Anyone who has climbed DRT with a friction saver knows it reduces friction dramatically compared to tree bark. A pulley likewise is dramatically better than a friction saver. A couple of days ago I decided to actually measure the friction of a few devices.
Partly I was motivated by the diagram below, from Petzl, part of a large amount of info they supply if you buy one of their pulleys. The pulley in this case is a so-called rescue pulley, a model with ball bearings and low friction.
From the diagram it appears that 1.1 lbs are needed to lift 1 lb of load, which is an efficiency of about 91%. It is interesting that they show the efficiency of a carabiner in the same setup to be only 50%.
I used the same setup for my measurements. The load was a 20-lb dumbbell, the rope was a 1/2 in climbing rope, and I was standing on a pretty good bathroom scale to take the measurements. To get more bang per setup, I did the Petzl-style measurement while pulling down on the rope, but then took another measurement while slowly lowering the weight. It was the difference between these two measurements, in lbs., that I recorded; later I combined them mathematically to get the efficiency.
The measurements turned out to be more reproducible than I expected: for a given device I could take measurements 4 or 5 times and record the same difference every time within a pound or less for high-efficiency devices, and within 2 or 3 pounds for the low-efficiency devices; the averages in the table should be a bit better than that.
The results are shown in the table below.
The results surprised me. The first surprise was the poor performance of the micro pulley, which I had climbed under a number of times. The other pulleys waste 1/3 as much energy. Second, the aluminum rings, at under 50% efficiency, were much worse than I imagined. This would mean that footlocking or hip-thrusting under the rings would cost you 4 feet of work for every 3 feet of actual height gained. No wonder SRT feels so clean and efficient.
Partly I was motivated by the diagram below, from Petzl, part of a large amount of info they supply if you buy one of their pulleys. The pulley in this case is a so-called rescue pulley, a model with ball bearings and low friction.
From the diagram it appears that 1.1 lbs are needed to lift 1 lb of load, which is an efficiency of about 91%. It is interesting that they show the efficiency of a carabiner in the same setup to be only 50%.
I used the same setup for my measurements. The load was a 20-lb dumbbell, the rope was a 1/2 in climbing rope, and I was standing on a pretty good bathroom scale to take the measurements. To get more bang per setup, I did the Petzl-style measurement while pulling down on the rope, but then took another measurement while slowly lowering the weight. It was the difference between these two measurements, in lbs., that I recorded; later I combined them mathematically to get the efficiency.
The measurements turned out to be more reproducible than I expected: for a given device I could take measurements 4 or 5 times and record the same difference every time within a pound or less for high-efficiency devices, and within 2 or 3 pounds for the low-efficiency devices; the averages in the table should be a bit better than that.
The results are shown in the table below.
The results surprised me. The first surprise was the poor performance of the micro pulley, which I had climbed under a number of times. The other pulleys waste 1/3 as much energy. Second, the aluminum rings, at under 50% efficiency, were much worse than I imagined. This would mean that footlocking or hip-thrusting under the rings would cost you 4 feet of work for every 3 feet of actual height gained. No wonder SRT feels so clean and efficient.
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