by Anonymous_Coward » Mar 13, 2014 3:05 pm
This is a great discussion, but I feel like some things are starting to get confused. A normal double figure eight is not usually considered a load distributing/equalizing knot. Yes, it will slowly try to equalize under cyclic loading and unloading (like someone climbing the rope), but it is not a true self-equalizing knot. The legs are more or less "fixed" and the loops must be adjusted properly in order to equalize the load. I do agree that the knot would be in danger of failure if one of the loops were cut and then the knot was subjected to cyclic loading. But....
How many times have you ever seen the loop of a double figure eight get cut? I have used these knots hundreds of times and I have seen it exactly zero times. It is a nice learning exercise to talk about it, and realize the failure modes, but it is very unlikely to happen in reality. If you set two bolts, and then one or both of the double eight loops have to bend over a sharp spot, then you have either set the bolts improperly, or you are using the wrong knot, or both. The reason to use a double eight is to share the load between two anchors, usually bolts. The redundancy is there to protect against anchor failure. If we routinely worried about knot loop failure, then all knots would have two independent loops.
bradyfl, could you point us to the reference that says a self-adjusting double eight would fail because of anchor failure or loop failure? The self-adjusting double eight that I am familiar with would fail because of big loop failure, but not failure of one anchor. I don't think they are referring to a run-of-the-mill double eight here, but a true self-adjusting one. The one I know has a carabiner involved that joins the big loop and the little loop.
I do not agree that a (normal non-equalizing) double figure eight knot would fail in the event of one anchor failure. If that was true, then the whole operation of setting two bolts and using this double loop knot would be for no reason. Think about a double eight Y-hang off of two bolts. Imagine this rope is loaded, and then one of the bolts pops out of the wall. With the knot still loaded on the other bolt, what possible circumstances would cause the now unloaded loop with maillon, hanger, bolt, and probably chunk of rock attached to pull back through the loaded knot? I don't see how it is possible.
Andy Armstrong
American Carbide Council