By Daryl Allan From: Sierra Vista, AZ Jan 28, 2009
If leading this crack, be advised the rock isn't stellar and may blow open causing gear placements to fail. Check out this pic of a cam I caught Meghan on after it blew the crack apart yet miraculously held. It blew open with such force that a projectile from it hit and sliced my hand.
based on the photo, it looks as if the cam was placed behind a 1-1.5" thick flake. generally speaking that's too thin to support the outward force a cam places on the rock during the fall (approx 4x of the force exerted on the piece). note that the inner lobes held in the region of the placement where the supporting rock was much thicker.
part of the problem, as you pointed out, may have been rock quality. additionally, had the cam been placed deeper (so that all four lobes were pushing against the thicker rock) this probably wouldn't have happened.
that seems to calculate correctly - i think the BD cams are around a 13.5 degree cam angle - the outward force is 4.35x according to this equation.
i had calculated it by some roundabout way a while ago, but didn't have the formula on hand here at work! thanks. :)
By Daryl Allan From: Sierra Vista, AZ Apr 29, 2009
I would have sat between both of you poindexters in trigacalcuometry and cheated like a filthy rat. Then again, forgo my blabbering and lead yer peaches off; as many have done. I watched Jimbo stretch this thing out on next to nothing.
In all seriousness, though, the original message is getting lost behind all the math ... the moral of the story: place cams in beefy rock.
By Mike Diesen From: Sierra Vista, AZ Apr 30, 2009
So you're saying if I take a large fall that generates 6kn of force on my cam then the cam generates 25kn of force on the rock. Rocks tough so that doesn't bother me but remembering my college physics day this means that the rock generates an equal force back on the cam (25kn). Makes me wonder how a cam rated at 12kn holds a large fall. Maybe that is why I'm always more comfortable with a bomber nut placement.
cams are rated for the downward force placed on them, not the outward force they exert. each lobe must be able to exert 1/4 (or 1/3 for a tcu) of the total outward force. so in the situation you describe, each lobe would have to support roughly a 6kn load. cams are tough!
it wouldn't be easy to generate 6kn in a typical leader fall, though, even when using a grigri, falling with little rope out, and coming close to the ground. jeff fassett and i measured an average of 4.9kN generated in a 10' leader falls with just over 20' of rope out. if using an plate (which i'd strongly suggest over a grigri for traditional climbing) the same falls generated an average of 4.0 kN.
of course, this will vary depending on the rope characteristics, mass of the climber, etc. or, if you're doing multipitch climbs and take a factor 2 fall on the anchor, that's just gonna hurt. :)
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