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Is it safe to girth-hitch nylon webbing through my harness tie-in points and then shorten the webbing via an overhand loop (assume it is too long) and clipping the non-bight part to the bolt in the rock? I am wondering if the knot can come undone in this case. I understand the knot would not come undone if the bight was clipped in, but my question is about when the non-bight part is clipped in.

Picture below: example

I know that a Purcell Prussik is a better option if I have a cordelette, but let's assume that I don't.

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  • Why use a sling at all if it's the wrong size? Use your rope and a clove hitch and viola you have a anchor exactly the correct length.
    – user2766
    Jan 8, 2019 at 13:15
  • 1
    A tip: do not use an overhand, but a bowline on a bight or an alpine butterfly. Those are easy to untie. Jan 8, 2019 at 15:45
  • @Liam I am not using a clove hitch because I am preparing to rappel (single pitch sport climb) and thus the rope needs to simply be run through the anchor rings so that, once I have rappelled, I can just pull the rope through the rings. Jan 8, 2019 at 18:14
  • An overhand is perfectly fine in this scenario and actually probably the safest knot to use @QuantumBrick . For example a figure eight can roll, a bowline is easy to tie incorrectly and a alpine butterfly is only safe if the loop is clipped. "Those are easy to untie" ermm..no not really.
    – user2766
    Jan 8, 2019 at 22:29
  • @Liam I disagree. The bowline on a bight is one of the easiest knots to both tie and untie, and the butterfly would do the job too. I use the bowline on a bight (BB) for fixing both hauline and lead rope; when rope-soloing I trust a BB as main and butterfly as second; I attach haulbags to the swivel with a BB; I create on-spot belays with a butterfly; in self-rescue I attach each element in a tandem with a butterfly; since BBs can serve as bunny ears I often create rope belays with them; I abseil with huge weights by attaching haulbags to my Petzl dual evolve with BBs. The list is infinite. Jan 8, 2019 at 22:41

3 Answers 3

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You should worry about the opposite: An overhand knot on nylon webbing will get really tight and hard to undo once no longer in use. That's why I rather use a figure-eight in this scenario (double bowline would be even better for undoing, but a bit more cumbersome to tie). This setup is somewhat standard practice for abseiling here in Switzerland, where you tie a 120cm sling to your harness, make a knot at about 2/3 length and attach your backup (prusik) to your harness, the braking device (e.g. tuber, eight) to the lower loop of the sling (same position as in your image) and the biner for self-securing in the outmost loop of the sling (undone when abseiling, attached when at belay-station). So plenty of anecdotal evidence that they definitely don't slip.

And if you don't trust that: Would a failure be catastrophic? No! The loop would just extend to it's full length, you are still clipped into the "main" loop so you wouldn't get disattached. That's also why this setup is ok even with dyneema slings. Again anecdotal evidence says by not shockloading, they just pull tight (and in case of the narrow slings very annoyingly, potentially almost irreversibly tight). Tests have shown that under high stress, they do creep.

tldr:
It won't come undone, it will get really tight. And even if it did (it does not), that wouldn't be a catastrophic failure.

As to breaking of the nylon webbing (not in the question, but just to be thorough):
The rule of thumb is to assume a knot weakens the nylon by 50% (common climbing knots lie below that). A nylon sling as a loop typically has 22kN breaking strength, i.e. 11kN on each strand (as a ring is always equally loaded, as long as you don't fix your biner with a clove hitch or something). Now one strand has a knot in it, meaning now the breaking strength of the knotted strand is 5.5kN and the weakest strand breaks first, so 11kN for the entire sling.

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  • If the knot slipped, wouldn't the webbing extend and thus be shockloaded, which could be catastrophic? Jan 6, 2019 at 5:47
  • If the slipping were frictionless, aka the knot become undone at once, then yes. I can't imagine that, but I know my lack of imagination isn't a good argument. However I mean it doesn't slip at all, so why should it slip without friction. And even if it would, it doesn't come undone (kind of cyclic here, but really the point of discussion is moot).
    – imsodin
    Jan 6, 2019 at 7:55
  • @imsodin First: A slipping knot generates friction ie heat, which is a very bad thing for webbing. Do not use loose knots as a means of shock absorber! Secondly: A figure eight rolls much more easily than an overhand, making it a terrible choice for this application. (For the same reason that we dont use a fig8 for an EDK.)
    – Guran
    Jan 8, 2019 at 6:48
  • @Guran I removed that bit, it is contentious indeed and I am not aware of any tests. The reason why I think a slipping knot is not problematic is, that the heat is not generated at the same spot of the webbing, so heat does not accumulate at one spot. The figure of 8 problem is not a problem: It can roll exactly once and I never observed it with body weight alone. Fig8 to join strands when abseiling leaving at 0.5m on the loose ends is a documented standard in Switzerland and there have been no accidents reported due to it (not using it for an "EDK2 is ok, we don't do "death knots" here :) ).
    – imsodin
    Jan 8, 2019 at 8:10
  • @imsodin Ok. There certainly is a lot of “security by rumor” when it comes to climbing knots.
    – Guran
    Jan 8, 2019 at 8:14
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This is perfectly fine. Not ideal perhaps, but perfectly fine.

One caveat though: Never climb above the anchor when clipped in with a static sling. Especially a knotted sling. And if you do - do not fall.

There is a famous video from dmm demonstrating what a dropping weight can do with a knotted sling. https://vimeo.com/27293337

But take that with a grain of salt. Your body is not a steel ball. Still, a fall onto a static sling hurts.

So TL;DR version: Shortening the sling as in your pic is fine. Climbers do this every day. In any situation where the knotted sling is bad, an unshortened sling is at least equally bad.

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The hodgepodge of confusing information just keeps growing online. DMM drop tests of both Dyneema and nylon runners used this way showed how a knot of any kind weakened so a fall factor of 1 would break Dyneema.This is not extreme or severe, just dropping three feet on a three foot sling. Nylon knotted held FF1, but broke in a FF2. Better to use a shorter sling, or double this one, and clip to both the harness and anchor with no knots in between. PAS are incorporating dynamic rope and some slipping mechanism, for both length adjustment and energy absorption - a Purcell sling is a safe means as well, with similar benefits without mechanical pieces.

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  • Please note that a FF1 is still plenty. In fall testing, a metal weight is used while in reality the human body will absorb a lot of energy. Also a FF1 is pretty unlikely for belays. If the belay is on a ledge, one will hardly create full length fall. In a hanging belay, such a fall already requires to climb up in the first place and then fall. Thus, a FF1 at the belay can be easily avoided. Of course, in terms of shock absorption it is safer to use the dynamic climbing rope but that can have other disadvantages
    – Manziel
    Nov 21, 2023 at 7:41

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