So you’re looking for a new rope for your next climbing adventure. How do you make sure it’s strong enough for what you need.
A climbing rope’s strength is measured in units of force called kilonewtons (kN). A kN is equivalent to about 100kg at rest, so a 100kg climber hanging from a rope is putting 1kN of force on the rope. The rope’s packaging specifies how many kN it can withstand.
Here’s how to figure out what all those numbers mean, along with how different elements of the rope affect it. We’ll also go over the difference between single ropes, half ropes, and twin ropes.
kN and Impact Force
You may have noticed that the above definition of a kilonewton specifies that it’s 100kg at rest. That’s because it’s a unit of force rather than a measurement of weight. The force a load applies to a rope changes as that load moves.
The rope changes, too. Ropes used for climbing are usually dynamic ropes. That means they stretch and change under stress: the diameter shrinks and the length increases. This elasticity helps spread the energy of the force throughout the rope and stretch out the amount of time the fall takes. That’s part of how much impact force a rope can take.
1kN is 100kg hanging at rest, but what happens if it starts to fall? Suddenly, that 100kg is putting a lot more force on the rope, depending on how quickly it’s falling. Think back to science class for this one: f=ma or force equals mass times acceleration. The mass is 100kg. The acceleration is the change in its velocity.
So if you were hanging onto your climbing surface, and your rope was a little slack so that all your weight was on the climbing surface, suddenly letting go would be an acceleration that put the force of gravity suddenly against your rope. Your mass would be multiplied by the acceleration of gravity to get the force of the fall.
The equation f=ma usually deals with grams and Newtons rather than kilograms and kiloNewtons, so doing this equation in relation to a climbing fall might require some conversions. On the bright side, it’s an easy conversion to make: one kilogram is the same as a thousand grams, and one kilonewton equals a thousand Newtons.
Now we need to cover the fall factor. When a climber falls, the fall factor is the proportion between the length of their fall and the length of the rope between the belayer and climber. The highest fall factor is a factor of 2, which is when the climber falls twice the length of the rope.
To get the fall factor, you divide the fall length (how far the climber fell) by the rope length (the length of rope between the belayer and climber). Well, that’s the theoretical fall factor.
For the actual fall factor, you need to replace the rope length with the effective rope length. Effective rope length is how much rope is in play. Friction in belay points prevents the rope from stretching across its whole length. The fewer belay points, the longer the effective rope length will be, and therefore the smaller the fall factor. (Source)
So, for the actual fall factor, divide the fall length by the effective rope length.
With all that math out of the way, here’s what you should be able to expect from a rope that’s in good condition.
Single ropes are tested with 80kg loads with a fall factor of 1.77. They have to withstand 5 of those falls in order to pass the safety check. The first of those falls has an impact force of 12kN. That’s the maximum impact force for single ropes. Twin ropes have the same maximum impact force, and half ropes have 8kN as their maximum impact force. (Source)
Diameter and Length
Among all of a rope’s features, its diameter plays a big part in how much it can hold. The bigger the diameter, the more weight it can hold. Most single ropes have a diameter of 10.5mm, but they can vary between 9.4mm and 11mm. Half ropes are usually between 8mm and 9mm, and twin ropes are between 7mm and 8mm.
However, not are ropes are made the same way. A 10.5mm rope from one brand might have a different weight capacity than a 10.5mm rope from a different brand, even if they have the same length. Every manufacturer is slightly different, so the best way to find out the strength of your rope is to know its impact force.
Does a Thicker Sheath Make it Stronger?
One of the things that makes brands different is the way they attach the sheath of the rope to the core. Every rope has two parts: the core is the center of the rope, and the sheath is the outer layer. Typically, most of the rope’s strength comes from the core, but the thickness of the sheath varies, just like the rope’s impact force varies.
But does a difference in sheath mass entail a difference in rope strength? Well, no, not really. At least not as far as impact force.
When comparing three ropes that were all 9.8mm in diameter, it turned out that each of the three had 8.8kN of impact force. Two were Sterling ropes, and they had a sheath mass of 35%. The other was Edelrid, with a sheath mass of 40%.
Here’s another comparison, this time with two ropes that were 10.2mm in diameter. The Blue Water one had a weight of 64g/m and a sheath mass of 34%. The other rope was from Beal, with a 66g/m weight and 35% sheath mass.
These data points show that not every sheath is equal. Every manufacturer uses a different sort of weave to make their ropes, and different weaves contribute a lot to the diameter and density of the rope, and thereby its strength. So, in terms of weight capacity, the sheath isn’t a reliable variable.
