Snapping Slings (gear geek post)

This post is about safety. I know, WTF? (Boooorrring!) But there’s a first time for everything, and contrary to all indications otherwise, especially given my banter about Disaster Style, sometimes I try to be smart. It’s not always easy, what with my low IQ and all, but Disaster Style is all about living. By trying to be smart, and stacking the odds in our favor when doing such life-affirming things as climbing, it’s simple: we stick around long enough to keep doing it. Helluva deal. Along those lines, let’s get to it: slings. Excellent recent info from DMM (video here; article here), with test results from climbing slings, warrants examination.

The key with this sort of data, in my mind, is this: How does it apply to the real world? One could be forgiven for looking at a chart of numbers, and videos from industrial drop towers, and tuning out. I see two major factors from their tests, confirming what we already know:

* Elasticity – huge difference between nylon and Dyneema slings – as noted by the guy in the video (Graham “Streaky” Desroy, I’m told), a typical leader fall might only generate 4–7 kN. That’s with a climbing rope, which stretches to absorb and distribute impact. But even a very short fall with something more static can generate catastrophic impact forces. Dyneema stretches about as much as a steel chain. Nylon stretches more. This doesn’t mean to avoid Dyneema slings, as they’re great in many ways. It means be aware of the situation and application.

* Shock Loading – pretty much always a bad thing. It receives a lot of attention with anchors, with many considerations about the best setup regarding equalization and shock-loading (summary: trying to pre-equalize and tie-off an anchor rarely achieves true equalization, so if one piece fails, the others get a shock-load anyway; sliding-X type configurations are less likely to fail in the first place; bomber pieces are the most important in any anchor). Yet we shock-load gear when we take leader falls, and pieces rarely blow-out. Why? See above – elasticity of a climbing rope. Put this info together, and you see that static shock loading – like falling directly (no rope between) onto a sling equals disaster.

Note that DMM’s drops were done directly onto the slings – no climbing rope attached. The dynamic properties of a climbing rope save our asses. So, when would these tests apply? When you’re clipped-in directly to a piece of gear or your anchor, like using a sling as a daisy chain at belays (or simply using a daisy chain; the results here would seem to apply). For example, if you’re clipped-in with a 60cm sling (thus 30cm long, since it’s a loop), if it’s not taught, like you’re standing on a horizontal ledge, it could dip just 15cm (about 6 inches) down and back up to you, and so if you fall off the ledge you’re taking a factor-one fall onto a static piece — check out the forces. That’s a direct free-fall, but wow. Enough to break a piece of gear, possibly the sling, and your body. Yikes.

Take-home message: don’t clip-in directly with a Spectra or Dyneema sling and jump off the ledge. Duh. Sometimes, though, we might slip off the belay ledge, or climb above the anchor to mess with something above (like adjust the top piece of the anchor) and easily slip and shock-load.

For protection pieces? I don’t see this as any reason to ditch your low-bulk, lightweight Dyneema slings and draws, because you’re clipping the rope – the greatest shock-absorber in the system – into the sling/protection piece. I’ve long carried one or two nylon quick draws or runners on my rack, though, for the very purpose of minimizing impact force on a sketchy piece or my first piece off the belay (when your impact force from a fall would be much greater, due to less rope out). Seems a negligible contribution to rack weight, and maybe just a tiny bit of extra saftey. I figure I do enough stupid shit already, and so, when I can, I should stack the odds in my favor.

Interesting side note: In DMM’s drop tests, a knot in the Dyneema sling greatly weakened the sling, of course (we’ve all long known that the bends of knots make any material weaker than when straight/unknotted), and it snapped like a twig in the drop, breaking at about half the force of the unknotted sling. The nylon sling also recorded a lower force, but – but but but – the sling didn’t break. Huh? Yeah, it actually reduced impact force, without the sling breaking, in one of the tests. How? Most likely due to nylon’s elasticity allowing the knot to act as a shock absorber, dispersing the force, making it less of the sudden shock-loading, catastrophic jolt that snapped the Dyneema sling. Whew, the mind spins a bit.

