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madflash
Nov 14, 2003, 6:44 PM
Post #51 of 59
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Registered: Oct 25, 2002
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Oh god, just don't let the gym rats get there hands on them. I don't want to see those idiots out at the crags taking two hours to make it up my favorite 5.8 warmups. But then again, maybe they will get all their cams stuck for me to pirate. Hell yeah, let all the gym rats get their hands on these beautiful new toys for me to find in my beautiful cracks. Yes!Yessss!!!!YEEESSSSSS!!!!!!!!!!!
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on_sight_man
Nov 14, 2003, 7:31 PM
Post #52 of 59
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In reply to: I checked it out, but fail to see how a cam with a constant cam angle has variation in the amount of surface contact depending on retraction. I'm far from a math whiz, so I'm probably missing something. Could you be more specific as to where the model predicts that overcamming will provide less surface contact?
Yes, well, actually I was just wanting everyone to look at the picture. The math he uses to decide how much surface area is on the rock is the formula for a cylinder with a radius of 1/2 the crack width. So for HIS math, it doesn't matter how far the cam is retracted. The radius for his equations though is NOT in fact 1/2 the crack width.
Imagine where the cam lobe touches the rock. The rock wall makes an angle with the tangent to this curve which is fixed no matter how far the cam is retracted. That's the definition of the spiral right? If you look sideways at the situation (like the picture) you can approximate the curve as a circle whose radius depends on how far the cam has been retracted.
Now imagine you've got two cams, one bigger than the other, both using the same angle mentioned above. The difference between the bigger cam retracted 98% and the smaller cam retracted 65% is that in the smaller cam, the circle you use to approximate the curve has a larger radius, so more metal will touch the rock.
http://www.stoneseeker.com/...shape.definition.gif
Edited: Those circles should all be much bigger, but you get the idea... Also another note. He DOES actually have the radius changing in his elastic model and shows a chart of the maximal force versus the radius of the cylinder approximation. What the chart shows is the maximal force goes down as the cam is retracted (the radius gets smaller)
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on_sight_man
Nov 14, 2003, 7:54 PM
Post #53 of 59
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In reply to: Finally, someone points out that there is nothing less safe about "over" camming a unit. The cam angle stays constant. But they are a b*tch to remove. I dropped (not literally) a partner over his tendancy to do this.
Again, that's just not true (see above argument) The angle may remain the same, but the spiral is tighter so there's not as much surface area. A smaller cam placed in the lower extremes of it's retraction tolerances may be better.
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crotch
Nov 14, 2003, 8:02 PM
Post #54 of 59
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In reply to: Imagine where the cam lobe touches the rock. The rock wall makes an angle with the tangent to this curve which is fixed no matter how far the cam is retracted. That's the definition of the spiral right? If you look sideways at the situation (like the picture) you can approximate the curve as a circle whose radius depends on how far the cam has been retracted.
OK. I'm with you when it comes to overcammed units having less surface contact than the same unit with less retraction. Is there any comparison between radii of those imaginary circles for a tightly cammed unit vs. a smaller unit that is at mid range for the same crack width? How do the radii compare?
Does the radius of the circle depend ONLY on how far the cam has been retracted, or does it ALSO depend on the size of the cam?
This has been enlightening. Thanks.
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joshy8200
Nov 14, 2003, 8:17 PM
Post #55 of 59
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If an overcammed unit is so incredibly hard to clean...ie there's literally in most cases no way to get it out besides pulling it out with a truck...surface area smurface area that sucker is SOLID.
Now in all honesty I can picture this surface area issue. But I feel like that would be when you placed the cam in a baby smooth, oiled down crack. If you overcammed it in such a placement, yeah I could see the potential for it sliding out...whereas the 50% retracted would have better contact.
But in a real piece of stone...the surface area of a few fractions of a millimeter squared I think are negligible. A larger cam trying to expand in a smaller crack just feels a little more solid than that 50% cammed piece that still moves a bit.
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cedk
Nov 14, 2003, 8:35 PM
Post #56 of 59
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Registered: Oct 31, 2001
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The "over-cammed units are going to pull" argument is rubbish. Using the logic that an over-cammed #2.5 friend is deadly, a #2 friend that fits the crack perfectly must therefore be dangerous as well. Of course that is not true.
As has been mentioned about 6 times already, the reason over-camming should be avoided is because the placements are hard to clean.
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ep
Nov 14, 2003, 8:43 PM
Post #57 of 59
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Registered: Jul 23, 2003
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It's an interesting model, despite the many assumptions and simplifications. It's pretty hard to argue with the idea that, all else being equal (perfect smooth parallel sided crack, no teeth on the cams, etc.), a larger and wider cam will have greater maximum holding force. Sure.
But when it comes to placing an actual piece, the relative difference in the maximum holding force throughout the useful range of a cam isn't calculated. Instead the author says:
"These estimates do not necessarily reflect actual performance; rather, the force estimates are plotted to show the wide range of maximum forces that devices might be expected to sustain."
So maybe the difference between overcammed and optimally cammed is so small as to be negligable and of no interest to climbers. Or maybe it is significant and could mean the difference between a piece holding or failing. The model doesn't answer the question. Pull tests would be interesting to see.
Until then, I'll keep doing pretty much what Brutus does.
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trenchdigger
Nov 14, 2003, 9:24 PM
Post #58 of 59
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Registered: Mar 9, 2003
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Crotch wrote:
In reply to: Does the radius of the circle depend ONLY on how far the cam has been retracted, or does it ALSO depend on the size of the cam? Does the radius of the circle depend ONLY on how far the cam has been retracted, or does it ALSO depend on the size of the cam?
Assuming both cams have the same camming angle (which I believe all of the same brand do?), the surface area in contact with the rock for two different sized cams in exactly the same placement will be the same. In other words, if you want more surface area in contact with the rock, choose a shallower placement (in a flaring crack) or wider portion of the crack.
Another thing to keep in mind is that friction force is NOT a function of surface area. Granted, the grip of a cam is not pure friction between smooth surfaces. My point is that the only difference more contact surface area makes is in the pressure on the rock under the cam. lobes. I guess you could say greater pressure under the lobe is more likely to cause the rock to crumble and the placement to fail, but I would guess the difference in holding power due to this is not significant.
As was mentioned by others, overcamming makes pieces hard to remove. That's the primary drawback. But if you're limited on your selection of cams an the ideal crack placement means your piece will be somewhat overcammed, put it there rather than in less-than-optimal (too flaring/uneven/chossy) spot where the cam lobes are properly extended.
~Adam~
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on_sight_man
Nov 14, 2003, 10:00 PM
Post #59 of 59
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In reply to: OK. I'm with you when it comes to overcammed units having less surface contact than the same unit with less retraction. Is there any comparison between radii of those imaginary circles for a tightly cammed unit vs. a smaller unit that is at mid range for the same crack width? How do the radii compare? Does the radius of the circle depend ONLY on how far the cam has been retracted, or does it ALSO depend on the size of the cam? This has been enlightening. Thanks.
Hmmm, now THAT'S a good question. A post below came up with the crux of the answer and I may have to um, rethink my thinking (that is, admit I'm wrong :) depending on the answer. If on a smaller cam, the angle used in creating the spiral is the same, then the tightness of the spiral is the same (since the radius is the same), and the surface area is the same whether it's a big cam tightly retracted or a smaller one, loosley retracted. I don't know the answer to that.
I may be mistaken here. My thinking comes from comparing the tight spiral of a cam to the loose spiral of the same cam and then assuming the looser the better (surface-area-wise) I wonder whether companies use the same angle for all the cams?
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