Meaning of MBS of microcams

So I got a little bit of time on my hands and wondered how much you can tell about the real-life strength of a microcam when looking at mininal breaking strength. Thought this is useful to share for information regards and to unveil possible flaws of my ideas.

So my scenario originates from me owning a Z4 0.2 and wondering if I should prefer the Dragonfly 2 over the Z4 0.1 bc it's 6 over 5kN nominal breaking strength. The data I used are either official information on the cams and from this link: https://www.blackdiamondequipment.com/en_US/qc-lab-big-cams.html

The Z4 0.2 goes down to 10.4mm, so my usecase for the 0.1 will never exceed around 10.5mm placements, which means the worst usecase is 66% retraction of the Z4 and 43% of the Dragonfly respectively.

The testing of BD shows average peak load for a pretty small sample size, so it's not very scientific, but the best I could find. When approximating the average peak load for a 66% retracted #0 you get around 8kN (quadratic approximation). The one of the 0.1 should be bigger, so I'll treat it as equal for a conservative approximation. Furthermore, BD claims Lobe Invert as the cause of failure for all microcams, which intends, that the main factor of their strength is how much range they have left before inverting.

Now comes the most unscientific part, but regarding the way engineering works it should be acceptable due to the most probable idea that both products have similar safety factors and all the numbers are pretty close to each other.

The microcams get tested at 50% retraction and get their mbs according to 3-sigma-method, so 99.7% of failures occur above the nominal mbs. The data from BD in the link above is average peak load, which is 1.5kN (30%) higher for the Z4#0 under the same conditions. If you subtract that 1.5kN from the 8kN approximation found for the worst usecase of the 0.1 you get around 6.5kN "nominal" mbs in the worst case scenario (66% retraction) considering the actual range I'll use the product in. For the Dragonfly I know from the official data that the nominal mbs at 50% retraction is 6kN. My worst usecase would be 43% retraction, so the actual mbs for my worst usecase will be lower than this.

Note that the actual nominal mbs of the Z4 could also be like 5.8kN, which most certainly must not be rounded to 6, but to 5kN, which makes the result more conservative. Same for the DMM.

Bottom line for me is: For microcams range overlap between sizes beats nominal breaking load in many situations for real-life usage.

Thoughts?

I've yet to see a cam fail, it's always been the placements.

You're overthinking it.  In actual practice these types of placements are so finicky to micro-contours in the rock that they kind of have to be perfect to be confidence-inspiring at all for holding a lead fall.  You're asking a lot of an incredibly small contact surface of aluminum.

I do aid climbing and frequently have 0.2 size cams fail to hold body weight on placements that are just a little bit off ideal.

Don't choose your micro-cams based on rated breaking strength, pick them based on other things like how well they retract (all of them feel stiff to retract by virtue of simple geometry, but some are better than others) stem flexibility, sling type, etc.

If you're looking to protect super thin stuff, I would recommend practicing with micro brass nuts.  Those types of placements are just more reliable.  I would take a well placed 5kN brass nut over a #0 cam any day.

Sam and Gary nailed it.  Don't get caught up in the lab #s it's really not applicable to real rock.  I used to get excited thinking about 6 vs 7kn etc. 

Then saw a video on forces falling + talked to a BD engineer talking about getting around 7-9 kn on small .1s.  

Pulling out is the concern.   Place it well

Garry Reiss wrote:

I've yet to see a cam fail, it's always been the placements.

Do microcams have the same failure mode, cable at thumb loop, as larger cams when placed at 50% retraction?

BD Break Test

Edit: Found the answer

The Z4 #0 failure mode is inverted lobes.

I'm well aware that there's a lot more to consider when it comes to real-life application and that there are more important factors. Nonetheless the mbs is a number that is more often referred to compared to range overlap when building a rack. To me having a more retracted microcam (more range left to being tipped out) is definitly safety-relevant, as the possibility that it stays put is higher if something crumbles a tiny bit under the high pressure.

For the brass nuts I'm not really sure if they're truly superior for my situation. I'm placing gear almost exclusively in sandstone and over here flakes and parallel holes are far more common than good crack placements in that small sizes. My assumption is that the cam exerts less force on the rock, but on a smaller area, considering camming angle versus the angle of the nut faces, so I don't quite know what's gentler to the flake. But indeed there'll be placements that'll be more suitable to them.

From my limited experience the brass nuts must be well seated with plenty of contact, I have seen one brass micro nut, marginal placement, pull through the rock constriction as the brass deformed between the hard rock. In western sandstone, the rock could be more likely to break.  

Yeah, brassies are less useful in sandstone; they work great in granite and basalt micro cracks.