TFTC #46 Analyzing Forces on an anchor point

Sherrilltree May 30th 2025

With so many advances in modern tree care equipment, the most uncertain part of a climbing system is often your anchor point in the tree. Experienced arborists consider lots of factors when choosing a tie-in point, including limb diameter and geometry, tree species, and health of the tree. An often overlooked consideration is that HOW you anchor your rope can have a dramatic impact on the amount of force you place on that tie-in point. Let’s say you weigh 200 lbs with all your gear on. You’ve identified a number of safe tie-in points on the tree but aren’t sure how you should anchor your climbing system. Let’s explore some common options and how they change the force on the anchor point.

Ropes and vector forces are complex. To simplify, we’ve used pounds (lbs) instead of newtons (N) to measure force and are ignoring the effects of friction**. An efficient pulley will be closer to these numbers than a friction saver or natural crotch, but no pulley can be 100% efficient.

Note: All figures represent potential peak forces at the tie in point in the tree.
*Downward force in this illustration on A, B & C occurs at the tie in point and not the entire tree.
All arrows show direction of vector forces.

In a typical Moving Rope System (MRS/DRT) like the one on Rope A, the rope goes up and over a tree limb and back down to your climbing system. The anchor point is supporting 100% of your weight - 200 lbs - divided evenly between the two legs of the rope.

In the simplest Stationary Rope System (SRS/SRT) like Rope B, the rope is tied around your anchor point and does not move. This is called a canopy anchor. It’s simple to set from the ground and can also be retrieved from the ground by using a tag line or the tail of your rope (shown here with just a knot). Again, the anchor point is supporting 100% of your weight, now with the full 200 lbs on one leg of the rope.

Comparing A and B, they both put all of your weight on the tie point in the tree, and the force is coming straight down, at least on ascent. However, A is further out along the limb, creating a longer lever and higher force at its union with the trunk. Which would you choose?

Rope C shows a typical base anchor. The rope goes over the limb and is anchored to the trunk of the tree, either with a cinching knot or a dedicated base anchor. You can imagine that you are now the 200 lb load, and the trunk of the tree is “holding” you with 200 lbs of force. As with our simple pulley example, both legs of the rope are under 200 lbs of tension. At a narrow 11° angle, those combine to 398 lbs at the tree limb, very close to the theoretical 400 lbs with the ropes parallel. The force is directed between the two legs of the rope, in this case nearly straight down. This system gives you the benefits of a base anchor, but nearly doubles the force on the tree compared to Ropes A and B.

Rope D is redirected over three limbs before being anchored to the trunk. In addition to having an interesting effect on the forces involved, this technique allows you to climb past the first union and have a new tie-in point ready to work a different area of the tree. Carefully planning preset redirects (or predirects) like this can make traversing a large tree much simpler. Ensuring all limbs redirected through can support the force generated by the climber at any given moment. They also add a level of security - if one limb or union fails, it may not be catastrophic like it would be if you were canopy- anchored to that limb.

Rope D is both spreading that force over three limbs and directing those forces down through the limbs in compression. Rope C is adding downward tension on just one point. Which of these systems would you rather use?

Common Angles and Forces

Rope Angle	Anchor Force (200 lb load)
0°	400 lbs
15°	396.6 lbs
30°	386.4 lbs
45°	369.6 lbs
60°	346.4 lbs
75°	317.4 lbs
90°	282.8 lbs
105°	243.4 lbs
120°	200 lbs
135°	153 lbs
150°	103.6 lbs
165°	52 lbs

**All of the math is being done with a theoretical zero friction environment, versus actual real life applications.

Base anchors and forces

Another popular SRS technique is a base anchor, shown on Ropes C and D. Base anchors use the base of the tree as your anchor rather than a limb in the canopy. This allows for simple retrieval and doesn’t require you to isolate your tie-in point like with Ropes A and B.

To better understand base anchors, let’s first think of a very simple pulley system like the one in this illustration. With our theoretical frictionless pulley anchored above, you’re holding a 200 lb weight aloft, which means you’re pulling down on the rope with 200 lbs of force. Each leg of the rope is under 200 lbs of tension, and the pulley is holding both for a total of 400 lbs on the anchor.

If you were to move away from the load and open up the angle of the rope as it passes through the pulley, both legs of rope would still be under 200 lbs of tension, but the forces on the anchor would change in both magnitude and direction. The direction of the force is halfway between the two legs of rope. See examples of angular forces in the Common angles and forces chart.

Next time you’re looking up into a tree considering where you’ll anchor your lifeline, think about the different ways you can configure your system and how that will affect the forces on the tree.

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