Update: Carabiners can break your suspension rope!

Posted on Sat, 20 July 2013


In line with my claims of being "only human" and being prepared to admit my mistakes, I accept the initial tests were inaccurate due to stupidly measuring the load between the fixed point and the carabiner, not between winch and 'biner. OK, it's official, I'm an idiot! At least, I spotted it before anyone else pointed it out me :-) I blame it on a school curriculum that didn't include physics. Of course, if I happen to need to strike up a conversation with a dead Roman in his native language or become a Finnish newsreader (apparently, Finland have been quoted as being "Like the boy at the party with cheese straws stuck up his nose, it has been caught doing something vaguely disturbing", a simile which amuses me), I might be better equipped.

As I had already proved in my earlier experiments that the 'pulley' creates friction which absorbs some of the energy. Thus, the load is greater on the side from which the energy is exerted. Doh! These experiments showed that even a slow lift of 10kg needed you to input around 17kg to lift it over a 'biner and this increased to around 24kg in a fast 'drop lift. It also showed that a well lubricated rope over a large diameter ring needed less effort than a dry rope over a carabiner.
 
Anyway, I ran some further tests using what I expect to be my least strong rope, bar the 4mm, Asanawa 5mm, i.e. the very loose laid Japanese style rope that is barely 5mm. I carried out 5 tests over a 'biner. In each case, the rope broke at the 'biner at between 57kg-74.1kg. I went to repeat this using a wooden ring. The first run broke the rope at the ring at 106kg but two subsequent tests caused the rope to fail at the knot at 82.5kg and 101kg respectively. At that stage, I abandoned testing as I will need to find a way to prevent to rope breaking at the knot to get true comparisons. Although, one must consider that any rig is only as strong as its weakest point.
 
I recall a heated debate on this point with a student who was concerned that the chains forming part of a suspension rig where not 'rated'. In spite of us applying the 'two big men' test to a single piece of chain with no distortion, let alone signs of failure, he was worried that it was dangerous even though we had doubled the chain. Whilst he claimed various qualifications in physics and rigging saefty, he failed to appreciate that he should be a lot more worried by the use of natural fibre rope of unknown strength, i.e. the weakest link. He attempted to justify this by claiming that the rope would fail tantalisingly one strand at a time just like the movies, thus giving the hero time to save the day. No, when natural fibre goes it does so with a bang. Even when I demonstrated this, he claimed it to be "all smoke and mirrors". Hypocritically, the same gentleman had photos on his Fetlife profile showing one of his suspensions from an office lighting system which, he claimed, he had tested with a couple of chin-ups...so a good reliable scientific test, then! Bear in mind, that the load created on the suspension point will be much more than the load lifted and many times more when friction or fast acceleration is a factor, as any 1st year physics student should know.
 
The good news is that the actual breaking load was around 60kg or better on a single rope when the measurement is taken in the correct place between 'pulley' and the 'pull', even with what I suspect to be the least resilient 5mm rope in my range. If, and it is an IF, we can assume this gives us 120kg on doubled rope in perfect condition, it still leaves a very small margin even with a Japanese size model. I expect tighter laid rope, like my Tossa, to perform much better. All will be revealed when I get around to re-testing. Anyway, I will certainly not be doing dynamic lifts without a much stronger main line regardless of Nina being much the same weight as some of the lighter Japanese pros. Take what risks you like with your own safety but it is negligent to gamble with your partner's life. If they are not aware of the factors, it cannot be said to be 'informed consent'.
 
Using a ring clearly reduces friction and the larger radius is less damaging to the rope, to the extent that the main weak point becomes the knot thus reducing the chance of a break. Given that the knot now becomes the weakest point in some cases, it might be more realistic to factor this in to give a real world result. What use is it knowing at what point the rope would have broken had we eliminated the knot? I can't think of a knotless way to attach a suspension line. Until we all start utilising such an attachment, I guess we should take account of the weakest point in reality as that's what leads to an accident, not theoretical ones.

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