Author Topic: Dyneema shock/safety cords  (Read 3608 times)

Offline rhyst

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Dyneema shock/safety cords
« on: June 11, 2018, 09:18:00 am »
I've seen some SRT set ups using a dyneema shock/safety cord to attach the hand jammer rather than the dynamic rope. This seems like quite a tempting idea to reduce bulk but is it safe? The name 'shock' cord does imply that perhaps some stretch is good, but how much shock absorbing can ~1m of dynamic rope actually do? To me it seems that the majority of the shock absorption will come from whatever rope you are dangling on but I also I assume there is a good reason we generally have dynamic rope.

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Offline pwhole

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Re: Dyneema shock/safety cords
« Reply #1 on: June 11, 2018, 10:15:57 am »
but how much shock absorbing can ~1m of dynamic rope actually do?

With a barrel knot at each end, and assuming a stretch factor of 10% (as it's so short), I would guess a 1m dynamic link would stretch about 10cm under a shock load, which is probably enough to walk away from a FF1. With Dyneema, possibly not.

Online mikem

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Re: Dyneema shock/safety cords
« Reply #2 on: June 11, 2018, 12:54:20 pm »
If you are just using it as a connection to the hand jammer then the load will also go onto whatever length of rope is between the jammer & the next anchor - if you are using it as an additional cowstail, then don't use dyneema...

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Offline marysboy

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Re: Dyneema shock/safety cords
« Reply #3 on: June 11, 2018, 02:59:56 pm »
You are right to be wary. For me the primary purpose of the hand jammer safety cord (shock cord) is to resist a fall with reasonably impact loading. Fall onto hand jammer is foreseeable when clambering at pitch head, fall whilst swapping over chest jammer at rebelay, chest jamer slips or opens, bolt failure. Any of these falls could easily happen at pitch head or near anchor therefore pitch rope is not guaraneed to be available to absorb the impact. So a fall is foreseeable and it is possible there will be little other shock-absorbing material in the system, increasing the shock load on you, your jammer, the remaining rope and the anchor. Here some stretch is a big benefit in reducing impact and therefore peak force.

A 'non-stretch' safety cord does not seem to provide any advantage as this is not loaded in normal use for progression, such as footloops are. (NB static footloops made of dyneema are not expected to resist a shock load in normal use).

So whilst dyneema safety cord might be strong enough, it would not provide much shock absorbancy in event of fall which is its primary purpose so this is a major disadvantage. Whether this is 'safe' is for you to decide, but on this count alone it defintely seems less safe than the conventional rope option.

If you wish to persue a low-bulk kit, there are several alternatives worth considering. Simplest is perhaps just to swap your existing safety cord (i dont know what this is made of) for thin (8/9mm??) safety cord. This could be 'static' caving rope (better stretch than dyneema and cheap and easily integrated with footloop) or dynamic climbing rope (better stretch still).

Extending the dynamic safety cord idea, you probably already carry dynamic rope in the form of cowstails, so could eliminate the safety cord altogether and simply clip hand jammer with a cowstail. Good shock absorbancy and zero bul. But potential for fortgetting and/or dropping jammer. This method is described in Alpine Caving Techniques, which is well worth a read for other SRT/rigging ideas.

Offline marysboy

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Re: Dyneema shock/safety cords
« Reply #4 on: June 11, 2018, 04:21:47 pm »
thinking further on this element of your post:
how much shock absorbing can ~1m of dynamic rope actually do?
actually 1m of dynamic rope with two knots will absord quite a lot of energy, softening the catch and reducing the peak load. this is why most people have dynamic rope cowstails. principles of fall factors and reducing peak load is an aspect of SRT safety that is something of a fundamental to anyone learning SRT.

bear in mind that jammers have toothed cams and will damage the rope at much lower forces than typical resin anchors will withstand. so you need an even lower peak force on your jammer than your cowstails. for example the petzl instructions for the Basic hand jammer state "Shock loading must absolutely be avoided when close to the anchor." incidentally they also suggest clipping it with a cowstail.

lastly in the event of a fall it is worth considering whether you want a soft catch. i understand people have broken their back falling into a sit harness. if your chest jammer has slipped/come undone at the pitch head, would you want to give the hand jammer best possible chance of holding? what would the impact be to your caving mates below you if you are incapacitated? worth considering.

