I was just at the Tennessee Wall and wound up climbing some wet cracks. Do cams hold signifi- cantly less in wet cracks than in dry cracks? Is a nut a better idea or is its holding power equally compromised by water?
—Lee Kennedy | Raleigh, NC
Cams rely on two things to hold themselves in place: the coefficient of friction between the metal and rock surfaces, and
the pressure of the cams pushing against the walls of the crack. The outward force of the cams depends on the shape of the cam lobes (the cam angle),
and this mechanical action won’t be affected by water. The material of the cam lobes and the composition of the rock determine friction.
When you fall on a cam, the very instant you load it the friction provided by the metal takes precedent over the lobe’s outward force. If the cam slips
at that precise moment, it can fail. If the cam holds, the massive outward force applied by the lobes takes over, and at that moment the strength of
the rock becomes a factor, but more on that later.
Cams lobes are all aluminum (except for the steel lobes on the Omega Pacific Link Cams), and the various manufacturers use different aluminum, and some
change up materials even within their own sets. Black Diamond, for instance, uses three types of aluminum for Camalots. The different aluminums have
different frictional properties, so right off the bat we have a problem deter- mining the degree to which cam placements might be affected by wet rock.
There are just three main types of rock—igneous, sedimentary and metamorphic—but there are hundreds of subdivisions of rock. These variations
make it impossible to calculate the precise holding power of a cam since rock hardness and friction will affect the cam’s ultimate strength. And if
you don’t know how strong cam X is in placement Y, then you can’t know how much or little a placement is affected by water.
We do know generally that rock such as quartzite, polished granite and basalt will have a lower frictional value than gritty rock such as the sandstone
at the T-Wall. So we can expect a cam in Southern sandstone to grip better than a cam in the marbleized quartzite of Devil’s Lake. We also know from
practical experience that wet rock of any type is slicker than dry rock, so we can expect cams in wet placements in any type of rock to be more likely
to slip than cams in dry placements. The precise reduction in holding power is, however, unknown and unknowable—as noted earlier there are just
too many variables. Furthermore, the holding power of cams is greater than the ultimate strength of their slings and cables—even if a cam in
wet rock loses some of its holding power, it is still more likely to fail due to breakage than slippage.
Perhaps more important is the fact that wet porous rock is weakened by moisture. The article, “Laboratory impacts into dry and wet sandstone with and without
an overlying water layer: Implications for scaling laws and projectile survivability,” published in Meteoritics and Planetary Science, noted that wet
sandstone is 27 to 30 percent less resistant to impact than dry sandstone. A report in the Journal of Geophysical Research showed that the compressive
strength of wet sandstone is 60 percent that of dry sandstone.
The scientific mumbo-jumbo supports what we have seen in real life: Porous rock such as sandstone and limestone are more apt to break or crumble when wet.
The physical reliability of the rock at the T-Wall is probably your biggest concern. On a good, dry day, cams have been known to rip right out of sandstone
(notice the “tracking” grooves gouged into some of the more popular lines at Indian Creek). On a wet day not only does the sandstone have lower friction,
but as noted above, it has been proven to be up to 40 percent weaker.
If I were you, and given a choice between a nut and a cam placement in wet rock, I’d opt for the nut. A nut’s greater surface area will make the rock less
likely to break, and a nut in a nice constriction isn’t as friction-dependent as a cam. Gear Guy has spoken!
This article was published in Rock and Ice issue 204 (September 2012).