We are taught to use the rope to tie in to anchors, and know that a rope’s shock-absorbing abilities are reduced when you use a shorter piece of rope. But just what percent of stretch advantage does a rope have over webbing when used to connect you to an anchor? And does the Dynamic Elongation figure on a rope hang tag apply when you use just a few feet to tie in to the anchor. Last, how much does typical webbing stretch?
—jeffpfleger via rockandice.com
It would be easier to digest the Columbia Boulder than to untie your Gordian Knot of a question, but I’ll do it anyway.
Is dynamic elongation a factor when you use just a few feet of rope to clip in? Yes. A stretchy rope is a stretchy rope, regardless of its length. More
rope will mean more stretch, meaning that the six feet of rope you tied in with won’t stretch as much as the 8.25 feet of rope used in the CE drop
test (bearing in mind that impact force also plays a large role in rope stretch—higher impact forces result in more rope stretch). Since the
forces on a belay anchor are usually relatively low compared to the CE test, your piece of anchor rope isn’t going to stretch nearly as much.
But “not nearly as much” doesn’t mean “nothing at all,” and using a dynamic rope to clip into an anchor is, in fact, critical! As you hinted at, webbing
doesn’t stretch much relative to rope, and Spectra and Dyneema webbing doesn’t really stretch. Since there are so many types of webbing, and all rope
models stretch differently, it is impossible to say precisely how much more rope stretches than webbing. Just know that it does and shut up.
illustrate, I once conducted drop tests using slings, daisy chains and a rope in a short-length situation, as you’d have at a typical belay. I raised an
80-kilo weight (CE standard) about 2.5 feet and dropped it. The nylon slings and daisy chains often broke, or broke or bent the carabiner. But with a dynamic
rope, the anchor carabiners and rope hardly blinked. Gear Guy has spoken!