which carabiners for anchors

[Chocolate fireguard]

Trawling through some results obtained over many years of work by Bob & myself with the now dismantled rope drop test rig at the Bradford Pothole Club, I present the following as an honest attempt (by me) to fit numbers to the problem.

The following data came from drop tests done with semi static 9mm rope using a steel test mass.
Sorry I have no figures for dynamic rope.

I have used 6kN because the European Standards for various safety harnesses use a peak force of 6kN with a steel mass as one which most people should survive without injury. More detailed information can be found in https://www.hse.gov.uk/research/hsl_pdf/2003/hsl03-09.pdf .

Pulled to 6 kN, a Fig8 knot will release about 13 cm of rope and will absorb about 250 J of energy (that includes the energy absorbed by the released rope).

Pulled to 6 kN a metre of rope will stretch about 10% and absorb about 270 J of energy.

A long cows tail might consist of an 11 cm krab, a pair of 12 cm Fig8 knots with 40 cm of rope between the knots.

At 6 kN that 75 cm cows tail will stretch to 11 + (2*12) + (2*13) + 40 + 4 = 105 cm. So call it 1 metre.

And the energy in the cows tail will be (2*250) + (0.4*270) = 608J. So call it 600 J.

This means that a (steel!) 60kg caver will be stopped by this cows tail after a FF1 drop from an anchor, with a maximum acceleration force of 6kN. With 600N added to that, of course, because the rope will also be supporting the weight.

I have it in my mind, but can’t find the reference, that work done with volunteers (probably at forces lower than 6kN) suggests that about a third of the energy is absorbed by the body.

If that extends to 6kN then it is reasonable to believe that a 90kg person would experience the same 6kN acceleration force as the steel caver on that drop. I have no evidence, but I think that at forces well below 6kN the movable, squashy and bendy bits of the body will have done all they can.

I think most cavers use barrel knots to hold the krab. There is some data to show that while they produce a lower peak force than both a Fig8 and an overhand knot, they do release more rope in tightening up.

 

[mikem]

The only thing I can't find is whether knots naturally recover some elasticity over time or not (this would better indicate how much of the difference is made by retying/loosening knots) - whilst the French did tests 24 hours apart & 10 minutes, they seem to have been on the same rope (unsurprisingly forces increased after each shock load), rather than different samples of same material.

Interestingly a loose barrel knot only pulled 1 to 1.5cm of tail into the knot & was a better shock absorber than a preloaded one, suggesting that retying doesn't create that much of a risk of them coming undone (the same isn't necessarily true of fig8 knots which can work loose, particularly so on stiff or frozen ropes - not that that is a problem that you have to worry about for the centre knot).

The report in Speleology (4sport.ua link) suggests that two separate cowstails are slightly safer than having both on a single centre knot (as the future shock absorbency of one isn't then affected by falling on the other). However, having a loose end at the bottom creates a risk of that knot coming undone, so is better still to use one rope & have a short loop between the two (which is also very useful as a really short cowstail on hanging rebelays), but does mean more bulk on the harness attachment. Similar effect can be achieved by tying an overhand knot with both cowstails & a loop coming out of it - presumably this would have even better shock absorbency, but a higher risk of coming undone if not tightened sufficiently.

Another thing the French brought up is that "the concept of Fall Factor does not apply to the Cow's Tails made with one or several knots. In fact, we have seen (p. 5) that a Fall Factor 1 more accurately corresponds to a Fall Factor 1.75 for a short Cow's Tail and to a fall Factor 1.45 for a long Cow's Tail. We note that in all the tests on Cow's Tails with knots, the shock load is lower with the short side of the Cow's Tail than with the long side of the Cow's Tail. Therefore in this specific circumstance there is a lower shock load for a higher Fall Factor." This is explained by the fact that the short length means a shorter drop & thus less potential energy needing to be absorbed - so always clip in with the shortest required length.

The above HSE report says "Table 1 indicates that an energy absorber operating at 6kN maximum is probably best suited for workers of body weight 80kg to 100kg, in fall exposures of 4m (FF 2.0). In the range of body weight 50kg to 80kg the worker would be advised to seek an energy absorber of 4kN maximum. In the range of body weight 100kg to 140kg the worker would be advised to seek an energy absorber of 8kN maximum." Not sure that this would apply in the case of cowstails (see previous paragraph), but does suggest that some tests should cover a range of masses, rather than just standard 80 or 100kg loads.

