Knots decrease sling strength - one may surprise you
image: Jürgen Reinmüller, @alpinstil.at
This nice diagram is from Jürgen Reinmüller, @alpinstil.at.
Jurgen is an Austrian mountain guide who is publishing a how-to technical book, with the publication date planned for spring 2026. (To get an reminder email when the book is available, sign up through his Instagram Bio.)
He posts nice diagrams from the book on his Instagram, and this is one of them.
This should be fairly common knowledge for most climbers:
A sewn sling gets its full rating (22-ish kN) when clipped end to end.
If you basket hitch the sling, you share the load among four strands rather than two, so the strength is approximately doubled.
If you tie a knot in the sling (or more than one knot, as it turns out) you reduce the strength by about half.
So far so good . . . but that last example that made me scratch my noggin when I first saw it.
That seems like it would be closer to 50%, because you have 2 knots in the sling, right?
Well, turns out this diagram is correct. You're putting the load on one strand rather than two. Then, you're tying knots in that one loaded strand rather than two. Taking half of the material and reducing the strength by half gets you to around 25%.
How do I know Jurgen is right? Because I sent it to my buddy Ryan at HowNOT2, and he was nice enough to do a break test for me.
The video is below.
Ryan ran several tests. Two of them used a dynamic sling, which slipped in the testing machine so he didn't get a good result. The video below is of a nylon sling, which is a little grabbier and didn't slip.
The normal breaking strength that sling should be around 24 kN if you pull it it end to end; it broke around 6 kN.
Jurgen, thanks for teaching me something new, and I definitely look forward to reading your book.