The Anchor Point
Ideas that hold up under load
Hey Reader
Most rope courses teach an anchor angle rule, whether it's climbing, canyoning, rope access or rescue. The usual version is under 90 degrees for two-point anchors. My recommendation is tighter: keep two-point anchor angles at 60 degrees, with 45 to 60 as the sweet spot. Whichever number you use, sooner or later a student asks where it comes from, and "that's the rule" doesn't hold up for long.
The answer is trigonometry. SOH CAH TOA. If you haven't touched it since school, that's fine. The calculator on your phone does the hard part.
On a symmetrical two-point anchor, both legs carry the same load, so you only need to solve one side. Take half the load and half the anchor angle, then draw a right-angled triangle on one leg. The hypotenuse represents the tension in the leg. Half the load is the adjacent side. That's CAH, and rearranging gives you:
HYP = ADJ / COS
Run a load of 1 through it. At 60 degrees each leg carries 0.58. At 90, it's 0.71. At 120, each leg holds the full load. Past that, the numbers climb faster than most people expect.
I've put the working on the website: an intro to SOH CAH TOA, a way to rearrange the equation, worked examples from 15 to 150 degrees, and a quick reference table you can take into your next course.
Two-Point Anchor Angles: The Simple Trig Behind The Rules
Grant
