Should the leader clip the belay anchor?

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drizzle123

Trad climber
Jul 20, 2017 - 05:21am PT
rgold,

Could you please show the maths for your break-even point calculation? I can't figure it out.

It seems your model would undermine the Chariot Belay method, so what method do you use to mitigate things? Leading through to the first piece of the next pitch?

What do I mean by mitigate things? I guess there are two considerations, fall factor and peak force on anchor. I don't really understand how they are related or which takes priority, but maybe your maths will clear that up.

Thanks for all your informative posts!
rgold

Trad climber
Poughkeepsie, NY
Topic Author's Reply - Jul 20, 2017 - 08:40am PT
Drizzle, in a few days I'm off to the Tetons for three weeks and might not be able to reconstitute those calculations until I get back, as I have many things that need doing before I leave. In any case they can't be posted here; I'll send them via Supertopo email. If you don't get anything by early September, best to message me, as I may simply have forgotten.

The connection between the quantities you mentioned is that, in the classical rope model, the fall factor determines the rope tension and the rope tension determines the top anchor load, which is approximately 70% higher than the rope tension.

If the leader falls on the anchor, then as has been noted several times already, the only mitigation strategy is to decrease the fall factor by moving the belayer further away from the anchor. (The fall factor is decreased because moving the belayer adds rope to the fall-catching system and so makes the "L" in H/L larger while the "H" remains the same.)

Something that hasn't been part of the discussion is that the dependence on the fall factor ends as soon as the rope starts to slip through the belay device (if a device that allows such slippage is used). Once that happens, it is the length of the fall that is critical and not the fall factor, and what this means is that catching short FF2 falls is going to be "easier" than catching long FF2 falls if your belay method allows for rope slippage and you are equipped to handle it.

And speaking of rope slippage, there are two possible phases. In the first phase, the rope slips through the device but not through the belayer's hand; the brake hand is pulled to the device while gripping the rope. In the second phase, once the belayer's hand has no further opportunity to travel, the rope slips through the belayer's hand. It is possible that the first phase slippage absorbs enough fall energy to stop the fall, indeed I think this happens a fair amount, and when it does the belayer reports no rope slippage because no rope ran through their hand.

This suggests a belay habit which I rarely see in the field, which is keeping the brake hand as far away from the device as possible, say down by the hip. My unscientific observation is that most belayers have their braking hand almost on top of the device, which means they get no phase-one advantage for a severe fall.
guyman

Social climber
Moorpark, CA.
Jul 20, 2017 - 08:53am PT
rgold.... have fun in the Tetons.

drizzle123

Trad climber
Jul 20, 2017 - 11:56am PT
rgold,

I found your paper on standard equation for impact force. Using the parameters you laid out earlier, and the tension equation from that paper, I calculated a break even point at .596 fall factor when clipping the anchor (just solving for r assuming the peak anchor loads are equal across both situation). That's an even more dire outlook for clipping the anchor.

Anyway, I really appreciate your work, particularly the revelation that a factor 0 fall imparts 2x body weight on an anchor. That was very surprising!

My takeaway is to focus more on the eventuality of ff2, and less on marginal methods of avoidance.

Thanks a ton!



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