Field Analysis: How Much Force Does a Top Rope Fall Produce?
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20_kN

Sport climber
20 kN Land
Topic Author's Reply - Aug 27, 2015 - 02:10am PT
There is a huge difference between knotted and knot with Dyneema. Watch this video.
http://dmmclimbing.com/knowledge/slings-at-anchors/
As you can see here the dyneema should be ok at 7kN, but I wouldn't be overly comfortable with the margin since the overhand knotted dyneema failed at 10.4 kN in the example. Although a 10kN fall is likely gonna have you pretty messed up even if you don't deck.


While true, it's also worth noting the test used a 2-piece anchor. If you are building a cam nest with an 8' Dyneema sling, you may have more than one run of Dyneema in the mater point, which if you do will increase the strength.

Also worth noting is that they used a rigid mass in a scenario with no dynamic element. Adding a dynamic element (i.e. a human and dynamic rope) would very likely increase the strength of the knotted Dyneema relative to the video's results.

The problem is that Dyneema has a very low melting temperature (about 290F). Dyneema starts to lose strength with exposure to any level of heat, which means the mere fact that the sling was shockloaded so quickly, and not more gradually such as with a dynamic rope, very likely increased the temperature of the Dyneema enough that it affected the strength of the sling.

DSM, the manufacturers of Dyneema, in their own literature, openly claim that SK-75 Dyneema has a 22% strength loss just by the time it hits 140F. In addition, DMM found that frozen Dyneema slings with knots in them were substantially stronger than ones at room temperature for this very reason.

http://dmmclimbing.com/knowledge/knotting-dyneema-vid/

But probably most imporant is DMM's own research. In the same video I just linked DMM did a static pull test of a knotted Dyneema sling and then they did a FF2 drop on it. In the static test, the sling held 18kN. On the FF2 it only held 12kN and it failed.
overwatch

climber
Aug 27, 2015 - 07:55am PT
you guys must be a riot at a party. This forum constantly impresses me with the level of intelligence and caliber of thinking, especially to a door kicker like me. Thanks for the info
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