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.
