Pull Rope 6mm Double Braid in Knots and Devices

Last updated December 2, 2022.

• Pull rope
• Double braid
• Tagline
• Breaktests

Double Braid 6mm Pull Rope from SGT Knots is a high strength and relatively inexpensive tagline that has been used as the final tagline for a number of 500m+ projects in the US over the past few years. It is rated for 2700 lbs (12 kN) and costs roughly $220 for 375m of tag. One of its main advantages is it comes with spliced ends, commonly believed to be full strength, which negates the strength reduction due to knots that other taglines suffer from.

The tagline is the subject of current interest because it recently snapped in a progress capture during the Paradise Highline Community mile long US record attempt (more about the failure below). Although it is quite strong, its practical limit was found in its performance in progress capture devices. This testing has been performed in order to provide reliable data on the breaking strength of this cord in the widest variety of progress capture devices possible, along with common knots. Hopefully this will provide better information for planning and on site decision making for big rigs, as well as facilitate better failure analysis of the US mile project.

Double Braid Pull Rope is significantly cheaper than 6mm cord, and is rated about 60% stronger (compared to 7.3 kN Maxim cord). There is no published weight for Pull Rope, although it is likely heavier than 6mm accessory cord. It comes with spliced eyes on both ends, which are commonly believed to be full strength despite there being no claim to that effect from the manufacturer. Because it is full polyester, it is also quite low stretch. It has been seen as a great tagline for large projects due to it’s low cost, high strength, and suitability for use in powered winches.

During the Paradise Highlines US Mile (1670m) attempt, the Double Braid Pull Rope snapped while pulling the webbing across. At the time, there was roughly 1200m of Slacktivity Y2K in the gap. On the static side, the Pull Rope went through a kootenay pulley, then a Petzl Mini Traxion, before going into a gas powered winch. During the hold which preceded the failure, a Micro Traxion had been added between the Mini Trax and the winch but was not yet holding tension. The Mini Traxion was intended to be a backup in case the winch slipped, and in order to pass the soft shackle connections between sections of Pull Rope, a pulley system attached to two in-line handled ascenders was used to get the soft shackles from in front of the Mini Traxion to behind the winch.

During holds, the winch engine was having difficulty turning over with so much tension on the line (estimated at up to 4kN depending on the wind). As a solution, the static side team decided to remove wraps from the winch (from 4 wraps to 2), releasing tension onto the Mini Traxion. This is very dangerous, as this report of a winch incident indicates, but was the solution that the team decided on during the rig. They had performed this operation succesfully on an earlier hold, but at the time of the incident, as soon as wraps were removed from the winch the tagline snapped at the Mini Traxion, shooting violently out from the cliff, breaking a carabiner attached to the kootenay, and causing 1200m of webbing to drop into the trees below.

The system used was not ideal, and the failure could have been prevented with some small changes, like moving the backup progress capture to behind the winch or using a different method of reducing tension on the winch to help it turn over. However, it seems the system that was used was chosen because of the high strength rating of the Pull Rope, and a lack of awareness about the limitations of progress capture devices. Hopefully, this testing will shed light on the forces involved during the actual incident, and help provide useful data on the strength of used Pull Rope in devices.

This testing was performed using Balance Community’s break test machine taking readings at 500 Hz. Tests were performed by Philip Queen, Ana Lincoln, and Logan Henning. Tests were performed on roughly 2m long segments of Pull Rope, and were terminated on one end in a figure eight unless otherwise mentioned.

This testing would not have been possible without the generous donation of the remainder of the snapped section of pull rope by Vova Kozlov.

Capstan and spliced eye values are both weaker than expected, with a capstan to capstan value of 6.79 kN, instead of the 12 kN rating, and a spliced eye value of 5.46 kN. Lower than expected capstan values likely indicate the cord was significantly worn, while low spliced eye values likely indicate the spliced eyes were never full strength.

For knots, the Figure 8s tested were stronger than the weaker spliced eye at 5.6 kN. Alpine Bends and Double Sheet Bends were both weaker than figure 8s, but similar to each other, at 4.6 and 4.7 kN respectively.

