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Standing Tension Versus Working Tension

Introduction

Some slackliners believe that for a given tension the amount of sag subjected to a slackline from a slackliner will remain the same regardless of the type of webbing used. An argument in favor of this theory would state that trigonometry equations dictate sag and not webbing elongation. Others believe that the webbing does play a major role in determining how much sag a slackliner will subject to a line. Supporting arguments include the notion that increased elongation translates to increased length, which translates to increased sag. As I continue, it will become apparent that the true product of slackline sag is, in part, a blend of both of these theories.

 

Theory

To start I must define two terms: in-use tension and standing tension. In-use tension refers to the tension on the anchor when a slackliner is standing on the line. Standing tension refers to the tension on the anchor presented by the tensioning of the pulley system when the slackliner is not standing on the line.

The in-use tension subjected to a slackline anchor is a function of three main parameters: the slackliner’s weight, the amount of sag subjected by the slackliner at a given point, and the length of the line. Three constants exist with regard to these parameters: an increase in the weight of the slackliner yields an increased anchor load, decreased sag yields an increased anchor load and increasing the line length for a given sag yields an increased anchor load. This rule model forms the backbone structure that the first theory explained in the introduction is based off. You can follow the parameters of this model using the calculator posted below.

Slackline Calculator

However, although the above is accurate, one critical parameter that slackliners need to understand is that the mathematical model used to calculate the tension on the line is only relative to the in-use tension. Because we are measuring the anchor load after the webbing has already elongated, anchor load is not a function of webbing elongation following this theory.

Nonetheless, webbing with higher elongation will sag more for a given tension than lower elongation webbing will. Consider a slackliner who steps up on a chain fence and imagine that, clearly, the chain will not stretch. Next, consider the slackliner swaps the chain for an untensioned bungee cord. Clearly, the slackliner would fall to the ground, and the ground would support the majority of the slackliner’s weight. These scenarios form the basis for the second theory expressed in the introduction.

So, how can both theories be right? They can both be right because they reference two different phenomena. The mathematical model references the condition of the line after a slackliner steps on it, and the other theory references the condition of the line before a slackliner steps on it.

 

Test Methods

The methods applied in this study are quite simple. I started the test by stretching a 103’ milspec line across two trees. I tensioned the line and walked the line a few times to stabilize the line. I then fine-tuned the standing tension to 838 lbf. After, I sat in the middle of the line, I measured the deflection and I measured the average in-use tension. Lastly, I swapped the sample out for the next sample and repeated the test.

The setup The setup

I used five webbing types in my study, in order from highest to lowest elongation: milspec tubular (nylon-6 tubular), type-18 (nylon flat), Mantra MKII (polyester flat), Green Magic (polyester flat) and Kevlar (para-aramid flat).

Webbing Samples Webbing Samples

 

  • Tree-to-tree slackline length: 103’ (+/- 1 ft)
  • Standing tension: 838 lbf (+/- 2 lbf)
  • Load weight: 153 lbs (+/- 1 lb)
  • Load position: Within 1.5 ft of middle
  • Height measurement accuracy: +/- 2 in
  • Load acquisition and averaging accuracy: +/- 5 lbf.

 

Results

The first graph shows the load experienced by the anchors when subjected to the testing criteria listed above. It is worth noting that high-elongation lines are well able to absorb small load fluctuations caused by wind or other influences whereas low-elongation lines are not. The graph displays this observation clearly.

 

Standing Tension Results Standing Tension Results

The next graph displays the total sag of the line in relation to the load subjected to the anchors.

Sag Amounts for Various Webbings Sag Amounts for Various Webbings

The final graph illustrates the anchor height requirement for a particular webbing type as well as the difference between the standing tension and the in-use tension. Interestingly enough, the in-use tension difference for the milspec tubular line is less than my weight (153 lbs load, 120 lbf increase), but the Kevlar-line increase is 383% of my weight. This information sheds light on why it is ultra-critical to use exceptionally strong components when highlining on polyester or high-tech webbing types.

Anchor Height Requirement Anchor Height Requirement

Conclusion

Low-elongation webbings will produce less sag than high-elongation webbings for a given standing tension, but at the price of increased in-use tension. Overall equal, slackliners can expect to utilize lower-spaced anchors with lower-elongation webbing types.

