Knots in Nylon Sling to Protect Marginal Placements

Kyle Tarry wrote:

While this is true, the act of converting energy to heat isn't what makes a screamer useful in a climbing application.  The fact that the energy is converted into heat simply means that you don't get rebound (which isn't that critical for this application, I would argue).  For example, a perfect spring wouldn't convert any energy into heat, and yet could still be more effective at limiting peak force than a perfect damper, which converts all the energy into heat, depending on spring rate and damping coefficient (i.e. the shape of the force curve).  I'm rambling, but the point is that conversion into heat isn't actually that important, what's importantly is the shape of the force curve.

A screamer "works" (i.e. lowers peak force) by spreading the force out over more time/distance so you can get more area under the curve.  The tradeoff is that the extension also adds total energy to the system, so it's a balance.

Yes, you are rambling, and I can't say that I follow along in your ramble.  I don't know what "spreading the force out over more time/distance" is supposed to mean.  If a given load comes on the rope *later* (as would happen if a screamer converted no energy to heat, but only delayed things) then the force would be *higher*, not lower.  The only positive effect a screamer has is by "absorbing" some of the energy of the fall.  And as I said, for most significant falls, it's a negligible amount.

GO

David Kerkeslager wrote:

EDIT: Forgot to mention the caveat that dyneema behaves very differently from nylon when knotted, and probably isn't appropriate for this application.

DMM tests show that knots in Dyneema reduce peak forces more than nylon due to it being more slippery than nylon.

https://vimeo.com/74573767

baldclimber wrote:

DMM tests show that knots in Dyneema reduce peak forces more than nylon due to it being more slippery than nylon.

https://vimeo.com/74573767

Yes, but that video is the sequel to another video where they found that knots in dyneems reduced the breaking strength to levels which make me uncomfortable.

David Kerkeslager wrote:

Yes, but that video is the sequel to another video where they found that knots in dyneems reduced the breaking strength to levels which make me uncomfortable.

Why?  You correctly pointed out in your first post that any strength reduction from a knot leaves a sling that is still much, much stronger than a "marginal placement". What is going to fail first?  Your now ~11Kn dyneema sling or the "marginal placement"?  The video even makes the point that the knots reduced the peak force, and none of the slings (Dyneema or nylon) broke at anything but quite high forces.
You're worrying about the wrong thing :)

Regarding that paper, and associated graph, near the start of the thread.... The test was run with a 15cm drop. Six inches! Extrapolating that data to a lead fall scenario is... problematic.

Even for the case of falling onto a sling clipped from you into the anchor, which the paper (and that DMM video) attempts to address, the steel mass they use is a poor model for a human body on these scales: skin and connective tissue stretches, muscles extend, the spine compresses, capillaries rupture (aka bruising), blood sloshes, etc. All of these effects will reduce peak loads more than a knot tightening.

Just grow some balls and don't place shit gear

Serge Smirnov wrote:

I was comparing knots to using a lower-impact-force climbing rope (but I now realize that comparison is irrelevant for the scenario David had in mind).

Good point about damping - damping makes the element closer to a constant-force device.  So maybe knots are better than rope.

well, in the case of the rope damping is also important and is supplied by the friction in the weave of the strands as they try to elongate.  we always tend to think of the rope in terms of the spring component, but it really takes both aspects to optimize the system.

Kevin Mcbride wrote:

Just grow some balls and don't place shit gear

ha ha, leave it to a 9 year old to lay down the law...

Kyle Tarry wrote:

You started this thread about how to use protection rated at 3 kN, and yet a dyneema sling with a knot that still holds 10-20 kN (as much as any well placed cam or nut) makes you uncomfortable?

Fair point. Experiments show knotted dyneema failing at <10kN, but the marginal placement is definitely the weakest part here.

Kyle Tarry wrote:

Here are 3 different sets of data from DMM that don't have a single breaking point below 10 kN:

http://dmmclimbing.com/knowledge/knotting-dyneema-vid/

http://dmmclimbing.com/knowledge/slings-at-anchors/

http://dmmclimbing.com/knowledge/how-to-break-nylon-dyneema-slings/

I'm sure somebody somewhere could get one to break below 10 kN, but that's not a normal, reasonable situation, and being concerned about this isn't rational or realistic.

Is there a single reported case of an accident caused by a sling breaking (not due to rockfall or cutting)?

It's certainly conceivable. DMM tested with brand new slings, which isn't the case for most of the slings on most of our racks. If you're taking a high energy static fall on a fuzzy dyneema sling,  I wouldn't count on it holding.

CLIMB -SAFE - CLIMB -SMART = CLIMB  FOREVER!

no wonder that your climbing doesn't improve, your scared out of  your mind, by things that do not often, if ever matter.

  go across the Hudson river and climb up some route I opened  on the Palisades. . . 

 over-thinking and looking for ?(It is after 3am, I'm bonking' here) what are You looking for? VALIDATION?

Alan Doak wrote:

Regarding that paper, and associated graph, near the start of the thread.... The test was run with a 15cm drop. Six inches! Extrapolating that data to a lead fall scenario is... problematic.

Even for the case of falling onto a sling clipped from you into the anchor, which the paper (and that DMM video) attempts to address, the steel mass they use is a poor model for a human body on these scales: skin and connective tissue stretches, muscles extend, the spine compresses, capillaries rupture (aka bruising), blood sloshes, etc. All of these effects will reduce peak loads more than a knot tightening.

In the scientific community it is common practice to aim for repeatability in experiments. Including human analogs, although possible, would likely be extremely expensive. And, it is somewhat irrelevant - article examined effects of various knots as compared to benchmark. 

In falling fall factor is more important that fall distance, IIRC it was 0.25 for this experiment. Furthermore, even with 0.25FF, 0.15m drop load was peaking at, roughly, 12kN. Anything higher would start breaking equipment. The article focused on static rope - the 0.25FF is very significant.