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Discussion in 'Tech Tips and Gear' started by nathanslc, Mar 15, 2016.
Could you be more specific, Gordon? Which thing?
Whether more friction is really needed under the specified condition or not. Use the spring scale to take fatigue out of the equation and to measure the actual difference in braking force. You'll probably want to run the rope from the brake device through a leg-loop biner and pull with the spring scale in between. Might be most accurate to move the scale around to find a local minimum, probably in a direction orthogonal to the rope right at halfway.
You could also add weight in the control condition, 100' drop with 100' to go, so that at a given friction setting a slow descent occurs with only rope weight as the braking force. Then rap with that weight down 200', stop, rerig, let go, and see what happens. Since you're so crotchety, I'll point out in advance that 'let go' implies a bit of common sense rather than a literal, pedantic, interpretation. A bottom belay could replace that common sense. The setup is going to be a bit fiddly, although you could do it in your garage with a weight added to the rope ...
Are you suggesting that psychological factors should be removed because you are a Vulcan?
I didn't suggest that other than as an implied subset of fatigue. But it's a good idea unless the test subject is a Vulcan ...
Tom, you are interpreting the question correctly. I am curious to know if anyone else feels the same at the end of a 300'+ rappel? I always feel that I need to have a higher friction setting than I would normally have on a 100' rappel. I don't know if it is fatigue, psychological or both. I was originally thinking there was some physics that I did not know about such as rope stretch or heating of belay device. After being on a few more long rappels since this post, I have tried to be more aware of the fatigue factor and think you may be correct.
...could the rope diameter be shrinking as we descend???!!!!
The rope above you will be at one diameter, because it has your weight on it.
The rope below you will be at (or very close to) the "resting diameter".
The difference between those diameters when using a tightly woven static rope is going to be quite small.
There is a perceived decrease in the diameter of the rope as the rappel gets taller. There is a LARGE perceived decrease in the diameter of the rope when one needs to ascend it. The rope itself is indifferent to these activities.
Weight of the rope below you makes a difference too. You start out on a 300 footer with the full length of the rope below you which adds a bit of weight help to the brake hand. As you descend, there's less of it, so, less braking.
Thinking about the problem and reading through the responses, something occurs to me that hasn't been mentioned yet.
I think the rappel device could be growing due to thermal expansion, and all the accompanying distances and radii that the rope passes over get larger and therefore more gradual.
This is the only meaningful mechanical difference that I can think of between a 100' rappel and the last 100' of a 300' rappel- the heat. Yes, you'll have SLIGHTLY more tension in the rope above the device on the 300', but that seems pretty negligible. I did some cursory googling on relationships between temperature vs. dynamic coefficient of friction, but all the results were fairly inconclusive. If there wasn't any observable sheathe bunching by the end of the rapp, that doesn't seem to be a likely scenario either. And I'm assuming for the sake of discussion we're continuing to discount fatigue and the like.
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Thermal expansion coefficient of 7075 is 23.6 micrometers per meter per degree C. Given 100 degree C change in temp (kinda high), you would see a .23% change in dimensions. (My math is somewhat rusty, jus' sayin'). So I doubt thermal expansion would have any noticeable effect.
I see no reason "to discount fatigue and the like".
Hmmm you're right once you do the numbers that's pretty negligible. Scratch that as a theory!
I thought we were limiting the discussion to mechanical/physics explanations, my mistake! Fatigue certainly seems to be the most reasonable explanation.
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Get on top of the mechanics first, understanding/appreciating the physics comes with experience. No two rappels are the same, even on the same rope at the same drop. Although most variances (same rope/drop) are negligible when matched with experience..."oh, so your doing that today, okay, then I'll do this" (subconscious dialog with the rope/gear).
You don't have to actually do a 300' rappel to be prepared for one. Find a 25' drop. Take a couple of gallon jugs of water and hang them on the end of the rope (1 gallon = 8.33 lbs). Now that you're experiencing the physics of a 300 footer you can work on the mechanics - 25' off the deck. Change it up, rinse and repeat. A great training event for experience of all levels.