![]() In the first, or what the CAI calls the inertial phase, the brake hand is pulled up to the belay device. I have slightly oversimplified the discussion of rope slippage here, because there are, in reality, two phases of rope slippage, one of which does not produce burns. The CAI tests also indicate that there is enormous variation in the performance of experienced individual belayers, so much so that it was hard to get useful data from repeated trials. ![]() How much rope runs depends on the friction in the system, the amount of friction supplied by the belay device, and the strength of the belayer's hand. Both sophisticated mathematical modeling and extensive practical testing by the Italian Alpine Club (CAI), as well as an extensive historical record of belay testing in the US in the 50's and 60's make it clear that if the belayer is exposed to a high fall-factor situation with few sources of friction, there can be significant amounts of rope running.ĥ. The net effect is that many climbers can climb for years-or even an entire career-and catch many leader falls without ever having the rope run.Ĥ. Even when the fall factors are larger, there are usually enough sources of friction in the system dissipating fall energy so that it is rare for most belayers to experience the rope running. The great majority of climbing falls have low fall factors and do not produce very large loads at the belay device anyway.ģ. The result is that more fall energy has to be absorbed by the rope running and so the predictions of rope running are probably on the high side.Ģ. The Petzl simulator (and most theoretical models) use a relatively simple equation that either ignores or only partially accounts for friction through the carabiners and against the rock.
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