Carl Dec is a certified rock instructor and guide centered in Moab. He owns Red River Adventures and was my boss while I raft guided in 2006. We’ve stayed in touch since then and he sent me a note after reading my entry on building anchors.
In most cases, Carl uses a knot master point when building an anchor. He pointed out a study done by DMM in which slings were shock loaded. DMM explains the testing in an educational and interesting video. I encourage you to watch it here.
To summarize, DMM tested factor 1 and 2 falls on 48-inch dyneema and nylon slings with a 175 lb weight. Because nylon stretches more than dyneema, the impulse of a fall occurs over a longer time and the maximum force on the sling is less. Dyneema has a greater strength to weight ratio, but is less able to handle shock loading. Though the nylon slings performed better, both types of slings broke under certain realistic conditions. Tying a knot was shown to significantly reduce a sling’s strength. In any case, slings, personal anchor systems and daisy chains are not meant to be shock loaded.
Many of the issues with slings have arisen because of climbers clipping directly into a piece with a sling, climbing above the piece and then falling. Indeed, in the video and accompanying article, that and similar misuse seems to be the focus.
Carl was curious to hear my thoughts on the forces that would result on a sling from a piece blowing in an anchor with a Magic X master point.
Without spending hours on research, here are my thoughts.
If the belayer is secured via a clove hitch, the maximum force on the remaining anchor will not be as great compared to when the climber is attached with a daisy chain or another sling. If you are belaying someone up a climb, he falls and an anchor piece goes, a clove hitch will ensure the impulse felt by the remaining anchor will be lessened by the rope stretch. Even if tied in with a short amount of rope, the stretch will still greatly reduce the maximum force felt by the anchor.
In the case of securing to the anchor with another sling, it’s harder to quantify. The impulse will be more severe, but there will be some lessening of the maximum force due to small stretches in the additional sling, the harness and the pieces in the anchor. If we assume that these factors don’t lessen the impulse at all, the question is how far does the anchor extend. In the worst case – a two point anchor in which one piece blows, the sling will extend about 1/2 a sling length. With a 48 inch sling (the same length used in the video), that’s 24 inches for a factor ~0.5 fall. This should still be well within the sling’s holding power though it may cause you quite a shock (literally and figuratively). The climber who peels in either case is still attached to the system by the dynamic rope and will thus not cause a huge force.
Maybe the take away message is that if an anchor is built such that a blown piece will result in a huge extension, a knot is a much better master point than a Magic X.
Though I didn’t go into detail, this was something I alluded to in the aforementioned anchor post. If I have a sub-par piece in an anchor, I won’t include it in the master point setup. I’ll extend it separately and in that case it’s only there for catastrophic backup. Though the Magic X setup is worse in the case of a blown piece, the self-equalization it allows makes a blown piece significantly less probable. If built properly, either the knot or Magic X will give safe anchors.
I tend to use dyneema just because of the weight/clunk factor and because the in-seam sewing and narrow body make them perfect for extendable draws. Dyneema is also more abrasion resistant. Based on the results of the DMM testing, nylon slings would be preferable for any case in which a sling could be shock loaded.
Thanks to Carl Dec for sending along the video and for good discussion.