Why is Fall Factor Relevant?

[Toast]

Before everybody rushes off to try and explain Fall Factor see this.

 

Every time I ask this question, I get somebody rushing to explain what FF is. I understand the concept, but I don't understand why it's relevant. Let me rephrase that... Seems to me, what's most relevant is shock force.

 

Let me draw out an example. I'm on a short leash, 6". I stretch to place an omni, I slip, I fall 12". Fall factor = 2 in this case. Shock force, modest, maybe 2 Kn. That's enough to rattle me, maybe throw out a bad back or induce a case of whiplash.

 

Now, a more extreme example, I lead 10' up, slip, and fall 20'. Fall factor = 2. As the Thai say, same, same... but different. This time, the shock force is significantly higher, maybe 9 kN. That's enough force to break gear or possibly hurt myself.

 

I'm not trying to discount the importance of understanding FF, but it seems to me the thing I should be most concerned about is shock force. That's a tangible number that's gonna tell me if I can really hurt myself. Am I missing something here?

 

[Dru]

The fall factor determines the force on your piece. A 100 foot fall on a fall factor 0.5, 200 feet of rope out creates less force on your piece than does a 6 foot factor 2 fall on a 3 foot daisy chain.

 

In via ferrata, where the rungs are 5 meters apart and you are clipped in with a 1m leash, people took factor 5 falls (5m fall on 1m of line) and broke their biners and died. thats why they make special shock absorber screamer type things for via ferrata now.

 

thats right a 5m fall broke a biner due to high fall factor.

get the idea...

 

the "shock force" thing you mention has nothing to do with it. laws of physics do not change just because you doubt them.

[Gordonb]

The "shock force" has everything to do with it, but Dru you are right the fall factor is most important because that is how you would calculate the "shock force". The lower the fall factor the longer the rope and anchors have to absorb the energy of the fall and thus the "shock force" is less.

[iain]

I've never heard of a load on an anchor as a "shock force" before. There's only one force involved, and that's from the climber's mass accelerating. The only variable is the time over which the force is applied to the anchor. Force/Time is controlled by the amount of elastic response in the rope system, which is directly related to fall factor.

 

 

Also, if I understand the ferrata stuff correctly, the carabiner is subjected to some horrific torsional forces when falling on a ladder like that. The point still seems to be valid though.

[Bronco]

iain said:

Also, if I understand the ferrata stuff correctly, the carabiner is subjected to some horrific torsional forces when falling on a ladder like that. The point still seems to be valid though.

 

not to mention a completely static system

[freeclimb9]

Fall factor is simpler to calculate. Next question?

[Bronco]

Toast,

 

If you climb with out a rope it becomes much less complicated.

 

So, can someone shed some light on kN ratings for me? Say a rope is rated for a 8.2 kN impact force. I understand that's the force it could transfer to the anchor, but then what, the rope breaks?

 

 

[Dru]

everyone compelled to voice opinion here please scan your B.Sc. in physics and post it to this thread

[catbirdseat]

Toast,

 

Here is a handy force calculator for you. Calculate Fall Force. Note the Impact Force for a 9.8 mm Maxim rope is 10.5 kN, a 10.5 mm is 9.43, and a 11 mm is 9.62 kN. This is surprising. I would think that the smaller the rope the lower the Impact Force would be.

 

Using this calculator, if I (weight 195 lbs) were to climb 10 ft above my anchor and then placed one piece of pro and then fell 5 feet, my fall factor would be 0.5. If I were using the 9.8 mm Maxim rope the force on my pro would be 10.5 kN, which is enough to pull out most nuts and even bust some hexes. Therefore you want your first pieces to be really bomber, or else place them close together (or just don't fall).

[fern]

scan yours

 

was it that Beal catalogue that had all the nifty

equations and stuff? yeah study that. IT IS THE MOST

IMPORTANT WAY TO LEARN TO BE A SAFE CLIMBER!!

[shuksan]

This might make fall factor clearer (or not):

 

1. Before the fall, the climber has some amount of potential energy, and zero kinetic energy E1.

 

2. After the fall, climber has a smaller amount of potential energy E2=E1-E, and zero kinetic energy.

 

3. During the fall, the climber developed some kinetic energy, which was all absorbed by the rope stretching. Also, some of the climber's potential energy was directly absorbed by the rope stretching. The total amount of energy absorbed by the rope's stretching was equal to the change in potential energy, E.

 

The energy E which must be dissipated by the rope stretching is equal to the climber's mass multiplied by the distance of the fall. The important figure relating to the rope is how much energy it must dissipate per foot of length, so if D is this figure:

 

D=mass*fall distance/rope length

 

If we assume a "standard climber", the important figure is fall distance/rope length = fall factor.

 

Why is the energy dissipated per foot of rope important? Because the model of the rope says that you have to pull harder on a rope to dissipate higher amounts of energy per foot of rope. Basically, the simple model says that the peak force of the fall is directly related to how much energy per unit length the rope must dissipate. In long falls, this force acts over a longer time, but is no greater, if fall factors are equal. Of course this model is very simplified and does treats carabiners as perfect frictionless pulleys, so it leaves out a lot of forces we know are real.

 

Sorry if this is unclear, but I hope it helps. Back to work!

[catbirdseat]

Obviously, I'm not going to be doing fall force calculations while I'm climbing. I want to have an intuitive sense of where the danger is. Instinctively, we regard being higher as having more danger, but that is not the case. We are in the greatest danger shortly after we leave the belay. What is so freaky about Goran Kropp's accident is that it occurred late in the pitch when forces should have been low.

[Dru]

http://www.petzl.com/petzl/publicActiv?id=GLAC#

[Geek_the_Greek]

Toast, the problem with your hypothetical analysis is the "shock force" numbers you threw in. In theory, your factor 2 falls, be they 1 foot or 20 feet, will have the same "shock force" (force transmitted to the anchor/belay). The rope is a big rubber band! A longer rubber band (more rope out) sucks up more energy than a shorter rubber band! That's why a 100 foot factor 2 fall transmits the SAME "shock force" or whatever you want to call it, as a 10 foot factor 2 fall.

 

(In reality, I can't imagine a 12 inch factor 2 fall - your ass would be in your belayer's face and all that.)

 

By the way, forces in a factor 2 fall on a normal single climbing rope will be closer to 10 kN than 2 kN (but depends on the rope's impact force).

 

Sorry, but if you don't get this, you don't understand fall factor.

 

Yadda yadda via ferratas blah blah - valid points, but far from standard climbing scenario, so rather moot.