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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.
Shock-loads 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 shock-load forces during dynamic movements, such as leash falls on a highline. The shorter the slackline, the more apparent 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 dynamometer attached to both lines. Our second run had the dynamometer 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 dynamometer.
Test 1: Dyno on full system, no dynamic anchors.
Test 2: Dyno on only backup, no dynamic anchors.
Test 3: Dyno on only backup, dynamic anchors.
Test 4: Dyno on full system, dynamic anchors.
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 shock-loaded 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 shock-load. 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 shock-load 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 shock-loads it's receiving. Thus, it will shock-load at a higher value. Without the dynamic anchors, the mainline does most of the shock absorption 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.
Doing this experiment has brought up many issues that need to be tested. Some future studies that we are planning on doing are:
Feel free to comment below. I will do my best to answer any and all questions you may have.