Written by Sayar Kuchenski



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9 thoughts on “Standing Tension Versus Working Tension”

  • Hey Sayar,

    could you add the stats for the webbings you used to the article? Especially the strech? This should correlate directly to your data. (It won't be a linear relation though)

  • So clearly, we can no longer use the trigonometry equation to achieve a comfortable tension guesstimate. Also, what are the thoughts on bouncing hard on a longline of say Spider Silk and the tensions that manifest? It now almost makes me feel uncomfortable when walking an ~800ft longline with 12ft of sag while bouncing hard. It seems like our gear is being subjected to a lot more force than I once thought. Great article!!

    • On the contrary! The standard trigonometry equations that we have been using in the past still hold true. However, you must remember that what it is calculating is the amount of tension the line has while you are standing on it. To find the tension without a walker on the line is a bit more difficult and will change depending on what type of webbing you are using.

      This article shows that there is a rule of thumb that you can use across the board: The lower the amount of stretch your line has, the more tension you will add when you stand on it. That means that in order to achieve the desired tension while standing on your line, you will have to have a HIGHER standing tension on a line that has MORE stretch.

      Let's look at an example with known values. Let's say we have two 500 foot lines, one on Type 18 MKII and the other on Spider Silk MKII, both with 1,000 pounds of tension (while someone is on it) and a 150 lbs person that wants to walk these lines. The trigonometry equations tell us that we have roughly 18.8 feet of sag on these lines. Now, let's take a look at what the standing tension would be:

      We know that at 1,000 lbf, Type 18 MKII stretches 7.13% and Spider Silk MKII stretches 1.6%.

      Now, consider how much LONGER we will be making the line when we stand on it:

      By the Pythagorean Theorem we have

      (250 * 250) + (18.8 * 18.8) = 0.5 * (Line Length While Standing In The Middle)^2
      = 2 x SQRT((250 * 250) + (18.8 * 18.8))
      = 501.4

      So, we have increased the length of our line by 1.4 feet, or 0.28%.

      Now things will get a bit interesting. Assuming a linear stress-strain curve, we can plug this value into the charts for both webbings to get an approximation of what the standing tension is.

      With the Type 18 MKII we know that while we are standing on the line, the total amount that the line has stretched is 7.13%. The original 500 foot line with no one on it would have only stretched 6.87%. Looking at what the tension is for Type 18 MKII at 6.87% gives us roughly 963.5 lbf. This means that our body weight only added 36.5 lbf to the line!

      Doing the same thing with the Spider Silk shows that we only need 827.8 lbf standing tension to achieve the same sag. This means that to get the same sag at the same length using these two types of lines, you will need to apply MORE tension to the Type 18 compared to the Spider Silk.

      This phenomenon is more visible at shorter lengths or with larger amounts of sag. It all depends on how much longer you are making the line when you stand on it. As the lines get longer and the length/sag ratio gets larger, the difference between webbings becomes smaller and smaller.

      So, to answer your question, you should not worry much about bouncing hard on a 800 foot line with 12 feet of sag. The trigonometry functions will give a very close approximation to the tensions you are seeing during these dynamic events on such a big line.

  • Joseph Schlosser - January 29, 2014 at 11:53 am

    does the information from graph one indicate that a higher stretch line will sway less with wind?

    • Russell Phetteplace - January 29, 2014 at 11:53 am

      I don't think it's indicating that it will sway less, but rather that those oscillations from wind create a higher load fluctuation on a lower elongation line. I other words higher sway on a line that has less additional stretch to give means higher load increases. Whether lower elongation lines oscillate more is a interesting question, and one which is made more difficult by the additional variable of weight. I find Mantra moves very little side by side with T18, and I've always attributed that to weight, but I could definitely be wrong.

  • Panagiotis Athanasiadis - January 29, 2014 at 11:54 am

    Guys, what you do is helpful and interesting, but nevertheless all these graphs are predicted by elasticity theory, supposing one knows the tension-stretch curve of the line in hand, or (less accurately) by assuming a linear behavior and getting an estimate of Young's modulus. One important thing to consider is that the quoted differences in tension (standing versus in-use, and from one line to another) get increasingly smaller the longer the line is. You can go out there and test it: for a 100m longline with a given sag / in-use tension, the standing tension (elsewhere referred to as pretension) is pretty much the same for all webbing types. That is because the additional stretch that the sag brings is very much smaller than the stretch applied initially to reach the required pretension.

  • dude you guys are smart as hell, i just like to slackline this is too much math i just kind of skipped through the article to the conclusion then i read the comments. maybe its because im still in highschool but shit...

    • Dude, The reason for all this math is that some slackliners rig high off the ground. (highlining)
      Understanding tolerances and limits keeps them alive.

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