In reply to nb:
> What concrete evidence is actually out there about a well-tied fig8 failing catastrophically?
By the dearth of answers to my questions, I can only conclude that either (1) everyone has got bored and f*cked off or (2) the video below is the most reliable source of information we have concerning the overhand vs fig8 debate. It shows a fig8 capsizing once at 167kg, again at 535kg and then blocking until the rope breaks at 750kg kilos. The overhand rolls off the end of the ropes with a high value of 530kg. (Remember when abseiling your weight will be equally shared between the knotted side of the ropes and the unknotted side, so you can basically double these values.)
Start watching at 3min30.
As for accident reports, I have yet to find an attributable eye-witness account of a well-tied fig 8 knot rolling off the end of a rope.
In the reports I found which were critical of using a fig8 to join ropes, it is only ever assumed that the knot was tied correctly. The conclusion drawn is that the knot must be weak when some rare and poorly-understood circumstances arise.
Now personally, if someone has done 1000 abseils on a knot and then on the 1001st it fails, I don’t instinctively think, ‘the knot must be flawed’, I think, ‘they must have f*cked-up badly’. Maybe this thought-reflex comes from a deep awareness that I too may f*ck-up badly one day.
Interestingly when a French climbing instructor fell to his death while rappelling at a crag in 2001 (the only French example I could find), the authorities concluded that the knot must have been tied badly, rather than that there was an intrinsic problem with the knot. Maybe the French are more willing to accept the fallibility of the human condition - they did define existentialist angst after all!
So, when these accidents started getting reported around 2000, the French response – coming through federations or government institutions - focused on educating people on how to tie the knot properly. The UK/USA response - mainly generated by the media or by hearsay - simply warned that the knot was dangerous. Interestingly this excellent BMC article on abseiling does not issue any warning about using the fig8 to join ropes.
https://www.thebmc.co.uk/abseiling-not-the-quickest-way-to-reach-the-ground
All this begs the question, ‘could this whole death-knot theory be a huge, collective figment of the imagination?’ Martin Hore’s comment certainly raises issues about the reasoning behind it.
While the ENSA test should by no means be regarded as ‘the word’ on abseil knots, it is the most credible info I have seen so far and seems to corroborate the results here
https://user.xmission.com/~tmoyer/testing/EDK.html but with a very different conclusion.
It will be interesting to see the results of the BMC tests, particularly with regards to icy ropes. There are 2 issues that I see with icy ropes:
- There is less friction and so rope slippage will probably occur at lower loads.
- The knot will be more difficult to tighten.
I use my teeth to tighten knots on icy ropes. The jaw is amazingly strong and teeth grip so much better than fingers (I’ve even used them on my ice axes when my arms have given out!). There may even be a case for tying a back-up knot sometimes, although I’ve never felt the need to do so myself.
PS Here’s an even easier way for people to do their own DIY testing. Tie the end of a rope into an anchor (could even be a strong branch), tie a 20 cm fig8/overhand loop just underneath and then another loop just underneath again. Hang your mates from the lower loop (you only need 2½ mates now that all the weight is on a single length of rope). The middle loop will act like an offset fig8 joining 2 ropes but you stay attached to the same length of rope – easier and safer!
If you want to properly load the knot, tie one end to a tree-trunk and the other to the tow bar of your car. Don’t forget to take video!
To test icy ropes, take a short length of rope, soak it in water, shove it into the freezer compartment of your fridge, wait a few hours and off you go.