Rope Impact Force and Gear Load

Jake Jones wrote: After rereading it, I misspoke, or worded incorrectly, and I can see how my message could be misconstrued. What I meant to say is that in any fall, the piece that holds the fall will have more force subjected to it than the climber. So, if the stated max impact force of the rope (which, as you mentioned Jim is fairly arbitrary) is say- 8.2kn, and the climber actually feels 8.2kn from the fall, the piece that holds the fall will feel more force than 8.2kn.

Jake you're correct in the sense of a random fall (force on gear > force on climber), however the "impact force" in the UIAA sense is defined as a very specific fall I guess, so:

"Impact force: The amount of force, measured in kiloNewtons, that the first UIAA fall puts on the falling object. "

NC Rock Climber wrote:The one thing I have not seen tested is a Mammut Smart. They claim that it is the best of both worlds; assisted locking and some slip to minimize impact force. It would be great to see some data to prove or disprove this... Geir? 20 kN? Anyone?

the smart will allow some slip in a fall ... god knows ive taken and caught enough whippers on em

on the note of ropes ... ropes with lower impact forces tend to be much more stretchy even with body weight ... those are reflected in the static elongation numbers ...

even with smaller falls the felt force on the climber can be perceptively less ... i find it hard to say definitively that has no translation into less force at the last placement ...

if one were to take an extreme example ... a bungee cord would likely have much less force at the last placement, while a dyneema cord would have much higher, compared to a climbing rope

will it make a difference? ... who knows ... but it having gone from harder catching ropes to softer ones ... im not going back ... its much easier on the back and heels for repeated whippers, even with a dynamic belay ...

I want to reiterate what Jim said. Belayer behavior introduces so much variation in results (even if the same belayer is used over and over) that the far smaller variations due to something like rope impact rating can be almost entirely obscured. Testing protocols sufficient to minimize the confounding effects of belayer variation are at best impractical, making questionable the results of amateur testing with few controls and inadequate number of trials.

As for rope models, every one of them is based on springs and Hooke's law. The standard model is indeed the least sophisticated and is probably only good for getting an idea about certain comparisons, rather than extracting realistic numbers. The CAI models use a damped spring to model each segment of the belay chain and account for friction over the carabiners. They claim excellent fit with experimental data, and they argue rather convincingly that the model is actually better than experiments whose confounding variation obscures the effects of changing elements in the system.

Models like the Pavier model use a combination of springs and dampers. These arrangements bear little conceivable relation to the reality of the rope system, but are justified by the observation that their results fit experimental data. (This seems to be a typical approach in materials science, however.) It is hard to know how much faith to put in such models when they are used on data sets that are not similar to the ones used to find the original coefficients, since there is no direct argument that the model represents any intrinsic physical properties of the system.

I'm sure Beal won't like my saying this, but my suspicion is that, variations in belay and system stiffness being what they are, the actual UIAA impact value is not very important when it comes to the load on the top piece. But I should add that a lower UIAA impact level is, according to the increasingly maligned standard model, an artifact of a lower "spring constant" or "elastic modulus" and, as such, will have load-mitigating effects throughout the range of falls and not, as some have suggested, only for the highest fall-factors.

Jake Jones wrote: It's ok Greg, I'm all for learning, and not much for pissing contests. What I typed is my general understanding of it. Correct me if I am incorrect. I'll be grateful.

I'm not much of a pisser on the proj. So sorry if I offended you. Haven't really read the other post here so sorry if I'm repeating.

Impact force explained:

When a climber falls, kinetic energy is accumulated. The longer the fall and the greater the mass, the more energy is accumulated. All of this energy must get absorbed by different components of the system for you to live. If your rope is made of steel it will absorb very little energy. Other components of the system must absorb the energy including your body. Ouch. Hard catch. High impact on your body. You will get hurt. If your rope is made of bungy cord it will absorb a lot of energy. Yum. Soft catch. Low impact on your body.

Climbing rope impact force explained:

This is not arbitrary. It is a specific test with a specific mass and specific fall factor. It provides information about a rope's ability to absorb energy.

