Two-Point Anchor Angles:
The Case For Building In Error
This post looks at the anchor angle for two-point marginal anchors with a fixed focal knot. We challenge the “smaller angle is better” rule, showing that it can overload one leg entirely once a rigging error or load movement enters the picture — and propose a built-in error range instead. (Not covering floating rigging like equalettes or magic X.)
Context
For any anchor and most definitely when joining two marginal anchors, there is a hierarchy of thinking:
- The first priority is to build strong anchors with a sufficient margin, considering the worst-case loading.
- The second priority is sharing the load as evenly as possible between the two anchors.
- The third priority is considering what happens if one of the two anchors fails.
If we do a great job at #1 and #2, it will be less likely that a single anchor point could fail.
A key part of sharing the load equally between two anchors is considering the anchor angle.
For many years, as long as I can remember, there have been a few ‘rules’ that the anchor angle for two-point marginal anchor systems (with a fixed focal point) needs to be as small as possible. However, the maximum is 90 degrees.
The concept here is that with marginal anchors, you want to decrease the load on each point to maximise the overall system strength.
This concept seems right and follows logic.
What if these ‘RULES’ were wrong and led you to have a higher likelihood of overloading one of the anchors or even having the whole load on one?
Let’s get into it.
Terminology
Before we begin, let’s get some terminology down so we speak the same language.
Marginal anchor points
An assessment that a single anchor point cannot hold the entire load. Several marginal anchors are joined together to form a suitable anchor system (multi-point) with sufficient margin. For example, bolts, threads, small trees, snow stakes, rock protection or ice screws. In general, anything placed is considered marginal.
Compare this with a bombproof anchor, where a single anchor point is sufficient, such as a large tree and a large, well-secured rock.
Anchor angle
The anchor angle is the 2-point V-shaped internal angle formed by the rigging.
Factor
The factor is the proportion of load applied, where the load equals 1. Rather than using percentages, we need to convert them to factors to make our life easier for multiplication, as the load could be any number. A factor is the same value as percentages but represented as a decimal. For example, a factor of 0.6 is the same as 60%, a factor of 1 is 100%, a factor of 1.3 is 130%, etc.
Focal point
The rigging from multiple anchor points comes together, and a suitable knot on a bight is tied for a fixed focal point. The focal is the point where you want to work. As such, start the anchor rigging process with the end in mind by identifying and marking the focal point.
The traditional way of thinking
Many think about anchor angles logically and linearly (for two-point marginal anchors with a fixed focal point). There is no allowance for error. Considering the numbers in the diagram, table, and graph below, having the ‘RULE’ that anchor angles need to be as small as possible makes sense. There is less load on each anchor point for a smaller anchor angle. However, these numbers assume that you have rigged perfectly and share the load equally between the two anchors.
Anchor angles/factor diagram
Rounded to 2 decimal places up to 120 degrees
Anchor angles/factor table
Rounded to two decimal places
Anchor angles/factor graph
Rounded to two decimal places
The numbers above assume that you have perfectly aligned the direction of load, the focal point, and the centre point between each anchor (C), and it stays that way—see the diagram below for an example.
Let me state again that traditional anchor angle thinking assumes YOU ARE PERFECT.
The world is not perfect, & neither are you
Two main things can happen with a fixed focal 2-point anchor rigging.
- You did not rig correctly. You did not judge the direction of the load with the focal point and line it up with the centre point of the two anchors.
- You did rig it correctly at the start. However, the load moved left or right and went out of alignment. This movement can happen in a 3D world when we rig. We need to consider left and right (and often up and down) movement.
Consider these two situations:
A. Off-centre shift—because of how you tied the focal point, the tension is more on one side (of the two-point anchor system) than the other. In this example, the anchor on the left will have more tension.
B. Off-angle move—as a result of the load moving. Once again, in this example, the anchor on the left will have more tension.
Let's allow for an error
What if we built a 5-degree error into our working equations?
Let’s assume that at some stage, we will be out by 5 degrees (which is not that much).
Let’s assume we are not perfect at rigging and the load moves around.
So, let’s add these numbers to the equation—we are shifting off-centre or moving off-angle by 5 degrees to the left (or it could also be to the right).
Note: The numbers are the same for shifting off-centre and moving off-angle.
5-degree error off-centre shift / off-angle move
Anchor angles/factor graph
Rounded to two decimal places
These numbers are above graphed (5 degree error off-centre shift / off angle move) compared to perfectly shared anchors.
By assuming we are not perfect and building in an error, we get a different view of two-point anchor angles for marginal anchors.
The graph above shows overloading above 75 and below 30 degrees on the LHS leg and an ideal range (sweet spot) for rigging anchor angles of 45-60 degrees when we build in a margin of error of 5 degrees.
The ideal range (sweet spot) for two-point anchor angles is 45-60 degrees with a built in error.
Conclusions
1. Smaller angles can increase the load
You need to change your thinking about using the traditional ‘smaller anchor angle is better’.
A smaller angle significantly increases the load on one anchor leg if you are not perfect.
For example, it seems acceptable at a 15-degree anchor angle if you perfectly share with 0.50 (50%) on each leg. However, if the load shifts or moves by 5 degrees, you have 0.84 (84%) of the load on one anchor leg.
15-degree anchor angle
2. A 90-degree anchor angle is not okay either.
It seems acceptable if you perfectly share with 0.71 (71%) on each leg; however, if the load shifts or moves by 5 degrees, you have 0.77 (77%) of the load on one anchor leg.
This number is too high for a two-point fixed focus marginal anchor.
90-degree anchor angle
3. Have some angle in the rigging 45-60 degrees
Having some anchor angle in the rigging allows us to be less than perfect, and that’s a good thing.
The ideal range (sweet spot) of 45-60 degree anchor angle allows us to be 5 degrees off while keeping loading on our anchors to an acceptable level of 65-66% at worst on one leg.
45-degree anchor angle
60-degree anchor angle
4. A maximum range
You have some margin for error if you don’t judge the anchor angle correctly (between 45 and 60).
If the anchor angle is down to 30 or up to 75 degrees, the loading will not increase significantly (0.68-0.70).
Don’t start with a 30-degree anchor angle as okay; if you get this wrong, the tension increases dramatically under 30.
30-degree anchor angle
75-degree anchor angle
Summary
Let’s build in an error for two-point fixed marginal anchor angles, assuming we are not perfect at rigging and the load moves around.
- Smaller angles can increase the load.
- A 90-degree angle is not OK.
- The ideal range (sweet spot) for two-point fixed marginal anchor angles is 45-60 degrees.
- The maximum range for two-point fixed marginal anchor angles should be not more than 75 degrees and not less than 30 degrees.
- Carefully plan and implement the anchor rig to get even loading on each anchor point.
Its better to be a little bit wrong than a hole lot wrong
Video
For more detailed information:
Rope Rescue & Rigging Field Guide (3rd edition)
for cave, canyon, alpine and rock
Provides easy-to-reference practical reminders on essential field techniques for teams and individuals training and responding to rope rescue incidents. 200+ pages, 250+ drawings, A6 size, waterproof paper
Disclaimer
SUMMARY: This post is not an instructional guide. Use at your own risk. We assume no responsibility or liability for any errors or omissions. Any testing undertaken was under controlled conditions with a limited set of equipment. The views, information, or opinions expressed in the post are solely those of the author. For the full disclaimer, click HERE
