QC Lab: To Screamer or not to Screamer?Wednesday, Septembre 7, 2016
A couple of months ago I climbed the North Face of the Eiger with some friends. On one particular pitch above the Waterfall chimney, I was climbing some harder mixed moves right above the belay. My gloved hands groped a rounded rail, while my crampons scraped on marginal limestone edges and a thin layer of verglasse. The single piece between me and my belayer was a half-pulled-out, bent over piton. As I moved further above this piece, questions formed in the back of my mind: "Will that manky pin hold?" "Should I have a Yates Screamer on that thing?" "Do Screamers even help?" Luckily, I didn't have to find out the answer to any of these the hard way. As soon as I got back, however, I decided to do some drop tests in the lab to give myself some rough answers to these questions.
I roughly re-created the climbing scenario I described above in the Drop Tower, as shown in Figure 1 below.
- The "climber"-an 80 kg (176 lb) mass which runs on frictionless tracks-was tied into a 9-foot (2.74 m) section of 10.1 mm dynamic rope with a figure-8 knot which was pulled hand tight
- The mass was raised 4.5 feet (1.37 m) above the first "piece of gear"-a quick draw clipped in to the load cell --for measuring the forces generated in the fall
- The other end of the rope was tied to the "belayer"-a fixed point 4.5 feet (1.37 m) below the piece of gear-with a figure-8 knot which was pulled hand tight
- The "climber " mass was dropped from this height on to the single piece of gear resulting in Factor 1 fall of 9 ft (2.74 m) with 9 ft (2.74 m) of rope in the system
- A new rope was used for each drop, but it should be noted that this set up is designed for repeatability and simplicity, so it does not account for all dynamics in the real world belay
- Figure 1: 9-foot (2.74 m) Factor 1 Drop Test Setup Schematic
Test Variables and Hypothesis:
We decided to do this same drop using an example of all of the possible "quickdraw" configurations while holding all other variables (knots, biners, new rope, drop height) the same for all drops to test the effect of the dogbone on the force that the piece of gear sees. We did the same 9 ft, Factor 1 drop on the following samples with the same Hotwire on the rope end and a Quicksilver on the gear end:
- Steel Chain
- Dynex Dogbone
- Econo Nylon Dogbone
- Variwidth Nylon Sport Dogbone
- New Standard Screamer
- Old-Version Standard Screamer
We hypothesized that the steel chain would not stretch at all, resulting in the highest force to the piece of gear. We presumed that the Dynex runner would behave about like steel, while the two types of nylon runners would stretch absorb slightly more energy, and ultimately that the Screamers would absorb the most energy to reduce the peak force by upwards of 50% when compared to the baseline peak force established in the drop with the chain.
Results: 9-foot Factor 1 Fall on to 1st Piece of Gear
From the data above, it became clear that we were not too far off in our hypothesis: the Screamers did reduce the peak force on the first piece of gear, while nylon absorbed a bit of energy, and Dynex behaved similarly to steel chain. The Screamer did reduce the peak force by 14-17%, or a reduction of 1.4-1.8 kN.
We figured that the very severe nature of a Factor 1 fall very near the anchor is perhaps not showing us the difference between Screamers and other dogbones as well as it could because the forces generated in such a fall are WAY above the activation force for the Screamer. We decided to repeat the experiment with a lower fall factor to get peak forces that are less severe and more typical of a real climbing scenario.
Revised Test Setup-Lower Fall Factor
I repeated the experiment on a new sample of each type of dogbone and another new Standard Screamer. Again, a new rope with hand-tied Figure 8 knots at each end was used for each drop.
This time, I set up the fall scenario with the piece of gear the same 4.5 feet (1.37 m) above the belay, while the climber's waist was 1 foot (0.30 m) above the piece of gear as shown in Figure 2 below. This created a 2 foot (0.60 m) fall on 5.5 feet (1.68 m) of rope, or Fall Factor 0.36.
Results: 2-foot Factor 0.36 Fall on to 1st Piece of Gear
With the Factor 0.36 drop, the force generated was generally around 40% lower than the Factor 1 drop. These lower peak forces allowed us to see the difference between each dogbone type and the benefits of the Screamer more clearly. In this case, the dynamic stretch of each dogbone was noticeable over the static steel chain. Again, the Screamer was able to reduce the peak force by 1.6 kN, but this contribution was very considerable as a percentage, reducing the peak force that the piece of gear is subjected to by 26%.
The forces generated in falls right above the belay, where very little dynamic rope is in the system are very high. Forces in the neighborhood of 10 kN applied to your piece of gear will be very hard on small stoppers, micro cams, and screws. You should do everything in you power to prevent falls like this when possible! Our limited testing does show, however, that a Screamer (or similar energy absorbing device) could reduce the peak force that the piece is subjected to by up to 26%. This may be enough to keep that manky pin or marginal nut in place. Remember, our testing is not the real world, and using knots instead of a real belay device, as well as a rigid mass instead of a squishy human body are factors to consider which may make our results different than the manufacturer's claims. The bottom line is: they do work to reduce the peak force applied to the piece of gear in the system.
Other considerations for scenarios such as this should also be to always give a dynamic belay, and if you should take that whipper, come back to the belay and switch ends of the rope! The rope is the most key piece of energy absorbing equipment. If I would have done these falls in succession on the same rope, you would see forces go through the roof-with a Screamer or otherwise. That's a set of tests for another day...stay tuned!