FAQ- Climbing Ropes explained
By Jon Griffith
The climbing rope is the founding material that makes our passion for the mountains and crags possible. Quite apart from the unity and bond that it represents between climbers it also a quite literally a life line that serves to help us push our limits in the mountain safe in the knowledge that we have a back up for when axes pop, forearms cramp, feet slip and gravity wins yet again. From clothes lines and thick hemp ropes to the super light 7.7mm aptly called ‘Ice Thong’ we have and will always trust our lives in them. However, ask your average climber what the fall rating on his rope is and what the Impact force means and he will probabably guess a number between 5 and 10 for the first and look at you dumbfounded about the latter. The UIAA standards allow us to exhibit a kind of blind faith in ropes allowing us to concentrate on what concerns us the most of all- the price! Having said that I am totally at fault too as it took me years of climbing before I even started to think about the stats that come with a rope. For me the two most important things were weight and price and the rest…well that was for the UIAA to decide right?
Luckily for us the UIAA has pretty strict guidelines on not only climbing ropes but all climbing gear. As it might interest some of you to find out exactly what those numbers on your rope actually translate to I thought I would try and explain them in easy to digest english!
Back to Basics
So where do we start? Well the most important thing to think about when it comes to ropes is the force that you generate when you are falling. Whilst Sir Isaac Newton may have realised the power of gravity by an apple falling on his head, we tend to only realise its full effect as we are plunging straight down a cliff face.
For every kilogram that we weigh (ok of mass) we exert a force on the ground that we stand on of 1kg x the strength or force of gravity. The multiplying factor here is actually 9.8 which is close enough to 10 for my maths. So if you weigh 70kg then the force that you are currently exerting on the ground below you is 70 × 10= 700 newtons. In the climbing world we tend to use kilonewtons instead which is a 1000 newton measurement- hence a 70kg person can also be said to exert a force of 0.7kn. To put it into context. If you were hanging directly off an anchor, for example, you would be exerting a total force of 0.7kn on it.
“Ah” I might hear you cry “I’ve seen those ratings before” and indeed you have. Splattered all over our climbing gear are kn ratings. Our climbing gear is built to withstand certain forces that you can exert on it and by thinking a little more in the future you can start to use your gear with more care and confidence. However I’m not going to get bogged down into that now.
So we’ve covered what kn means.
What does a dynamic rope do?
The reason we climb with dynamic ropes is because they stretch. Due to this stretch they absorb a lot of the force of the fall- and I really mean a lot. This makes a huge difference to the safety of the climber for two reasons.
The rope is the very first thing in the ‘chain’ if you will. When you fall, the rope absorbs part of the force, then the left over force is transferred into your top runner. The fact that your rope has absorbed such a large amount of the force means less is transferred to your last bit of pro meaning that it has a higher chance of holding (because at the end of the day this is the weakest point in the chain). Secondly the breaking force of the human body is 12kn. If your body is subjected to shock forces above that then it will most likely break in some kind of back or neck snapping way. A dynamic rope helps avoid this. These two reasons are why people never lead climb with static ropes.
The International Union of Alpine Associations (UIAA website) is the governing body which tells us whether or not gear is safe to use and until what force it is safe. It has standard tests that certain gear has to pass to be able to achieve the UIAA seal of approval. To be honest gear lacking in the UIAA, CE, or Sigma 3 symbols are just not worth buying.
For a rope to pass the UIAA test it has to undergo 4 specific tests…
1. Fall Rating
Half ropes have to pass a minimum of 5 controlled leader simulated falls. These falls are factor 1.77 falls so they are tested to close to the maximum fall that a climber can take (a factor 2). For those interested, a 55kg weight is dropped from a height of 2.30m above a preclipped karabiner. The amount of rope extending to the simulated belayer from the karabiner is only 30cm so that the set-up looks like the image on the left.
The weight is dropped with a 5 minute rest inbetween tests. The diagram on the side is a little out of date though as the test has now been modified so that the rope passes over a 0.75mm metal edge instead of through a karabiner. 3 sample ropes are tested and the weakest one of the three becomes the fall rating. The UIAA has a minimum fall rating of 5.
It should be noted though that a factor fall of 1.77 is very high and chances are that you will never even have such a high factor fall all your life. So do not interpret this rating as the maximum amount of times you can fall on a rope before you have to chuck it. Also remember that taking huge whippers do not necessarily induce large fall factors (it is often the exact opposite) so dont just think that because you have taken 3 whippers on your rope that you should be starting to think about retiring it. For more information about fall factors you can click here
Obviously, though, you will be looking for a rope that has a high fall rating as, in theory, it will last you longer. Theoretically thicker ropes will have a higher fall rating than thinner ropes, as you would expect.