(some of these factors might not be of interest to the OP but i thought it worth airing for anyone browsing or searching in future)

Online Mark Wright

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Re: Dyneema shock/safety cords
« Reply #5 on: June 11, 2018, 04:58:49 pm »
For me the primary purpose of a safety cord on the top ascender is so I don’t drop it. If I’m passing a re-belay or other mid-rope obastacle or getting off a rope at a pitch head then it would (should) be protected by a dynamic cow’s tail clipped into the re-belay or other appropriate attachment point.

I use 5mm accessory cord for my safety cord.

Mark

Offline marysboy

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Re: Dyneema shock/safety cords
« Reply #6 on: June 11, 2018, 07:50:17 pm »
For me the primary purpose of a safety cord on the top ascender is so I don’t drop it. If I’m passing a re-belay or other mid-rope obastacle or getting off a rope at a pitch head then it would (should) be protected by a dynamic cow’s tail clipped into the re-belay or other appropriate attachment point. I use 5mm accessory cord for my safety cord. Mark

worth noting that petzl (for example) recomend that users do not rely on a single jammer. in the usual SRT cavers' case this means the chest jammer would (should) be backed up by another full strength attachement point, e.g. a safety cord rated for a fall.

https://www.petzl.com/GB/en/Sport/Using-only-one-ascender-?ProductName=CROLL-L&Familly=Ascenders

beal 5mm cord (for example) should resist 650kG but would not be rated for a fall.

Offline Hammy

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Re: Dyneema shock/safety cords
« Reply #7 on: June 11, 2018, 08:37:28 pm »
Mark Wright has the issue nailed here.

My footloop and safety cord is a length of 8mm static which has untieable knots (bowlines on bights - themselves shock absorbing) so I can use a full length piece of cord if necessary for creating a jigger if all else fails.

Offline cavemanmike

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Re: Dyneema shock/safety cords
« Reply #8 on: June 11, 2018, 08:50:38 pm »
Forgive me for being ignorant but I thought a knot was a week spot in a rope

Online Mark Wright

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Re: Dyneema shock/safety cords
« Reply #9 on: June 11, 2018, 08:58:47 pm »
The main reason why Petzl make the recommendations they do is to cover people who have a particularly poor climbing technique or decide to completely ignore the criteria for removal from service due to excessive wear of the cam.
If a harness and associated ascenders, cows tails etc. are correctly fitted and not overly worn then the likelihood of a chest ascender coming off the rope whilst ascending is pretty much zero.

A 5mm accessory cord with knots tied in it will be considerably weaker than 650kg. I don’t use it for falling onto, I use it to stop me dropping by top ascender. No accessory cords are rated for falling onto.

Mark

Online mikem

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Re: Dyneema shock/safety cords
« Reply #10 on: June 11, 2018, 11:10:10 pm »
The only time I've had a croll come off the rope was when it pushed up against a handled jammer at just the right angle to force the release switch down - you notice pretty quickly...

If you are using the connection cord as a cowstail, then it should be with the krab rather than the jammer.

Offline Fulk

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Re: Dyneema shock/safety cords
« Reply #11 on: June 11, 2018, 11:13:01 pm »
Quote
the likelihood of a chest ascender coming off the rope whilst ascending is pretty much zero.

I wouldn't argue with that . . . but I was once climbing the 'Double-Bucket' pitch in Notts Pot when the spring of my Croll (which on subsequent inspection appeared to be in good condition) broke, as a result of which the device slid down the rope until I my descent was stopped by the saftey link to my foot-jammer. As it happened, I was making my way out of the cave alone, as I'd sliced my hand open on a sharp flake of rock, so you can imagince that I wasn't very happy. I made further progress up this pitch by standing up (as normal) and then pushing the Croll's cam in manually – which worked, but was rather slow.

Offline andrewmc

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Re: Dyneema shock/safety cords
« Reply #12 on: June 12, 2018, 08:27:13 am »
If a harness and associated ascenders, cows tails etc. are correctly fitted and not overly worn then the likelihood of a chest ascender coming off the rope whilst ascending is pretty much zero.

