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Is 6kN really the magic number?


Eric D · · Gnarnia · Joined Nov 2006 · Points: 230

There is no hard and fast rule, just people's opinions.  I have taken many lead falls on 4kn and 5kn pieces and have never once broken a piece or pulled one of those tiny pieces.

bkozak · · Sterling, VA · Joined Jul 2016 · Points: 70
Greg D · · Here · Joined Apr 2006 · Points: 867

Yoi 2 are talking gibberish.

  • If a rope's rated max impact force is 8kN, then in that test with a 1.77 fall factor and 80kg, the top piece will see more that 13kN.  Even if it was a real human, the top piece must see a greater load than the climber.  
Patrik · · Unknown Hometown · Joined Jun 2010 · Points: 30

Jeff G wrote: I've never understood the "free vs aid" piece.  All pieces can be used for free climbing.  Maybe you shouldn't whip 30 feet on a #2 rp but it can certainly hold a fall when placed above your head or at chest level.  Often times that's what you need for the crux moves.  Silly to think you wouldn't bring the smallest gear needed because it might not hold a big fall. 

Some others have also chimed in claiming there is no hard limit. Yes, there is a hard limit (as given by Jim Titt). 

And the hard limit has nothing to do with the FF2 fall as many are alluding to previously. It is more relevant to look at the dreaded FF0 fall (as Jeff implies). A piece that can (barely) hold body weight (aid) is not guaranteed to hold a FF0 fall. The simplest way reasoning about it is that if you let the belayer lock off as you hangdog, a rough estimate is that the top piece of gear will feel the force twice that of your body weight (with friction and other details it might be in the range 1.6x or 1.7x). The "standard" UIAA climber is assumed to be 80kg, equivalent to 800N. A piece rated for 1kN (=1000N) could (possibly) hold the body weight of an aid climber (800N), but would (likely) fail if you hangdog on it (2x800N = 1.6kN or with some friction losses: 1.6x800N = 1.3kN). A FF0 fall generates a little more force on the top piece than gently hangdogging on it, so that's why the limit for lead gear is higher than this theoretical 1.3kN or 1.6kN. With some high school physics assuming the rope is an ideal spring, ignoring friction losses, and ignoring any effects of the dynamics of the belayer, I think (anyone with a more recent background of physics/math might correct me here) the maximum force on the top piece of gear turns out to be 4x your body weight (3.2kN for the "standard" UIAA climber) at a FF0 fall. Yes, I know, too much theory. But at least it gives some understanding on where these numbers come from.


Jim Titt wrote:
King Tut · · Citrus Heights · Joined Aug 2012 · Points: 430
bkozak wrote:

//vimeo.com/24894498


Um, that ain't no whip braj. :D

More like a cozy little plop.

Ska Ggs · · Unknown Hometown · Joined Dec 2015 · Points: 10

I used this in a Presentation once, Most falls I take are just over 5kn for a fatter guy like me, but even so. 5 feet past my last placement, and You're gonna bet that is well over the 4kn's worth of force. http://ferforge.tripod.com/Srt002.htm

David Kerkeslager · · New Paltz, NY · Joined Jan 2017 · Points: 108
Ska Ggs wrote:

I used this in a Presentation once, Most falls I take are just over 5kn for a fatter guy like me, but even so. 5 feet past my last placement, and You're gonna bet that is well over the 4kn's worth of force. http://ferforge.tripod.com/Srt002.htm

Wow, Tripod still exists. So much nostalgia and a little bit of horror at the bad poetry websites I made in middle school.

EDIT: That calculator is also wrong. I put in 45ft of rope and 15ft since last anchor, and got FF=1.333. FF=h/L where h = 2*(distance since last anchor) and L = length of rope, so the correct answer should be FF=.667

Jim Titt · · Germany · Joined Nov 2009 · Points: 490
David Kerkeslager wrote:

I'm curious about how these numbers were chosen. I'm not disagreeing with you, I'm just trying to get an idea of how well the data these numbers come from represent real life use cases.

