Why Climbing Falls Are Seldom Severe, And How To Keep Them That Way

I’m always surprised by people’s differing attitudes toward lead protection. Some feel that even a large piece of well-placed pro in great rock should never be loaded — the classic “leader must not fall” mentality. Other climbers take repeated whippers onto small gear in suspect rock because, “This stuff is made to take this kind of beating.” As in most situations, the reality is somewhere between the extremes. Much has been written about impact force and product ratings, but what does this mean when you’re run out and strung out?

The Theory

Climbing gear strength is measured in kiloNewtons (kN), a measure of force equivalent to roughly 225 lbs. Typical carabiners have a breaking strength of about 24kN, or 5400 lbs, but most lead protection breaks at significantly lower loads. The standard rope-certification test involves dropping an 80kg mass approximately five meters, on a section of rope about three meters long. The top piece of lead protection in this scenario experiences almost double the force exerted on the rope, so if this fall were to occur in the field, loads approaching 20kN — enough to break even the strongest cams — could in theory occur. Yet gear failure is rare in the field. So what’s the deal?

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Fall Factors

"Tech Tips - Trad - Stacking the deck, to avoid hitting it"

The “Fall Factor” gives an indication of a fall’s severity, and is calculated by dividing the distance the climber falls by the length of rope between the climber and the belayer. A Fall Factor of two is the harshest, and the lower the Fall Factor number, the less severe the fall (assuming you don’t hit a ledge, the ground or a protrusion.) A climbing rope absorbs energy by stretching, and more rope in play means more energy absorption capacity. When it comes to determining the severity of the forces in the system, the length of rope that catches the fall is just as important as the length of the fall itself. In the lab test, the type mandated by the CE and the UIAA, the fall is almost twice the length of rope, giving a Fall Factor that approaches two — the maximum possible. This kind of fall is dangerously harsh even when short, say falling 60 feet on 30 feet of rope, but fortunately it’s extremely rare for a leader fall even to attain Fall Factor one; to do so, the falling climber must fall the same distance as the rope out, that is 50 feet of rope are in between the leader and belayer, the leader has to fall 50 feet. In practice most climbers clip a bolt or place gear every six to 10 feet, making it difficult to achieve a Fall Factor of one although in theory it is possible, especially when gear rips. Low Fall Factors are one reason why loads in the field don’t reach the hypothetical 20kN mark, but there are other factors of importance. In practice most Fall Factors are well below one.

The Analysis

Other factors come into play to soften the impact on the system include:

  1. Rope slippage through the belay device. This happens regardless of how hard the belayer locks off. Auto-locking belay devices slip less, but they still let the rope slip, reducing the overall force generated by a fall.
  2. The belayer is slightly lifted during a fall. This effect is magnified when the belayer “jumps” into the fall (i.e., soft catches), as can be common when sport climbing.
  3. Both climber and belayer’s harnesses and bodies slightly deform during the moment of impact, absorbing a bit of energy.
  4. Dynamic climbing ropes absorb an amazing amount of energy, meaning they’re very effective at minimizing the forces on your lead protection, and on your body.

Situations That Can Increase The Impact Force

  1. Falling past the belay station on a multi-pitch route, which can indicate a fall factor greater than one. Falling past the belay is possible when the leader runs it out—placing gear as soon as you lead off the belay will eliminate this potential as long as that gear doesn’t fail.
  2. Successive falls in a short period of time on the same rope. This increases impact force because the rope temporarily loses some of its stretch after a fall, that is, the needs time to recoil. After a fall, give the rope a few minutes to regain its stretch before you start climbing again.
  3. Belaying directly off an anchor instead of the belayer’s harness. This practice increases Impact Force because it eliminates the “give” of belayer in the system.  It’s better to belay off your harness and then clip the lead rope through the belay anchor as the leader’s first piece of pro.
  4. Actual climbing. Many pieces of climbing gear have breaking strengths it or below the 6 to 9kN range, specifically smaller nuts, micro cams, and carabiners that are cross-loaded or loaded with the gate open. Keep this in mind when climbing above thin protection, manky placements, or when the consequences of gear failure are dire. When gear fails, the Fall Factor and Impact Force will likely increase.

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Tips to Help Keep You Safe

  1. Back up small and dubious protection, or better yet, equalize several pieces using RENE (Redundant, Equalized, and No Extension) when possible.
  2. Watch out for carabiners with sticky gate action, and placements that might pry open the gate. A carabiner with its gate open is only about one third the strength of a closed-gate carabiner. A closed carabiner with the spine loaded over an edge can easily break, even in a moderate fall—use a sling to move the carabiner away from the edge.
  3. Never catch and weight the nose of a carabiner on a sling, hanger, or cable. This can cause it to break under body weight, as there is a tremendous amount of leverage on the spine of the carabiner. Even with small protection, actually breaking gear is quite rare. It’s far more common for placements to fail from being placed poorly or from rock breaking, than from exceeding the rated strength.

Paul Tusting, M.E., has worked as an outdoor-industry engineer, testing and designing new climbing equipment for five years. He’s been an active climber for over 10 years.

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