For some time now, and for those of us sad enough, there’s been over a beer debate about what sort of lanyard should be used (if any) to connect a hand jammer to a harness in caving SRT. So, spurred on from a thread on UKCaving I decided to do some back yard testing. Before I go any further than this I want to highlight I’m not aware of any injuries (or worse) from a fall onto a lanyard or hand jammer in SRT; if anyone is aware of any incidents I’d love to hear from you to further this pooling of knowledge.
The illustration below highlight some of the various suggestions from manufactures or text books for a lanyard between a hand jammer and harness.
When I first started out with SRT I used a short length of 9mm dynamic rope as the lanyard between my hand jammer and harness, thinking that should I fall onto the jammer the dynamic rope should catch me! However as I’ve come to understand the damage a toothed jammer can do to a rope it’s made me re-think how I set up my personal SRT kit, and what I teach.
Few facts and figures to be aware of…
- Toothed jammers (well part of the standard) must not damage a rope with a 4kN load put on them , and have shown to damage ropes (click here or here to watch examples of rope failures) if exposed to forces greater than that
- Knots (as a general rule of thumb) reduce a ropes strength by around 50%, although
- If falling onto a knotted rope the knot will absorb some of the impact, some studies suggest a barrel knot and figure of eight knot yield the lowest peak force in drop test. However once the knot has tighten up (i.e. already fallen on) that absorption is reduced
- Cavers (or climbers) exposed to an impact (i.e. a fall into a harness) of 6kN, or more, are likely to be injured
With the above in mind I set about some drop tests…
- Test mass of 80kg as this is the mass used in many of the standards (although arguably not many cavers! I weighed myself with all my caving kit on and dry I weighed 85kg 🙁 )
- To look at this I conducted more drop tests with both 100kg and 85kg but only on some of the ropes/cords as I had a limited supply and wallet!
- My SRT lanyard between my harness and hand jammer is 65cm long tied with a figure of eight at one and and barrel knot on the other. For the purpose of these tests I used the same knot configuration. Each were tied and tensioned by hand with a 3 finger width tail on each knot
- I figured it would be difficult to fall more than half the length of the lanyard (fall factor of 1) if stepping up onto a foot-loop then falling so based the tests on that
- I tested a range of rope types, cords and lanyards commonly used;
- 5mm Dynema cord (rated to 12kN)
- 6mm DMM accessory cord (rated to 8.8kN)
- 8mm Beal Antipodes accessory cord (18kN)
- 8mm English Braids cord (unknown)
- 9.6mm DMM Dynamic (full weight) rope (dynamic rope so only the impact force in 80kg FF1 drop tests given at 8.7kN)
- Petzl Spelegyca Nylon Stitched Cows-tail (22kN)
- Test method:
- A load cell was secured to a tree (via double strands of 11mm semi-static rope tied with a double Fishermans knot, which had been subjected to several drop tests prior to the final run of testing so unlikely to be absorbing any significant amount of the impacts)
- A 11mm Semi-Static rope was fixed to the load cell as the host rope, and a Petzl Basic hand jammer fixed to the rope. A new section of rope was used for each lanyard type, however the jammer was not moved or rope replaced for each round of lanyard drops
- In earlier tests I played with a variety of host rope diameters and types but the variables were endless!
- The lanyard was secured to the jammer with the mass attached to the other end of the lanyard
- The jammer was fixed to the host rope 30cm below the knot securing the host rope to the load cell
- The lanyard was loaded and haul mechanism attached to the mass (hung for in the region of 1 minute)
- The mass was then lifted so that the knot attached to the mass was horizontal to the knot at the base of the jammer (an approx fall factor of 1), and dropped
- Each cord was dropped 10 time
- Lanyards were measured before and after the test
- Peak impacts were recorded with a Rock Exotica enForcer load cell with the trigger setting engaged at 2kN, this takes 4,000 samples per second for 4 seconds (one second before the trigger value of 2kN is reached and 3 afterward, don’t ask me how!) and averages it out to 500 samples a second
Here’s a chart mapping out the results of the drop tests with an 80kg mass.
Looking at the results it’s possible to see how in all but the Spelegyca (which is a stitched nylon cows-tail designed to rip if exposed to a significant load to ensure an impact does not exceed 5kN, although its worth highlighting this only works once! as once the stitchings strips higher impacts are likely) the first impact was significantly less than all of the subsequent drops. With all rope types after a few drops the 4kN threshold (at which the toothed jammer may damage a rope) was reached.
The 5mm dyneema cord yield the highest impact, nearly 6kN.
The Spelegya remained the most consistent, but that being a relatively high impact.
Another observation was how much each of the cords (again with the exception of the Spelegyca) had stretched following the tests. This was really noticeable across all cords tested…
- 5mm Dynema Cord: Extended by 15cm
- 8mm Antipodes: Extended by 19cm
- 8mm English Braided: Extended by 10cm
- 6mm Accessory Cord: Extended by 16cm
- 9.6mm Dynamic Rope: Extended by 30cm
In all cases both knots were impossible to undo. Both knots had been tied with 3 finger widths of tail, by the end of the tests that had shrunk to on almost consistently 2 finger widths.
Any conclusions drawn from these tests need to bear in mind these were some back yard tests rather than anything scientifically robust. The results and observations can help shape our decisions together with other evidenced experiences and knowledge. That said I’d suggest…
Don’t fall onto jammers!
But if you do, and survive, don’t do it again!!
It would make sense to replace your lanyard periodically depending on use, and relax the figure of eight knot (relaxing the barrel knot is probably not a good idea if you use one as they creep) every now and then, in much the same way as we do with our cows-tails. Knots that cannot be undone by hand are likely to have been subjected to significant loads, and are therefore unlikely to absorb any impact of a fall (i.e. time to replace them!)
From the cords tested anything less than 6mm are unlikely to be suitable. Dyneema bungee cord is also not suitable. If choosing between the cords tested, based on these not particularly scientific tests, the 8mm Beal Antipodies delivered the lowest peak impact on the first drop test, closely followed by the 6mm Accessory cord, although the dynamic rope performed fairly consistently after the initial drop test. The Dyneema cord and Petzl Spelegyca each delivered peak impacts above the threshold of the toothed jammer so wouldn’t be my first choice in lanyards, and arguable dangerous.
I should also highlight all the ropes used were new, and so the slippages in knots likely to be more (yielding lower peak impacts) than older, dirtier or wetter ropes/cords. Wet ropes have shown to yield greater peak impacts, so the figures highlighted above need to take that into considerations (in reality we’re likely to see higher impacts than observers here)
There are advantages in using a single piece of rope or cord as both the lanyard between the jammer and harness as well as the foot-loop for some improvised rescues, so 6mm cord, or the Beal Antipodes would be my preference.
Make sure you have at least 3 finger widths of tail on your knots, as in loading them the tails retreated!
Be prepared for your lanyards to get longer in use, as the knots bed in and I assume cord extend. Dynamic rope extend more in use than the accessory cords (note the 8mm Beal Antipodies is considered an accessory cord), which may be an issue for some. If using dynamic rope it’d make sense to use it for the link between the jammer and harness only, and use something less dynamic as a foot-loop.
Finally, there’s a justified argument that a human falling, compared to a static mass (in this case a 60l drum filled with water and 20kg of concrete weights) is different. Tests have illustrated (click here for recent tests by Petzl) that impact tests with people (poor soles, but fair play for volunteering!!) yield lower impact that static weights, although they are still high enough to potentially damage host ropes with toothed jammers.
Again; Best not fall onto your jammers!