The weakest link
I have been doing some testing recently to see how to avoid rope breaks since it is the weakest link in our suspension rig. Other than rigger error, if somebody gets dropped, it will be the rope that let’s them down, literally.
At first, I thought finding how much tensile load a rope would take was the key to understanding the risks. After all, rated rope is measured in breaking strength. However, we know that rope is weakened by knots and other factors. So, the important thing is to discover what makes it fail due to the way we use it and how to minimise that risk. What my tests try to do is to see how rope (jute, in this case) fails in real life during suspensions.
My rope test rig
I recently bought a professional 1,000kg load cell and control module. This records an accurate peak load, i.e. how many kg it took to break the rope. This is anchored to one end of a scaffold tube frame and, at the other, there’s a hand winch, such as you often see in dungeons. This allows various configurations. The most relevant being when I rig it as for a suspension: Rope tied off to load cell, with the usual suspension bight. Rope goes through a carabiner, back through the bight to the winch. Crank the winch till the rope breaks and note the reading.
Main takeaways, so far
1) The bight itself almost never fails during a suspension lift. See linked videos to see why.
2) The tensile strength of your rope is largely irrelevant as it can break at much less
3) It is the bight that usually causes the failure. Minimise friction, e.g. use a carabiner or double bight
The fuller story
I’m compiling my findings here as this is being discussed in several threads on Fetlife so it’s getting a bit hard to follow. Anyway, I have heard there’s at least one rope person not on Fetlife 🙂
Disclaimer: I’m not a materials test engineer but MP, another Fetlifer, is and has separately confirmed my general conclusions