Webbing slippage in low-tension highlines

Webbing slippage in low-tension highlines

Authors: Philipp Gesing, Lisa Bretagne, Thomas Buckingham

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Hypothesis

Webbing slips incrementally and continuously with each leashfall on low-tension highlines. We observed this during a highline session in Bern, Switzerland. On different occasions webbing stopper knots were dragged inside the weblock after a relatively high number of leashfalls.

 1  2  3

Pictures: A 5-hour session on a 47m highline with 15 leash falls and a couple of catches showed visible slippage in the AWL as indicated by the movement of tape

We suggest that high amplitude load cycles caused by dynamic events lead to webbing slippage in a weblock; specifically the rapid de- and increase in tension during leashfalls and catches on low-tension highlines.

Experiment

We rigged a miniature highline to determine the cause of the slippage. The highline was 3.5m long and the leashfall was simulated by dropping a 2.5kg brick into a 1m long leash.

4Across the different trials we varied weblocks, webbing, installation methods and tension. We measured the amount of webbing exiting and entering the weblock after each trial to determine slippage.

Weblocks:

  • Lynx 2, 3 and 4 Vari
  • Zilla 2 and 3
  • Sladlock Power
  • Yang

Webbings:

  • Marathon
  • Sonic I

Installation methods:

  • ‘Single’ – Single wrap
  • ‘1.5’ – 1.5 wrap
  • ‘Double’ – Double wrap

Tensions:

  • ‘Low 1’ – No base tension (~0kN, minimal visible sag)
  • ‘Low 2’ – Hand tension (~0.25kN)
  • ‘Low 3’ – Mild tension via soft-release (~1kN)

The tensioning side weblock was the Lynx 4 Vari across all trials.

Trial 1 – Determine whether all weblocks show slippage

We interchanged the static side weblock. For this trial the webbing was Marathon, the installation method was ‘single’ and the tension was ‘low 1’.

Trial 2 – Determine whether slippage is continuous

We raised the number of leashfalls to 100. For this trial the static side weblock was the Lynx 2, the webbing was Marathon, the installation method was ‘single’ and the tension was ‘low 1’.

Trial 3 – Determine whether installation method affects slippage

We interchanged installation methods on the static side weblock (Lynx 2). For this trial the webbing was Marathon and the tension was ‘low 1’.

Trial 4 – Determine whether webbing affects slippage

We interchanged webbings. For this trial the static side weblock was the Lynx 2, the installation method was ‘single’ and the tension was ‘low 1’.

Trial 5 – Determine whether tension affects slippage

We interchanged tensions. For this trial the static side weblock was the Lynx 2, the installation method was ‘single’ and the webbing was Sonic I.

Results

Slippage is expressed in mm. All indicated trendlines are linear approximations. The comparison between the amount of webbing exiting and entering the weblock indicates that the slippage measured is actual slippage and not webbing elongation inside the weblock itself. Webbing elongation is present, but minimal (< 5mm).

Trial 1 – Determine whether all weblocks show slippage

image(1)All tested weblocks show slippage.

Trial 2 – Determine whether slippage is continuous

imageSlippage is continuous and linear up to the tested number of falls.

Trial 3 – Determine whether installation method affects slippage

image(2)The installation method affects slippage. Our data is not conclusive as to whether ‘1.5’ and ‘double’ slippage is continuous and/or linear.

Trial 4 – Determine whether webbing affects slippage

image(4)All tested webbings show slippage. However, the data is inconclusive as to whether and how webbing affects slippage.

Trial 5 – Determine whether tension affects slippage

image(6)The tension affects slippage. We conclude that higher tensions decrease slippage.

Discussion

The data indicates that our hypothesis is true; the simulation of a leashfall in low-tension highlines causes webbing slippage.

The following considerations should be taken into account when rigging low-tension highlines:

Webbing uninstallment

The tests and prior experience indicate that webbing can accidentally slip out of the weblock, leading to catastrophic failure of the highline setup.

We suggest the use of stopper knots to prevent this failure mode.

Weblock damage

There is a possibility of webbing stopper knots being dragged into the weblock due to slippage, thereby acting as a wedge pushing the sideplates apart, ultimately leading to catastrophic failure of the device well under it’s indicated minimum breaking strength.

65

Left picture: Overhand knot dragged into the weblock (unfavourable)
Right picture: Springer knot dragged into the weblock (unfavourable)
We suggest the use of stopper knots with a different point of contact than the weblock (i.e. a shackle).

7Barrel knot on the shackle. No slack between weblock and shackle (favourable)

Webbing damage

The tests and prior experience indicate that webbing is damaged by slippage in certain weblocks.

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Left picture: Slippage damage due to a single catch on a low-tension waterline
Right picture: Slippage damage in Lynx 2 (top) vs Lynx 4 (bottom) in our experiment

Low base tension

Highlines with low base tension are especially prone to slippage, therefore additional experience is necessary and caution is advised.

Future research

We suggest to concentrate future research on the following questions:

  • Installation method (Is double wrap linear?)
  • Is slippage present at all tensions or is there a limit?
  • Is absolute slippage webbing dependent?
  • Is absolute slippage load dependent?
  • How does weblock design affect slippage?
  • What are safe and effective webbing stopper-knot techniques?
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