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I'm tipping out, man
eli poss wrote: I'm curious, is there load testing data for tipped out cams (in test bench placements)? |
apoet wrote: Yes, literally have of the data for cams is for tipped out cams. The UIAA tests cams in a tipped out position and a more retracted position. They must hold 5kN and I'm pretty sure they have to hold whatever they're rated to in both positions.
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Most people know that a cam with a constant cam angle has the same holding power whether it is retracted 1% or 99%. What BD is saying is that the margin of error becomes greater when a cam is retracted less than 25% (cam can walk into wider area, rock can break, flake can flex, etc. Hence, they don't recommend it. When a cam is retracted more than 90%, its holding power is just fine, same as 50% retracted, same as 60% retracted. But, it might be difficult to remove. |
eli poss wrote: Is english your second language? |
eli poss wrote: 3/4 bMax (25% retracted) is nowhere near 99/100 bMax (1% retracted). The tests show a very different "tipped out" than what you claimed upthread. With ideal materials that don't bend or compress under pressure, sure, the force will always be the same. But I suspect at 1% retraction, even the steel plates might bend enough to reduce holding force, and the UIAA tests do not disprove that. |
David Kerkeslager wrote: You're confusing holding power with wiggle room. Wiggle room is how much the rock and break/flex or how much the cam can open up due to walking before it umbrellas. Holding power is whether there is enough friction and outward force for the cam to hold in a particular placement. |
David Kerkeslager wrote: I disagree wholeheartedly. All I want to see is the *actual* range of a manufacturers cams. That's the only reasonable way to compare apples to apples. I don't give a damn what some manufacturer considers "effective" range, and I'm sure this varies between companies, so it's a totally useless number to compare cams to cams. I know how to place a cam; I've never seen anything more stupid than the red-yellow-green dots on that Metolius cam. And I know how to compare the actual size ranges between cams to know where they fit on my rack, and what to toss in the backup rack because the range sucks (I.E. most Metolius) |
eli poss wrote: You're trying to separate holding power and "wiggle room" into two different concepts, but in real life you can't calculate holding power without taking into account wiggle room. If the cam is 1% retracted, then the steel plates on either side have to flex by only 0.5% bMax for the cam to expand to 100% bMax at which point your outward force is whatever spring force the steel plates are exerting back into the cam lobes to keep the outward force and inward force at equilibrium. Unless you have some evidence that proves otherwise, I'm going to guess that force is a lot less than the outward force a cam would exert at 75% bMax, which is going to significantly affect the holding power. I'm not confused about what you're saying--it's just that what you're saying is unrelated to how cams behave in real life. |
GabeO wrote: Yeah, that's basically what Kyle Tarry said upthread, and I think it's a good point. I've changed my opinion since the post you're quoting. :) If there were some objective way to determine the effective range of the cams, that would be more useful then the full expansion range, but given it seems Metolius' numbers are basically educated guesses, they're too subjective to be really useful. |
GabeO wrote: haha ya'll got any of that exaggeration? Gabe, that's a fair criticism: Metolius has a smaller range than BD and most other cams for that matter (I think metolius uses a 13.25 degree cam angle? can anyone confirm?). That said, there's more to a cam than the range. I guess my question for you would be: why do you feel strongly about how those numbers are reported? I guess ideally the practice of reporting the range is standardized, but like you said: you know how to place a cam, so you're probably just using the stated range numbers as a guideline if at all, right? I don't remember the last time I referenced a cam's range. It's trivial to look at the cam and know what you're working with. |
Dave Alie wrote: If I'm considering buying a new cam, then no, it may be "trivial" for you, but I don't have the eye to be able to distinguish by squeezing the trigger that this cam has a mm smaller range than that cam. That is what I want from the manufacturer - to tell me the actual (not effective) range of the cam. Then I can compare and contrast. And IRL, those numbers make a huge difference between two cams that are "equivalent", but one just won't fit while the other fits perfectly. Or one is way tipped out while the other is still pretty reasonable. This is particularly an issue in splitter cracks, where you can't just poke around until you find the sweet spot for the cam you have in hand. It either fits well or it doesn't, and finding that it doesn't when other "equivalent" cams do (but you placed them already) - well that kinda sucks when you're a ways from the anchor and trying to figure out whether to gun it or back-clean. |