 

[Bob Mehew]

For me the most significant point in the French Cows Tail report was highlighting the point about crabs making Fall Factors meaningless in many caving scenarios. We should really be talking in terms of energy 'released' by the falling caver and how the rope absorbs it.

The only thing I can't find is whether knots naturally recover some elasticity over time or not (this would better indicate how much of the difference is made by retying/loosening knots) - whilst the French did tests 24 hours apart & 10 minutes, they seem to have been on the same rope (unsurprisingly forces increased after each shock load), rather than different samples of same material.

In a caving fall scenario, there are three parts, the knot at the top, the rope between the top and the caver and the attachment of the caver to the rope. I will ignore the attachment and focus on the rope and knot.

The rope absorbs energy by extending. It is worth noting that kernmantle rope is better than hawser rope because as the sheath is extended, it compresses the core thus enabling the cords in the core to better share the load. Although nylon can creep under load, it is reported as being fairly resistant. (I have to admit my attempt to measure it was confounded by temperature related expansion and contraction.) So apart from the what I would expect to be a very small fraction of the energy going into a permanent rearrangement of the cords, most of the energy will be released as the rope relaxes back, similar to a spring.

The knot is far more complex. First, the knot is held together by friction of one part of the rope on another a number of times. Simply put, when the knot is stretched by loading it, that friction is overcome and part of the rope within the knot is 'released' both into the loop and into the active part of the rope. (And yes it is not a simple sliding action, rather we have seen it stick for a while, then slip releasing rope, then sticking again. This has been seen to occur several times in one fall arrest both by high speed camera and by force measurement.) That movement is permanent and the energy used to overcome the friction is left in the rope as 'damage'. In 'stretching' the knot, most of the bends will become smaller. Clearly, the strain response varies across the radius of the bend. In lifting the load on the knot, the amount of rope released by the knot will not be drawn back into the rope. And I would expect that to a first approximation, the bends will hardly increase in radius, certainly not back to their original radii. So no I don't see there be any appreciable relaxation in the knot on lifting the load.

Hence why I have said for many years relax your cows tail knots after use as that is the only way you can get the knot back to its original shape and hence maximum energy absorbing capacity and minimising peak force experienced. (OK I will hold my hand up to having stepped back from saying undo and then retie them, save for when you have fallen on the knot. )

 

[mikem]

I guess that even if you were tied in directly the short lengths involved would still have less influence than the knots. How much they recover also depends if damage actually occurs within the rope itself, as cowstail knots will be repeatedly tied in the same position, which will rarely happen in a rigging rope (I know it won't be catastrophic damage, but may be enough to affect the results).

Also of interest is the fact that many climbing walls are now using semi static for top roping, as the dynamic requirements are lower & they were getting more injuries from people dropping slightly further (potentially onto the floor or other holds)

 

[Mark Wright]

(OK I will hold my hand up to having stepped back from saying undo and then retie them, save for when you have fallen on the knot. )

A 'Thorough Examination' of rope used in the workplace would always include untying the knot. A cow's tail put forward for such an examination with termination knots that that were so tight they couldn't be untied would fail the examination and the cow's tail rope would be cut up into unusable lengths and suitably disposed of.

 

[mikem]

"Relaxing" the knot involves getting it to the point where it can be undone.

Bob (or Chocolate), did you do any testing on knots that had been loosened / untied & retied?

 

[Bob Mehew]

Bob (or Chocolate), did you do any testing on knots that had been loosened / untied & retied?

I am fairly sure we did not do so. Unfortunately I did not create a searchable index of what we have done and the alas the rig is no more. (But we are planning to build a replacement rig near Matlock.)

 

[mikem]

Having just dug out 3 retired cowstails that have been hung on, but not shock loaded (although one was inherited when I was given kit & not used by me, so can't be sure of that), I can confirm that barrel knots (once krabs removed - which can be a challenge) & big central overhand were relatively easy to untie, whilst krab end fig8s were more difficult, & central 8s really challenging (in fact the one that wasn't mine is still refusing to come undone!) The sheaths are flattened in places that were within the knot, but bend radiuses are still pretty much the same.

