A is for Angle


Anchor angle is too wide

The angle is far too wide.

As you likely already know, the ‘A’ in the rock climbing anchor building mnemonic acronym ‘EARNEST’ is for “Angle”. The angle between the legs of your anchor shouldn’t be too wide, right? Certainly less than 90°, and ideally less than 45°. The angle formed by the outer legs of the anchor in the photo above are too wide. This is a bolted three point anchor, so it’s not likely to fail. But it’s odd that the bolt on the right is so far off from the two on the left. I mean who bolts a route to make this scenario likely? This anchor could be improved simply by using longer cord and extending the power point closer to the edge of the cliff. The further away the power point is from the bolts, the smaller the angle becomes.

Location: Unknown.

Why does the angle matter? Intuitively, it makes sense that regardless of the angle created in constructing a two point climbing anchor, each of the two anchor points will carry half (50%) of the load. But this isn’t true. The wider the angle, the greater the load each anchor point has to bear.

Take a look at this graphic from the AMGA Manual. You have to create a narrow 20° angle between the anchor points to have them each bear 50% of the load. At 45° the weight distribution only increases to 54%, at 80° it’s 70%, and by 120° each anchor point is bearing 100% of the load! If you really want to geek out on the physics involved, check out this trigonomic analysis of tension in climbing anchors.

Climbing anchors angle

Relative weight distribution on a climbing anchor determined by angle

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3 thoughts on “A is for Angle

  1. Forrest

    A longer cordalette would be the obvious way to go for this one, the rope doesn’t want to rub on the edge like that.

    Also tying in to all three points uses up a lot of cord. Whoever set this anchor could have tied into two bolts, which would still be perfectly safe if they’re modern, and then used a sling to back the anchor up from the third bolt. 🙂

    Reply
  2. pmonks

    It’s the Pinnacles in central CA, which has some of the chossiest rock I’ve ever climbed on (super hard cobbles embedded in compressed ash). Bolts can and do fail there, in part because bolts can only be placed ground-up, which makes placing proper soft-rock bolts (i.e. glue-ins) difficult.

    Upshot: I sure as shit wouldn’t overload bolts there in the way shown in the photo!

    Reply

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