsort is a tiny slice out of one of the fibres in a cord and that it is under tension along the fibre – the tension is trying to rip the fibre apart. If you were now to squeeze the fibre this would be imparting a tensile force along the line of the fibre – in other words, squeezing the fibre actually increases the tension in the fibre at the point you are squeezing it. In the case of the Liquorice Allsort, if you squeeze hard enough, the middle eventually splits, allowing the bowed out sides to move away from one another. With a cord under tension, the addition of squeezing it either on one side or circumferentially adds to the existing tension, and at some point the combined tensile forces break the fibre.

To add substance to this perception, I have just completed some tests using the Adjustable grip hitch tied in 2mm Polyester cored braid. By carefully noting the position of the cord before the break occurred, it can be seen that the bulk of broken fibres emanate from the heart of the four round turns which encircle the loaded line running through the heart of the hitch. The extreme compression presumably added extra tension to the load tension and at the point that the additional tension existed , the threads snapped and pulled out as the cord failed.

If this mechanism is at work in a purely compressive knot, then what combination of forces might be at work in the first tight load shedding turn that is the weak point in most ‘conventional’ knots?

Derek

2

There’s some papers at
http://allaboutknots.com/papers.html
about knot strength and types of breaks.
This author distinguishes between breaks in natural fiber, and synthetic fiber where melting can be a greater factor. He also distinguishes between failures in ‘bowline’ type knots (most types), and core-and-wrap knots (e.g. double fisherman).

In bowline case, he argues that most failures occur at the entry to the first curve.

This curve distributes the heavy load unevenly, causing the knotted rope to fail at the entry point.
http://allaboutknots.com/pdf/7_breaking.pdf

paulj

3

It would bode better for these theories were evidence consistent with them.
In the case of the Bwl, there was testing (actual, not mental) done of them and
an effort via high-speed camera to pinpoint the place of failure: it was (as one
should think with but a moment’s reflection!) somewhere in the nipping turn of
the Bwl; it appeared to be somewhere in the collar (i.e. of the SPart) of the Fig.8
loopknot.

It’s also apparently worth reiterating that many cited studies are (1) poorly done
and reported, and (2) of natural-fibre cordage (if the study is old). For the most
part, the break point isn’t noted–e.g., EVEN as to which rope in a Sheet Bend
parts: that’s an asymmetric bend, so there might be a predicable weaker part,
but I’ve never seen this mentioned (it would e.g. have some implications for
whether the bend got stronger with one or the other side being thicker–and
this seems relevant as the bend is often recommended for diff.-dia. ropes).

In some broken laid rope that I had tested long ago, insofar as I could determine
from trying to trace back the position of the broken strand or two (in all cases,
the testing was stopped upon the failure of a strand (or two), so pieces were
intact end-to-end (loopknots & bends)), the broken strand was on the CONCAVE
side of a bend, where it was COMPRESSED.

I’ve read accounts of breaks coming well outside of the knot–e.g., in solid tape
presumably of some considerable (3 tons, say) strength tied in an Overhand knot
(LK, I think). I’ve seen photos of smallish stuff (half inch to quarter inch) in nylon
where the breaks appear to be within the knot.
Again, if the misc. home testing we are trying to encourage can just give better
data on Where…? , we will be advancing the science of knotting–nevermind
some ranking (which needs often close tolerances of various factots, notably
cordage and force(rate)).

–dl*

4

What material was this rope?

In a sense, all strands experience tension (lengthwise stress), compression (radial inward stress), and bending. The outer ones experience more tension, and less compression. The inner ones have a bit less tension, but higher compression. I’m getting rusty on continuum mechanics terminology, but there are names for the test case where you just apply stress along one axis of the test object, and for the case where you apply both the stress in one direction, and stress radially.

I can imagine a thorough study that first looks at actual knots to see where the failure occurs, and then simulates those conditions in a smaller structure, such as a length of rope looped around a metal bar. A further step is to find or develop a computer model, possiby using finite elements, to look at the strains and deformations in a knot like structure.

paulj

5

The rope was 1/4" laid nylon. I’ve also a broken Oh. loopknot from a
lobster-pot bridle in which one side is broken, and that seems to be
at the concave part (7/16" laid PP). And one time I broke (partially,
again) some smaller (3/16"?) natural-fibre (manila/sisal) rope around
a hook, and there too it was the compressed strand, as best I could tell.

