Chaining locking carabiners
Googling seems to show that the conventional wisdom is that it's not a good idea to chain carabiners together:
Another thing to avoid is chaining carabiners together to lengthen a connection in the system. If subjected to a twisting motion they can unclip themselves. S. Peter Lewis, Toproping
Avoid chaining carabiners in succession, because they can twist, which weakens them and can open a gate. -- Freedom of the Hills, p. 170
Is there any reason not to chain carabiners together if they're locking biners? The specific application I have in mind is extending my ATC for rappelling. The FotH quote says that twisting can weaken the biners, but I don't believe this is realistically anything to worry about in a rappel setup, as opposed to an anchor.
What I've been doing previously is to girth-hitch a dyneema sling through the tie-in points of my harness, and tie a butterfly knot in the sling in order to shorten it a bit. I'm not happy with that technique because (1) I've learned that a girth-hitched dyneema sling fail in lab tests under surprisingly small loads (small fall factors), and (2) it can be a pain to get the butterfly knot out.
I tried replacing the dyneema sling with a nylon one, but the nylon one is too fat to make it practical to tie a knot in order to shorten it.
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Carabiners are designed to be strongest in tension. That's why the biner has two limits -- closed (where the forces are distributed lengthwise among two sides of the biner) and open (where the remaining arm is in tension and torsion.
When you twist two biners together, you are putting rotational forces on each biner that they weren't designed to take.
And while you are correct in that rapping doesn't usually put the kinds of dynamic stresses on biners that anchors do, if you lose half your anchor, it may be a different story.
Short answer is it probably will work, but you could just go buy a quickdraw and be done with it.
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The technique I learned through AMGA is to extend your rappel using a full length nylon sling. Girth hitch one end through your hard points, tie an overhand knot halfway through the sling then clip the remaining end back to your belay loop with a carabiner. You can then clip your ATC carabiner through both loops (over the midway overhand knot) and set up the rope. An autoblock or prusik can be attached below the ATC via your belay loop.
See this Mountaineers article for more details on the set up.
Its a pretty slick set-up that I use frequently. You avoid the mess of attaching a third-hand via the leg loop and instead have everything right in front out you. Also very useful when doing multiple rappels because you can unclip the biner from your belay loop and attached it right to an anchor while still being set up in your rappel (since it's redundant).
As far as linking carabiners is concerned, the one time I do this is when I am managing a topside belay and have set up a large lockers as my masterpoint.
This post was sourced from https://outdoors.stackexchange.com/a/7465. It is licensed under CC BY-SA 3.0.
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