Break testing my homemade caving harness

I use a homemade harness for caving, rather than any commercially available one. This tends to alarm some people, as one's harness is a critical, single-point-of-failure piece of life support equipment. It tends to alarm more people from the rock climbing world than from the caving world, as caving has much more of a DIY/homemade gear culture due to its niche nature, with fewer large gear manufacturers mass-producing any and all gear that a caver could ever want.

The other week, back in May, I had the chance to pull my harness to failure and determine its breaking strength. AKS Tower Supply, a local business selling rope access/work-at-height equipment in Albuquerque, has one of these break testing machines, and opened it up to the public for one night for members of the New Mexico Mountain Club to come and break test all of their old, worn out, or otherwise questionable gear. I was super eager to test a bunch of my old and worn out caving gear, especially the homemade gear, altho there were a lot of people with stuff to break so I only had one chance to break something. I chose one of my homemade harnesses, which was 1 year old but heavily used.

It broke at...drumroll please...actually, it didn't break! At 30.52 kiloNewtons, or 6861 pounds, the steel carabiner connecting to to the break-test machine broke instead of the harness! That was a very pleasant surprise.

A line scale reading 30.52 kn, connected to a partially-broken harness

The line scale after the break test, connected to the semi-broken harness.

As the steel carabiner snapped, it cut both the leg loops of the harness, and damaged the waist loop without fully severing it. So the harness was not even fully broken (altho I certainly won't be using it ever again!)

A partially-severed harness, with both leg loops cut and the waist loop partially cut

The harness after the pull test. The leg loops got severed by the sharp carabiner as it snapped, and the waist loop was damaged but not fully severed.

For context, a force of 7 kN in when falls start to become dangerous. Much higher than that and there's a real chance of breaking your pelvis or other bones as the harness crushes you. So 30.5 kN is way overkill. As it should be—harnesses should be stronger than the human bodies that they protect, because harnesses can and do wear out over time and become weaker. (So do humans, I guess...). The UIAA standard for harnesses specifies that harnesses should hold 15 kN. The standard for connectors like slings and quickdraws specifies 22 kN—higher because the rope running from the belayer to the gear then down to the fallen climber creates a 2:1 pulley system, altho with friction between rope and the carabiner it tends to be more like 1.5:1 in practice. So my harness tested well above both of those standards.

A carabiner broken into 2 pieces

The broken steel carabiner from the test.

I break tested it by pulling, on one side, both harness attachment points, and on the other side, the waist loop and both leg loops, approximately equalized. This is somewhat similar to how a harness is actually loaded in a fall—the attachment points are connected to the rope, and your bodyweight is approximately equally distributed among the waist and leg loops. The steel carabiner actually broke below its strength rating of 50 kN, probably for 2 reasons: (1) the 1.5" webbing didn't sit super well in the bottom of the carabiner, and was loading it somewaht close to the nose, a configuration where carabiners are weaker; and (2) this carabiner had been used for many break tests and may have experienced some metal fatigue.

I've made a few of these harnesses for some friends of mine, and they were all super psyched to hear the break test numbers, as they were trusting their lives to my homemade harnesses without them having any official strength ratings besides my assurances that they were "super good enough". I use a homemade harness, rather than a commercial one, not primarily for cost reasons, but because I like my homemade harnesses better. My harnesses are a fixed size custom fit to myself, with no adjustment buckles. This means less snaggy things sticking out of the harness to get stuck, and thus more durability, as the webbing that sticks out where the buckle is tends to wear out first due to it constantly rubbing against the cave walls. No metal buckles nor extra webbing for adjusting the fit also means the harness is lighter and packs down smaller. I also put extra gear loops on my harness where I want them. No commercially available harness has these features (and it would be hard for a manufacturer to pull them off, because the lack of adjustment buckles means the harness has to be custom-fit), so instead I just make my own.

I use a standard doestic sewing machine (Brother LX2500) that I found at a thrift store. It sews thru 3 layers of seatbelt webbing with a #18 needle with no problem. The webbing I use is 1.5" polyester webbing from Strapworks, with a strength rating of 4800 pounds (the harness has multiple strends sharing the load, so it makes sense that it broke way higher than that). I use size 69/Tex 70 bonded nylon thread with breaking strength 11 lb. Each harness attachment point (there are 2 of them) gets approximately 300 stitches. Multiplyingh 11*300*1.5 gives 4950 pounds per attachment point (the factor of 1.5 comes from the fact that each stitch has 2 strands, but they tend to not be full-strength because the thread has a sharp bend at the stitch, and 1.5 is a common fudge factor). So 9900 pounds, or 44 kN, total strength for pulling at both attachment points, theoretically. I don't think I'll ever break test a harness with a machine that can go that high, so that number will forever remain a theoretical prediction. And lately I've been using size #92/Tex 90 nylon thread instead, with a 15 pound breaking strength, as my sewing machine has no trouble with that thick thread. So the strength of the harnesses as I make them now may be even higher.

Close-up of harness stitching

Close-up of the harness attachment points stitching after the test. The stitching looks unaffected.

Huge thanks to AKS Tower Supply and the New Mexico Mountain Club for putting on this event! It was tons of fun and very informative. We also broke a bunch of old, sun-damaged webbing collected from various rappel routes in the Sandias that night, and it was alarming to see how weak some of it was. Also—AKS tower supply stocks some caving gear, such as Petzl Simples, Basics, and other essentials. It's pretty rare for any in-person store in the US to stock caving gear. So if you ever need to buy some caving equipment in Albuquerque, check them out!


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