2. Impact Force
The impact force measurement is the resulting force that is transfered onto the end of the rope- ie you. If you look at the diagram above again, you can see that the peak force generated by the 1.77 factor fall can be no more than 8Kn for a half rope. Basically the lower this figure is the better, not only because it reduces the force on you but also because it will reduce the force on the top runner- ie the lower the number the more force the rope absorbs. The flip side of this is that the rope will invariably have to stretch more to absorb more force. Now whilst this might not sound like a bad thing it can be. Situations can arise where you want the least amount of stretch in the rope- eg falling close to the ground, when top roping, etc. This is a point that is worth thinking about. If you are used to sport routes then you arent going to be worried about minimising the force on your top runner as it’s a bolt, and in addition the more the rope stretches the more you are going to have to climb back up if you fall. On the other hand if you climb alot on badly protected terrain then the extra stretch may not be a worry if it helps keep the top runner from ripping out.
There are two measurements of stretch. One is a static test- ie if you simply weight the rope and measure how much percentage stretch there is. The other is a dynamic test which measures the maximum stretch when the rope takes an actual simulated lead fall on it. The maximum amount of dynamic stretch allowed is 40%, and static is 12%. To put this into context if you are seconding on 50m of rope and weight it, if it has an 11% stretch then you will ‘drop’ down 5.5m- now that’s quite a long way down!
Stretch test, image © UIAA
4. Sheath Slippage
Ropes are made up of a core and a protective sheath that covers it. Poorly constructed ropes can end up with the core and the sheath not stretching in sync with each other (ie not acting as a single unit but more like two seperate ones, which is essentially what they are). However this leads in ‘lumpy’ areas of the rope as the sheath gets bunched up in certain parts and stretched out in others. The reason why this happens will be explained further on.
Testing the sheath slippage, © UIAA
Putting it all together
So now that you understand the different properties of a rope its time to put them all together. The first step to take is to quickly explain how a rope is made- even if you think you know this you actually probabaly dont so bear with me.
© Wild Country
All ropes are constructed in pretty much the same basic manner. It’s obvious from the diagram above how you start off with the basic yarn which is composed of multiple filaments and then keep adding and twisting until you get to the ‘core’. However the really interesting part comes into play right at the beginning of the process. The reason ropes are dynamic is because the filaments that form the yarn are twisted and shrunk. Its the equivalent of taking a piece of string and holding one end tight whilst twisting the other. After a time it will start to fold back on itself and it gets much harder to turn (see image below). The string itself will appear to have shortened but if you then load it the amount of extra stretch it takes is much higher than if you hadnt twisted it. By applying this principle to the filaments, you can radically alter the dynamic properties of any rope that you make.
Mike Kann was kind enough to explain this further (and this is the really important bit):
The impact force is a measure of how much energy is absorbed by the rope. As energy absorbtion occurs over a time period (even if its nanoseconds), you can reduce the force transmitted to the rope by lengthening the period over which the energy of the fall is absorbed. This is the same theory as used in crumple zones. The fact that you lengthen the absorbtion period means that impact force on the piece of protection is reduced, making the piece less likely to fail. Seems like a win win situation until you come to actually build the rope.
So you’ve got your twisted core. Easy. The problem is that you then need to hold the cores together with the sheath. The sheath needs to elongate by the same amount that the core elongated otherwise the cores will not be able to fully extend. So you need to weave the sheath to cope with this – which is where the problem lies. The easiest way to do this is to make it looser. The problem with making it looser is that you increase the core/sheath slippage and you end up with floppy bits on the end when you abseil on the rope. Secondly it means that it gets abraded more quickly. Making the sheath fibres more slippery helps (which is how Mammut have made the Serenity so small – new Teflon coating) so dry treatment helps. However you will always end up with a rope which does not last as long, even if you manage to find a weave that prevents excessive core slippage.
So there you have it, a rope with less impact force should in theory be less durable due to its sheath construction. That is the compromise that you have to take. So if you find yourself leading often very dicey ice pitches then a half rope with a lowest impact force is what you are going to be looking for. Luckily ice does not abrade ropes very much and so you can get away with it, however if you find yourself doing alot of rock climbing then you should look for a rope that is a bit hardier or else it will not last very long!
After all that what are the compromises?
As always in the climbing gear world you have to compromise. So here are the rope ‘compromises’ you should be thinking about:
weight vs fall strength
weight vs dexterity/ thickness of the rope
fall impact vs durability
fall impact vs stretch
Special thanks to Mike Kann for his invaluable engineers mind and to the UIAA