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Gear Test - Leash Fall Simulations

Welcome to the Gear Testing section of Slack Science. These types of posts will include experiments that we do using gear that we sell here at Balance Community. I will write up a scientific report for each test that we do and post it here along with a video of the actual testing. Feel free to comment down below.



Shockloads during a leash fall on a highline built on double Vectran webbing.



A report on the effects that leash falls have on a highline made from double Vectran. We go into the effects that using dynamic anchors has on the loads during a leash fall on the entire system (both lines), and just the backup line.



Recently, the lengths of highlines have been exploding. For this reason, many slackline retailers have been exploring new options for webbing to be used that uses high-tech fibers. These new webbings have significantly lower stretch than traditional slackline webbings, which are made from nylon and/or polyester. This combined with a much lighter weight makes these webbings very appealing for all slackliners. Because of their low stretch, these webbings are subject to much higher shockload forces during dynamic movements, such as leash falls on a highline. The shorter the slackline, the more apparant these peak forces become.

In this experiment, we setup a highline consisting of 2 strands of a prototype Vectran webbing, with tension on both the main line and the backup line (using separate pulley systems for each). We also tested how much of an effect that dynamic anchors had on the peak forces on the full system as well as just the backup line.



First we setup a 118 ft. long highline between two trees consisting of 2 strands of a prototype Vectran webbing. We tensioned the mainline and backup line with separate pulley systems. Our first run had the dynomometer attached to both lines. Our second run had the dynomometer only attached to the backup line. After these two tests, we a dynamic anchor to the whole system on the static side, which was a piece of type-18 wrapped between two shackles 5 times (10 strands total).

The way we simulated a leash fall was with a set of weights and a tree stump attached to a leash, which we lifted up a tree that was directly next to the middle of the highline. We would lift the weights about 1 foot above the line, to simulate where most of your weight would be on a leash fall, then we would cut it loose and watch the peak force on the dynomometer.



Test 1: Dyno on full system, no dynamic anchors.

  1. Standing Tension: 1,000 lbf
  2. Peak Force: 4,292 lbf

Test 2: Dyno on only backup, no dynamic anchors.

  1. Standing Tension: 304 lbf
  2. Peak Force: 722 lbf

Test 3: Dyno on only backup, dynamic anchors.

  1. Standing Tension: 296 lbf
  2. Peak Force: 998 lbf

Test 4: Dyno on full system, dynamic anchors.

  1. Standing Tension: 1,256 lbf
  2. Peak Force: 3,916 lbf



We got some interesting numbers from this experiment. A few things happened that were not expected. First off, in our first test, we had an increase of 429.2% on the total tension during the leash fall! This is much higher than we were expecting to see. A few things that I thought could have caused this high value were the length of the line, the distance of the fall, and the way the weight was distributed on the leash fall. The shorter a low tension line is, the less room there is for the fiber to stretch. When the stretch of the whole webbing is only 2-3%, this becomes significant. A highline that's twice as long will have twice as much room for stretch, which could decrease our shockload by as much as 50% (needs to be tested).

Also, we saw a 237.5% increase on just the backup line during the leash fall. During this test we did not know the tension of the mainline, so this could have effected our results significantly. It's good to know that not only the mainline is taking the force during a leash fall. If we assume that we had roughly the same tension on the backup line in our first test (as well as the same mainline tension), we could safely say that the mainline only shockloaded to ~3,500 lbs from a 700 lbs load, which is an even higher percentage increase, but less total load.

Adding the dynamic anchors seemed to benefit the entire system, but increased the backup shockload. We cannot be certain of this because we did not know the individual tensions on the main and backup lines during test 4, nor did we know the mainline tension in test 3. If we had roughly the same tensions during both tests, this means that the mainline only increased 300% with the dynamic anchor (~1,000 lbf standing tension, ~3,000 lbf peak force). If that's true, then the dynamic anchor is very effective in lowering the peak forces on the mainline while sharing the shockload more with the backup.

This theory somewhat makes sense to me as I have seen the stress-strain curves for nylon. During the leash fall, the mainline is starting to slow the load down first. At the same time, the dynamic anchors are stretching. As soon as the backup starts to receive some of the load, the dynamic anchors have already stretched quite a bit, which has lowered the peak force in the mainline. Now that the dynamic anchors are stretched, the backup line doesn't have any dampening to eliminate the shockloads it's receiving. Thus, it will shockload at a higher value. Without the dynamic anchors, the mainline does most of the shock absorbtion during the leash fall before the backup has a chance to take any of the load. The fact that the mainline can stretch more during the leash fall with the dynamic anchors causes the backup to take more of the shockload.

