Testing the breaking strength of a Permanent highline setup

The highline permanent setup

In February 2021 we rigged a 75 long permanent highline setup in Stans, Switzerland. This slackline was south-exposed and there was almost no shade – it was permanently exposed to UV radiation when the sun was shining. The highline was up for 7 and a half months straight. This also means that it was up during the summer months, when there is more UV radiation.

Almost every day, there were highliners on the line, sometimes for up to 8 hours in one day. Those freestyle highliners (amongst them Tim Odermatt, Arne Lauwers and Samuel Volery) kept sessioning in the same 10 meters and took about 5’000 leash falls during that time. The mainline was a SLACKTIVITY – LSDTube. The backup an old dyneema webbing with an initial breaking strength of about 60kN. In the middle, there was a sewn connection between the mainline and the backup, increasing the safety of the setup. On one side the line was locked with a sewn end loop. On the other side a weblock was used. This weblock was a weblock from another brand, that we do not want to specifically mention here.

The same leash had already been hanging on the previous setup that was up for 9 months. Therefore it had seen about twice as many leash falls. A rough estimation is, that it has seen 10’000 leash falls. We estimated these numbers also with the help of a leash fall counter that we’ve used for 2 weeks – counting every single leash fall over that period of time. The leash was the green leash of SLACKTIVITY. In this setup, 2 aluminium leash rings were used.

Permanent highline setup breaking strength after 7 months in the sun

After 7 months we decided that it was finally time to take the line down and rig a new line. The ISA recommends to use e.g. the pinkTube for a maximum of 6 months of permanent rig (180 days of use). Unlike the pinkTube, the LSDTube is not yet a certified webbing for highline use, but we hope to get a certification for this webbing soon. For 5-6 months the line felt great and we were able to stick countless new tricks on it. During the last 1-2 months the highline did not have a powerful nice bounce anymore. But it felt kind of dead. Some people say, that if such a line is derigged and re-rigged a few months later, the bounce would be back. We do not know, we have not tested it. Some of the creeping that happens is for sure reversible, but also there is irreversible creeping happening and also the UV light causes irreversible damage to the line, so for sure the line would never feel new anymore.

After taking off the tension, the line felt like cardboard. In the middle section where all the leashfalls have happened, it was clearly bent downwards and felt fluffy on top. That part was also totally exposed to UV light. We started to break test this part. The MBS (minimum breaking strength) of a new LSDTube is 25kN. We’ve done 3 tests at the most used part and measured 18.87kN, 17.49kN and 18.19kN. Clearly less than new but still reasonably high. But definitely a good time to exchange the line. We did not conduct any maths upon these results, but simply wanted to know the actual values.

In the next test we tore apart a part of the line that was also totally exposed to UV light but had not seen many leash falls. The line there was not bent and not fluffy but it had also lost its colour. We’ve measured 19.73kN. We only did one test. Surely not enough to make a clear conclusion. This value was higher than in the sessioning part. It looks like the UV light is responsible for the main weakening of the highline and the countless leash falls only added a little to the weakening. Clearly, it is possible to weaken the webbing with one single leash fall beyond repair, if you e.g. have a sharp object on you!

Damaged webbing in the weblock

On the tensioning side of this permanent highline setup, the webbing was fixed in a weblock. This weblock was not from Slacktivity. There were some traces of heavy wear on the webbing. We do not yet know the mechanism why this wear has happened. It looks like some friction has occurred on a length of about 10mm. We need to find out more on this mechanism in future. We wanted to go public with these results to inform the highliners of this possible weakening and to regularly check their webbing for traces close to the weblock. We hope to have more information about the reasons of the wear. And also, whether the seaHorse also causes wear or not. To find out, long test cycles are necessary and this is why we do not yet have any results and answers on these questions.

Anyway, the webbing got weakend down to 11.72kN in such a part. We were only able to test one damaged part as the rest of the torn pieces were too short to test again. This value is extremely low and it is dangerous to use such a webbing with such severe damage.

The damaged parts were distributed over a distance of about 1m. Probably the line got worn out in a new place after every tensioning process. And every time we pulled out the damaged part.

Weakening of the leash

The leash still felt pretty soft and not too stiff after all that time. But we also have to state that it was stored pretty much in the shade underneath some branches and that in other permanent highlines the leash could get stiffer or more weakened as in our tests. We never opened the figure-8 knot of our leash. During more than 16 months the knot stayed at the same place and has taken about 10’000 leash falls.

In the break tests, first the mantle (leash webbing) broke at 8.53kN and shortly after, the rope broke at 8.64kN. The rope broke at the knot. A new leash held 16.53kN also in a figure-8 scenario.

At last we break tested a single leash ring. The rings had lost all the eloxation between the two rings. Also some aluminium powder already came off between the rings. One single heavily used leash ring held 38.38kN.

Sewn end loop and intermittent connection

The intermittend connection held 11.33kN. We do not know how much it would have held when it was new as we didn’t conduct any test on this particular setup. The sewn end loop held 21.78kN. But it was partly in the shade and therefore the value could not really be compared to the rest of the results. Therefore we do not want to discuss these results at all.

Conclusion

We clearly recommend to use ISA certified webbings. We also recommend to stick to the rules of the ISA about the time of retirement of the webbing. Upon the certification, we also clearly recommend to always use setups with an intermittent connection to increase your safety! The tests show, that the ISA label seems to be reasonable with the time of use.

The only worrying results are the damages in the weblock. More tests need to be conducted here. We will for sure conduct such tests in future. But any tests by the ISA or also by other brands will help to analyse the problem and find out more about it in future.

For the leash, we for now recommend to exchange it after a maximum of 360 days of use. We will also keep testing our leashes and might adapt the recommendations upon those new tests.

Anyway, besides those results we should never forget, that one single leash fall or other incident might damage any of your gear to a point where it should immediately be sorted out.

Written By:

Samuel Volery

SLACKTIVITY Slacklines co-founder, managing partner and team athlete, Samuel Volery olds multiple highline world records (1900m – September 2019) and has a spectacular and versatile style when performing highline-shows.