The answer to this question will probably stun most climbers. A lot more force happens in a fall then most climbers would probably expect. In Rock & Ice issue #133 Tyler Stableford wrote an article about fall forces on a sling-shot (or pulley) style top-rope anchor system. Here’s the skinny on the tests that were featured in this article. The climber weighed 200 pounds, the belay was static, there was no slack in the rope, and 35.5 feet of rope was in the system. This resulted in fall forces ranging from 700 to 800 pounds on the anchor. Another test was reviewed in this article that was identical to the previous test, except there was 4 feet of slack in the system. The results were forces ranging from 1300 to 1500 pounds on the anchor. That’s a force of up to 6.67kN. To put this into perspective, you can think of this as the amount of force to blow a  Black Diamond #00 C3, a BD #3 Hex, a BD #5 stopper, or a BD #5 micro stopper.

Why were the forces so much greater than the climbers weight? The reason is because of the pulley effect. In Craig Luebben’s book Rock Climbing Anchors: A Comprehensive Guide he explains that in a sling-shot style system, the force on the anchor is about 1.66 times the weight of the climber. The climber’s weight is applied to the anchor plus 2/3 of the climber’s weight is being applied to the anchor from the belayer. The belayer only applies 2/3 of the climbers weight because of rope stretch, friction in the system, and other real world factors. In addition, a more static belay and any slack in the rope when the climber falls will apply more force to the anchor.

When leading the highest placed piece will always receive the greatest force. Which, as John Long states in his book: Climbing Anchors: Second Edition, there is research to suggest that the highest real world force that a climber can exert on a piece is somewhere in the range of 5.5 to 8.5 kN. Nonetheless, we still want to build an anchor that can withstand the worst-case-scenario of a factor 2 fall. If all of the pro rips on a climber, then a bomber anchor that can withstand a factor 2 is obviously a very necessary component of the climbing safety system. If you build a belay anchor that is perfectly equalized, then each piece may only need to hold about 2 or 3 kN, and most climbing gear is capable of this. To learn more about building safe climbing anchors, refer to our article, “What is the safest way to set anchors?”

In summary, your anchors (whether they are single point lead anchors, or multi-point belay or top-rope anchors) should always be as bomb-proof as possible. If they aren’t bombproof then consider setting more than one and equalizing them with a Sliding X (even mid-pitch, if you have to!), or one of the other equalization methods that we discuss in our article about anchors. If you need to descend from a sketchy anchor, it’s better to rappel than to lower, because when you rappel the pulley effect won’t increase the load on the anchor. If leading and you can not get a bomber piece of gear then you can add more pieces to the system and equalize them for added security.  

Petzl’s website has an excellent resource that demonstrates how much force happens for different fall scenarios. You can check out the Petzl fall force simulator at: http://en.petzl.com/petzl/SportConseils?Conseil=56&Activite=14.

Rock & Ice, John Long, and Craig Luebben have not yet endorsed the opinions described here by Splitter Climbing Gear.