What the sheath does do is make the rope more durable. A rope with a larger sheath mass can withstand more wear and tear. For example, those Sterling ropes with the 35% sheath mass could withstand 6 UIAA falls, while the Edelrid example with 40% sheath mass withstood 7. So while it might not hold more weight, a thicker sheath will make your rope last longer.
You have to be really aware of how many knots you’re tying in your rope because knots can decrease the amount of strength your rope has.
A common knot in rock climbing is the double figure-eight knot. A rope with this knot only has between 66% and 77% of its full strength. The barrel knot, which is used to tie on the carabiner, reduces the rope to between 67% to 77% of its strength. Double overhand knots can bring the rope’s strength down to 58% to 68% of its strength.
Basically, any knot will weaken a rope, but this is less of a rant against knots than it is to tell you just to be aware of what the knots can do to your rope. The knots won’t weaken your rope to the point that it’s unusable: the rope is stronger than the knots you can tie in it, so it’s still safe to use. It can just support a little less.
Falls and Rope Testing
In addition to stating the impact force, the rope’s packaging should also show how many falls it can take. I’ve seen single ropes that can take 6 falls, another that can take 7, and another that could either take 8 or 9.
Every time you experience a fall, your rope is weakened. That’s why the ropes are tested to see how many falls they can withstand in a laboratory setting. Single ropes and half ropes have to withstand 5 falls, and twin ropes have to withstand 12. These standards are set by the UIAA, which is the International Climbing and Mountaineering Federation.
They test the rope by dropping a certain weight with a fall factor of 1.77, which means the distance from the drop point to the low point of the fall is 1.77 times the length of the rope. That sort of fall won’t usually happen in a real-life climbing situation. The rope is tested until it breaks, and that’s the number that ends up on the label as the UIAA fall rating.
But how did they come up with that process? Well, it wasn’t always like that. in the 1970s, they determined that the worst-case scenario for a fall on a real-world climb would be a factor 2 fall with a mass of 80kg. So, they decided that every rope should be able to withstand at least one fall like that, and they started testing ropes that way. (Source)
This applied to half ropes and twin ropes as well as single ropes, where half ropes and twin ropes were tested individually. That is because when a climber using half ropes or twin ropes falls, they fall primarily on only one of the ropes. The other rope offers some support, but it’s not an even spread. Therefore, every rope has to withstand a worst-case scenario fall.
The problem arose when they realized that testing a rope once wouldn’t be enough to ensure safety. To fix that, they decided to start testing the ropes with multiple smaller falls. Half ropes and twin ropes are tested with loads of 55kg. Single ropes are still tested with 80kg loads, but a 1.77 fall factor is different enough from a factor 2 fall that it’s already a significant decrease.
Different Types of Rope
So far, we have gone over a lot of things about ropes, but I’ve referred to them as “single ropes,” “half ropes,” and “twin ropes” without really saying why or how they’re different. Let’s tackle that now since it is an important distinction when it comes to climbing in practice.
Single ropes are the thickest, and they are used by themselves, hence the name “single.” This is the simplest rope system to work with because there is only one rope to keep track of. For that reason, it’s the most common rope used in the majority of climbing situations.
Since they’re always used by themselves, with no other rope to spread out the impact with, they are designed to hold multiple falls. Along with that lack of redundancy, the disadvantages of this type of rope are that you can’t do a full-length rappel unless you have two ropes, and that rope drag can present a problem on wandering routes unless you use a half rope system.
A half rope is made to be used in tandem with an identical rope, in a system where each rope is anchored to its own point. Each half rope can withstand a fall on its own, but they are thinner and less durable than single ropes.
This is the most complicated system to learn and work with. It’s hard to determine how much weight one of these ropes can hold by itself because it’s supposed to be used with another, and the weight is distributed unevenly between them.
Half rope systems are popular for difficult rock climbs because they offer good protection and redundancy.
A twin rope is a thin rope that’s used with another rope at the same time. While it sounds similar to a half rope, there is a big difference in the system setup: both twin ropes are attached to the same central equipment. Both ropes bear part of the load, but the amount of the load either rope is carrying can shift during the climb.
Unlike with a single rope, you can do a full-length rappel with a twin rope system. You also get the redundancy of having two ropes bear the same load. That, plus the fact that they make the lightest double rope system, makes them great for alpine climbing.
One important thing to keep in mind is that your rope is only as strong as your weakest bit of gear. Even if you had a rope that could withstand 50kN of force, it will do you no good if your belay gear can only take 5kN. In that hypothetical situation, anything more than 5kN would overwhelm the system. The belay would break and the rope would be useless.
So, don’t stop with your rope: make sure your climbing gear is in good condition as well!