How do knots affect cord and webbing strength?
Tying a knot in rope, cord or sling decreases the strength. (While it varies depending on the material and the type of knot, a conservative rule of thumb is about 50%.)
Is there much of a difference in strength loss between cord and webbing?
What about nylon vs. Dyneema?
What about a standard anchor configuration with a knotted sling?
Do multiple knots in the same cord decrease the strength more?
All interesting questions, let's find some answers!
Strength loss from knots in webbing and cord
Here’s a nice diagram drawn by IFMGA Guide Georg Sojer @sojercartoon from an article by German Mountain Guide / Bergfuhrer Chris Semmel of the German Mountain and Ski Guides Association (“Verband Deutscher Berg und Skiführer” or “VDBS”). It’s from a German climbing magazine, here’s the original.
Interestingly, it shows that cord is slightly stronger than webbing. Also note that when a larger loop is doubled over, even with a knot, the strength is twice that of the unknotted material.
image: https://www.outdoor-magazin.com/klettern/basiskurs-alpines-klettern/
What about nylon vs. Dyneema?
Great question. Lots of people think if you tie a knot in Dyneema, it's somehow magically going to break. Maybe in a drop tower with a concrete block, but highly unlikely in a real world climbing scenario. Some testing from the German company Edelrid gives us some answers.
Edelrid tested the strength of both “polyamid” aka nylon, and “high-density polyethylene”, aka Dyneema, in 3 ways: 1) a single strand, 2) in a sewn sling, and 3) a sewn sling with a single overhand knot in the middle.
Here are the results.
You can see the strength of the nylon sling is higher in every case. In terms of strength by weight Dyneema might be stronger. However, the nylon sling is made of more material and it stretches about three times as much, so it can hold a higher load.
The Dyneema sling with an overhand knot broke at 11.2 kN. This is still significantly higher than anything you would see in a recreational climbing scenario, so I personally don't have any problems doing it. Having said that, if you want to use your slings at full strength, avoid tying knots in them when possible.
screen grab from: https://youtu.be/0SqHwymGxfM
Here's a nice video from Edelrid showing their testing procedure. (I recommend browsing through the entire series of knowledge base videos from Edelrid, there are some gems in there.)
My friend Ryan Jenks, founder of the YouTube channel HowNOT2 did some very interesting testing comparing “tech” cord (Sterling VT-X) with a Dyneema core, to standard nylon cord.
The Minimum Breaking Strength (MBS) the tech cord was significantly higher than a comparable diameter 6 mm nylon cord.
MBS of VT-X cord: 15 kN
MBS of plain nylon 6 mm cord: 8.8 kN
The VT-X cord is rated a bit less than twice as strong as the nylon cord.Ryan broke each cord 5 times, with a figure 8 knot on each end.
8 to 8 average, VT-X: 8.6 kN
8 to 8 average, nylon 6 mm cord: 6.6 kN
The VT-X still tested a bit higher, but not very much higher. Then, he looked at those break test numbers as a percentage of the MBS.
VT-X: 57.6%
nylon 6 mm cord: 75.6%
Isn't that interesting? Tying figure 8 knots in the VT-X cord breaks about half of the rated strength, but the same knot tied in nylon cord breaks about 3/4 of the rated strength!
This tells us that tech cord loses a larger percentage of the rated strength when you tie knots in it.
If I understand Ryan's explanation of this, this is because the tech cord, being very static/non-stretchy, does not elongate at the outer radius of the bend in the knot. The nylon, because it is stretchy, loses less of its strength in the knot. Cool!
So, for me, the takeaway is that 6 or 7 mm nylon cord in just about any recreational climbing situation is probably gonna be fine.
Having said that, using higher strength tech cord for anchor building gives me a warm fuzzy feeling, extra kN and it only cost like $1.20 a foot, so I'm gonna still roll with my V-TX cordelette, which I especially like for snow and glacier travel.
See all the results at his video below.
How do knots weaken slings in standard anchors?
Here's a common anchor scenario. 120 cm Aramid / kevlar sling rated to 22 kN. It’s doubled, with one arm clipped to each bolt. A overhand knot is tied for the master point.
How strong is this anchor?
Scroll down for the answer.
The answer is B, around 22 kN, the original strength of the sling. Why? Because the sling is doubled before the knot is tied.
Doubling the sling also doubles the strength to about 44 kN. Adding a knot reduces the strength about 50%, down to around 22 kN.
Same principle applies to a girth hitch anchor, here are made with Dyneema.
Next time you hear somebody saying “OMG, don't use a girth hitch, it weakens the sling by 50%!” remind them of this.
This is confirmed by the (always awesome) testing by Ryan Jenks at HowNot2.com. His testing showed the girth hitch master point breaking at around 26-28 kN. (Check out his video here, start at 7:10 and go to 9:00.)
A couple of screen grabs are below.
Do multiple knots weaken cord more than one knot?
Let's have a look at some screen grabs of gear testing from the Alpine Club of Italy (Club Alpino Italiano, or CAI). The CAI has been break testing gear like this for decades, and they have some interesting videos about their studies.
I paid to have English subtitles added to one of them, and these screen grabs are from that video. You can watch the video below, the 7mm cord breaking is in the first few minutes.
(Note to engineers: the values shown in the video are in “kgf”, aka “kilograms-force”, which is kind of strange. I converted the values to kN, because that's a more common unit of measurement.)
Full strength test: no knots, broke at 13.7 kN
Note how the cord is wrapped around a large, round metal drum. This means there are no sharp bends in the rope that are created when you tie a knot. The drums allow the full strength of the material to be tested.
This is the rated strength of the cord. If you're shopping for 7mm cord, and the technical specifications say that it's rated to about 13kN (which it is), this is what they're talking about, and probably how it's tested.
(If you're curious about strength ratings for all other kinds of climbing gear, I made a spreadsheet that covers most of them. You can see it here.)
One knot, broke at 7.7 kN.
That's roughly half the strength of the unknotted cord.
Four knots, broke at 7.6 kN
The strand with 4 knots broke at about the same force as the strand with 1 knot. This clearly shows that multiple knots do not decrease the cord strength multiple times. (Yes, it's pretty unlikely you would tie something like this in real life, but it proves the point)
One knot tied in a loop, broke at 17.8 kN
Let's look at a more realistic scenario: a strand of cord tied in a loop with a knot. Good news, the most common way to use cord in climbing is super strong!
Cord tied into a loop, then one knot added to loop, broke at 15.2 kN
This one is a little strange. I would've expected this to break somewhere around half of the cord tied in a loop, or around 9 kN. But it's clearly a lot stronger than that. This is why real life testing is so interesting compared to the theoretical stuff!