My ideal experiment would be to give a bunch of climbers a bunch of load cells and have them trad climb for a month with load cells on all their gear, tracking the forces/fall-factors of all their falls, but obviously the cost of the load cells alone would be cost prohibitive. :D

No idea why except that the nut standard is considerably older than the one for cams and usually the values (for all climbing equipment) are a combination of some vague theoretical number and historic experience about what works.

We did a project logging sport climbers for a summer, it´s actually quite cheap if you use data-logging decelerometers. No, the information isn´t going to be made public!

David Kerkeslager · · New Paltz, NY · Joined Jan 2017 · Points: 108
Jim Titt wrote:

No, the information isn´t going to be made public!

  

DrRockso · · Red River Gorge, KY · Joined Sep 2013 · Points: 339
King Tut wrote:


No, all you need is a test rig and someone to right down the numbers to find the rope's max impact force.

If a rope is rated for say, 8kn max impact in a FF2, then any factor 0.5 and below will only be 2kn etc. This is most leader falls when there is a significant amount of rope out (napkin math).

What! lol. Do you have evidence to back this theory up? This is not an accurate way of calculating forces in falls, number one you're not even accounting for the most relevant factor the mass of the object falling, two, I'm not sure the relationship is linear. According to your model if someone (Let's say they weigh 200lbs) has a 100 feet of rope out and takes a 50 foot whipper then it's only going to put 2kn (450lbs of force) on the gear, I suspect it would be much greater. There used to be only calculator to estimate this, though I can't find one at the moment.  

King Tut · · Citrus Heights · Joined Aug 2012 · Points: 430
DrRockso wrote:

What! lol. Do you have evidence to back this theory up? This is not an accurate way of calculating forces in falls, number one you're not even accounting for the most relevant factor the mass of the object falling, two, I'm not sure the relationship is linear. According to your model if someone (Let's say they weigh 200lbs) has a 100 feet of rope out and takes a 50 foot whipper then it's only going to put 2kn (450lbs of force) on the gear, I suspect it would be much greater. There used to be only calculator to estimate this, though I can't find one at the moment.  


Napkin math assumes the relationship is linear. Having taken a 100' fall with 200' of rope out it is remarkably gentle I assure you. :)

I also weigh north of 200# :P.

Ryan Swanson · · Unknown Hometown · Joined Jan 2018 · Points: 50
DrRockso wrote:

What! lol. Do you have evidence to back this theory up? This is not an accurate way of calculating forces in falls, number one you're not even accounting for the most relevant factor the mass of the object falling, two, I'm not sure the relationship is linear. According to your model if someone (Let's say they weigh 200lbs) has a 100 feet of rope out and takes a 50 foot whipper then it's only going to put 2kn (450lbs of force) on the gear, I suspect it would be much greater. There used to be only calculator to estimate this, though I can't find one at the moment.  

F= m*a in simple terms

F=mass * acceleration + damping coefficient * velocity + Spring rate * displacement  in correct(er) terms.  Here are some numbers if you are savvy to a mass spring damper system.

Jay J · · Euelss · Joined Jun 2011 · Points: 5

Do people still use Screamers?  I liked to used those when clipping small pieces.

Eric Boesch · · Unknown Hometown · Joined Oct 2012 · Points: 55

6kN is not a magic number. It has simply been observed in practice that 6kN gear usually holds. Even 5kN gear usually holds, but its failure rate even when well placed may well be several times higher. On the other hand,  I vaguely recall an old Chris Harmston quote to the effect that 10kN rated Black Diamond pro never failed in the field.

A new UIAA rated rope can generate forces up to 12kN in an 80kg drop test. Add in the near doubling of force that can occur when a rope is slingshot 180 degrees across a carabiner, and the theoretical magic number for gear safety is 20kN. Why does reality turn out better than theory here? Here are a few plausible excuses.

1. Most effective fall factors are well under 2. However, the force does not decrease linearly. If you wanted to oversimplify, decreasing as the square root, so FF 0.5 represents half the peak force compared to FF2, is closer to reality.

