Dyneema sling as anchor?

Kees van der Heiden wrote:

The chain is at your feet. Say, you are particularly clumsy that day and fall down the overhanging wall, while you were standing on the edge, tethered with a dyneema schling. That would be fctor 2, wouldn't it? What I always do in this particular situation is stay low and keep the schling taut. It's also not a really narrow ledge and you can move down a bit until your hips are more or less equal with the chain, so it isn't that bad, but still, gives me creeps now I think about it.

Edit: Oh, I see now my own writing wasn't very precise.

Yeah, might want to edit to avoid confusion.  You cant generate a ff2 if you are already below the anchor.  

If you are above the anchor and tether yourself into the anchor and then fall, that would be a ff1-ff2.  

Harumpfster Boondoggle wrote:

With the 2' leg and additional 2' the load is tied to you have 4' of rope for a 2 foot fall for a FF 0.5 test plus some absorption by the knots (getting us a substantial reduction in forces).

You're still not calculating it right.  If you have 2x 2' legs in parallel, you don't get to count 2' of additional material.  Those two legs behave as though they were twice as stiff, since it's 2 springs in parallel (ish, we can also factor in the trigonometry of the included angle, but let's not get too fancy, especially since anchors should have relatively small angles anyway).  So, the net effect is that the anchor behaves the same as 1' of free single rope.  That's 2/3 = FF 0.67.

With regard to the knots, if you want to take them into account, then you also need to take into account the knots on the climber's tie-in and tether, so the initial scenario wouldn't be FF1.  For sure knot slip reduces force, but we shouldn't ignore it in 1 scenario and take it into account in the other.

There is no doubt that a rope anchor will result in less force on gear than a sling anchor.  The question is to what extent, and it's definitely not 50% (like you claimed).  It might be 30% in some rare circumstances, and in most more like 5% (i.e. inconsequential).

Kyle Tarry wrote:

You're still not calculating it right.  If you have 2x 2' legs in parallel, you don't get to count 2' of additional material.  Those two legs behave as though they were twice as stiff, since it's 2 springs in parallel (ish, we can also factor in the trigonometry of the included angle, but let's not get too fancy, especially since anchors should have relatively small angles anyway).  So, the net effect is that the anchor behaves the same as 1' of free single rope.  That's 2/3 = FF 0.67.

With regard to the knots, if you want to take them into account, then you also need to take into account the knots on the climber's tie-in and tether, so the initial scenario wouldn't be FF1.  For sure knot slip reduces force, but we shouldn't ignore it in 1 scenario and take it into account in the other.

There is no doubt that a rope anchor will result in less force on gear than a sling anchor.  The question is to what extent, and it's definitely not 50% (like you claimed).  It might be 30% in some race circumstances, and in most more like 5% (i.e. inconsequential).

I think you might still be getting it wrong as well.  The fall factor is still the same for the climber.  They still fall 4ft on 2ft of sling.  The force on the anchor however is just divided between two separate anchor points.

And a dynamic rope tether will be a “softer” fall force on the climber (and anchor) than a relatively static dyneema “daisy” tether, but I still wouldnt advise falling on the rope either.  There is not a lot of rope to absorb the fall so it’s going to hurt.  So I’d agree that the difference is nowhere close to 50%.  I’d lean more towards the 5% as well.  But the rope would not break.  But a dyneema tether on the other hand.., 

PS to the OP:  

Just for clarity, you might consider changing the title of your post to:
Dyneema sling as personal tether?

There is nothing wrong with using a dyneema sling to build a belay anchor.

There's a lot of speculation and extrapolation in this thread. I don't pretend to be an expert on this subject, but in all my experience with testing ropes, rope devices, and rope systems, there's one certainty: things don't always behave like you expect they will. Knots, rope construction, humidity, temperature, carabiner style, and tons of other variables change things. We can compare spring rates and fall factor math all day, but it doesn't necessarily mean the system will follow those rules.

To this point, I repeated the previous drops I did, but replaced the sling with a piece of dynamic rope of the same length as the sling. Instead of sling>carabiner>rope tether>mass, it was rope tether>carabiner>rope tether>mass. Same drop (1m). Peak force was 6.1kN. Not half - not even appreciably less. This is ONE SAMPLE. The other tests were ONE SAMPLE. I wouldn't make any conclusions from them except to say "huh, I guess as long as I have dynamic rope in the system, it probably doesn't matter much what I'm using for an anchor". Maybe I'll repeat them all with FF2 drops later if I have time, just to see what happens.

Zephyr Feryok wrote: There's a lot of speculation and extrapolation in this thread. I don't pretend to be an expert on this subject, but in all my experience with testing ropes, rope devices, and rope systems, there's one certainty: things don't always behave like you expect they will. Knots, rope construction, humidity, temperature, carabiner style, and tons of other variables change things. We can compare spring rates and fall factor math all day, but it doesn't necessarily mean the system will follow those rules.

To this point, I repeated the previous drops I did, but replaced the sling with a piece of dynamic rope of the same length as the sling. Instead of sling>carabiner>rope tether>mass, it was rope tether>carabiner>rope tether>mass. Same drop (1m). Peak force was 6.1kN. Not half - not even appreciably less. This is ONE SAMPLE. The other tests were ONE SAMPLE. I wouldn't make any conclusions from them except to say "huh, I guess as long as I have dynamic rope in the system, it probably doesn't matter much what I'm using for an anchor". Maybe I'll repeat them all with FF2 drops later if I have time, just to see what happens.

100%.

I will not be using anything except dynamic rope for PAS though, when rappelling.