UKC

Was enjoying some slate trad at the weekend, but I was a bit unsure of how much I could trust cam placements.

I'm aware you need to treat cam placements on limestone with a degree of suspicion as some cracks can be very smooth and don't provide enough friction for them to engage properly. 

Slate is also very smooth by nature, so I was concerned cam placements on it need to be treated with similar suspicion?

Anyone aware of an incidents of seemingly well placed cams ripping on slate?  Or is the friction better than it would seem?

Sketch out the forces acting on the cam and you will find that whether it slips or not is independent of the force applied to the cam, the frictional coefficient, the angle of the crack, and the camming angle are all that matters. Therefore, if you place a cam and give it a firm tug, if it holds then it should also hold in the event of a fall, and if it slips then it will slip in a fall. In the real world though you want to confident that it's not going to walk a little, possibly only mm, to a place where the angles are such that it could slip

Wow, I'm going to sound like a right old git... but a few wired hex may come in handy 

That's all very well, but in practical terms I know there are placements on my local limestone crags where a Totem will hold but a Dragon won't even though they're made of similar metal and have a simllar camming angle.

The same might apply on slate. Or not; Totems' superiority on limestone might be due to the independent lobes engaging better in irregular pockets, which wouldn't matter in parallel-ish slate cracks

Totems also fit into slate shot holes unlike other cams that are too wide (the old option was tricams). I haven't fallen on a totem in a shot hole though.

I have first hand witnessed cams failing in slate on two separate occasions. One a red dragon and on another occasion a red and green dragonfly both ripped during a fall. All these cams had been placed by competent people and pull tested normally.

I have also taken a pretty large whip on a red camalot on a slate route, which was big enough to deform a lobe, but the cam held.

Make of that what you will. 

This sounds correct but I have personally experienced a successful pull test of a cam in limestone followed by an unsuccessful body weight test. Maybe the cam slipped an unnoticeable amount in the pull test and seemed OK. 

I wouldn't be shocked if something funny in the real world happens that a simple free body diagram doesn't account for, maybe the stem could catch on the rock and help dislodge the cam in actual fall but not in the pull test, or the frictional coefficient isn't as constant under different loads as is assumed. Maybe I should have caveated my answer further

You're making an assumption that the rock, or the cams lobes (this should require higher loads than would occur in most falls) doesn't fail/ deform/ fracture in some way (which is sensible most of the time!).

The outward force applied to the walls of the cracks by the cam's lobes will be proportionate to the force pulling on the stem of the cam, as such the contact stress between the cam lobes and the rock may exceed the compressive stress capacity of the rock when heavily loaded in a fall causing the cam to pull out when the placement passed a routine tug test when the cam was first placed. This would be more relevant on something like grit or sandstone due to the greater disparity in load capacity between the patina and the sub-surface layer. I'd imagine slate's sheet like structure might result in some differences in certain orientations?

Interesting. I get the logic, however when aid climbing on smooth super solid granite I've had cams hold during small bounces only to rip when tested harder. And I can't imagine just giving them a quick flick like I do when free climbing then marching to the top of the ladder.

Two comments. 

(1) Totem cams have a smaller camming angle than any of the other models.  Their unique design allows them not to pay a range penalty for this small cam angle.  The small cam angle means a Totem can hold in placements whose coefficient of friction is not sufficient for other brands.  But the cam angle in question is only slightly smaller than the other smallest cam angle product, which is the Metolius cams, so there is a reasonable question whether the theoretical advantage is detectable in practice.  Anecdotal evidence I've seen and personally experienced suggests there is an advantage. I'd prefer Totems or else Metolius cams for rock types that are smoother than granite.

(2) If the only factors affecting whether a cam holds or not are the load to the cam and the coefficient of friction between rock and cam surface, then it is true that a cam that holds a small load will also hold a big load, which means that jerk-testing should be predictive.  In real-world placements, there are at least two additional factors that could be in play.  One is that the rock surface might shatter or crumble or flake under the cam loads, which would create a lubricated surface with possibly inadequate coefficient of friction.  This might not happen at low testing loads but then show up in a fall arrest.  The second issue is what happens to the cam structure itself under load. Deformation of the cam surface and eventually a shear yield failure of the cam mean that the math describing the ideal cam-crack wall interaction goes out the window, and again the jerk test no longer predicts the outcome.

So the jerk test is valuable--especially if it fails--but cannot be taken as a guarantee of security, and short of tossing sandbags off the crag to simulate actual leader falls, the climber will have to live with some uncertainty.

As we always have.

I haven't tried them personally but I'd be interested to know people's experiences with WC Friends on these smooth rock-types as they are meant to have some kind of extra grip coating on the cam lobes which was presumably designed to solve this issue.

Not been on the Slate yet but I've had plenty of solid-looking limestone cam placements rip out on tugging; doesn't half mess with my head game!

It’s not extra grip coating, at least on mine, it’s no coating and unanodized to give more grip that an anodized lobe.

My Dragons and Totems are like this too, my C4s are anodized, or at least were before it wore off.

I like to find a little lump or divot in the rock to put at least one lobe in/against… can be quite tiny, just anything that might cause the cam to bite before it can slip. There’s usually some kind of imperfection you can find, even on slate… a sharp tug to seat it, just like you would a wire, should stop it walking.