Forces in Slacklining: Calculating Anchor Loads

1
Length of the slackline
m
2
Weight of the user
kg
3
Sag under load
m
4
Slackline type

Expected force on the anchors:

kN with static load & kN with dynamic load:

During slacklining the anchors are subjected to enormous forces

When you are dealing withslacklines, you are also inevitably dealing with physics. And in particular, you are dealing with the forces produced when the line is tensioned.

When a slackline is tensioned and used, forces are exerted on the system as a whole, but also on the individual elements. It is important to be familiar with the forces which develop. Finally, it is important to know whether the individual components can withstand the forces, and thus whether the system is safe – or not.

Even though the breaking loads are stated on all commercially available materials, you should be familiar with the forces which come into play when you are using slacklines and how these can change with different uses of the line.

The vocabulary: Forces and units

With physics calculations it is important to always define and qualify the terminology beforehand.

What are forces?

In slacklining the forces are given in kilonewtons (kN) or sometimes also in decanewtons (daN).

Non-physicists often lack the conceptual understanding of how large a Newton is, to say nothing of how it relates to a kN. The following is a popular example using a block of chocolate: If one lifts a 100g block of chocolate, a force of 1N is acting on the hand holding it. If this is extrapolated to a slackliner weighing 80kg, then a force corresponding to 0.8kN is exerted on the slackline.

Mass and forces (values rounded)
Mass Forces
1 kg 10 N = 1 daN = 0.01 kN
10 kg 100 N = 10 daN = 0.1 kN
100 kg 1000 N = 100 daN = 1 kN
1000 kg 10 000 N = 1000 daN = 10 kN

Breaking load vs. Load capacity

Since slacklining - broadly speaking - developed from climbing sports, the materials and terminology used will be familiar to climbers. If you use products intended for industry, or from the hardware store, you may encounter some confusion. The reason for this is largely because of the different ways that load capacity or breaking strength/breaking load are described.

The strength specifications on materials designed for mountain sports always refer to the breaking load, that is the load under which the material “breaks”. On the other hand, products intended for industrial use will specify the load capacity, which refers to the permissible load during usage. This is much lower that the breaking load. The result is that the thinner slings intended for climbing have a breaking load of 22kN (2.2t), while the more substantial slings sold for industrial use have a load capacity of 500kg (5kN).

Industrial products also specify the safety factor, which indicates how much the breaking load exceeds the load capacity by. An industrial sling with a safety factor 7 and a load capacity of 500kg therefore has a breaking load of 3500kg, that is 35kN, making it significantly stronger than the thin Dyneema slings.

Which forces play a role in slacklining?

When using a slackline system, the slackliner doesn’t produce force with their mass alone. Force is also introduced into the line during tensioning, and this force is known as pre-tension force or pre-tensioning. Another important factor is the reactions forces, which are formed in the system as a result of the movement of the slackliner, when their weight stretches the line and creates an angle.

Forces during sag: It’s the angle that matters

The sagging of the line in relation to its length is significant to the forces in a slackline system. In this case: The smaller the angle, which forms at the anchor point between the straight (unloaded) line and the slanting (loaded) line, in comparison to the length of the line, the greater the load on the anchors.

If we assume that the slackliner is standing still in the middle of the line, the calculated values correspond almost exactly with the measured ones. This means that, using the length and the sag (or the angle) of the line. as well as the weight of the slackliner, we can make a prediction about the forces which come into play.

Slackline physics - sag and angle
The forces are dependent on the sag (D) or rather the angles α and β of the loaded slackline

The resulting forces are also highest when the load is in the centre. That is why the calculations are based on this scenario.

Sag and force on the anchors (source: “Slackline“ by Fritz Miller & Daniel Mauser)
α β Sag (D) in a 10m Line Multiple of the load on the anchors Force on the anchor with a person weighing 80kg
0.5° 179° 0.04 m 57.3 45.8 kN
178° 0.09 m 28.6 22.9 kN
1.5° 177° 0.13 m 19.1 15.3 kN
176° 0.17 m 14.3 11.4 kN
2.5° 175° 0.22 m 11.5 9.2 kN
170° 0.44 m 5.7 4.7 kN
7.5° 165° 0.66 m 3.8 3.0 kN
10° 160° 0.88 m 2.9 2.3 kN
15° 150° 1.34 m 1.9 1.5 kN
20° 140° 1.82 m 1.5 1.2 kN
25° 130° 2.33 m 1.2 1.0 kN
30° 120° 2.89 m 1.0 0.8 kN

The formula

By 2006 the safety research of the DAV had already compiled their findings on forces in slacklining into a formula. This allows the forces in a system to be calculated quickly and uncomplicatedly. Since the weak points of a slackline system are almost always in the anchors or the attachment points of the line, we generally concern ourselves with the forces which act on these points (“anchor load“)

F = (L x W) / (S x 400)

F = force on the anchors (in kN); L = length of the slackline (in m)
S = sag under load (in m); W = weight of the user (in kg)

Dynamic loads

So far we have been concerned with a static load – the slackliner standing still. But the slackliner would usually be moving continuously, bouncing on the line, and more advanced practitioners would even be jumping. Because of this the dynamic loads must also be considered. How the reaction forces increase with dynamic loads, depends on the tensioning of the line. Here are two examples which illustrate the difference.

A fairly short line is relatively loosely tensioned, with a tensioning of around 1kN. When the line is loaded with a person, the forces on the anchors increase. If the person begins to bob, the forces can reach 5-6kN.

An 8m line is tensioned fairly tightly with a tensioning of 6kN, like it would be for use as a jumpline. If the slackliner stands on the line the values increase by at most 1kN. But if the slackliner starts jumping the reaction forces can reach values around 9-11kN.

Guidelines for increasing the reaction forces:

  • With conventional slacklines the reaction forces in a system increase by a factor of around 1.5.
  • With jumplines or weakly tensioned lines the forces increase by a factor of 1.5-2.
  • With longlines the increase of force under dynamic loads is minimal and therefore not practically relevant.

Practical experience

Since calculations function best when they are combined with empirical and measured data, we want to give you some rules of thumb to get you started:

  • True “slack” lines are only slightly pre-tensioned, and therefore generate no appreciable forces.
  • When you use conventional equipment for setting up moderate longlines (with tensioning under 10kN) and standard jumplines, you don’t really need to worry about the resulting forces.
  • With conventional slackline materials the critical values are actually only reached when lines are very short and very highly tensioned, and even then only when several people use them.
  • When using longlines, the highest forces are generated through the tensioning of the line.

Attaching the slackline to trees

In most cases trees are used as anchors. It is first thing you should consider is that the trees you choose must be stable enough to support the loads which will be produced. Secondly, you should also consider the fact that damage to trees is not as easy to repair, for example, as damage to material. Trees in parks are often repeatedly used for slacklining, and their bark is put under a lot of stress. You should always ensure that you use a tree protector underneath the attachment. It is also advisable to use systems, which “chafe” the tree as little as possible, and which also distribute the load over a larger area.

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