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What Do Those Numbers Mean?

OK, so these numbers are theoretical, laboratory-based calculations that do not take some real-world factors in to account.  Some of these factors include rope stretch and friction, which can further reduce the forces exerted on each anchor point.  It is important to understand this general idea, and to apply it in a real-world setting using common sense.  Always find good placements in solid rock.  Use multiple different crack systems.  If the only good protection points are too far apart, lengthen your anchor to reduce the angle.  Never exceed 120°, and for good measure try not to exceed 90°.  Strive for angles of less than 60°, and your anchors are going to be bomb-proof!  If in doubt, back it up or beef it up! 

Stone Adventures - Joshua Tree Rock Climbing Guides - Rock Climbing Anchor Forces and Angles
Degree of angle between 2 anchor points
Amount of total load applied to each point

50%
10°
50%
20°
51%
30°
52%
40°
53%
50°
55%
60°
58%
70°
61%
80°
65%
90°
71%
100°
78%
110°
87%
120°
100%
130°
118%
140°
146%
150°
193%
160°
288%
170°
574%
180°
infinity


Angle of fulcrum
Load applied to anchor

28%
10°
31%
20°
33%
30°
35%
40°
38%
50°
40%
60°
43%
70°
46%
80°
50%
90° (typical clifftop)
53%
100°
57%
110°
61%
120°
66%
130°
71%
140°
76%
150°
81%
160°
87%
170° (slight bend)
93%
180° (no fulcrum)
100% (full load)


In the picture above, you can see that there is a 110° degree fulcrum utilized.  This means that if the climber weighs 100 pounds, only about 61 pounds would actually be transferred to the anchor (these calculations do not take rope stretch and friction in to account).  Fulcrums are great on paper, but in the real world they have a drawback that should not be overlooked- friction over the edge point.  Climbing ropes stretch, and sometimes climbers move from side to side (think pendulum swings and traversing routes).  This can create a friction point at the fulcrum that can damage gear, and even cause total failure! Think ahead and take caution to protect your rope or anchor from rubbing over the edge.  Remember that even when a climber is not swinging from side to side, the stretch of the rope can still cause the same abrasion effect at the fulcrum point.  Watch this video by Petzl where the rope cuts after 9 falls (rope stretch) and 5 pendulum swings!

Fulcrums

Have you ever wondered the best way to configure your rock climbing anchor?  Joshua Tree rock climbing guides Stone Adventures has your answer!


When building your rock climbing anchor, it is best to reduce the angle between anchor points as much as possible.  Why?  The smaller the angle, the less load is transferred to each anchor point.  Let's look at some numbers for a 2-piece anchor:


What Should I Do in the Real World?

Stone Adventures - Joshua Tree Rock Climbing Guides - Rock Climbing Anchors and Fulcrum Forces

What are fulcrums you say?  You are probably in the same boat as most other climbers if you don't know what fulcrums are, what they can do for your anchor, and their dangers.


In the rock climbing world, we most often utilize fulcrums when setting up an anchor on the top of a cliff.  Try to imagine a nice vertical cliff with a flat top and a 90° edge to top out on.  If you set up an anchor on the top of the cliff and then toss your top rope over the 90° edge, you have utilized the cliff's 90° fulcrum!  This bend in the rope greatly reduces the forces applied to the anchor.  Here are some theoretical numbers:

If you weren't the greatest math or science student, don't stress!  We will attempt to explain.  Let's assume you have a rock climbing anchor with 2 anchor points (an anchor with 2 cams, for example).  When considering where in the rock to place your protection, it the best to reduce the angle as much as possible, or in other words, place your pieces closer together.  Another way to reduce your angle is to lengthen each leg of the anchor.  The longer each leg is, the smaller the angle will be.  Looking at the calculations in the table, we can see that once the angle is more than 120°, we will have more force applied to each anchor point than the overall load applied to the system!  That means that if a 200 pound climber sits on the anchor, each anchor point will take more than 200 pounds of force!  Yikes!  A poorly-built anchor really can be dangerous.

Forces on Rock Climbing Anchors

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