How strong is it?- KNOTS (Part 1)
This section of my website is where I satisfy some of my curiosities (and hopefully some of yours) about the actual breaking strength of anchors, gear, and knots.
If you're new to this part of my website , I suggest you start at the overview page, and then bounce back here.
What's on this page?
How strong is a Dyneema sling girth hitched to a belay loop?
How strong is a poorly dressed figure 8 knot?
How strong are friction hitches?
How strong is a basket hitch in Dyneema?
How strong is a stopper knot / barrel knot as a rappel safety?
How strong is a figure 8 knot in climbing rope, tied end to end?
How strong is a “shoelace wrap” as a tree anchor?
1 - How strong is a Dyneema sling girth hitched to a belay loop?
More than 13 kN.
Source / testing: YouTube, How do you know yours is safe? (about 12:40)
This is a pretty standard way that many people attach a personal tether to their harness. Lots of people freak out about this, often citing the Todd Skinner accident. Is it dangerous? The belay loop broke at 13+! The sling was fine!
Takeaway:
Avoid permanently girth hitching a sling or tether to your loop. Remove it when you're done for the day so you can reattach it to a slightly different spot next time.
Inspect your harness regularly and retire it when it shows any significant wear.
2 - How strong is a poorly dressed figure 8 knot?
About 12.4 kN (properly dressed, about 12.7 kN)
Source / testing: YouTube, Is your figure 8 knot going to kill you? (about 2:52)
Climbers often obsess with tying the perfect figure 8 knot, one that doesn't have any crosses or twists in it. Giving proper attention to your knots is a good thing, but it turns out but having a figure 8 with the strands crossed is just about as strong as one that’s tied perfectly.
With a figure 8, there are other more important things to pay attention to, such as:
The size of the loop (about fist sized, not much bigger)
Having an adequate tail of about 6 inches / 15 cm
Snugging down the knot by pulling hard on each of the four strands, one at a time, so it's a tight, cylindrical shape, not loose and flat. AKA, “Dress it and stress it.”
These are the things that can cause you problems. Having said that, I still encourage you to learn how to tie it right.
Why is it important for knots to be tied and dressed properly?
Easier to check
Usually easier to untie
Less likely to loosen up unexpectedly
Less likely to roll
May be stronger (but not always, as we see here)
3 - How strong are friction hitches?
Varies a lot, see below.
Source / testing: YouTube, Prusik, Auto Blocks, and Klemheist Break Tests
There are LOTS of variables in this, such as diameter of the rope, how new or slippery the sheath is compared to an old fuzzy rope, the material and type of friction hitch you’re using, etc.
Having said that, here’s some real world testing from HowNOT2. (The test results below are from the caption of this video, shared with permission.)
5MM ACCESSORY CORD
Prusik 8.16 kN and 5.36 kN
Klemheist 7 kN-ish Slips and locked off and broke at 11.62 kN
Auto Block: 3 wraps - 0.36 kN Slips / 4 wraps - 3.62 kN Slips
7MM ACCESSORY CORD
Prusik 7.9 kN and 5.18 kN
Klemheist 3.3 kN Slips
Auto Block: 4 wraps - 0.8 kN Slips
HOLLOW BLOCK 14KN MBS
Prusik 5.84 kN
Auto Block: 3 wraps - 0.42 kN Slips / 4 wraps - 1.38 kN Slips
4 - How strong is a basket hitch in Dyneema?
About 56 kN
Source / testing: YouTube, You aren't supposed to tie a knot in this (about 2:18)
A basket hitch typically doubles the strength of the material you're using, because it divides the load four different strands rather than two.
This is normally overkill in recreational climbing, where you're never gonna see much of a load passed around 9 kN in any situation.
But if you need to rig something with absolutely maximum strength, it's good to keep in mind how you can increase the strength of your soft goods by making a basket hitch.
Here's a link to my longer article on this.
Sewn Dyneema sling rated 22 kN, end to end pull, broke about 28 kN.
Same sling, rigged with a basket hitch, broke about 56 kN, about double!
How strong is a stopper knot / barrel knot as a rappel safety / backup?
If you tie two figure 8s in climbing rope and pull it end to end, where does it break?
How strong is a “shoelace” knot tied around a tree? (Instagram lost their mind when this was first posted . . .)