 

[oldfart]

Omniknot

xkcd.com

 

[mikem]

Thanks to Tim Dobson for finding this - loading caused by lead falls, comparing person to steel weight on dynamic & the weight only on semi, static & canyoning ropes:

 

[Fjell]

Thanks to Tim Dobson for finding this - loading caused by lead falls, comparing person to steel weight on dynamic & the weight only on semi, static & canyoning ropes:

OK, thats’s the sort of concrete data point I’m looking for. He highlights the point that short drops at FF1 will hurt more than long ones despite the same peak force (takes longer to get there). And that realistically you really need to not get anywhere near FF1 on caving rope. Which means (for instance) not having short distances between rebelays unless you double bolt (like less than 5m for me).

Also someone who takes sampling rate seriously. Back in the day when I was blowing shit up for laughs and possibly academic research, I used a storage oscilloscope for exactly that reason.

 

[Chocolate fireguard]

Yes, a good video.
But even his short drops were much longer than a caver would take on cows tail in a FF1, and that's where the energy absorbed by the knots matters.
I remember being told about storage scopes. Didn't Noah have a pair of them?

 

[Fjell]

Yes, a good video.
But even his short drops were much longer than a caver would take on cows tail in a FF1, and that's where the energy absorbed by the knots matters.
I remember being told about storage scopes. Didn't Noah have a pair of them?

The thesis was typed up on an Amstrad PCW, the latest thing at the time, and I seem to recall a golf ball printer. All of which was a step forward on my A levels using punched cards for a machine with ferrite core memory where each bit was clearly visible to the naked eye.
The oscilloscope worked really well. Started recording when you fired the detonator and surprisingly actually captured all the shock wave from the pressure gauges. At least I was surprised. The mod was slightly suspicious what it was all about when I was picked for interview, but I got my first. Fake it until you make it.

 

[mikem]

He did say that his next plan was to see if short rope factor 1 falls were actually worse than long ones, but we already know that they aren't, from the French testing. In a short fall you don't build up as much momentum, so less energy needs to be absorbed (skydivers reckon it takes 12 seconds to reach terminal velocity, but that is with body horizontal, whereas we are normally vertical).

 

[Fjell]

He did say that his next plan was to see if short rope factor 1 falls were actually worse than long ones, but we already know that they aren't, from the French testing. In a short fall you don't build up as much momentum, so less energy needs to be absorbed (skydivers reckon it takes 12 seconds to reach terminal velocity, but that is with body horizontal, whereas we are normally vertical).

I think he was saying (from actually doing it) that short drops hurt more at the same FF because of (effectively) the higher rate of deceleration and relative shorter period to get to peak force. Like using an impact driver. If I got it right he jumps off cliffs using a long semi static sometimes?

 

[mikem]

He's comparing factor 1 falls on 5 & 50ft lengths: "...they actually generate *spoiler alert* I THINK the same number, and I have that video coming out soon, after I do a few more tests to verify that."

IF the loads are the same then the shorter rope decelerates more abruptly & greater force in transferred to the climber, but the French research shows this not to be true & it can't be just due to the knots, as they are same both times.

 

[mikem]

Drop tests on long rope cowstail, sewn one (not as bad as he expected) & energy absorber - at end, with suggestion for safer aid climbing set up at start. Unfortunately he doesn't test short cowstail:

 

[Fjell]

This is a US safety board graph where they state that 60g for 3ms should be the limit for chest loading (seat belt). It is related to crush zones (more relevant to steel cars than elasticity), which would be the same thing as rope elasticity. It makes more sense to me that there should be a g limit for humans, which is not the same thing as max load (but is obviously related). Those force curves the guy has at high sampling rate could be converted to acceleration.

 

[Ian Ball]

Drop tests on long rope cowstail, sewn one (not as bad as he expected) & energy absorber - at end, with suggestion for safer aid climbing set up at start. Unfortunately he doesn't test short cowstail:

Top dude that guy, if I were Irata training and Spanset were busy, I'd head Sheffield way

 

[mikem]

The problem is that working out the DECELERATION for different ropes is extremely complimacated!