As for tension distribution, hmmm, if the compressed fibres are fairly
locked in postion as outer ones can stretch over top of them, there
might be a good bit of tension in them, too–getting little feed/relief
from farther-in-the-knot parts (i.e., the effective length is reduced?).

One can also see in knots effects of torsion–especially conspicuous
in braided rope where the strands going one direction all arc upwards
in obvious over-relaxation, their counterparts going the opp. dir. being
rigidly tight–and that’s close to a 50-50 split! (With laid rope, ALL of
the fibres/strands will be so torqued.)

–dl*

6

Evenin? all, evenin? Derek

First and foremost Derek your use of food is perfect for ME.

As previously indicated I am IGNORANT in the subject of cord / rope rocket science. Until recently I knew nothing of the subject other than what I was taught and before the guild came into existence.

I Tied a knot using a particular cord and knot?. because that?s what the book said. There were no explanations why. On the other hand I have seen : what when and where cords break?which goes back to the second sentence ? there were no explanations why ?

MY EYES ARE BEING OPENED TO A NEW SCIENCE

There?s plenty of books out there with the suffix: X, Y or Z ? for dummies ? Not for this subject though.

Being an outsider?perhaps I have an alternative view on things or.. just a fresh pair of eyes. Daft, ignorant or stupid?..you chose ?

This leads, no pun intended, me onto my first question. How do manufactures test their product ? More importantly how do they attach their ? test sample ? to a test rig?

Unfortunately I have put on my engineers hat??. In that respect cord /rope testing should begin with the manufacturer. What standard do they follow: BSI, ISO or another ? What percentage of batch production do they test ?

That?s without considering the test subject?s properties: Nylon, polyester, sisal, manila and others.

Final question. Who?s spaghetti did you compress Derek : ASDA, Tesco, Sainsbury or Waitrose ? Apologies I couldn?t resist that example of cost / quality / taste / ;D

Final plea???.not to much anti aircraft fire please. I did work on aircraft made of canvas and string, not these ultra modern contraptions.

Regards

DerekMW

7

I believe rope samples are secured in the test rig by wrapping them around a large diameter bar.

Consider, for example, how line is attached to a winch. A fitting of some sort is secured the end of the line (a compression fitting), and this is screwed to the drum. This anchors the end of the line without having to take up much of the load. The load is taken up by multiple turns around the drum. A search on ‘amsteel blue’, an line commonly used for 4x4 winches will give more details.

Also, a well made eye splice is supposed to be nearly as strong as the line.

For my hand testing with 50lb fishing line, my standard is an eye splice, which is easy to make in hollow braid. Nearly as good is one the hitches that wraps around the anchor (a carabiner) multiple times.

A search on ‘logging amsteel’ turns up a number of papers based on a study done at Oregon State on the potential use of this light but strong synthetic line as a replacement for steel wire in the logging industry. This line has very low stretch, and also a low melting point, so knots weaken it more than most other lines. The OSU papers allude to their intent to test one knot, but they don’t name it.

paulj

8

Hello All,
in a fit of inspiration I typed “knot testing” into the search engine and here is just one of the many hits that came up.
http://www.caves.org/section/vertical/nh/50/knotrope.html
It has a picture of the rig testing a knot and some data, rope break versus knot break, etc.
Seems to me

Dave Richards, Technical Director, Cordage Institute
706 Lakeview Dr., Sugar Land, Texas 77478

should, might, could be of some assistance with any testing.
Perhaps a Guild member of good standing would consider making contact.
Cordage Institute hmmm.
2 cents
SS

9

I have stated this before but I’ll reiterate it again.

I use ropes and knots in life support applications - where a failure of a knot (or the rope) would lead to a catastrophic outcome (eg death).