Mass =80kg or 176 lbs
Fall factor =1.77

This is a severe test done with a static belay and a solid block. No harnesses, no body etc to absorb energy. This Is near worst case scenario. It provides you with information. It does play a role in what force will your top piece have to withstand in a fall. All else equal, the top piece will have to withstand greater force with a rope with a higher impact force rating.

The impact force rating on your rope is important in high fall factor fall. But plays a less significant role in low fall factor falls.

All ropes' impact force increase with use and age.

Fu%k! What was the question?

This is the question though, is the spring constant really a constant? The older tests showed it was near enough but ropes have moved on a lot since those days.
Might have to think about testing that after the weekend if I can get a `modern´ rope to play with.

Jim Titt wrote:This is the question though, is the spring constant really a constant? The older tests showed it was near enough but ropes have moved on a lot since those days. Might have to think about testing that after the weekend if I can get a `modern´ rope to play with.

Yes the spring constant remains constant.

But, if you increase the impact force, the constant changes. Therefore, it Is no longer constant.

Ropes do not behave quite like ideal springs.

Geir wrote:Here are some numbers that might be helpful: A friend and I measured the peak force on the top piece in four scenarios: Average peak force on the top piece when taking 10 foot falls with 50 feet of rope out: 820 pounds with a GriGri, 435 pounds with an ATC. Average peak force on the top piece when taking 10 foot falls with 30 feet of rope out: 1030 pounds with a GriGri, 900 pounds with an ATC. These measurements were done with a 10.5mm dynamic line, the climber was 150 pounds, the belayer was 170 pounds. Of course this is not definitive but it helps corroborate some of the other figures people have presented here.

This is not really pertinent to the discussion. Your data shows that the top piece sustains a higher load with a grigri vs an atc. This has been proven over and over again. The queston here is how does a ropes impact force affect the load on gear.

Done tests?

Eric Krantz wrote: Jake you're correct in the sense of a random fall (force on gear > force on climber), however the "impact force" in the UIAA sense is defined as a very specific fall I guess, so: "Impact force: The amount of force, measured in kiloNewtons, that the first UIAA fall puts on the falling object. "

Yes, impact force rating is not arbitrary. And the top piece will experience double the load if there were no friction at the top piece biner.

Example: with a frictionless biner at your top piece, the belayer must resist with an equal and opposite force of the lead fall force in order to stop the fall.

Hence, the top piece will experience twice the force the leader experiences. If the leader is experiencing 8kN, the belayer must resist with 8kN. The top piece will experience 16kN. Huge!

Because of friction at the top piece, these numbers will be much smaller.

Btw, 8kN on the climber is quite high.

Jim, I seem to recall some tests, maybe by Blue Water, that suggested that the spring "constant" depended at least to some extent on the fall height and not just on the fall factor.

There are certainly results that indicate the spring constant is increased by rope fatigue. A number of those tests might be skewed by a failure to control for the effect of knot tightening though.

An interesting aspect of our progress or lack of progress in understanding rope behavior is that the concept of a fall factor is a direct consequence of modeling the rope as a spring. If that model is really seriously off, for example if the results I vaguely remember are significant and the height of the fall, independent of the amount of rope out, really matters, then almost everything written about anchor loads in the last forty or so years may have to be rewritten, and the relevance of the current UIAA tests will have to be re-evaluated.

for practical applications its interesting to note that some manufacturers, like beal, indicate that their ropes will meet their impact force number even after multiple falls in succession

consider a real world hard climbing, trad or sport, scenario ...

youre on this 12 something project with clean falls ... you whip ... you clip in short to the piece to give the rope a chance to rest for a bit ... you dont lower back down because yr trying to work off the moves ... you unclip short ... and you try the move again ... and you whip again .. over and over again

its a very common scenario ... and as we all know ropes get less stretch each successive fall ...

i used to be fine with less stretchy ropes when i was climbing easier stuff ... but for stuff im working out on lead and whipping over and over again, i now prefer softer catching ropes ...