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Check out what he says about the Basics. |
David Kerkeslager wrote: Nope. According to basic cam theory, holding power is two attributes that are determined by the camming angle. First, the coefficient of friction between cam lobe and rock must greater than or equal to the tangent of the cam angle. The cam angle also determines the ratio of force applied to cam stem : outward force exerted by cam lobes. Those attributes are what is commonly referred to by holding power. Now I'm not suggesting anyone goes out and places tipped out cams. Rather, I was simply dispute a claim made by somebody that cams have different holding power at different retractions. AFAIK, only totem cams do that because they don't have a constant effective camming angle. Wiggle room, on the other hand, is (max range - crack size) - ~.01mm Here's a link with some basic cam theory for those who want to geek out. Of course all of this goes to complete chaos once the cam is actually loaded and both the cam and rock deform on a micro scale. The physics of what occurs after the first split second the cam is loaded is a PhD level topic and nobody really has any good answers yet. |
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eli, you should check out Asimov's The Relativity of Wrong. What this discussion comes down to, I think, is that we're talking about models of camming action that are both wrong (but to differing degrees). Your model is a good-enough approximation of how a cam works when it isn't extremely tipped out (like in the UIAA tests), but when a cam is tipped out to 1% contraction, it's no longer a good-enough approximation, because other factors become more important. Think back to physics 101: if you have a 1kg book on a table, the book exerts 9.8N force on the table. If you take the book, still on the table, and drop them both in a vacuum near the earth, the book exerts 0N on the table while they are falling. The reason for this is that in the first example, the table is exerting 9.8N upward force on the book. For the book to exert the 9.8N force on the table, there has to be an equal and opposite force from the table. Similarly, for the cam to exert outward force on the rock, there has to be an equal and opposite inward force on the cam from the rock--which there won't be at 1% contraction, because the rock is compressible. What Kyle said upthread is correct, your model makes sense for experimentation (i.e. in the UIAA tests you described). But if we're talking about holding power at the extremes of a cam's expansion range, it definitely changes when it's extremely tipped out, because the rock can't exert enough force back onto the cam lobes until it has compressed. |
eli poss wrote: David is right - for the same reason's that the transient behavior of a cam is difficult to model (as you point out in the latter part of your quote), the basic model of cam behavior is not a particularly accurate representation of how any cam will behave at 99 % retraction, for a variety of reasons. |
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Sure, flex does play into it, as I mentioned earlier in wiggle room. However, unless you're looking for a topic to do a PhD, you probably aren't going to come up with a model that incorporates flex due to the sheer variability of different rock types and even micro-variation within a specific rock. At some point you have to just realize that placing tipped out cams is bad and that cams with a larger expansion range will give you more wiggle room. Theory is far less useful when it doesn't/can't inform practice. That being said, it's still fun to geek out on stuff even if it may not be very useful. |
eli poss wrote: Of course I won't come up with an exact value for how much the rock will compress, for the reasons you mentioned. But we know the direction that the rock flexes (away from the cam), so that gives us + or - direction that he holding force will go when the cam is tipped out: the holding force will be less. It also gives us another piece of information: since we can guess rock compresses a roughly consistent distance for the same force applied, that distance will be a greater percentage of the expansion range of smaller cams. So the holding force begins to fall at a lower percent of expansion range for smaller cams. This theory can totally inform practice: it tells us why we shouldn't place very-tipped out cams even in very solid rock, and why smaller cams are more susceptible to being tipped out. It's not necessary to "just realize" anything. :) |
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Do you guys even fall on cams? It sounds like you guys like to split hairs and debate minutia, often inaccurately. I challenge you both to go out and whip. Take photos of your placements. Video your falls. Whip. Whip. And whip some more. Then, come bs on mopro. |
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OMG. The flex is in the cam axle not the test jig. Watch the video I posted up thread. Rock doesn't compress at the force we are talking about. Unless you're placing cams behind flakes. What it does do is granulate. |
Greg D wrote:
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