I have extensive mountaineering, rock climbing, abseiling and vertical rescue experience spanning some 24 years. I am active in climbing and average 3 days per week over 12 monthly periods (basically every weekend and also midweek).

Therefore, I have a keen interest in the IGKT and all matters pertaining to knot theory - my life literally depends on my skills with respect to knot tying and rope management.

With greatest respect to other users of ropes and knots, I would support any work that expands our collective knowledge in this area. In particular, vertical rescue teams who are in the business of saving lives could also benefit from advancements in knot theory.

Other user groups such as fisherman, decorative knot tyers, do not put their lives ‘on the line’ in quite the same context. This might spark a debate but I’m going to go out on a limb here and say that exposure to falls from height ranks higher in terms of apparent and immediate need of accurate and reliable knot data.

Just 2 days ago, I took a 10-15 free-fall off a steep and unrelenting cliff in a remote area in Australia. I’m obviously still alive otherwise I would not be posting on this forum.

Why am I still alive?

Because I use and place a great deal of trust in my knot tying skills. Other climbers also rely on my skills - eg my belayer/second who could die with me if I make a serious error in establishing anchors (using various knots).

On some of the cliffs I climb, I am so high up that I would have a few seconds to ponder my fate during a free-fall plunge to the ground - they say its not the fall that hurts, its the sudden stop at the end.

…

Anyhow, just thought I’d share a little of who agent smith is and what my interest is. So I’m basically saying sorry if I express zero interest in decorative knots or knots to secure a line to a fishing hook…

Without doubt, the dominant rope used in life support applications today is the ‘kernmantel’ rope (yes Dan, thats how the German word is spelt). Hawser or other laid rope is simply not used or is used only in a narrow range of applications (eg roof access on pitched roof surfaces).

Where am I going with this, you may ask?

In a nutshell:

  1. The guild should already be aware of kernmantel ropes - ie two part ‘sheath & core’ construction introduced in the 1950’s by a German company called Edelrid.
  2. Kernmantel ropes are broadly divided into two (2) categories as follows; i) Dynamic ropes and ii) low stretch ‘static’ ropes.
  3. There are well defined standards in existence for rope manufacturers and how they test. These standards are: i) EN 892 for dynamic ropes and ii) EN 1891 for low stretch ropes.
  4. The EN standards are European but have in fact become the world-wide defacto standard. Yes, even USA rope makers implement the EN standards (and CE marking) in their test regimes.
  5. The EN standards specify wrapping and then clamping the rope around a pin (or bollard) to anchor it for specific tests - this is not a new concept - its been around for over 20 years. Some may refer to this type of ‘knot’ as a ‘tensionless hitch’.
  6. The EN standards specify the use of a figure 8 loop to form a connective interface between the drop mass and the test rope - the tying method is interesting and worth looking at by IGKT members (the tying method creates a form where the rope strands all lie parallel to each other and the knot can lie perfectly flat on a level surface). The knot is, quote; “hand tightened”.

I support DerekSmith in his quest to find answers to knot related questions. However, I would only have a specific interest in knots that are used in life supporting applications. This means the type of rope used in DerekSmith’s test ought (in my humble opinion) be ‘kernmantel’ rope. Again, to be consistent, both EN 892 dynamic and EN 1891 low stretch ropes should be tested for comparative results. Also, one brand/model of rope should be used from each category for consistency. It gets expensive if DerekSmith or anyone else uses several different brands/models to build a data base of results across different test specimens. The most common diameter of rope is 11.0mm. Many tree climbers and USA vertical rescue teams use half inch (12.5-13.0 mm) ropes, but this is not the norm if you look at the whole world usage patterns.

