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Understanding Climbing Fall Forces on Small Gear


Original Post
Keithb00ne Boone · · Las Vegas, NV · Joined Jul 2013 · Points: 115

As a single pitch sport climber I never paid much attention to equipment and fall ratings. Now that I have been trad climbing for a short period, I am trying to understand the fall forces on small gear. One climb I have been enjoying is a finger crack for the first 20 feet and will only accept my Metolius 0, 00 and a DMM #6 brass. I am able to get gear in every 5 feet, but if I, a 165 lb climber, falls 2 feet above my first piece, a solidly placed Metolius 0 that is rated at 5kn, would it blow? The force calculators say that a 2' fall with only 8' of rope out is almost 10 kn's on a dynamic rope. How do you process all this information on a climb? Please be kind.

Dylan B. · · Unknown Hometown · Joined Mar 2006 · Points: 521
Keithb00ne wrote:As a single pitch sport climber I never paid much attention to equipment and fall ratings. Now that I have been trad climbing for a short period, I am trying to understand the fall forces on small gear. One climb I have been enjoying is a finger crack for the first 20 feet and will only accept my Metolius 0, 00 and a DMM #6 brass. I am able to get gear in every 5 feet, but if I, a 165 lb climber, falls 2 feet above my first piece, a solidly placed Metolius 0 that is rated at 5kn, would it blow? The force calculators say that a 2' fall with only 8' of rope out is almost 10 kn's on a dynamic rope. How do you process all this information on a climb? Please be kind.
It's pretty complicated. Here are to basics to remember:

(1) The rating of the piece of protection is the force at which the piece will break, not necessarily the force at which it will pull from its placement. The placement can be quite variable.

(2) The rating on your rope is the maximum force your rope will impart onto the climber in a two-meter, factor 2 fall with a 180 lb deadweight on the end.

This fall force calculator puts you at just over 5.25 kN using the data you supplied. Not sure where you got 10 kN.

EDIT: As pointed out below, the rope rating is force on the climber, not force on the top piece of pro. Post corrected.
Scot Hastings · · Las Vegas · Joined Apr 2013 · Points: 35

What force calculator are you using? 10kN seems a bit high for your example.

For this discussion, it's important to differentiate the force on you as the leader vs the force on your pro.

The UIAA fortunately tests ropes to measure the force on you as a leader. This is based on a the UIAA standard fall which, for single ropes, is something like a factor 1.8 fall with an 80kg weight. 80kg is 176lbs, so a bit more than you weigh. 1.8 is also a very big fall. Most ropes are in the 7kN range for this. Given your setup (factor 0.25 fall), I would expect the forces to be much lower than 7kN. Someone smarter than I can possibly calculate a more exact number for this.

The force on your pro is slightly different, though, due to a concept known as the pulley effect. When you fall, your belayer (or self belay anchor) offsets the force on you as a leader minus some friction. The result is that both you and your belayer pull down on the anchor, thus leader to a higher total force on the anchor than you experience as a leader. In a frictionless system, this would be 2x the leader impact force. Friction reduces the force on the belayer, though (intuitively this makes sense as the belayer feels less force than the leader), and thus reduces the total force on the top piece. The exact amount is dependent on a bunch of factors (rope drag, carabiner size, belayer weight/anchor, etc.). I've heard estimates in the 1.66 range, though, which seems to me to be a good starting estimation.

So, in your example, I would estimate the total force on the top piece to still be below 7kN, probably by a couple kN. Again, others may be able to give a more precise calculation.

You could always build an multi-piece anchor for your lower pro where the FF would be the highest. You could also use something like a screamer.

Scot Hastings · · Las Vegas · Joined Apr 2013 · Points: 35
D.Buffum wrote: (1) The rating of the piece of protection is the force at which the piece will break, not necessarily the force at which it will pull from its placement. The placement can be quite variable. (2) The rating on your rope is the maximum force your rope will impart onto the top piece of gear in a two-meter, factor 2 fall with a 180 lb deadweight on the end. This fall force calculator puts you at just over 5.25 kN using the data you supplied. Not sure where you got 10 kN.
I could be mistaken, but I thought the UIAA data was measured at the leader (not pro) and was for a slightly smaller (FF ~1.8) fall. I briefly tried to look this up before my post, but came up short.

