Playing with force. A bit of unscientific trivia.

I just spent a long weekend up in the far northern reaches of Michigan with the folks from Arbor Quest. The get-together was all about coming up with ideas on how to let disabled climbers enjoy the thrills and chills of riding along a modified version of a zipline. The result was a system that works but that is in need of a whole lot of tweaking before time to actually put a disabled person on the ropes. What we did mostly was to sit around and “overthink” the entire process. Once the “overthink” has been balanced with a bit of “underthink” they will have themselves a good system. No problem there!

However, as part of the sitting, talking, thinking, and testing we had ourselves a couple of very neat dynamometers to play with, Dillon EDJuniors, that had been rented for the occasion. As soon as I saw those I knew that we were going to have fun looking at some numbers that I had been thinking about for a while.

I have often wondered, while during a climb, how much additional dynamic force is being placed on an anchor as I jiggle, wiggle, and bounce on the way up. I wasn’t worried about additional force on the rope, harness, and other gear. I wanted to know what’s happening on the anchor.

So. We hung a dynamometer from a limb on a piece of webbing. We hung a nice pulley on a carabiner below the dynamometer, and then ran a rope through the pulley. One end of the rope was tied off to a ground-level-tie-off, the other was to be climbed on.

The first thing I did was to hang like a dead-weight from my RAD on the climbing side of the rope. Dennis Furlong, up above and monitoring the dynamometer, reported the force on the anchor to be 367 pounds. Since I weigh “about” 175 pounds, and since I had the weight of the rope, harness, and other hardware to consider, that coincided nearly exactly with what physics said it should be: My weight plus the weight of the other stuff doubled.

Then I started climbing, after Dennis had “zeroed” the equipment. After ten rapid strokes on RAD the maximum peak force on the anchor was shown as 430 pounds. This meant that the dynamic force exerted on the anchor had increased by 67 pounds. This was not nearly as much as I had expected, so I am now feeling real safe about climbing RAD.

Next, I repeated the climb, on RAD, doing what I call a “gentle” climb. A “gentle” climb means being careful not to “plop” back down in the harness after each stroke. The maximum peak force reading was now 370 pounds, meaning that during the climb I had only increased the force by 3 pounds. That verified my theory that climbing “gently” places a lot less force on an anchor than does the normal herky-jerky way of climbing.

After that, I tried pulling myself up as high as I could reach and then doing an intentional bounce on the rope, trying to see what the maximum force would be if a climber was playing around on the rope while climbing. The maximum peak reading for this was 537 pounds, or slightly more than 3 times my weight.

These numbers, dutifully recorded by Patrick Dimmer, show that the act of climbing does indeed place an additional force on an anchor, above and beyond the dead weight of the climber. This, of course, is something that we all understand intuitively, and goes right along with what physics teaches us. But, now that I have actually created and seen the numbers for myself, I feel a whole lot better about it.

Next, since we still had a lot of time to play, we set up an SRT route with a limb cinch, and tested the dynamic force exerted by a fast, but controlled, stop during a grigri rappel. Bob Remenap and Dennis combined to rig the system and I headed up the rope to do the rappel. Opening the grigri for full speed I let myself drop about ten feet then put on the brakes. Maximum peak force exerted on the anchor was 597 pounds or 3.4 times my own weight. Remember that this was a limb cinch; with a ground level tie-off the force would have been twice as great, almost 1200 pounds. Interesting…and a bit scary!

With help from the dynamometers we also “proved” that placing a setting over multiple limbs, with a ground-level-tie-off reduces the force on the “primary” or highest anchor limb.

Other tests were done on normal DRT and the SuperSystem with Norb serving as test dummy. I do not have those numbers but it was all interesting nevertheless.

The experiment above was not done in any sort of scientifically credible manner and subsequent tests by others in the group showed quite a bit of variation in the numbers. But it was a lot of fun actually putting these things to a hands-on test and confirming all the things we had already been taught and already knew to be true.

It was a fun excuse for doing a bit of climbing that none of us would have done otherwise.

It wasn’t rocket science at all!

Great report! Indeed your numbers affirm what many of us have suspected, a smooth climbing motion decreases dynamic loading. Great info on the hard stop on the Grigri, the same applies for rappel, keep it smooth.
-AJ

After looking at thoss hard stop numbers, just think what a five foot free fall would do! Even a 2 foot free fall for that matter.

Interesting test Joe, thanks for sharing.

Do you think the force exerted on an anchor would differ with the type of rope used? Dynamic opposed to static for instance? And would a rack exert more or less force than the grigri with a sudden stop.

Would climbing systems other than RAD have different numbers?

I would love to see more numbers on these things. This is interesting.

Probably, because a dynamic rope or any rope that offers "stretch" should absorb some of the force created by dynamic movement. I say "probably" because until I try it, and read the numbers out, I'm not sure.


I don't think so. The dynamic force is probably a function of the suddenness of the stop.


Again, probably. Some systems are, by nature, a bit more herky/jerky than others. Or maybe it's just a matter of how smoothly the climber is climbing.


Definitely! Not only is it interesting to see the numbers, it's also a lot of fun!

If Ron would break down and buy himself a dynamometer (Dillon EDJunior= $1,125!) we could all have a gathering in his yard and play with the numbers all day long. Where's your GearLust, Ron?

If I had $1,125 to spend I would be headed to On Rope. If we are going to be testing with Ron's new dynamometer then we need more equipment to test. Right?

Hey! I'm going broke keeping you guys apprised of stuff as it is!

A dynamometer, hmmm, I did get my longevity check and it would cover that. Heya wait a minute JOE - YOU"RE AT IT AGAIN!!!

You conned me into that Scarab thing and now I think I can't climb without it. Hmmm, wait, that's a good thing though, right?

$1100 for a Dyno - yeah right - as if............who sells them?

http://www.cranescale.com/products/dynamometers/details/?productid=3354

Call me when it comes in!

Very interesting reports, thanks for sharing.

My wife (HexNut) has a couple of dynamometers, one is a Dillon. She uses them to measure the tension of guy wires. It is a possibility that she would loan one of them for some research at the rendezvous if anyone is interested...

Definitely! Bring the thing! The only problem will be that everybody will want to test everything and we will spend all our time arguing over "Who's next?'.

I would love to repeat the tests we did in Michigan and see if different rope types offer different results and if different systems exert different levels of force on anchors.