For toothed progress capture devices, the Petzl Pro Traxion performed the best at 3.9 kN. Unlike in the Farmer Boy testing, the Micro Traxion (3.2 kN) outperformed the Mini Traxion (3.1 kN), which also slipped at 2.9kN in the same test. The Edelrid Spoc was also tested, and matched the Mini Trax at 3.1 kN. The Micro Trax was tested doubled, and performed significantly better at 4.33 kN, the best of any of the toothed progress captures. We also attempted to test the Mini Trax doubled, and the one sample broke at 4.11 kN, but wedged in between the sideplate and the pulley, and more testing seemed to risk hurting the device.

A second set of tests was done on the Pro Trax with an extra meter or more of space between the termination knot and Trax in order to see if the short space between the knot and device in the tests was influencing the breaking strength. The results were not significantly different (3.9 kN for the short distance, 3.7 kN for the longer distance), indicating the short throw distance used in these tests was not problematic.

We also tested a chained Mini Trax and Micro Trax to try to replicate the Paradise Mile failure and see if chaining toothed progress captures does improve their performance. The tagline was loaded into the Mini Trax, and then the tail was loaded into a Micro Trax about a foot away and at a shallow angle from the direction of pull. This broke at 5.1 kN, showing a massive benefit to chaining progress captures. This test does not adequately reflect the setup of the mile failure though, because the Micro Trax had not yet seen tension since nothing had pulled behind it yet.

Three configurations of handled ascenders were tested. A standard, single handled ascender broke at 3.3 kN. Two handled ascenders, chained in line with a carabiner, broke at 4.2 kN after slipping at 3.5 kN. The same two ascenders, connected with a sliding X in a short nylon sling instead of a carabiner, broke higher at 4.5 kN.

Of the three standard camming devices tested, the Kong Duck performed the best at 3.6 kN. This outperformed the Petzl Micro Trax but not the Pro Trax, showing that the general rule that camming devices preserve more strength than toothed devices is false. The Roll N Lock also slightly outperformed the Micro Trax at 3.4 kN, while the Wild Country Ropeman 1 broke as low as 1.6 kN. We also tested an old Rock Exotica cam, which we were not able to find a device name for, but which broke very high at 4.2 kN, but began slipping at 2.1kN. We did one test each of the Petzl Rescuecender (an older than current model), which broke at 1.6 kN, and the very cheap Nite Ize CamJam XT, which broke at 3.9 kN. The CamJam continues to impress for a device costing only $15, although its design makes it very impractical for use on bigger lines where knots in the tagline will have to be passed.

Finally, we tested three configurations of 4 wrap prusiks. The first was made out of a short sling of Pull Rope, which broke very high 5.2 kN, but started slipping as early as 1.7 kN. We also did one test each of two models of Sterling Hollow Blocks. The 6.8mm, rectangular style hollow block, slipped at .8 kN and never was able to hold more tension. The 5mm round style hollow block slipped at 2.7 kN, and also was unable to hold more tension.

In the worn Pull Rope, the spliced eyes have a small advantage over just terminating the sections in a Figure 8. The Figure 8 based Flemish Bend (untested) would be the strongest direct connection, although much more difficult to untie than an Alpine Bend or Double Sheet Bend, which were indistinguishable in this testing.

For progress capture devices, the Petzl Pro Trax was the best performing, followed by the Kong Duck. If doubling the tagline in the device is suitable for your project, the double wrapped Mini Trax outperformed all of the single wrapped devices. Whichever device is chosen, if it is possible to reach forces near 3 kN, using multiple devices in line (with tension pulled through both) will significantly increase your safety margin.

Two handled ascenders connected with a sliding X outperformed all other progress captures, and if there is a way to mind them (by having slight tension pulled from the front of the ascenders) they are the best option for high tension use of Pull Rope. Prusiks made of Pull Rope have a similarly high break strength, but have a tendency to slip badly. They may be a good, cheap backup option to the main progress capture, and could also serve as a tension limiting device.

For full break test results, see Pull Rope Full Break Tests. Note all values listed are the minimum across the samples from that test, not the 3 sigma MBS.