The list of knots that best fit within a ‘life support’ role can be narrowed down to a manageable amount of say eleven (11). I am sure Dan and others would speculate as to which knots/hitches are relevant and necessary in life support applications. I have my own theories on this…

Some interesting links:

http://www.iop.org/EJ/article/1367-2630/9/3/065/njp7_3_065.html
http://www.caves.org/section/vertical/nh/50/knotrope.html
www.hse.gov.uk/RESEARCH/crr_pdf/2001/crr01364.pdf [tests of 1 climbing, 3 low-elong. ropes …]
www.personal.strath.ac.uk/andrew.mclaren/KatherineMilne2004.pdf [Fig.8 LK & Bwl in laid & dbl.braid yacht rope] ? dead link!
www.gudelius.de/spst.htm [offset bends: fish+oh, grapevine, oob, in 8mm & 10mm climbing rope]
www.xmission.com/~tmoyer/testing/EDK.html [tests of offset bends variously dressed & set (Fig.8 & Overhand), also a grapevine]
www.xmission.com/~tmoyer/testing/High_Strength_Cord.pdf [incl. Fig.8 LK testing in hi-mod cored cord]
www.bwrs.org.au/bwr/research/index.html [and on this page are links to parts of a PDF report w/testing by David Drohan]

agent smith

10

Thanks for the reminder about the Oregon university’s studies of using HMPE
fibre ropes for logging–I owe them some thoughts, and esp. about “special” knots!
(They e.g. found a Bwl to slip if not stoppered; when stoppered, to break at 33%.)
Typically, their knot world is quite limited.

As for “kernmant ”, by demoncratic principles manifest via Google,
the former wins currently over 2-to-1 in citations. Taking a detour into AltaVista’s
Babelfish (not ‘Bable’) finds G (E-mantel) = “coat” and G (E-mantle) = “cover”,
and the full k. terms resp. as “kernmantel” (i.e., no change) & “core-cover”.
Interestingly, Merriam-Webster’s 9th Coll. (surely based on Web.3rd New Int.) gives
the derivation of “mantel” to an ancient ‘LE’ term, and that for “mantle” to an ‘EL’ one!
–perfect confusion!!! ::slight_smile: (which I’ll let sit and fester or simmer of blend, for a while)

Anyhow, yes, you’re do some off-line particular comments about some other document.
(which, quite sadly, begins with the obvious but oft’-parroted nonsense about [i]ABOK
containing some over 3,800 knots–not a good omen :-\ )

Dave Richards needs some innovative & insightful input, also on my to-do list. His hoped-for
testing of hi-mod ropes & knots has been delayed for personal reasons & general busyness,
but I think remains a hope. Here is a chance to get some real rope tested by those fancy
devices!

As for limiting the set of user knots in (name your application–climbing/caving/SAR are some),
while the stated advantage of manifesting the supposed “KISS” principle is given, I think
that this is to some significant extent specious: how can one know well a reasonable set
of knots but not understand and readily deal with others? --yes, it seems that it happens
regularly: users do their business well but with a few knots, and they do show bafflement
trying to step beyond this limited knot-world. --but it seems also reasonable to argue that
an understanding of a knot necessarily entails knowledge that will enable the person to
easily work with other knots and to handle new situations by modifying/adapting known
knots or knot components to solve the problem. I see this as using knotting skills
as opposed to merely indexing by problem into some set of knots to get a specific one.
The more rigidly defined an application, the more feasible some fixed set of knots will be.
But I’d really like to see better understanding than is currently exhibited. And with the
growing diversity of knottable media, this is probably a practical necessity, not a luxury.

–dl*

11

Evenin’ all,

Agent Smith…What became of the rope you took a fall on ? Similar to you I have only tied knots for a " surviavl "purposes. I’m only just getting into the decorative bit for financial gain…if there is one?

PaulJ. Thanks for the info on STANDARDS. In particular the point about how the attachment for testing is made: " large diameter bar "

SS369. Please keep looking.

Dan Lehman…I need to re read your posts…at the moment there a bit to deep for me. However there are a few points I can relate to… and sort of get my head around. Tension distribution is just one area. I do need to play with my food on this one, Spag bag for dinner tonight including strange looks for winding the spagetti differently at each mouthful.

Derek Smith…you can open my tin of spagetti hoops any day

Regards

DerekMW

12

Ultimately, he coiled it and carried it home to await the next adventure
(after he untied some knot).

:slight_smile:

13

Thanks for your concerns IGKT members.

Maths is my best friend here.

