Thursday, August 22, 2013

Vertical Darwinim: Slings 'n' Things

Recently, I was asked about the safety level of girth hitching a sling to a tricam in a top rope anchor. My gut reaction was that it is absolutely fine as I have done it many times before. Then that nagging feeling of doubt snuck up on me. Do I really know what I think I know?   So, I did my due diligence and found some research.

First of all, why would anyone girth hitch a sling to a tricam or other nylon slung piece of pro? Why not just use a carabiner?  The obvious answer is that it can get you out of a jam if you are short on biners and need to extend a piece. The less obvious situation is that your piece is positioned so that attaching a carabiner would position the biner on the edge of a rock. This is a very bad place for a biner as it may cantilever over the edge, possibly bending or deforming the biner. In this case it is much preferable to make a softer nylon to nylon connection and extend the piece around the corner. In the case of a cam a girth hitch directly to the stem will suffice, but in the case of a tricam, girth hitching nylon to nylon is unavoidable.

So, if that is the only option you have, is it okay? Will it greatly reduce the strength? I found some compelling and disturbing research posted on the black diamond quality control lab. As it turns out, girth hitching a sling to another sling can actually reduce the strength by up to 50%. (Just FYI, when it does break, it is usually the THINNER sling that breaks.) This shocking strength reduction might permanently turn you off to the idea of girth hitching slings altogether. But some simple math shows that there may be times when this practice is acceptable.

Let's break it down. We can assume that our master point carabiner can hold a maximum force of 22KN. We can also assume that a well equalized 3 point anchor should distribute this force among three points of protection leaving each piece responsible for roughly 7.33333333.... KN lets round up to 8KN to adjust for a low angle of attack. Note that a .75 Camalot is only rated to 9KN. Now consider taking a 22KN sling and girth hitching it to that Camalot to reduce its effectiveness by half.  The now 11KN sling is still stronger than the .75 Camalot which still more than handles the distributed overall fall force.  Then remember that this scenario deals with maximum forces and real forces in a top rope scenario are far less than 22KN.

In summary, in many cases even a weakened sling should be effective in sharing the load of a top roping climber. Furthermore, if the piece that you are slinging is rated to anything less than 11kn,  your piece will pop before your sling breaks. Of course all of this math is based on a new, full strength sling. An old sling will start at a disadvantage and therefore fail at a lower level of force. It is recommended that slings be replaced regularly as per manufacturers suggestions. (usually every 5 years)

I am currently looking into an alternative to girth hitching yet have not found any research to support my new strategy. Any engineers out there have a pull test machine that they would like to use to help out your climbing community?

What have we learned today?
  • Always make sure slings are in good shape and regularly replaced (about every 5 years)
  • Use a carabiner when you can.
  • If you cannot use a carabiner, directly girth hitch to the plastic cam stem.
  • If there is no cam stem, girth hitching is okay as long as you know your anchor can handle it.
  • Never girth hitch to a wired nut or hex. There are other ways to extend those pieces. Ask me about it.
  • Listen to your small voice of doubt. Do research. Ask questions. Know that you know.

Climb Smart MN is a grassroots approach to climbing education based on community and charitable giving. For information on donation based climbing lessons visit ClimbSmartMN and follow us on Facebook. With over ten years of climbing instruction, coaching, and guiding experience,  Chris Hesselbein strives to better the climbing community through personalized technique and safety education.


  1. Hi Chris, nice blog!

    Sometimes when I'm setting tri-cams and they are in a position such that a carabiner would bend over an edge in the rock, I will clip the biner to the upper gap in the sling (above the sewn tie in the webbing). What are your thoughts about doing that?


  2. Hey Sarah,
    That is an interesting idea. Thanks for the comment. I am glad to offer our community a place for open discussion. If anyone else out there has information about this topic, please comment as well. We are all in this together. In my research I found no mention of any testing done to check that strategy. The official instructions only indicate a clip in point at the end of the loop. I would tend to agree with the instructions. They were designed in a specific way to create the optimal clip in point at the end of the loop.

    Now, lets talk what actually would happen if you clip in above the stitching like you have described. In that situation, the biner would have to choose one side or the other to pull on, causing unequal rotational stress on the stitching above it(closer to the cam). Additionally the biner would place the stress directly on the stitching below it. In a high factor fall, that type of stress can cause stitching to fail (much like the daisy chain situation in my "Attachment Issues" post. But let's continue and say that the stitching below did break. If the biner is on one side, it will whip to the end of the loop where it should have been in the first place. If it is on the other side it could continue putting pressure on the next two sets of stitching. In summary, there is a lot of stitching between your biner and thin air, but clipping in at the designated "clip-in point" will sidestep any of those possibilities. The manufacturer's suggestion is always the most secure method.

    So, I guess it all depends on your system and what type of force you are applying. This short clipping method has flaws and so does the use of slings. As I stated in the post above, all climbing systems have inherent weaknesses. It is up to us all to know what they are and accept the risk that each weakness presents.

  3. Chris,

    I see what you're saying - how if it loaded the "outside" of the stitching it could possibly be out of the loop if it ripped. If they redesigned it, such that that upper portion was always in the loop (naturally in the direction of pull) instead of the lower actual clipping spot, I wonder if it would be a viable option. After your comments and looking at it, I think clipping the upper portion isn't wise. Fortunately I've only done this a couple of times and only on top rope with other bomber pieces!

    Another thing I've done with tri-cams and tricky biner placements is to wrap the clip loop webbing around the biner an extra time to shorten it up a few centimeters. I've read before that wrapping sling is ok, but sometimes I wonder how it changes the load on the webbing. Any thoughts on that?


  4. I have not seen anything saying that wrapping webbing or slings around a biner is poor practice. If anyone out there has any thoughts please chime in. I could get on board with that idea. I think that wrapped webbing would have a tendency to equalize. But again, I have not seen any research as of yet. Thanks again for the comments!