UKH

Hi,

Musing after reading through this thread: https://www.ukhillwalking.com/forums/rock_talk/dyneema_for_abseiling_anchor-74...

There are references in there to abseiling on different diameter ropes potentially creating a sawing motion, presumably due to the ropes having different rates of travel through the belay plate.

When I climb with two ropes I usually climb with an 8.9mm triple rope partnered with an 8.2mm half/twin.

I am aware of some potential/theoretical hazards of pairing ropes with different diameters:

1. knot rolling, mitigated with a well dressed overhand and a decent amount of tail, and/or a second overhand immediately behind it to back it up
2. abseiling off the end if one rope passes through the belay device more quickly, mitigated by knotting the ends of the rope (which I do as a matter of course).

However I hadn't heard of/considered the hazard of different rope diameters potentially exerting more friction where the ropes are threaded. Is this a thing?

Of course (1) the difference between 8.9mm and 8.2mm is marginal and (2) I haven't actually yet had cause to bail off a sling directly (I normally carry 6mm tat as a matter of course), but if there is a potential failure mode here (especially with a bigger discrepancy in rope diameter) then I'd like to be aware of it.

In reply to mcawle:

You have a couple of options, depending on height.

• if <1/2 a rope length high, just ab on the thicker one doubled and carry the other.
• if >1/2 a rope length high, you can arrange it (carefully!) so that you just ab on 1 side (the thicker one) and use the thinner one to pull it down.

In reply to mcawle: I've done a fair few abs in this situation, and a sawing motion is setup by differing rates of stretch. and loading/unloading during long raps.  

Just set up the abseil in a normal, safe way, and be aware this needs to be considered

In reply to mcawle:

Arrange the abseil so that you pull the thin rope to retrieve.

In reply to mcawle:

I would have thought that the 'sawing' is another potential hazard caused by one rope going through the device more quickly. The device will grip the thicker rope more, potentially 'pulling' it a bit and therefore creating movement through the sling. I'm not sure it's sawing in terms of a back and forth motion; it will likely be mainly one way.

I would be just as wary of this for 6mm tat! Maybe depends on the particular 8.9 and 8.2. There's definitely a fair difference between an iceline (8.1mm) and a typical 9mm rope.

In reply to Presley Whippet:

This has the advantage that the thicker line doesn't 'travel' as you ab, as the knot keeps it in place. However, it's easier to pull the thicker rope when retrieving!

In reply to mcawle:

Whether or not abseiling on one side only with some kind of knot block (or Krab block) is a good idea depends very much on the specific circumstances.

I abseil to get back down something, retreat, or as a pull through while going somewhere (but never ever for "sport" of it) and the rope getting stuck on pulldown is my biggest worry, especially when you have more pitches to descend. Normally I have very dissimilar ropes (like a dynamic and a semi static) or a rope and a tag line

Depending on what is your top anchor, it might be better to abseil on two ropes if they aren't too different as *if* it's safe to only have an "EDK" (a simple overhand well dressed and tightened with long tails) that's far less likely to get snagged on pulldown than an alpine butterfly and krab (or clove hitched krab if a chain ring) or whatever pulldown option you pick.

Ropes getting stuck can be a real pain in the azz, risky too in some situations

Edit: at risk of being obnoxious, make sure you remove the knot on pull through but have some kind of procedure/ritual that involves checking, double-checking and triple checking as most situations (except a tree 😁) that will get you stuck. Just before Christmas I had to rescue a rope for a student group as they left the knot and pulled it up and saw the knot going up just too late to grab the end, they didn't fancy climbing up the stuck rope as they couldn't see the anchor from the bottom. I jumared up a stuck rope last year and that is brown adrenaline in the trousers time, it shifted and dropped me 3 or 4 foot while I was climbing under an overhang, turned out to be safe, but really worrying times. Avoid these situations!

In reply to mcawle:

1. with a 9.8 and a 7.8mm large slippage can occur, as I've seen it! With closer diameters, not sure. It is more a creep in one direction, than sawing. (Going off question, this creep can be used with similar rope diameters, to useful effect, if you have enough rope ,as the last person down can make sure the knot is over the edge and away from cracks.)

2. If both people put their devices on the rope at the same time (on teether) the rope is unlikely to creep for the first person down. This means both ends get to the next station, however, the second can be exposed to creep.

3. Place the knot on the side of the thinner rope if possible. If not, due to multiple abseils, always use point 1 above.  

4. Probably best just to sacrifice a krab if no rap ring.

5. In my experience, if no ring/krab then pulling the rope can be quite hard, using a pull line can then be really tough if there is an edge below the upper station, or other sources of friction

In reply to mcawle:

Cheers everyone. My main interest was to get a view on the potential hazard - sounds like there may be something there to keep in mind. 

Thanks for the options to mitigate - as a rule I try to avoid abbing off a single strand with a blocking knot because I worry about it getting stuck, but I agree that it does mitigate the 'sawing' risk as a few people have pointed out.

The melting point of dyneema is still 145C or so... it would be great to know how much friction/heat is likely to be generated from what I would expect to be pretty gradual movement of the rope over the sling (albeit weighted movement)... I feel like it would be relatively low (compared with hauling 30-60m of rope quickly against it), and I've never heard of anyone coming to grief from this situation, although admittedly it's a bit of an edge case.

It would also be interested to understand how much it dissipates if ambient temperature is low e.g. cold alpine/winter conditions. Not that I expect anyone here to have that info off the top of their heads!

Nevertheless I'm probably now more inclined to default to leaving a krab or maillon on a dyneema sling in this particular situation, all else considered.

Re: the 6mm tat - yes agreed caution still to be exercised there although my understanding is that the melting point for nylon is a bit higher at 245C or so.

(Sources for dyneema and nylon melting points is a somewhat unrelated BD article: https://www.blackdiamondequipment.com/en_US/stories/experience-story-qc-lab...)

In reply to David Coley:

Thanks, good point about the knot. And yes, I think if I'm down to dyneema slings I'm probably just doing to be sacrificing a krab or maillon as well, depending on the situation.