That might be true for climbing a nice vertical rope. It's probably not true for messy traverses etc. Big wall climbers use two handled ascenders and routinely remove one as they ascend to pass pieces of gear on a climbing line (with backup knots). Those ascenders twisting off the rope has done in several big wallers who failed to use backup knots or equivalent.

In my more paranoid moments I have been known to clip a crab through the holes at the top of my handled ascender which stops the rope escaping when I have been using it for protection or assistance on steeply sloping traverse line or similar, or when rigging/derigging.

Offline caving_fox

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Re: Dyneema shock/safety cords
« Reply #13 on: June 12, 2018, 09:09:54 am »
Quote
then the likelihood of a chest ascender coming off the rope whilst ascending is pretty much zero.

I've once caught the handle of my croll with the knot of my handjammer's safety line, as I moved up the rope, causing the croll to disengage mid pitch  :o

This was with the 'old' style croll slightly bulkier with a more protruding handle than today's offerings, but it was still very unnerving, if ultimately completely safe. I wouldn't like to repeat the experience, especially if the safety rope wasn't load rated.
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Offline Bratchley

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Re: Dyneema shock/safety cords
« Reply #14 on: June 12, 2018, 09:16:56 am »
Realistically how much weight are you saving between a Dyneema 'safety' cord and 8mm dynamic rope? Very little - you're not climbing everest, you're caving. You'll collect more weight in dust and mud on your kit than you'll save, and you're potentially compromising safety.

Granted, you shouldn't be falling on your hand jammer, you should always have something else attached - but pitches aren't always simple or spacious and bolts/anchors aren't always bomb proof. Nothing is impossible either, a tight pitch with rock nodules has almost caused my croll to come off when squeezing vertically, for instance.

It's been mentioned that falling on jammers is bad, especially Petzl ones with very sharp teeth. It takes suprisingly little peak force to de-sheath a rope with a toothed jammer and with a fully static cord attachment it wouldn't take much to generate it especially if you were very close to a rebelay knot (again, you shouldn't ever fall on it, but lots of things *shouldn't* happen).

Its your safety and your choice, the argument that it's not actually a 'safety' cord is perfectly valid, but to me it seems an unneccesary thing to compromise on and target when looking to save such a miniscule bit of weight/bulk. I have my hand jammer on my long cows tail, totally removing the additional safety cord. Saved the weight, still dynamic.
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Offline rhyst

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Re: Dyneema shock/safety cords
« Reply #15 on: June 12, 2018, 10:05:50 am »
Thanks for the replies!

I currently have a length of  horrible 11mm dynamic that I stole from my club years ago) that is quite awkward to pack into an SRT bag. I have some 9mm dynamic to switch to as that seems fairly uncontroversial and I am quite convinced by not wanting to strip the sheath/break my hips.

I will consider the ACT method of cowstail clipping though. Combined with cord to prevent dropping that seems to have few disadvantages. Do people generally use a locking krab for this?

Offline Fulk

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Re: Dyneema shock/safety cords
« Reply #16 on: June 12, 2018, 10:37:38 am »
Quote
I will consider the ACT method of cowstail clipping though. Combined with cord to prevent dropping that seems to have few disadvantages.

Am I missing something here? If you have to combine your set-up with a cord to prevent dropping, what really is the advantage over using a spearate cow's tail and 'safety link'?

Offline rhyst

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Re: Dyneema shock/safety cords
« Reply #17 on: June 12, 2018, 10:49:57 am »
Quote
I will consider the ACT method of cowstail clipping though. Combined with cord to prevent dropping that seems to have few disadvantages.

Am I missing something here? If you have to combine your set-up with a cord to prevent dropping, what really is the advantage over using a spearate cow's tail and 'safety link'?

I think the anti-dropping cord can be quite thin as it will not take your weight at any point so there would be some bulk reduction without compromising safety. If I haven't thought it through do let me know!

Offline Bratchley

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Re: Dyneema shock/safety cords
« Reply #18 on: June 12, 2018, 11:05:32 am »
Quote
I will consider the ACT method of cowstail clipping though. Combined with cord to prevent dropping that seems to have few disadvantages.