Its not the length of the fall by itself that matters… you also need to take into consideration the length of the active rope out at the instant of the fall. there is a formula for this: (as it applies to EN 892 dynamic ropes).

Length of fall / Length of active rope out

This formula gives a mathematical ratio known as the ‘fall-factor’.

Any fall-factors of 1 or higher are classed as hard falls and counted towards the manufacturers specified fall capacity.

Any fall-factors less than 1 are ‘soft falls’ and do not count to toward the manufacturers specified fall capacity.

The highest fall-factor in climbing is 2. EN 892 drop tests generate factor 1.78 (which is very severe).

Dave Richards might be able to add his opinions here.

Obviously, counting the number of falls a dynamic rope sustains is important as they ship with a finite number as specified by the manufacturer. However, this is only one of several criteria used to determine if a EN 892 dynamic rope needs to be retired from service.

So what did i do, I did not retire my rope as I had 43m of active rope out at the time of the 10-15m fall.

Believe it or not, its the short falls near the start of a climb that are most severe - not the long falls near the end of a pitch where there is more rope available to disperse kinetic energy.

My rope is fine - and thats my judgement based on criteria.

But heh, thanks for thinking of me!

agent smith

14

Fall factor etc.

http://www.bstorage.com/speleo/Pubs/rlenergy/Default.htm

15

Evenin’ all,

Hi Agent Smith

Thanks for your response on what became of the rope.

Maths is not a strong point of mine; however I can relate to the fall factors you describe.

In my blinkered way I was wondering if there was somewhere you sent a rope after it had been subject to a fall similar to the one you describe: Research centre or likewise. In some way I’m stuck in the past. Anything used in anger or for its purpose, in my past work, was recovered and sent to a research establishment.

What I did not realise was there are degrees of a fall. Something else for me to think about.

I did say I am a novice in this caldron. Another way of putting it. Mouth open before brain’s in gear. However energy disapation is a familiar statement. Even more to dwell on.

As for making the judgement if a piece of KIT it’s safe or not. I would never question your experience and skill. Likewise I would not expect you to have questioned my judgements " when I had to make them…long ago"

Enjoy your passion

Derek

16

Are there unresolved knot strength questions when dealing with these two types of climbing ropes? One the references (the UK industrial one) said, all these knots tested at 50% or better, so as long as the rope is used with in that safety margin, don’t worry about the knot.

On the other hand, with high modulus (very low stretch) ropes, knots are more of an unknown. Some test at 30% or worse, with the rope either slipping out of the knot, or breaking due to strain-heating. It is bad enough that manufacturer’s like Samson say don’t use any knots on their Amsteel Blue (a popular rope for offroad winching). Maybe an innovative knot typer can find something that does work in this type of line.

Fishing and kite flying are other activities that used high strength line at close to its breaking strength.

paulj

17

Ah, another semi-myth of sorts. Climbers cite “fall factor” w/o bothering to remark
that the forces generated also depend upon falling mass, which in standard testing
by UIAA protocol is about 180#, but YMMV as per individual climber masses–women,
e.g., are typically much lighter (Lynn Hill is a small person, but for stature!). PMI did
a report on the loads generated by well heavier (maybe starting at 200#) masses,
and showed a significant narrowing of safety margins.

I also recall information shown in an old Chouinard catalogue in which although
the peak forces of long vs. short falls of equal FFs were also equal, their duration
was proportionate to the fall–i.e., long falls sustained the high load longer. OTOH,
the significance of hard-to-specify small shock absorbers such as body flexing and
so on looms more significant for short falls, where an inch or several is a greater
proportion of the full length (ditto for knot compression). One unseeming case of
severe loading comes when attaching to an anchor with a short and static sling,
reaching above that for some reason, and slipping upon it–short fall, big FF, and
really big impact forces!

(I’m not sure what Agent Smith means about falls w/FF>1 being subtracted from
a rope vendor’s count, but that does seem to be about a rule of thumb on “severe”,
and some vendors recommend subtracting the rope from use, not merely a count!
There was some discussion of this on RC.com, IIRC. --and the point about actual
forces vis-a-vis climber weight was raised then.)