Agreed that the strength of the rock around the placement could be an issue, though.
Dylan B. · · Unknown Hometown · Joined Mar 2006 · Points: 521
Merlin · · Grand Junction · Joined Mar 2006 · Points: 10
Keithb00ne Boone · · Las Vegas, NV · Joined Jul 2013 · Points: 115

I appreciate the technical side of my question, but how do you apply this in practical application? Are there any general rules you follow when climbing above smaller gear?

Merlin · · Grand Junction · Joined Mar 2006 · Points: 10
Keithb00ne wrote:I appreciate the technical side of my question, but how do you apply this in practical application? Are there any general rules you follow when climbing above smaller gear?
I'd say use one of the simpler fall force calculators putting in various fall distances then plug pieces that are safe assuming that's an option.
marty funkhouser · · Unknown Hometown · Joined Dec 2007 · Points: 30

Your math suggests that your 1st piece was 6' off the belay. I probably wouldn't bother with a marginal piece so low.

Greg D · · Here · Joined Apr 2006 · Points: 878

Some incorrect information above regarding rope impact force rating. This is merely a test result in a severe fall simulation with a 1.77 fall factor, 80 kg weight and a static belay on a brand new section of rope. This is very hard to duplicate in the field. Keep in mind all ropes lose elasticity with use and time. So this number would be higher if you retested this rope after lots of use.

Fall force calculators tend to be way off. There are many factors that determine the force on your top piece. Your weight. Your belayers weight. Fall factor. Rope drag. Elasticity and slickness of rope. The lower the impact force rating on the rope the more energy it will absorb.

A piece rated to 5kN well placed in good rock may be totally solid. If you feel your gear is marginal place gear at closer intervals if possible. Ensure your belayer is not putting unnecessary slack in the system. Keep rope drag to a minimum. Don't fall.

Kirtis Courkamp · · Golden · Joined Mar 2011 · Points: 378
junkfunnel.com/fallforce/

One of the best fall calculators out there
Merlin · · Grand Junction · Joined Mar 2006 · Points: 10
Kirtis wrote:http://junkfunnel.com/fallforce/ One of the best fall calculators out there
Better than the last one but still makes simplifying assumptions about the rope modulus if you look at the source code.
Jim Titt · · Germany · Joined Nov 2009 · Points: 490
Kirtis wrote:http://junkfunnel.com/fallforce/ One of the best fall calculators out there
Rubbish, it uses the wrong rope factor.
Phil Esra · · Unknown Hometown · Joined Apr 2010 · Points: 100

I think John Marsella's post above is the best answer to your question. Rather than doing mental calculations, you are better off doing whatever it takes to not fall. If that requires being on some other, easier route, that is your best option by far.

Small trad gear is really quite dangerous to fall on compared with a bolt, but it is the placement that is more likely to fail, not the gear--even in bomber granite. I don't carry tiny gear on free climbs, because it wastes time and gives a false sense of security. I have no interest in falling on a gray Metolius, and I don't think Metolius really wants you to either.

That's my take, anyway.

Rob Warden...Space Lizard · · Between Zion, Vegas, LA, an… · Joined Dec 2011 · Points: 115

this is entirely anecdotal... but, I have fallen 15+feet without rope stretch on a 00 purple C3 rated 6KN on a 9.4mm rope in Zion sandstone. no blow out. I have taken 20ft falls on 0 and 00 C3's and 1 TCU's no blow out. if that helps at all with your confidence on small gear. a nice bouncy rope and a soft catch do wonders on thin pro.

Jake Jones · · Richmond, VA · Joined Jul 2011 · Points: 1,734

I wish I could find it now, but I have seen a vid of a guy plugging a 000 C3 in a seam and whipping 20+ feet easily on it. He was a good distance up, and had ample gear in, so the FF wouldn't be that huge to begin with. The belayer is in the frame and it looks like there was a dynamic belay. Don't discount the importance of a dynamic belay on small gear. Most often we think of a dynamic belay in terms of the "soft" effect it has on the climber, but it can reduce forces on your top piece as well. A dynamic belay isn't always possible, especially if you're doing multi and off the ground, but it's worth considering.

