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hugepedro
Sep 28, 2012, 7:34 AM
Post #26 of 30
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Registered: May 28, 2002
Posts: 2875
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petsfed wrote: I played with both. Strength or weakness?
No, seriously, I'd say you're prolly ok climbing trad, but I might not want to climb with you and hear all about you're latest existential crisis.
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acorneau
Sep 28, 2012, 1:04 PM
Post #27 of 30
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Registered: Feb 6, 2008
Posts: 2889
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hugepedro wrote: When you were young, did you play with: 1. Legos, Erector Set, and Lincoln Logs (or something similar, I am old, so these specific toys may not be relevant).
Oh hell yeah!
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rgold
Sep 28, 2012, 2:29 PM
Post #29 of 30
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Registered: Dec 3, 2002
Posts: 1804
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I'm as or more fond of theory as the next guy, but the elementary theory of cam function is indeed so removed from reality to be of primarily academic interest. It is true, in the elementary theory, that a cam that will hold a tug should hold a big fall, but as has been pointed out over and over, that theoretical result bears no relation to a much more complicated reality.
The problems with the elementary theory begins immediately, with the assumption that there is such a thing as a "coefficient of friction." People write Ph.D. dissertations on the "real" replacements for the naive coefficient of friction concept, which may be appropriate for friction between a pair of highly-polished metal surfaces, but is of questionable relevance for relatively rough aluminum on even rougher rock. The entire concept of coefficient of friction is a convenient empirical observation, but is nothing like a universal physical principle.
Leaving aside the coefficient of friction quaqmire, and assuming some form of solid rock that does not crumble or dispense lubricating micrograins (I'm not sure there is such a form of rock), the failure mode of cams, as anyone who has watched a jig test knows, is due to shear stress failure. The cam surface flattens under pressure, and then the aluminum shears at the contact surface. As a result, in jig tests, one often observes aluminum deposits left on the jig after the cam has failed.
The instant cam deformation begins, which is to say once the lobe material has exceeded its elastic limit, the mathematics involved in describing cam behavior goes out the window and a new analysis is required. This means that, even if the elementary theory is approximately correct for tugging on or just weighting cams, it is no longer even theoretically relevant to cam failure under high loads.
An account of the elementary theory and the sheer stress analysis can be found at http://web.mit.edu/.../cams/cams.body.html
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cracklover
Sep 28, 2012, 3:33 PM
Post #30 of 30
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Registered: Nov 14, 2002
Posts: 10162
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patto wrote: cracklover wrote: In even the hardest and most parallel of desert sandstone, I'm absolutely sure that a hard fall on a small cam will cause it to track at least a little. Then the rock isn't hard or it is glassy smooth.
Have you ever climbed on good wingate? It sure doesn't seem like you have a clue.
In reply to: If a small cam (and by small I mean Metolius 0 or below) start tracking then its pretty much all over. If its "tracking" then you are going to have lubricating rock dust. If it is slipping or sliding then the lower kinetic friction will likely mean it will continue. If you are suggesting that a cam moving out and then stopping is common in cam behaviour then you clearly don't understand the mechanics here. Once a cam moves in the direction of fall then in all likelyhood its not going to stop. (Unless you are talking slight compression movement that is visible on all big cams on even a hand tug.)
As RGold and others have pointed out, you seem to be confused between simple principles and what happens when the rubber meets the road (or real cam meets real rock).
In reply to: cracklover wrote: In even the hardest granite, you sometimes must make do with a shallow cam in a pin scar. Expecting these to hold a hard fall is wishful thinking. A buddy of mine recently fell and ripped four such cams from their placements - all of which he thought were truck. Your buddy needs to assess his gear placements better.
Listen to this one more time: every single cam got a hard bounce test (that means it held somewhere in the 1 to 3kN range) but failed under the force of a real fall. According to you, in good Yosemite granite, such a thing should be impossible. My point, for the Nth time, is that there are many many placements that are okay, but not great. They will pull out before the rock or the cam is seriously compromised.
It seems like you have chosen to brainwash yourself into the belief that your gear will always hold a fall, unless that fall is big enough to destroy the piece. I clearly will not convince *you* otherwise, and for everyone else, I'm sure I'm starting to sound like a broken record. So on both counts, I'll give it a rest.
In reply to: Onlyoriginal asked a question and I answered by mentioning behaviour in strong cohesive rock. Sure if the rock is really shit failure can occur at lower loads that is long been recognised. But whats your point again?
The fact that your black and white version of events does not match reality, and does a disservice to new climbers who are looking for good information.
GO
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