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
—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.
To 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!