Hi Speed Long Rappels

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Hi Speed Long Rappels

Postby dutchcontractor » Sep 23, 2012 8:01 pm

As we all know, heat buildup in a rack is a combination of load, friction and speed. (and a few other minor variables)

MY instinct tells me that the faster you rappel the more heat builds up in the rack and you may get to a point it cannot dissipate the heat fast enough which results in rope glazing.

However, I was talking to a very experienced vertical caver telling me that when he rappelled El Capitan (2650') in 6 minutes, his rack was still cool enough to hold with bare hands.

Is this because he was spacing his bars so much that there wasn't enough surface contact to create enough friction for heat to build up?

Can anyone explain the science behind this? 6 minutes is very fast for that that height!
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Re: Hi Speed Long Rappels

Postby Scott McCrea » Sep 23, 2012 8:24 pm

Hmm. Six minutes is very fast for that distance. I guess it might be possible. I've seen people do sub-30 second rappels at Bridge Day, about 800'. The guy that died at El Cap a number of years ago, only three bars engaged on his rack, got the rack so hot it created orange-sized globs of nylon on the rope. There is an accident report somewhere. There is also video of Bill Cuddington rappelling Golondrinas very rapidly, probably about 30 seconds. I saw this at convention a few years back, but haven't seen it online yet.

As for the rack being cool enough to hold, that could be true. Racks can cool quickly if you gradually slow down and let some cool rope slide thru and absorb a bunch of heat. See Wm Shrusbury's rack heat study at Bridge Day in the '90s.
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Re: Hi Speed Long Rappels

Postby NZcaver » Sep 23, 2012 8:41 pm

Sustaining 7.4 feet per second for 6 minutes, and being able to hold it with bare hands afterwards?

With a normal long stainless rack I call BS. I suppose it might be possible if the rack was 4 feet long and used a ton of heat sinking aluminum or whatever.
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Re: Hi Speed Long Rappels

Postby Chads93GT » Sep 23, 2012 11:16 pm

I heard the same story from a guy at bridge day last year. His rack was about 3' long. It was massive. Probably the same guy?
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Re: Hi Speed Long Rappels

Postby dutchcontractor » Sep 24, 2012 6:51 am

24" rack, 4 bars.
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Re: Hi Speed Long Rappels

Postby Scott McCrea » Sep 24, 2012 6:56 am

NZcaver wrote:Sustaining 7.4 feet per second for 6 minutes, and being able to hold it with bare hands afterwards?

But, if you slow for last 50' or so and let the rope absorb a bunch of heat, the rack will cool a lot.
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Re: Hi Speed Long Rappels

Postby NZcaver » Sep 24, 2012 3:04 pm

Scott McCrea wrote:
NZcaver wrote:Sustaining 7.4 feet per second for 6 minutes, and being able to hold it with bare hands afterwards?

But, if you slow for last 50' or so and let the rope absorb a bunch of heat, the rack will cool a lot.

Saw you post that before. Interesting. Sounds like you may have tested this theory with a thermometer?
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Re: Hi Speed Long Rappels

Postby Scott McCrea » Sep 24, 2012 3:14 pm

NZcaver wrote:
Scott McCrea wrote:
NZcaver wrote:Sustaining 7.4 feet per second for 6 minutes, and being able to hold it with bare hands afterwards?

But, if you slow for last 50' or so and let the rope absorb a bunch of heat, the rack will cool a lot.

Saw you post that before. Interesting. Sounds like you may have tested this theory with a thermometer?

Wm had a couple racks rigged with temp measuring devices. The graphs he showed had huge spikes and dips depending on how fast the rappeller was going. I don't think he ever published the data, tho. He presented it at the 2000 convention in KY, I think. I also helped with an experiment using a thermal imaging camera at the '05 convention. It showed that the rope above the descender got nearly as hot as the device, but dissipated the heat quickly. Using those two experiences, I conclude that rope absorbs a good bit of heat away from the descender.
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Re: Hi Speed Long Rappels

Postby dutchcontractor » Sep 24, 2012 6:12 pm

Not the guy I talked to but here's another one.

http://m.youtube.com/#/watch?v=ahrPH6oP ... hrPH6oPAUk
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Re: Hi Speed Long Rappels

Postby NZcaver » Sep 24, 2012 8:54 pm

Scott McCrea wrote:I also helped with an experiment using a thermal imaging camera at the '05 convention. It showed that the rope above the descender got nearly as hot as the device, but dissipated the heat quickly. Using those two experiences, I conclude that rope absorbs a good bit of heat away from the descender.

Quite possible. I also remember reading somewhere that a portion of the heat created rappelling is through the compression/expansion bending action of the rope itself, which may be another reason the rope was so hot just above the moving descender. I can't find the reference, but it does introduce an interesting alternative to heat being solely a product of surface friction between rope and rack.
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Re: Hi Speed Long Rappels

Postby Chads93GT » Sep 25, 2012 12:04 am

Damn. That video. I guess el cap isn't a free drop after all. I notice he kicked off the wall quite a bit on the way down. I guess the rope doesn't wear on any rub points on the climb though?
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Re: Hi Speed Long Rappels

Postby dutchcontractor » Sep 25, 2012 11:50 am

Supposedly it really depends on the wind on how far away you are from the wall. That 3000 foot rope is a big sail.
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Re: Hi Speed Long Rappels

Postby Bob Thrun » Sep 25, 2012 1:31 pm

NZcaver wrote:I also remember reading somewhere that a portion of the heat created rappelling is through the compression/expansion bending action of the rope itself, which may be another reason the rope was so hot just above the moving descender. I can't find the reference, but it does introduce an interesting alternative to heat being solely a product of surface friction between rope and rack.

You may be thinking of a discussion that appeared on Cavers Digest, numbers 5509 to 5529, Dec. 1997 to Feb. 1998. There were several theories of heating and heat disapation presented there.
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Re: Hi Speed Long Rappels

Postby Chuck Porter » Sep 29, 2012 2:39 pm

This previous thread
viewtopic.php?f=5&t=6975&hilit=rappel+physics&start=45
should help. Check the first two replies on page 5. Especially Scott McCrea's. My lengthy explanation of the physics is indeed in Cavers Digest #5521, Topic 10.

The essence: Most heat goes into bending and twisting the rope fibers, not friction with the device.

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Re: Hi Speed Long Rappels

Postby KWW » Nov 15, 2012 9:03 pm

I have a close friend, whom some of you know, that is my "vertical mentor". He has repelled both El Capitan and Half Dome. I'm not sure what his speed was for El Capitan but he repelled the 2000' ascendable face of Half Dome in just under 6 minutes. Lets also keep in mind that no matter that the face of El Capitan is 2650', only 2400' of that is ascendable. So 7 minutes is absolutely doable! He has told me about popping bars off of his rack during ascension to limit the amount of heat building up on the rope. All be it, we're talking about some crazy ass kids at the time, but the fact remains that experience equals success. And YES, his rack was hot enough to burn him once he got to the bottom, and, he actually has a custom made rack that is much longer than anything you can buy. Not sure on the actual length or amount of bars.
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