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.