2. Most ropes are rated to peak forces well under 12kN when new, not exactly equal to it. Note that a beat-up rope that seems bouncy in a short fall may still exert an abnormally high peak force in a harder fall.

3. The stamped strength of UIAA rated rock protection is less than its average strength.

4. The climber may weigh well under 80kg.

If most of these advantages are not present, then gear that is normally adequate may fail.

This is pure speculation so I'm being vague, but over a decade ago there was a fatal accident that left behind an unusual pile of mangled gear. Most assume the gear's performance was compromised by underextended slings, and so do I. But I think it's also likely that other contributing factors included a heavy climber, a beat-up rope, and an effective fall factor possibly exacerbated by the same underextension.


Mark Straub · · Berkeley, CA · Joined Feb 2010 · Points: 25

I took a near-factor 1 fall onto a #3 BD nut, rated to 5 kN.  Saved me from hitting the deck.  I still carry the nut, which has a nice groove in it from where it contacted the rock.

· · Unknown Hometown · Joined unknown · Points: 0

II thought that reason why lead gear is tested to 5kn is because is because in a fall we do nay generate more than 5kn of force applied on the gear. It is stupid to certify gear that will break in a fall 

David Kerkeslager · · New Paltz, NY · Joined Jan 2017 · Points: 108
JulianG wrote:

II thought that reason why lead gear is tested to 5kn is because is because in a fall we do nay generate more than 5kn of force applied on the gear. It is stupid to certify gear that will break in a fall 

It's not stupid at all. Just because a piece of gear will break in SOME falls doesn't mean it will break in ALL falls. I'd guess a 3kN piece will fail in a lot of falls (haven't fallen on my white tricam) but I've placed the white tricam over my head so I can do a tricky sequence on short top rope, and in that situation, where the FF<0.1, I'm fairly certain a fall isn't going to generate 3kN.

Part of what you sign up for when you do trad is that you're not always going to have a 10kN placement available to you. In those situations, it's usually safer to place a 3kN piece than to place nothing.

And in case it isn't obvious, they can't just make stronger gear. These, for example, fit in some very small cracks, so small that there's very little material to even hold weight, and they have to forge the stoppers directly onto the swaged wire just to get it to 2kN for the smallest piece.

· · Unknown Hometown · Joined unknown · Points: 0
David Kerkeslager wrote:

It's not stupid at all. Just because a piece of gear will break in SOME falls doesn't mean it will break in ALL falls. I'd guess a 3kN piece will fail in a lot of falls (haven't fallen on my white tricam) but I've placed the white tricam over my head so I can do a tricky sequence on short top rope, and in that situation, where the FF<0.1, I'm fairly certain a fall isn't going to generate 3kN.

Part of what you sign up for when you do trad is that you're not always going to have a 10kN placement available to you. In those situations, it's usually safer to place a 3kN piece than to place nothing.

And in case it isn't obvious, they can't just make stronger gear. These, for example, fit in some very small cracks, so small that there's very little material to even hold weight, and they have to forge the stoppers directly onto the swaged wire just to get it to 2kN for the smallest piece.


Well right. But I'm talking about UAII certifications they only require 2kn for stoppers and 5kn. If industrial equipment is rater to 25 kn it is made to hold 50 kn. Maybe in climbing we generate only a max of 2kn on gear

 

David Kerkeslager · · New Paltz, NY · Joined Jan 2017 · Points: 108
JulianG wrote:

Maybe in climbing we generate only a max of 2kn on gear

No. Not trying to be rude here, but this idea is unambiguously and dangerously wrong.

John Wilder · · Las Vegas, NV · Joined Feb 2004 · Points: 1,535
JulianG wrote:


Well right. But I'm talking about UAII certifications they only require 2kn for stoppers and 5kn. If industrial equipment is rater to 25 kn it is made to hold 50 kn. Maybe in climbing we generate only a max of 2kn on gear

 

No, climbing doesn't follow the same rules as the industrial world. The numbers you see in climbing are true numbers, and do not take into account safety factors. 

Guideline #1: Don't be a jerk.

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