Rock Climbing Anchors (The Mountaineers Outdoor Experts Series) by Luebben Craig

Author:Luebben, Craig [Luebben, Craig]
Language: eng
Format: mobi
Publisher: Mountaineers Books
Published: 2013-01-02T23:00:00+00:00

This cam is horrendously unstable. A little rope wriggle will walk it right out of its placement.

A cam generates twice as much outward force as the downward load it holds, so in a fall it can easily pry off a block or flake. The falling rock could injure you, your partner, your rope, or all three.

CAM TESTING

In 1994 I conducted a number of strength tests on cams set in good granite and soft sandstone. These tests were conducted with a slow-pull machine, which is more severe than the fast loading and unloading in a real fall. In most tests the units failed at loads between 12.5 and 14 kN (2800 to 3100 pounds)—plenty strong enough to hold most real-life falls. The strongest cam managed to hold around 19 kN (4250 pounds) before it exploded through the soft sandstone rock; at this point the axle was severely deformed.

The weakest cam, however, held about 0.5 kN (115 pounds) before it broke a thin sandstone flake. Other cams failed at frighteningly low loads near or below 4 kN (900 pounds), if: the cam lobes were open too wide; the piece was set in a flare in soft sandstone; the piece was set near the edge of the crack in soft sandstone; or if the piece was set behind a loose block. A smart climber avoids placements like these.

In the soft sandstone many of the cams punched through the harder surface layer at around 7 kN (1600 pounds). Once this happened the cams would continue pulverizing the surface layer and slide out of the crack. When leading on soft sandstone you’re wise to set the cams fairly close together to minimize the length of a fall and the force created, and to have ample backup below in case a piece does fail. Belay anchors should be well-equalized as well. Another way to increase anchor strength in soft sandstone is to use Metolius Fat Cams, which have cam lobes that are much thicker than most other units. These spread the load over a wider surface area of rock, which decreases the pressure on the crack walls.

CAM DESIGN

Any cam design is a balancing act: weight against strength, “holding power” against expansion range, durability against cost. To choose the best cams, decide how you will use them and what features are most useful. Don’t base your decision solely on dollars; it’s better to choose the cams that provide the most security and utility for the lightest weight.

Cam Lobes. Cam lobes are the heart of a cam. The lobes’ curvature is designed to create the same cam interception angle (the angle between a line drawn from the axle to where the cam contacts the rock, and a line perpendicular to the crack wall—see drawing next page) throughout a cam’s expansion range. A smaller cam angle creates greater outward force on the crack wall, which increases its chances of holding in a flare, slick rock, or other irregular placement. Increasing the cam angle adds expansion range, but if the cam angle is too greedy, the unit won’t create enough friction to hold.

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