IME softer catching ropes also stay stretchy longer over their lifetime IME ... one important reason to retire a lead rope is because it doesnt stretch as much anymore ... a stiff and not very stretchy rope will get worse, a softer one will degrade as well but be usable longer in that regard

i have no theoretical mumbo jumbo ... just what i use in the real world ;)

Jake Jones wrote: Not at all man. Thanks for taking the time to elaborate. All I was really inferring was two things- that A) load on the top piece will always be higher than load on the climber, and that the rope's "max impact force" rating applies to the force on the climber, and B) that the rating of the rope's "max impact force" is likely to exceed the manufacturer's specifications because the test, although severe, is done on a new rope and the first fall. As the elastic properties of a rope wane with age, impact force increases. I understand that it is likely that no one knows if "max impact force" will be exceeded with an older rope, because it takes quite a severe and dangerous fall to produce forces that high. Not to mention, you would have to have some way of recording it. It could most likely be replicated with a static mass like the original test, but it wouldn't actually apply to a "climber". Regardless, an interesting discussion.

Sort of. But, some of the conclusions you are coming to are a bit off. Let me clarify.

Yes, ropes lose elasticity with time and use. So, all else equal, the impact experienced by the climber will go up with time and use compared to a brand new rope in an identical situation.

The max impact force is only a rating based on a specific test, with a specific mass with a specific fall factor with a STATIC BELAY. It is information only. It is not a maximum allowed in real life. I think you get this for the most part. But, it is nearly impossible in reality as you will see below to exceed the max impact rating..

Yes, the top piece will experience a greater load than the climber. In a frictionless system, the top piece will experience 2 times what the climber experiences. But, due to friction, the top piece will experience only about 1.67 times what the climber experiences.

Therefore, if a rope has a MAX impact RATING of 7.9kN and you manage to exceed this somehow (nearly impossible) and the climber experiences 8kN. Then, 8 x 1.67= 13.4kN on the top piece. Hmmm. Most trad gear is rated between 5 and 10kN. Ooops. Your gear exploded well before you got close the the MAX impact force RATING.

Most lead falls generate 3 to 7kN of force on the top piece. Even a severe lead fall generating 10kN on the top piece imparts only 6kN to the climber.

But, this is nearly impossible to achieve even on a sport route with bolts. WHY:

The impact force test uses a STATIC Belay. Climbers do not.
The impact force test uses a solid block. Climber are not. Bodies deform, harness give, knots tighten, belay devices slip a little. All this absorbs energy!

Edit: So, you would need a very high fall factor with a extremely fat belayer, using a grigri and a very stiff rope to exceed the MAX RATING.

Sorry if this is a little off topic... It seems close enough that a new thread would not be justifiable.

Having reached a comfort level in trad climbing where I'm starting to push my limits and risk taking falls, I have been reading about the force on the top piece and the possibility of it coming out. Realizing that even small fall factors can, in theory, generate forces approaching the strength rating of small cams, and notwithstanding rgold's comment that many factors besides the rope influence that force, I am starting to look at the "impact force" rating of ropes as a factor I *can* control.

To see what sacrifices (besides the obvious increased length of fall) are involved in using ropes with lower impact forces, I tabulated the specs for ~60 rope models from 4 popular manufacturers and found a few not-necessarily-expected patterns.

1) As pointed out elsewhere on MP, rope diameter does not really correlate with impact force. I.e., thinner ropes do not generally stretch more for the same force. Manufacturers appear to have ways of tuning the spring constant independently of thickness.

2) As also pointed out before, impact force ratings for half ropes are misleading - based on 70% of the weight used for single rope ratings (55kg instead of 80kg). I have seen this done for ropes as thick as 9mm. A good indication that this is the case (besides "rope type") is that the product "(impact force) * (dynamic elongation)" is out of whack. I.e., if most ropes take 8kN to catch the UIAA fall at 35% elongation, and one rope manages to use only 6kN at the same 35% elongation, it's a good sign that it was given less energy to absorb.

3) Impact forces for half ropes used as twins (clipped together) are not dramatically higher - not even by the square root of 2 predicted by the ideal spring model. They are, however, among the highest impact force numbers published - typically over 9kN (whereas all single ropes I looked at have impact forces below 9kN).

4) Each manufacturer appears to have its own target for impact force - one likes 7.5kN, another 8kN, and two manufacturers like 8.5kN. There is some variance within each manufacturer, but the centroids appear quite prominent (especially if we ignore the half rope ratings, which are tested with a smaller weight).

I'm not saying that any of this is terribly important in actual climbing. It does, however, seem important on rest days in front of a computer.