UKC

I know that when you knot a Dyneema sling you reduce the strength at the knot considerably, due to the friction generated as the knot tightens in a fall. my question is: Does the same principal apply to the overlaps when a 60cm sling is doubled or trebled up when used as an extendable quickdraw?

In reply to bikerrog: No

Al

In reply to bikerrog:

I would guess not, because there is no knots.

In reply to bikerrog:
Does the same principal apply to the overlaps when a 60cm sling is doubled or trebled up when used as an extendable quickdraw?


Knot if you are careful.

In reply to bikerrog:

I doubt you get anything like 2x rated strength for a doubled sling but I also doubt it's significantly weakened. IIRC Jim Titt had some data indicating there is potential for a loss of strength but I don't recall the details or it being alarming. I could be mis-remembering!

jk

In reply to bikerrog:I have a little DMM sling strength diagram in front of me showing a doubled sling as 200% as strong and knotted being approx 50% as strong.

In reply to jezb1: you have only one layer at the end of the loop, even if you fold the loop into two/three hence max 100% (not doubled but folded) but what happens where the folds of dyneema sit ontop of each other under compression and movement around the crab during a fall?... Although, the under layers can slide in sympathy, if you see what I mean

In reply to bikerrog:
I thought the weakening at a knot was due to the sharp bends within the knot rather than any movement as the knot tightens.
Doubled/trebled slings in extenders is pretty common so it must have been tried and tested by by thousands of climbers in thousands of falls.
You'd expect a doubled/trebled sling to take a load of about 44/66kN so unless there was a huge weakening of the sling, the caribiner would break before the sling. More realistically the rock or nut/cam/hex would fail or pop out.

In reply to elsewhere: all fair comments. I understood the damage was done by frictional HEAT generated as the knot tightend. Hence my concern as the layers of dyneema settle against each over the crab, and under tension

In reply to bikerrog:
You may be well be right about the heat/friction/tightening aspect.

In reply to bikerrog:

According to Jim Titt, the answer is yes, by up to 40%

http://www.ukclimbing.com/forums/t.php?t=412666&v=1#x5887472

In reply to plexiglass_nick: I think that description is referring to "larks footing".

Al

In reply to tradlad:

to my understanding Jim is indeed talking about doubling the sling over and not larksfooting, but perhaps ask Jim for clarification.

In reply to tradlad:

read further down the thread - ie. the discussion between Jim and speekingleesh.

In reply to plexiglass_nick: I was once climbing in the Ecrin and a French climber went out of his way to tell me that tripled slings were dangerous. I thought he was talking b*ll*cks but if it is indeed true that is a really serious concern as it is a recommended practice in many manuals and other instructional material. Personally I'm not going to get too concerned about it at this time, I only tend to use it in this way when I have run out of quick draws, but I will be interested to hear any opinions from respected parties, like Jim Titt. For the time being I remain unconvinced that there is a significant adverse affect.

Al

In reply to tradlad: Presumably this weakening would apply in the case of extendable slings such as those found on DMM cams also then?

In reply to tradlad:

He is discussing doubled up slings as used in 'slingdraws'. 60% of 22kN is still stronger than anything I'll be clipping to, I shan't be losing any sleep over it despite having at least half my quickdraws set up like this. It's an interesting counter-intuitive result though!

jk

In reply to hexcentric: If it is true yes and perhaps even worse as the sling is broader and there is less chance of it lying at the side of the other part of the sling as described in JT's note. If it is true there will be a few embarrassed authors and designers out there. As I say I remain to be convinced but bear in mind I climbed in the days when gear was very primitive so everything is an improvement compared to that.

Al

In reply to bikerrog:

You have to remember, that although it may be weaker, the nut, cam, hex, or rock will break first.

In reply to Dan Lane:

Quite so. I also agree with jkarren the results are counter intuitive and on first acquaintance a lttle alarming.

Al

In reply to plexiglass_nick: Thanks Nick, just read the thread you linked to so worst case realistic senario now is will 60% of 22kn be stronger than reasonably placed nuts etc which have 8-12 kn bs's.
I'm thinking that each of the system components will be approximately equal in strength even when doubled slings are used, so I dont need to worry about that aspect, just be aware of it

In reply to bikerrog: which is just what you other guys have said.. Doh! thanks everyone, glad I posted this, it's been educational