Worst-Case Scenario – A Factor 2 Fall
Let’s say I’m using a rope that has a maximum impact force of 8.6kN, and the leader takes a serious almost Factor 2 fall. The force on the belayer catching the fall would be less than that on the leader because of the friction on the carabiner through which the rope is clipped. How much less typically? If it’s 50 percent less, then the total force on the carabiner is 8.6 + 4.3 = 12.9kN, way less than the strength of the closed carabiner and also way less than the strength of the quickdraw. Is this analysis correct?
—Steve Pomerance, via rockandice.com
If you really want to know, read Richard Goldstone’s “The Standard Equation for Impact Force,” based on calculations by Richard M. Leonard and Arnold Wexler, in Belaying the Leader, Sierra Club Bulletin, 1946. Have your 13-year-old daughter explain quadratic equations to you and leave Gear Guy out of it.
But you asked, so I’m pulled back in.
First, if you take a Factor 2 fall you won’t have a carabiner in the system because you can only take a Factor 2 by dropping directly onto the belayer with no gear clipped between the two of you. With 20 feet of rope out you fall 40 feet before rope stretch. You can only take a Factor 2 on a multi-pitch route with no protection—if the route has gear (and it holds), you can’t take a Factor 2. Of course, if you fall on a single-pitch route with no gear you hit the ground. I call this a Factor 3 fall.
But, you said “almost Factor 2” fall. I’ll guess that you are working off the Factor 1.77 drop test used by the CE and UIAA to certify ropes. If you were to take one of these test falls, the impact would be such that you would spend the rest of your life in a diaper, unbothered by carabiner strengths.
In your question, you hypothesized whether the load would, due to friction on the carabiner, be reduced by half. On Earth, friction on the carabiner holding the fall will be a factor, but to what degree is unknown except in math models. The best estimate—the one posited by eggheads who stay up nights drinking diet Mountain Dew and punching numbers into calculators with key symbols known only to the Pharoahs—is that friction reduces load by one third.
We know based on practical experience that carabiner friction is real because a 150-pound belayer can “take” a 170-pound leader and the belayer will not get hoisted off the ground. Your assumption—that even a worst-case fall cannot load a carabiner to the point of breaking it—is like that nutty theory of evolution, mostly correct. CE standards require a 20kN minimum breaking strength along a carabiner’s major axis. Even with load multiplication on the anchor you’ll never achieve this high a load.
Don’t call your agent and cancel your life insurance just yet, however: A misused carabiner, like many a climbing marriage, can break. Cross load or nose hook that biner and it can snap even below the CE-required minimum gate-open strength of 7 kN. In the Accident Report in issue 212, we reported on a fall that broke not one, but two carabiners in a row.
Despite the recent bad news, Gear Guy can report that he once took a Factor 2.1 fall—170-foot fall on 80 feet of rope—which is possible if you screw up in enough ways. The rope was nearly blown into pieces (see photo), but the single non-locking carabiner that caught the fall, though bent, was like GG himself, unbroken. Gear Guy has spoken!
This article appeared in Rock and Ice issue 214 (December 2013).
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