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JosephH

Skinny dyneema slings - 2 years later...

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i'm 230 lbs - under what conditions could i put 11 kN of force onto a piece of pro? (not a rhetorical question - just curious and havne't taken physics in awhile :) )

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Yeah physics guys, could Ivan generate 11kN with a bounce test?

 

How about me (30% lighter)?

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Another good test might be having the shoulder length runner tripled up like I and many others I climb with. It'd be interesting to see if that fails at a higher or lower point compared to a fully extended runner.

 

should fail at a higher load than a fully extended runner. you have more strands sharing the load.

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Well....

F = ma so a 230 lb (104.5 kg) body that exerts a force of 11 kN

implies an acceleration of 105.2 m/s^2. This is obviously a lot higher than the force of Earth's gravity. Technically, they shouldn't be talking about how much force the thing could withstand, but what the yield stress (force per unit area) is. Most people know what a force is though and not a yield stress.

Even fewer seem to understand that perfect lab conditions are rarely, if ever, met.

 

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It's not the fall that kills you, it's the sudden stop at the end. Tis true.

 

(Warning: biologist attempting physics calculation - no warranties expressed or implied).

 

The punch line: If a 100kg mass falls 5 meters it reaches a velocity of 10m/sec. If it is stopped in 0.1 seconds the force required to stop it is 10kN.

 

Note that this assumes no energy is absorbed by the rope, the climber, the pieces, the sling, or friction. In practice, all of these things do contribute. Modeling them is beyond my meager capabilities.

 

Also note that if you increase the time of stopping the mass you decrease the force required to stop it. This is why dynamic ropes put lower loads on pieces and climbers than static ropes, ditto dynamic belays. This is also the reason why bumpers are designed to crumple on impact and why airbags are effective.

 

................

So...

 

Force is measured in newtons = kg(m)/sec-sec.

Indeed, force = ma (where a is acceleration = m/sec-sec).

Inspection of the units reveals that force is also equal to a change in momentum per unit time, sometimes called an impact force.

 

In other words force = change in momentum/stopping time = MV/T

Momentum is MV (v is velocity of the object, M its mass).

Stopping time is T.

 

Solve for V, the velocity of the falling mass:

V = F*T/M

 

Now plug in numbers: 230lbs is about 100kg. Let's say the stopping time is 100 miliseconds (0.1 seconds). Note, I have no idea if this is a good estimate for the stopping time. Force = 10kN (keep numbers simple for estimates).

 

V = 10m/sec.

 

Now, if a mass is dropped how far must it fall before it reaches 10m/sec?

 

In that case, the gravitational potential energy (Mgh) is converted into (read equal to) the kinetic energy (mv*v/2). Note that mass cancels out. All objects accelerate at the same rate, g.

 

Solve for h (height dropped) = v*v/2g

g = 10m/sec-sec. v = 10m/sec (see above).

Plug them in and you'll see that h = 5 meters.

 

So, a 100kg mass that falls 5m will be moving at 10m/sec. A force of 10kN will be required to stop it in 0.1 seconds.

 

 

Edited by Rad

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Further tests of girth hitched slings to slings/biners/whatever are not very interesting. Those test were in specific reaction to the failure John Sherman experienced, and just quantitatively verified the "Don't do it" motto.

 

What would be good is if we as individuals submitted slings with a recorded number of days exposure to outdoor conditions. Then we could test the new, the old, and the really old. That would give us all a reasonable guideline of when to retire "normal" looking slings. Industry is not likely to generate this data because "use" is such a subjective term.

 

also, I wouldn't believe that girth hitching a sling to a biner is stronger than the sling its self. Rather that with a sample size of 1, that sling was stronger that any of the other slings. ;)

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The falling forces really tie back to the fall factor and elasticity of the system. 11 kn is probably fine in a dynamic system (biners are 22kn, cams and other pro are usually rated to 14kn though). Might not be good as first piece on a runnout pitch where you could get a high fall factor, but 2nd or 3rd piece with a bunch of rope it and I imagine it would be fine. Probably a bad idea to use them as a personal anchor though where it could be subjected to short static falls.

 

Rock and Ice, or Climbing, or someone did a bunch of short drop static falls to see what forces were generated on gear(maybe a year ago?), and it really didn't take much of a static fall to hit 11kn.

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Even before this thread, I've decided to move away from the ultra skinny slings (edelweiss, mammut, etc.) that I've been using for a while now. The weight is trivial in comparison to slightly wider sewn slings. I frequently hang a long sling to reduce rope drag on a sport project and the really skinny ones don't hang with the gate oriented the way you want. The slightly wider ones hang with the gate the way you want. It is way over my head but take a look at the fixed slings on routes like Rude Boys at smith if you want an example. I held 5 ultra skinnys in one hand and 5 slightly wider slings in the other and the weight difference seemed to be less than one 100 calorie packet of GU. Plus the ultra skinny ones cut over sharp edges way easy. just my two cents. And for ice and alpinism, the water absorbtion seems to be about the same.

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