15 Acres of Adventure in the Heart of Leicester

Tuesday, 15 December 2015

How safe is rope?

written by Chris Murnin, Deputy Manager and Head of Ropes at Leicester Outdoor Pursuits Centre

“What if the rope breaks?” is one of the most frequently asked questions once you get a group onto the high ropes course, this is always met with the answer of “It won’t”, but rather than using the classic cliché your parents used time after time of “because I said so”, I thought I'd explain a little bit behind the science of the ropes we use.
The most common rope used at LOPC is what is called “Dynamic” rope of a thickness around 10.5mm. What makes this rope dynamic is its ability to absorb shock by stretching a little bit, (around 10-15%, but it is designed to be able to stretch up to 30%).
But what people really want to know is how strong the rope is. Rather than spouting off numbers and statistics I like to put it into terms of what it could lift. Well we know it can hold a full grown adult, but could it hold 2? Yes. What about 5? No problem! In fact we could support an entire group of 10 adults on one strand of the safety rope, but as you can imagine this would be impractical and hugely uncomfortable. The standard is around 22-30kN or 2200-3000 kg, that’s more than the weight of your average family car!

So now you are reassured by that knowledge, I'm going to drop a bombshell, every time you put a knot in a piece of rope it makes it weaker! The standard mountaineering knot- the figure of 8, reduces the strength of the rope by 20-25%, the Bowline (not used at LOPC) by as much as 25-30% and the clove hitch 35-40%!!
BUT!!!! Before you start panicking, it’s worth noting that even a 40% weakening on a 22kN rope leaves you with over a tonne to play with.
And it gets better, modern engineering practices dictate the use of a 'Factor of Safety' of at least 2.0 for anything that it is designed for, essentially, saving somebody’s life. This means that the ACTUAL design strength of your rope is closer to 60kn. This translates to 6000 kilograms.
So next time you get tied onto a rope, you can rest assured it's UNLIKLEY that it will break.