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chadmichael
Jun 4, 2007, 6:47 AM
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I recently had a well placed cam pull and would like to get some feedback on the causes. I've been climbing for over ten years, mostly trad, and have been falling on all sized of cams in all kinds of rock. I've never had a cam that seemed well placed fail.
The other day however, I had a well placed cam fail under body weight, twice in a row. I guess you just have to take my word for the "well placed" part of this, but suffice it to say that it was a 1.5' - 2" piece ( black diamon old style camalot -- thin cams ) and was a deep, non-flaring, midrange placement. And it came out as soon as I put weight on it.
The crack I was climbing is basalt and is full of bats. My supposition is that the bat urine ( or whatever the ooze onto the rock ) makes the rock slick enough that the cam slips before activating. The crack certainly seemed slippery. But I've climbed on slippery feeling rock before, granite often feel slipper.
My questions are:
Anyone ever hear of bat pee making a cam slip out?
Does rock that "feels" slippery to hand jams mean that a cam will slip?
How can i test the cam placement for slipperyness? I read on the web that if the cam resists a sharp tug, it means the friction is adequate for a full fall, does this sound valid?
Thanks in advance.
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ryanb
Jun 4, 2007, 7:42 AM
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chadmichael wrote: How can i test the cam placement for slipperyness? I read on the web that if the cam resists a sharp tug, it means the friction is adequate for a full fall, does this sound valid? Thanks in advance.
Unless the rock or cam breaks or deform the coefichant of friction between the metal and the cam should be the same weather loaded by tug or by fall, meaning (if i am remembering my freshman physics right) if there is enough frictional resistance to resit a tug it will hold a fall.
However most cam pullouts i have heard of (cams tracking out in sandstone for example) come from the surface of the rock pulverizing due to pressure and acting as a bunch of mini ball bearings and this test would not be good for that as a tug might not generate enough force to break the rock.
By the way, thin lobed camelots = higher pressure between rock and cam = higher chance of rock surface damage and pullout. I bet this is what you were seeing, though it may have been a layer of uria on the surface of the rock that was getting pulverized.
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medicus
Jun 4, 2007, 7:53 AM
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I think a layer of urea being pulverized is a bit far-fetched but who knows. If that specific area took layer after layer of urea being put on it, maybe... but I don't know about that.
I would guess that it had to do that specific rock's quality the area you were placing the cam.
Who knows though.
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chadmichael
Jun 4, 2007, 8:47 AM
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In reply to: However most cam pullouts i have heard of (cams tracking out in sandstone for example) come from the surface of the rock pulverizing due to pressure and acting as a bunch of mini ball bearings and this test would not be good for that as a tug might not generate enough force to break the rock. By the way, thin lobed camelots = higher pressure between rock and cam = higher chance of rock surface damage and pullout. I bet this is what you were seeing, though it may have been a layer of uria on the surface of the rock that was getting pulverized.
I understand what you are saying about sandstone. This however is the most bullet proof rock I've ever climbed on. The pulverized pee sound intersting. We're going to return and top rope the line so that we can do some testing and close examination.
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rgold
Jun 4, 2007, 4:29 PM
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In order for cams (with the more or less standard camming angle of 14.5 degrees) to hold in smooth parallel-sided cracks, the coefficient of friction between aluminum and the rock has to exceed 0.26. I think I've read that granite averages around 0.30.
To the extent that the oversimplified coefficienct of friction mathematics corresponds to the real world, the coefficient of friction requirement of exceeding 0.26 is independent of the forces involved, so if your cam resists a hard jerk, it should also hold a fall (unless the forces are high enough to break some part of the cam or pulverize the crack surface).
It seems to me that your ability to jerk a well-placed cam out more than once is already proof of the low coefficient of friction of the placement. Whether that coefficient of friction is a general feature of the basalt you climb on or is in some special way related to the presence of the bats could only be determined by repeated experimentation.
Metolius uses a cam angle of 13.25 degrees, giving the lower coefficient of friction threshold of about 0.24. Doesn't sound like much, but could be the difference between holding or not in marginally slippery rock.
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ryanb
Jun 4, 2007, 7:57 PM
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I was under the impression the cam resisted a tug and then failed under body weight?
I have seen some bat/rat-waste encrusted bits of rock...how bad was this one?
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paulraphael
Jun 5, 2007, 3:51 AM
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There are definitely cases where the coeficient of friction actually changes with pressure. The rubber in car tires is an example. If the pressure on the rubber goes past a certain point, the rubber's structural characteristics change and its coeficient of friction drops.
This is part of the reason sports cars need wide tires and rigid suspensions. The design keeps the the forces (gravity and inertial cornering forces) spread out, so they don't concentrate on the outer edge of the tires. This keeps the pressure below the point where the coeficient of friction drops.
Not the same mechanism as rock pulverizing into little ball bearings, but the same effect.
Who knows what's coating that rock, or what its characteristics are under pressure ...
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chadmichael
Jun 5, 2007, 7:13 AM
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The cam was not jerk tested before weighting. On the first attempt, the climbing was very hard and the placement perfect, so I would never have thought it needed testing. On the second attempt I was trying to ease onto it and have a look. I'll definitely take a look at the rock on a top rope.
I don't think its a matter of pulverization though. The ooze on the rock is not, at least at first glance, an accumulation of any visible size. The rock is just way slick. And the two climbs at this area with such slickness both have bats in them, and the slick parts on those climbs are on steep areas where the bats are kind of up in the cave so to speak.
At any rate, folks seem fairly confident in the jerk test theory of friction. I will certainly post back after the weekend when i check out the climb again.
By the way, here's a pic. Rad, don't you think? Well, I'll try to upload the picture somewhere else, it was too big.
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kixx
Jun 9, 2007, 12:58 AM
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Try to use a tri cam instead - this might work for a wet crack. They work well in wet and icy cracks for me... when an active cam won't.
I've been aiding on cams that came out of wet cracks under body weight like yours... uncool.
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chadmichael
Jun 11, 2007, 7:25 AM
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I've done some testing. It seems that the rock has to be excessively slick. I've only been able to reproduce the cam slipping out under the jerk test when the rock was perfectly parallel, perfectly smooth, & covered with a white enamal of rodent deposit. I've not been back on the bat pee crack yet to check. Anyway, I think slick isn't enough to make a cam fail, it has to be slick and relativey texture freee.
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stymingersfink
Jun 15, 2007, 8:50 PM
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the geo-goddess peeking over my left shoulder mentions:
In reply to: basalt is especially slippery or smooth, very fine-grained and will often sport a weathering rind on the surface.
there you go.
I wonder if the dominant rock type in the general vicinity of Metolius has had any influence on their choice of protection designs.
me wrote: What kind of rock they got outside of Bend?
geo-goddess wrote: I'm not sure that it's Modoc Plateau Basalt, but it probably is, or at least age equivalent to it... i'd have to look at a geologic map of the area to be sure, it's been too long since I've been up there.
Probably. Especially since one considers that Smith Rocks is right outside their back door. 
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