Is a small anchor angle better? Maybe not!
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In Anchor Building 101, you probably learned the importance of the angles between the “legs” of an anchor.
The general concept: The smaller / narrower the angle, the better load sharing between the legs.
For example . . .
With an anchor leg angle of around 30° or less, each leg gets approximately 50% of the load.
About the maximum angle you ever want between the anchor legs is 90°. In that case, the force going on each leg is about 70% of the load, not so good.
So (as the theory goes) keep your angles as small as possible in order to best distribute the load to each leg.
While that’s generally a good principle to follow, it turns out for ideal load sharing, it may not be that simple.
In fact, a smaller angle can at times actually increase the load on the anchor legs!
A recent web article and video (links below) by Grant Prattley, a New Zealand rigging expert with Over the Edge Rescue, shared an interesting theory and load cell testing that puts a new spin on this idea.
Short version: the "ideal" angle of 30 or so degrees works mostly in theory only, and not so well in the real world.
In real world applications, the load may shift a bit from side to side, or your original rigging is not perfectly balanced between the left and right anchor legs. Let's call this an “error.”
When even a small “error” of about 5% happens with a 30° anchor leg angle, it quickly shifts a majority of the load onto one leg of the anchor.
However, when this same 5% error happens with a 50° anchor leg angle, you maintain much better load distribution between each leg.
Grant presents a pretty compelling argument that the optimum angle for a two point anchor is between 45° and 60°.
Curious about those bolts in the photo?
They’re called Practibolts, and they are a great way to practice anchor rigging on just about any magnetic surface, such as your refrigerator or here, a dry erase board.
Does anchor angle really matter for most recreational climbers?
My short answer: not really. Bigger picture, it's usually more important to pay attention to the quality of your placements rather than the rigging you use to connect them.
If you have two good bolts, then the angle between them really doesn't matter.
If you follow the general anchor rule of “narrow angles are good, anything over 90°, not good”, and you'll be fine.
However, in some cases when load sharing might be important, like when, try as you might, your placements may be lousy, or you have a giant load like pulling your truck out of the ditch, paying attention to small rigging details like this might be important.
Sometimes I dive into more esoteric parts of anchor building on my website, and this is one of those times. =^)
I’ll leave you with that as the takeaway.
If you're interested in a deeper dive, including the math, check out Grant's entire video and article on his website.
(To be honest, given the number of clever engineer people who are attracted to climbing, I’m a bit surprised that it took this long for this to be recognized. But hey, credit to Grant, he took a close look at this and explains it thoroughly in the video and article.)
Want direct links to the detailed article and video?
How about a trick to use your fingers to estimate the angle of your anchor when you're actually climbing?
