RopeLab: testing a proposed use of a knot.

At the risk of appearing to be a RopeLab fanboy (which I am) here’s a really great experience I had on that web site.

I’m a member of RopeLab which gives me access to concise testing reports, rope physics information, and rope-work discussion, some of it climbing related but even the other stuff I find interesting. I was thinking about the cat’s paw knot and wondering if it could be useful for shortening climbing slings.

I’ve seen it used in industrial lifting slings and was impressed by the big static loads it would take. So for setting up multi-point anchors where there’s a need to adjust sling lengths I thought the cat’s paw might work. Access to testing for members of RopeLab is one of the reasons I joined, in particular to ask about this use of the cat’s paw. I wanted to know if:

• the knot compromises the sling strength
• it would cope with a dynamic load
• the opposing twists would bend the basket of the carabiner under load
• as it takes up under load would the knot absorb some of the force, if so could it be used as a makeshift ‘screamer’

What a great asset RopeLab is for the climbing community; to be able to access evidence based analysis of climbing safety and methods. It’s hard to piece together the best practice from all the history and hearsay (not to mention all the global variations via the web). For me, in coming back into climbing after decades it’s been tricky to figure out which methods to take up, so it’s great to have a place to go for sound advice.

cats-paw-knot
Cats paw knot unsafe for climbing – do not use it to shorten slings

RopeLab did some actual drop tests on slings with the cat’s paw knot, replicating a dynamic load, as-in a fall onto the sling with the knot. Pretty extreme, but it’s the kind of loads that are possible in certain situations, you know, those certain situations that accidents are made of. Now in my head the knot seemed like a good idea but the tests proved that the dynamic load slid the knot along the sling causing it to break: load plus heat generated by friction. This failure occurred in nylon and dyneema slings. There is a great slo-mo film of the tests is in the report, you can actually see what happens.

So the cat’s paw is unsafe for use in climbing, the end of the test report gives a clear conclusion (even if I get a bit lost in the physics of the test data). This result gives me a new perspective on internal movement and friction-heat in other scenarios of ropes, knots, and slings. I guess that’s where carabiners help out. The small cost of RopeLab membership might just have saved my life; how good is that?

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