The cited/linked reference makes assertions about the effects of the Fig.9 LK that
are contradicted by Dave Merchant’s [i]Life on a Line literature & postings
(on NSS’s forum caves.forums.org, OnRope ?), where the latter asserts that the
rate of loading–i.p., “shock” loading–leads to greater weakening in the Fig.9 than
in e.g. the Overhand LK or even Fig.8 LK. But here we see a case of not knowing
enough about the basis for these assertions, such as knot geometry & setting.

–dl*

18

Dan Lehman is right about the mass of the falling person.

I did not provide the full picture with fall-factors and dynamic ropes.

EN 892 dynamic ropes are further divided into 3 categories as follows:

  1. Single - where the rope is tested with an 80kg drop mass and must survive at least 5 EN 892 test falls (impact force must not exceed 12kN on first test fall)
  2. Half - where the rope is tested with a 55kg drop mass and must survive at least 5 EN 892 test falls
  3. Twin - where two rope strands are drop tested simultaneously (ie as the inseparable twins) with an 80kg drop mass and must survive at least 12 EN 892 test falls

EN 892 specifies a 5.0m free fall using a 2.8m rope specimen (hence the fall-factor of 1.78… 5/2.8 = 1.78).

The test mass in an EN 892 fall IS important. Ropes are essentially defined by what mass they were tested with. There is a trend to find the holy grail of lighter and lighter ropes (meaning manufacturers are striving trying to find ways to reduce the diameter of their ropes - thereby making them lighter). The upshot is that ropes are generally getting thinner and thinner (something I would like to do but age seems to be slowing me down somewhat). I would theorise that rope manufacturers are looking for new and more advanced synthetics to weave the holy grail of ropes - anyone for a ‘nano rope’.

If a climber has a mass greater than 80kg (as I do!) the physics of the fall may exceed the test parameters given in EN 892 standard.

Each EN 892 test fall generates a fall-factor of 1.78 - which is severe in anyones language.

The conventional wisdom is to reduce the number of falls a rope can sustain by a factor proportional to the amount by which the climbers mass exceeds 80kg.

For example, I have a mass of 100kg (yes, I’m a big boy). This means I reduce the published number of falls a particular rope can sustain by 80/100 = 0.8.

Therefore, if I buy a 10 fall rated dynamic rope, I reduce its effective fall rating from 10 down to 8.

So for me, I treat a 10 fall rated rope as if it were actually an 8 fall rope.

Other than that, it is true to say that as more and more rope is payed out through the belay device, the fall severity becomes less and less.

Its ironic that it actually gets ‘safer’ as you climb higher!

Try explaining that one to novices and/or non climbers!

agent smith

19

And, believe it or not, there is one–now maybe more–rope that has been certified as all three
–viz., Beal’s (aptly named) “Joker” (which is 9.1mm)
(and I also think that sizing by fractions of a millimeter is pushing things!).

The test mass in an EN 892 fall IS important. ... The conventional wisdom is to reduce the number of falls a rope can sustain by a factor proportional to the amount by which the climbers mass exceeds 80kg. For example, I have a mass of 100kg (yes, I'm a big boy). This means I reduce the published number of falls a particular rope can sustain by 80/100 = 0.8. Therefore, if I buy a 10 fall rated dynamic rope, I reduce its effective fall rating from 10 down to 8.

So for me, I treat a 10 fall rated rope as if it were actually an 8 fall rope. …
[Try explaining that one to novices and/or non climbers!]

I hope you aren’t “explaining” this “conventional wisdom” to anyone anymore–it’s dangerously mistaken!
You should read a version of PMI’s report (PDF–available in HTML, too) at
www.safeclimbing.org/education/Heavy_Climbers_Beware.pdf
My guess is that your 100kg mass would see fewer than 8 falls in a 10-fall-rated rope,
as fall #1 = 10.1 kn/2260# vs. 8.3kn/2040# for 80kg mass (e.g., for the PMI rope used),
and not only is that well more severe than for the lesser mass–perhaps “2” falls ahead(?)–,
each successive fall will be made with the same heavier mass, and thus accelerate the
deterioration over what the lesser mass will do–NOT merely keep a supposed “2”-fall lead.
(You could think about reducing the acceptable FF by taking 0.8 of it, but that would
give 1.368 and PMI show the equivalent impact force coming only lower–1.23 or so.)