Am I missing something here? If you have to combine your set-up with a cord to prevent dropping, what really is the advantage over using a spearate cow's tail and 'safety link'?

I think the anti-dropping cord can be quite thin as it will not take your weight at any point so there would be some bulk reduction without compromising safety. If I haven't thought it through do let me know!

So you'd end up with one cows tail and the 'anti-drop' cord going from you to the jammer, and then footloop from jammer? Three bits of cord seems a little silly when you can have two do an identical job with less faff for about 40 grams more.

EDIT: Or even less weight if you just use the long cows tail to jammer as I mentioned earlier. You asked about the karabiner, sorry i missed that bit. It's probably best to use a locking karabiner as you will then not run the risk of losing your jammer.
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Offline rhyst

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Re: Dyneema shock/safety cords
« Reply #19 on: June 12, 2018, 11:17:32 am »
You make good points. It's all very small optimisations anyway :)

Offline marysboy

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Re: Dyneema shock/safety cords
« Reply #20 on: June 12, 2018, 11:19:45 am »
Quote
In my more paranoid moments I have been known to clip a crab through the holes at the top of my handled ascender which stops the rope escaping when I have been using it for protection or assistance on steeply sloping traverse line or similar, or when rigging/derigging.

from your description this sounds like proper usage of the equipment (not paranoia). assuming the old-shape handled petzl basic jammer this would be standard practice as required by the manufacturer to avoid danger of death.
https://www.petzl.com/sfc/servlet.shepherd/version/download/0681r000004z4JCAAY

note the current-model 'small' basic jammer instructions differ here but still require special action on sloping ropes.
https://www.petzl.com/sfc/servlet.shepherd/version/download/0681r0000078a2AAAQ

this might also be of interest:
http://www.scanrope.eu/download/ascender/AscenderReview.pdf

lots of interesting and useful discussion here - particularly interesting to hear users' experiences of jammer failure (there are more than i had anticipated).

Offline Fulk

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Re: Dyneema shock/safety cords
« Reply #21 on: June 12, 2018, 11:32:11 am »
Hi Bratchley, Just to get this clear – if one uses the long cow’s tail to jammer (with, presumably, one other [short?] CT) then, when you reach the top of a pitch you have to unclip your CT from your jammer in order to clip into the traverse line or whatever before getting off the pitch. This means that momentarily you’re depending on one (chest) jammer, and that (if you don’t have an ‘anti-drop’ cord) you could drop your jammer while removing it from the rope.

(Apologies if this has been covered somewhere else on this forum.)

Offline Bratchley

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Re: Dyneema shock/safety cords
« Reply #22 on: June 12, 2018, 11:41:31 am »
Hi Bratchley, Just to get this clear – if one uses the long cow’s tail to jammer (with, presumably, one other [short?] CT) then, when you reach the top of a pitch you have to unclip your CT from your jammer in order to clip into the traverse line or whatever before getting off the pitch. This means that momentarily you’re depending on one (chest) jammer, and that (if you don’t have an ‘anti-drop’ cord) you could drop your jammer while removing it from the rope.

(Apologies if this has been covered somewhere else on this forum.)

Don't have any issues clipping in with the (shorter, but not silly short) cowstail. With a pantin this is also a very smooth process as the croll can be brought even higher and get you in a good position at the rebelay. I very rarely remove the jammer from the cowstail, once the traverse is clipped with the shorter cowstail the cowstail with the jammer on can be moved and clipped with the krab onto the traverse line with jammer still attached to the krab. If it's a very steep traverse line the jammer can also be on the rope and slid along aiding your progress.  :bounce:
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Offline Fulk

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Re: Dyneema shock/safety cords
« Reply #23 on: June 12, 2018, 11:50:27 am »
Thanks for clarifying that.

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Re: Dyneema shock/safety cords
« Reply #24 on: June 12, 2018, 12:50:30 pm »
I currently have... horrible 11mm dynamic
... I have some 9mm dynamic to switch to...

Sounds like a vast improvement to me! :)

If you think your gear is 'horrible' then I'm sure you wont have much confidence in it. People can tell you what to trust and what they trust, but the ultimate judge of that is you.

If you're not happy with a bit of gear, think about replacing it.
 Or if it's a suggested change to your gear, think carefully before replacing it.