In conclusion, I would like to say that there is a good reason why these new high-tech fiber webbings are significantly stronger than the standard slackline webbings: the will be seeing MUCH higher loads. If you are planning on investing in a high-tech fiber webbing, stick to the ones that are rated at the highest possible strengths (i.e. Spider Silk MKII and try and use them only for longer highlines as the loads will be significantly less.


Future Research

Doing this experiment has brought up many issues that need to be tested. Some future studies that we are planning on doing are:

  1. Test leash falls at different points in the line to see if the shockloads vary.
  2. Test different main and backup line tensions to see if the percentage increase changes.
  3. Test different length highlines to see if the peak forces are different.
  4. More testing with dynamic anchors to find the optimal amount of material for the biggest impact on peak forces.

Feel free to comment below. I will do my best to answer any and all questions you may have.


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7 thoughts on “Gear Test - Leash Fall Simulations”

  • am Damian slackline and practice for 10 years. usually used in higline passed back to the disposer rope leash tied to the harness half past ten. I tested a highline 50m long with more than 150 falls using the same rope leash and the same tape and with the same disposer. I can testify that the disposer is a great tool to decrease the impact force in the fall. is essential not to compromise the breaking loads of the stops anchor - slack - rope safe - leash - harness. . it would be interesting to work alongside the Highline held from 50 - 80-100m with and without disposer. in an average fall of 1.8 m is close to stop the dissipation of 1m, added 1.8 m + 1m. in case of fall in the middle of the line dissipation decreases almost to zero.
    hand in hand for keeping material advisors to the heart with cardio testing tool, as you can see from the video ... you can get from 180 to 200 heartbeats. not to be underestimated! personally now I can get 130-140 beats falling behind ... I have experience more than 200 falls.
    congratulations for the test and your professionalism. thanks and see you soon

    video: http://www.youtube.com/user/arcieridellaslack?feature=mhee#p/u/1/L-eMB6vxwMI
    foto: https://picasaweb.google.com/101410678538745165528/HighlineValleDe7Femane

    sito. www.arcieridellaslack.it

  • Christian Katlein - January 29, 2014 at 10:58 am

    The deviation between solid fallweights and a real human body is enourmous! Check research from Climbing rope testing on that (Mägdefrau, Schubert etc..) Human body reacts far more dynamic, so your peak loads will decrease. Rather interesting might be the potential increase on peak force in the leash, as already with dynamic highline system we measured up to 4kN. If you just double or triple that you reach the physiological limits for breaking your backbone...
    Thats why especially on Hightech Fiber lines i recommend leashed of dynamic climbing rope and nothing else! Using dyneema there can hurt a lot. Don't make your body the dynamic part of the system!
    Nice measurements by the way. Mine are unfortunately just available in german on my website

    • We speculated that a human body would react very differently than a solid weight. I think that the loads would be even less even though a typical human weighs far more than the weighs we used (120 lbs).

      We only use dynamic leashes on all types of highlines, no matter the material. Also, the loads are drastically reduced as the length increases. We typically are using Nylon (polyamide) for lines that are this long. The shortest line I would suggest rigged with High-Tech fibers is in the 50-60m range, where there is a lot more room for stretch to occur.

      Thanks for the comment, your input is much appreciated!

  • Terrence Kevin Fritz - January 29, 2014 at 10:58 am


    You do great work!!


    Thanks a bunch. I've learned a lot and have this question for you...

    The highest peak force you recorded was 4292 lbf.

    Does this mean any singular piece of equipment needs to have AT LEAST 5,000 lbf for what ever the weight of the "person" was...AND PREFERABLY and 6-1 ratio or AT LEAST 30,000 lbf...or...am I completely missing the point of all the equipment ratings for lbf and kN?

    Thanks again for all of your time, energy and knowledge you put into a sport and way of life to be more easily accessible to me.

    Muchos Gracias!


    • Hey Fritz,

      These tests were done using an extremely low stretch material. The forces we generated were unlike anything you would experience using standard materials like nylon or polyester. For this reason, I suggest only using the highest strength materials when rigging with ultra low-stretch webbing, like Spider Silk MKII.

      You are correct that you should always maintain a high safety ratio with your rigging. All hardware should have at least a 5:1 safety ratio between the peak force and the breaking strength. Sometimes this is impossible to do though, so having strong backups in place is absolutely necessary.

      I hope this answers your question.

  • What do you mean by dynamic anchors? Are you using dynamic rope?

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