That aspect aside, I wonder what your thinking about the significance of the fall rating
of ropes is? There are educated opinions that they aren’t as readily translatable into
retirement criteria as you seem to imply. (And for a while, at least, there was the issue
of different test labs generating different ratings–something the UIAA tried to clean up.)
But the figuring gets quite complicated by assessing falls & other history, and taking
into consideration actual mass/forces, for which there isn’t a lot of test data put out
by anyone to refer to. PMI made a start; it would be nice if they or others took this
insight into seeing the effects of an accumulation of such falls, both on the heavier
& lighter side of the 80kg standard.

–dl*

20

Thanks for remarks Dan.

You are entitled to your opinion as we all are - and I respect this.

However, I will make the following comments as a right of reply to your posted remark - “I hope you aren’t “explaining” this “conventional wisdom” to anyone anymore–it’s dangerously mistaken!”

Comments as follows:

  1. I’ve read the report from PMI a few years ago… and it never purported to give reductions to the maximum published fall rating of a dynamic rope.
  2. I have a very clear underpinning knowledge of the math involved.
  3. I stand by my theory regarding ‘rule of thumb’ reductions to published fall ratings on dynamic ropes.
  4. I have applied my own theory to myself over 24 years of continuous climbing - I have sustained countless numbers of falls - particularly while attempting new (unclimbed) routes on sight from the ground up. Obviously I am still alive, and I have never severed a rope (yet). Of course, this information cannot be verified by you - only by my numerous climbing partners over the years.
  5. I have yet to see a rope manufacturer provide any advice or informative data to heavy (more than 80kg mass) recreational climbers about how they should treat published fall ratings on given on ropes. PMI nor any other rope manufacturer have given such advice (see #1 above). I suspect that no such advice or data will ever be forthcoming (why? you may ask - legal liability springs to mind).
  6. Lets look at the data… a standard EN 892 test fall generates factor 1.78.
  7. It is not possible to sustain a fall factor 1 or greater on a single-pitch route (a factor 1 fall implies the lead climber has hit the ground - or ‘decked’ as we say).
  8. To sustain fall factors greater than 1, a lead climber needs to fall past (ie below) the belayer - for all intents and purposes, this can only happen on a multi-pitch route.
  9. I count any fall of factor 1 or higher as one (1) fall of the life of the rope. This is very conservative.
  10. I have a mass of 100kg. I then establish a proportionality factor by dividing the EN 892 figure of 80kg by my body mass of 100kg: 80/100 = 0.8. Therefore, if I purchase a 12 fall rope, I do the following: 12 x 0.8 = 9.6. This figure is then rounded DOWN to 9. I treat that rope as if it were a nine (9) fall rope. If I sustain nine (9) factor one (1) falls, I retire my climbing rope. Although I dont have any certified test data to prove my theory, I can attest that the rule of thumb has worked for me in real-world applications in the field.
  11. In summary:
    I advocate that any fall generating factor 1 or higher should be counted as one (1) fall towards the ultimate retirement of a climbing rope.
    I advocate that heavy climbers reduce the published fall rating of their rope by an amount proportional to their body mass - for 100kg climbers, this figure is a 0.8 reduction off the published fall rating (used in conjunction with counting any fall of factor 1 or higher towards rope retirement). All fractions are rounded conservatively - eg if a figure of 0.86 is calculated, this is automatically rounded down to 0.8.
    I am confident in my rule of thumb theory as it has been proven by empirical means over 2 decades.

agent smith

ps I forgot to add, I own a Beal ‘Joker’ rope. Haven’t fallen on it (yet)…

EDIT NOTICE:

I made a calculation error while typing quickly during the wee hours… see #10 above. I originally stated that I treat a 12 fall rated rope as if it were a 10 fall rope. This is incorrect. The correct reduction is nine (9).