It’s tempting to simplify, but I don’t think that these tests indicate any need to stop using lightweight gear, like Dyneema slings. Dyneema slings have superior abrasion resistance and, at least unknotted, are much stronger than nylon, and they absorb less water. Only idiots take a single piece of information (“that sling broke at XXkN!”) that’s part of a complex system and jump to a rash conclusion.

So, more take-home points? I’m no rocket surgeon, so don’t listen to me, but here’s what I figure:

* “Having slack in the system is bad news,” as Streaky says. Keep in mind that we’re particularly talking slack with a static system here – slings, particularly Dyneema slings. No rope incorporated. Clip-in with the climbing rope, and you’re fine.

* Don’t tie knots in Dyneema slings if they can be shock-loaded. They’re too static, it makes them snap. They’re fine at their regular length, though (so long as you don’t shock load them without a rope involved). What about Dyneema slings knotted for an anchor? I don’t know, good question. We’ve always known to avoid shock-loading anchors anyway, but this becomes especially pertinent.

* Shock loading something static is bad news (again). A very short fall can generate deadly force in a static system – just a couple of feet, like imagine you’re standing on a higher ledge, clipped-in directly to the anchor with a Dyneema sling, and your foot slips and you shock-load the anchor. Especially if you had a knot, maybe for length adjustment, in your Dyneema sling. See-ya! Even if you’re backed-up with the rope, the force you’d generate is likely enough to cause you internal injuries.

If I were to distill it all to a single useful point: if there is any risk at all of your shock-loading your piece or anchor, use the climbing rope – not a sling – to anchor yourself in. It’s usually simplest and fastest anyway. Reach the anchor and clip-in with the rope – usually a clove-hitch to the power point or a single bomber piece that’s connected to everything else (always thinking “what if this were to fail?”). Related point: when clipped-in directly to a bolt or piece of gear, only hang on it. If you’re going to boulder-up and work a move (shock-load risk), be sure you’re on the rope, not directly in with a sling or draw.

A crucial thought, along the lines of crucial general thinking: “What if this were to fail?” The associated question, of course, is “what is the likelihood of this failing?” If I’m at a hanging belay, and on hard terrain where a fall is more likely, I’ll be more vigilant with how I clip-in, and with backups. If I’m on an enormous ledge, one where a lighting strike wouldn’t knock me down the face, sometimes I won’t sweat being unroped (or I’ll at least allow myself a bunch of slack in my tie-in point). Just like how I don’t wear my seatbelt when I’m sitting on the couch, watching Cops every night.

OK, I’ve repeatedly repeated myself enough already. Use the gear correctly, with some thought, and you can go lighter safely – it all adds up, as grams become ounces become pounds. Just remember to think – problem solving is part of what we love about climbing. Going lighter means going faster, which means more climbing in a day, which means the day ends with time for margaritas. Really, it’s all about the margs.

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~ by Kelly Cordes on July 9, 2010.

5 Responses to “Snapping Slings (gear geek post)”

  1. Thanks for the reasoned post to counter the “dyneema eeevil!” stuff.

  2. One additional point for very short falls is that the human body flexes a fair amount. So when you’re hanging at the belay and your crampon slips off the crystal, the 10cm “shock-loading” is still not nearly as bad as a similar test with a static weight (block of metal). If you start falling a significant fraction of a meter onto a static sling, the shock-loading should quickly become apparent in the form of back pain and bruises around your harness line.

  3. I would be interested to see the results of drop tests using a daisy chain instead of open slings. I have heard that the bartacks in between each loop are rated at 2kn, so it seems like it might act as a load limiting factor, like a screamer

  4. Even when you do have a perfectly equalized anchor, once you loaded the slings the wrong way they can snap at any moment.
    I wonder if a cordelett or something similar is better.

    Ps: yes for the margs! I have a brand new herradura waiting for me

  5. Watched this video a long time ago and it made me worry about making sure I had no slack when anchored by a sling preparing to rap. Also made me wonder why I bother saving weight buying all those dynema slings when nylon is clearly safer. It’s a good video to share with your palls who insist on using dasies or a pass and not tying in with the rope. Even if the p.a.s.s wont fail think about the load on the anchor!

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