At the Jungfrau Slack last weekend, Jediah acquainted me with his idea of a shock-absorbing tensioning system for low-tension (and/or low-stretch) highlines. This method will hereby be referred to as J-Team-Method or JTM and is defined as follows:
The main line is tensioned with a 2:1 base system and a 3:1 multiplier. This yields a 6:1 theoretical mechanical advantage. Basically a Strongman. However, the base system is set to stay extended (over the edge, or at least a predefined length [future tests will determine this distance]) after tensioning in order to function as a shock absorbing unit in the main system. A dynamic rope may be used to enhance that effect.
Therefore the tensioning system is hard-pointed, which requires additional safety measures. In order to get a rough idea of what needs to be done, we performed some tests with Jed’s Enforcer, which is tiny and beautiful.
Since this is the proclaimed function of the entire idea, we wanted to see, whether a short length of dynamic rope in this configuration would actually reduce the shock loads. In order to do that, we rigged an 8m line on flat polyester (21kN, 40g/m, 7% @ 7kN) on triple hand tension (~0.64kN) and dropped a tree stump (~15kg) into a leash (~30cm).
Obviously, this is rather comical. We didn’t intend to get any quantifiable or usable results for real life application, but rather a qualitative answer to two questions:
Does a short length of dynamic rope absorb shock?
Yes. We varied the length of rope to determine whether it decreased the loads the system was experiencing. In the JTM (doubled up dynamic rope, because of the 2:1 base), we recorded 1.64kN (+- 0.1kN in 5 drops) for 1m of absorbing unit and 1.96kN (+- 0.12kN in 5 drops) for 0.2m of absorbing unit. Thereby we concluded, that a short piece of dynamic rope will decrease the shock loads. Further testing on a more realistic rig will be done soon.
Does a dynamic rope absorb shock repeatedly?
In order to answer this question, we dropped the weight 5 consecutive times with a 15s break interval. The results showed no trend. This indicates that the dynamic rope does not loose it’s shock absorbing abilities. However, you have to keep in mind, that since the rope is used to tension the main line, it does no longer have the massive shock absorbing qualities you might be used to when lead climbing. A lot of the stretch has already been pulled out by that point. We will perform the same test on static rope and see if it behaves equally well.
We learned that the JTM was capable of absorbing shock, now it was time to test, how well it performed as a tensioning system. We identified three variables that could influence the efficiency. The single pulley in the 2:1 base, which is directly connected to the weblock, the brake and the multiplier pulley. There are already enough tests out there that determine the efficiency of single pulleys in multiplier kits, so we decided to save some time and only test the first two variables.
In the first test, we used the VT and a CT Simple as a break (read THIS, if you are unfamiliar with the Sla’Knot), a 3″ SMC Single as a multiplier and two different base pulleys (12mm stainless steel shackle and a CT Single (2″ ball bearing)).
The choice of base pulley did not influence the maximum amount of tension one could reach. Results ranged from 3.72kN to 4.56kN depending on who was pulling, which indicates that the choice of base pulley is not one of efficiency, but one of strength, weight and rope friendliness. The 12mm stainless steel shackle didn’t allow the rope to move much under tension, which might lead to a failure of the JTM. Therefore we advise using an actual pulley in your base system, even if the efficiency does not matter.
In the second test, we tried to determine how much of a role different brakes would play in terms of efficiency. This time we rigged a 40m line on tubular nylon (Jormungand) with a 12mm stainless steel shackle as a base pulley, a 3″ SMC as a multiplier and 4 different brakes.
CT Simple with VT: 4.24kN
CT Single with VT: – (VT failed at 3.8kN, because the pulley didn’t treat the knot)
CT Sparrow: 4.16kN
Petzl GriGri 2: 4.01kN
Again, the choice of break does not greatly influence the maximum amount of tension one can reach. However, the more stretch you pull out of your webbing, the more energy you waste with inefficient brakes (all camming brakes, i.e. Sparrow, GriGri, Eddy, etc.). With the Petzl ProTRAXION (or any other member of the TRAXION family and the CT RollnLock) you could reach higher tensions than what we achieved with the VT, because the VT looses a little bit of tensioning way before the knot starts to grip. This can get annoying as fuck.
Real life implications:
4kN is a lot of tension. A lot, a lot. And we are talking about standing tension. This is easily enough for a 60m line in the park. If soft-pointed, the JTM can be a perfectly suitable lightweight tensioning system. However, if hard-pointed you will encounter some problems. The load on the brake is really high. Therefore you have to remove the brake and replace it with a munter hitch, which is then tied off. Especially when using a TRAXION this will be difficult. Jed and I spent the later afternoon trying some rescue methods, but with loads as high as 2kN on the brake strand, these begin to get impossible. If you exclude using the tree (since this is meant as a highline rig), it only gets worse. Isolating the brake is the biggest problem the JTM has right now. If you use the VT with the prusik and the pulley on different anchor points (or different holes on a rigging plate), this can be easily done. All other brakes require more effort and time.
We will upload a video on different approaches to this problem at some point, but right now, we have not yet found an entirely satisfying solution (Take a look at THIS and THIS, if you’re interested in brake rescues).
That will be all for now. If you’ve been particularly observant, you will have noticed that in the first picture, I made a sketch of the Buckingham tensioning method as well. We performed some tests on weblock-pulley efficiency (using a Hangover, highslide and a LineGrip to tension) and the results will be published here soon with some more detailed information on the Buckingham.
And don’t forget:
Always use sunscreen!