C4 with kinked stem: still safe?
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@Abram- Don't want to drift the thread too much, but in a nutshell there is enough clearance between the axles and the arc cut into the opposing lobes that when loaded the axles begin to bend and the outward force is reduced. This problem is compounded if the lobes are expanded enough to reach the divot cut into the axle clearance arc, which allows the axles to bend significantly and pull out well under rating. The one in the pics below took 4 attempts to get it to stick in the test fixture on account of the bending allowing individual lobes to slip and twist the cam into all sorts of uneven placements, and when I finally got it to stay in properly all it did was 9.15kN as the lobes pushed the axles U-shaped as they rotated to their full expansion and tipped out (at which point it slipped out of the fixture. Mind you, the cam was rated at 12kN. I did a couple more of them as well, and all of them did the same thing. I have video of it squirreled away somewhere that clearly shows the problem, so I'll post a link if I can find where I archived it. |
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@Abram- Found the video and tossed it up on YouTube for you. It's painfully obvious what's happening as the test progresses, and is why I think BD should recall them. |
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@Aric |
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Steve86 wrote:Yikes, that's pretty scary. Thanks for posting the video, makes the failure mode a lot more obvious.That it does. Sadly the only response I got from BD was a rather heated discussion with Russ Clune at the NRR a few years back, who was none too happy about me having the remains of the cams and an explanation of how and why they failed on display. He was of the opinion the tests were invalid because the cams being tested were used and had an unknown history, while I see them as good sample of what's currently out in the field (especially when the failure stems from a rather obvious design flaw). We ultimately agreed to disagree and I was asked a short while later to remove all of the BD gear from the display. BTW, when I first posted this on RC ages ago I stopped short of calling it a design flaw mostly due to not wanting to make trouble for the folks at BD. The discussion with Russ left a bad taste in my mouth WRT BD, hence my taking a harder stance nowadays. |
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I can understand why you'd feel that way. The "unknown origin" argument is a pretty weak argument considering the failure mode. If you had just popped a sewn sling or a trigger wire or something "soft", that argument would hold a lot more water. It's hard to imagine a set of reasonable or unreasonable circumstances that would affect the cam enough to fail in the manner displayed without serious predisposition to that failure mode. |
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Steve86 wrote:Speaking of which, is there anyway to access your old tests/videos?At the moment, no. Plan was to migrate all of it to TheClimbingLab.com, but the baby allows me little free time for that sort of thing. |
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Thanks for the extra info & video, Aric. That video is of the cam you mentioned that pulled out at a little over 9kn? |
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Abram Herman wrote:That video is of the cam you mentioned that pulled out at a little over 9kn?Yup. I don't recall what the other ones pulled at, but all were a good bit below rating and exhibited the same failure mode. On a side note, the only sizes I've tested were purple and green (.5 and .75) and I'd possibly be less grumpy about it if they were rated at 8kN rather than 12kN (as current designs usually are). But given the failure mode when placed at over 50% expansion and 12kN rating I've got a bug up my ass about it. -a. |
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How much force does it take to get the U-Stem to fail with a drop test? |
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Not to ask a loaded question, but does it matter how it's loaded? |
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Aric Datesman wrote:Not to ask a loaded question, but does it matter how it's loaded? (to not run afoul of Rule #1 I'll volunteer that I have yet to see data that supports there being a difference since the dynamic climbing rope by definition limits the impact force on the piece)I just assumed there was a difference and that you'd tested both ways. |
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@Ray- nope, only did a slow pull since that's how the UIAA tests cams. |
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We should re-title this whole thread "yet another way to make trad climbing scary". |
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Matt J wrote:How convenient, I was just going to ask the same question. I dropped this near new cam down a 200ft slab, it bounced around alot and landed in the dirt. I'm sure it should be retired, I hear cams get micro cracks and can just snap at any moment? Other than the obvious, there is no other visual damage, it still operates as smooth as it was before. It looks like one of the axles was bent/stretched. There is some play between the lobes but everything seems to be intact. What do ya'll think?Be happy to take it off your hands. |
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Bill Czajkowski wrote: Be happy to take it off your hands.I suspect that cam has died of old age by now. |