Brassmonkey · · Unknown Hometown · Joined Jan 2012 · Points: 115
Phil Esra wrote: Small trad gear is really quite dangerous to fall on compared with a bolt, but it is the placement that is more likely to fail, not the gear--even in bomber granite. I don't carry tiny gear on free climbs, because it wastes time and gives a false sense of security. I have no interest in falling on a gray Metolius, and I don't think Metolius really wants you to either. That's my take, anyway.
I have taken a 20 footer on a gray Metolius 00 MC with only 3 lobes engaged and it held. There was only about 30 feet of rope out as well so the forces would have been higher (I fell about 15 feet past my belayer, and was about 5 feet to their left). I completely disagree that small gear is a waste of time and a false sense of security. 5kn can be much harder to create than you think given all the variables in a trad system.
Ryan Watts · · Bishop, CA · Joined Apr 2013 · Points: 25

Calculating fall forces makes for an interesting discussion, but the reality is there are too many variables in the real world for that to give you a good idea of what's going to happen. Sometimes you take a 30ft whipper onto a microcam and it holds, sometimes you take a 10ft fall and it rips.

The main thing to realize is that it's usually the placement that fails (either rock breaking or the piece pulling out) rather than the piece itself. With small gear, especially being relatively new to trad, you really won't know if its good until you fall on it. So the best advice is don't fall.

Probably the next best thing to do would be to bounce test the piece (from somewhere you won't die if it rips). You can generate forces well above body weight with pretty short bounces. If it holds, at least it's not total garbage.

Good luck and don't die!

J W · · Las Vegas, NV · Joined Feb 2004 · Points: 1,637

Also keep in mind that Metolius in particular bases its cam ratings on rock strength- it assumes that the rock is weaker than the piece and rates its cams accordingly. Based on what I know from talking to the boys at Metolius and how they build their cams in general, I'd expect the true strength of their cams to be 2-3kn higher than rated for the little ones. (for the big ones, most of them pull 8kn higher than their rated strength)

rgold · · Poughkeepsie, NY · Joined Feb 2008 · Points: 526
Kirtis wrote:http://junkfunnel.com/fallforce/ One of the best fall calculators out there
According to what standard? That the picture is nice? As Jim says, its rubbish, and you don't need to inspect the source code (god knows what else might be wrong there) to figure out the calculator is worthless. The key is that the rope modulus is estimated by the elongation at 80kg, and this gives a seriously inaccurate result.

Assuming they get the math right, such calculators only stand a chance if they begin with the UIAA impact rating of the rope or the effective rope modulus, if you happen to know it, which seems highly unlikely.

The accuracy of such simple calculators, based on modeling the rope as an undamped spring, is not great; you can think of them as probably overestimating the worst-case scenario for most situations.

As far as I know, there is only one calculator out there that gets the theory right and so is the only one that deserves any attention at all, the one by Jay Tanzman at jt512.dyndns.org/impactcalc….

As for climbing over small gear, the crude theory based on spring models has little to offer. Do your best to place a lot (this often means stopping in places that are not comfortable) and try to climb down rather than fall going for it. Double ropes (properly handled by the belayer, something that cannot always be counted on) offer a significant advantage if you are managing to get overhead protection that is marginal.
BrianWS · · Unknown Hometown · Joined Apr 2010 · Points: 790

Place small pro well and equalize when in doubt. It's tough to break or pull gear when using it properly. Place it well and in good stone, and it will do its job - even the small stuff.

I've caught a fair share of fat-guy whippers onto the smallest C3 with no problems. I've seen larger cams deform and crack on relatively short falls. Pull-test data and computer modelling is cool and all, but kind of pointless for practical use. There are so many factors at play that determine gear safety and functionality that far outweigh a piece's Kn rating...

Guideline #1: Don't be a jerk.

Trad Climbing
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