Two Rope Tension System A-Frame Resultant Insight, Part 1

Written By: Lance Piatt

Let’s review some of the basic concepts starting with a term you’re going to hear a whole bunch: resultant. Resultant is a very important term when you work with a high directional.

Defining a resultant is what determines if your high directional is going to be stable or not. So when we say resultant, the full term is a resultant vector. A vector is a measure of force and direction. Here, we’re talking about resultants through the head pulleys. This is what’s giving compression and pulling down on our frame making it stable, so we know that force in and out of a pulley is going to be the same on either side.

We don’t need to worry about the magnitude or the amount of force in our resultant, we just worry about direction. We want the direction — that resulting vector — adding these two forces together to fall somewhere within the footprint of our frame or our pod. This keeps this compression pulling down and stable. In any system with just a single pulley through the head, we can always visualize that the resultant is this line through the center of the pulley.

You’ll notice with the setup we have in the next video, if we were to step back, we would see that the two resultants are coming down and landing almost directly in the middle of our frame. That’s what keeps it stable. One of the more common ways it changes is when we use this for an edge transition; we have an attendant out there, the attendant steps back and up, they move these lines up, and that resultant swings forward towards the front.

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We’re always going to have some movement, but one of the ways we manage that movement and keep everything in compression and anchored to the ground is by adding tension with a guying system. The guying system we’re using is three points and is more or less equally divided around our frame, which is optimal. On a 360-degree circle, optimal is 120 degrees between each one.

We’ve used a Petzl Jag which is a set of 4s and an overall really really simple system. When you pull on it, it adds tension. On the other side, we’re using what’s called a voodoo or a transport hitch. This is just a self-adjusting tensioning system. On the back side, we used a very common non-working 3:1 and dog tied it off.

We say it’s non-working because, at this point, we can’t pull on it, we can’t add tension, we have to untie it, and it doesn’t have any pulleys in it. It’s a very simple system but we want that friction that it offers us. We’re going to pull on it once, tighten it, leave it, and that’s the version of the non-working part. It’s still a 3:1 and would still function if we pulled it apart.

So there you have it! Three different guying systems that provide different ways of guying the opposing tension on the frame. They are all counteracting the compression and are designed so that the four vectors end up with a resultant falling right in the middle of our frame. That defines our stability here and later on in this course, we’ll move to a monopod or a gin pole. The difference there is when we don’t have the back leg, we don’t have that area to work with and our resultant needs to fall closer to the single pull.

If we had a tripod, this would open up a little bit wider and we’d have a big footprint in the middle for our resultant to land in. Just remember that a resultant needs to land within the space defined by our frame. If we have two legs, it needs to be in the middle.

Gear & Equipment We’re Using:

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Peace on your Days



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