impact force please explain

I'm shopping for ropes and looking at the mammut revelation 9.2mm. It looks nice and light, and currently 29% off.
here are the specs
Specs:
Diameter: 9.2mm
Type: Can be used as a single, half, or twin rope; dry-treated
Weight: 55g/m
Static Elongation: 7.2%
Dynamic Elongation: 31%
Impact Force: 8.7kN (single), 6.6kN (twin), 10.1kN (half)
UIAA Falls: 7-8 (single), ≥ 20 (twin), ≥ 20 (half)

Why is the impact force so dramatically higher when used as a half rope? I'm confused, shouldn't it be the same as when used as a single rope?

It should actually be LESS. As I recall, the dropped weight the UIAA uses to test ropes as half/double ropes is substantially lower than the weight used for singles. Maybe a typo, and it should be in reverse?

I think they have the twin and half rope impact forces reversed. Half rope force is lower than single because they use a lower mass in the drop test (single 80kg, half 55 kg). Twin is higher than both (single and half) because both ropes are tested together (in parallel) with a 80kg mass.

bealplanet.com/sport/anglai…

Since Revelations are rated for use as Single / Half / Twin ropes the following table contains three figures for various tests. Half rope testing uses a 55kg mass rather than 80kg as is the case for Single and Twin testing, hence the far lower value for Impact Force.

Diameter9.2mmWeight55g/mSheath Proportion36%Number of Falls7-8 / >20 / >20Impact Force8.7 / 6.6 / 10.1 kNDynamic Elongation31%Static Elongation7.2%Sheath Slippage0mm

Single/half/twin - 8.7/6.6/10.1

So the impact force quoted for a half is 6.6 not 10.1, taken from the site below, so I reckon there's a mix up somewhere!

dicksclimbing.com/products/…

I can't really discuss the particular rope, but, boy, can I generalize, or what?

A rope can be treated as an energy dissipating spring. Let's call it ideal since this is internet and things should be kept as simple as possible. And, let's look at force F hanging on that rope, here force F will be equal to the mass of climber multiplied by free fall acceleration which varies somewhat depending on diet. The rope has spring constant k. When subjected to force F, elongation is going to be L=F/k. Higher k will lead to lower elongation, higher impact force.

The question then rises - what if you take two identical ropes and do funky things with them? If one decides to stick to simple funky stuff, ropes can be tied in series - one after other, or in parallel - this is half/double rope configuration.

Well, then, what the F happens with spring constants? Thankfully, inquiring minds spent some time investigating this, and some soul created Wikipedia Entry On Series and Parallel Springs

But, you say, WTF?
I say - when tied in parallel, the rope system will have equivalent spring constant 2*k, resulting elongation for the same force will be (F/k)/2. As you may notice, elongation is half of that for single rope. Less elongation will equal a more violent stop, hence higher impact force.

So, ropes in parallel - less dynamic elongation, more impact force

In reality, one should look at energy dissipation in the rope system, and solve some differential equation, but this back of the envelope discussion is good enough for me.

amarius, I don't think this is what the op was looking for but there is a more complete discussion of the difference between impact forces from ropes used as twins (parallel) and halfs /singles for the same test here:

mountainproject.com/v/doubl…

Short answer: in the ideal lossless spring case, the impact force from ropes used as twins should be 1.4 times higher then the same rope used as a single.

Xam wrote:amarius, I don't think this is what the op was looking for but there is a more complete discussion of the difference between impact forces from ropes used as twins (parallel) and halfs /singles for the same test here: mountainproject.com/v/doubl… Short answer: in the ideal lossless spring case, the impact force from ropes used as twins should be 1.4 times higher than the same rope used as a single.

Yes, you are right. I attempted to give a simple explanation to what happens when two identical springs/ropes get combined in parallel - the resulting system is going to be stiffer, will result in higher impact force. I did not attempt to get the actual factors, not that the math gets any more complicated by a square root, it simply doesn't add to the story.

karl kvashay wrote:I'm shopping for ropes and looking at the mammut revelation 9.2mm. It looks nice and light, and currently 29% off. here are the specs Specs: Diameter: 9.2mm Type: Can be used as a single, half, or twin rope; dry-treated Weight: 55g/m Static Elongation: 7.2% Dynamic Elongation: 31% Impact Force: 8.7kN (single), 6.6kN (twin), 10.1kN (half) UIAA Falls: 7-8 (single), ≥ 20 (twin), ≥ 20 (half) Why is the impact force so dramatically higher when used as a half rope? I'm confused, shouldn't it be the same as when used as a single rope?

Those numbers are wrong

However the half rope impact force may not be realistic since it is tested with a 55 kg weight rather than 80 kg like single/twins as it assumes the second strand will take some of the weight

For more on this go here

willgadd.com/single-and-hal…

;)

bearbreeder wrote: Those numbers are wrong However the half rope impact force may not be realistic since it is tested with a 55 kg weight rather than 80 kg like single/twins as it assumes the second strand will take some of the weight For more on this go here willgadd.com/single-and-hal… ;)

Actually my understanding is that they use the lower mass not assuming the other strand takes the weight. Instead they originally required surviving 1 of the same 80kg test as single ropes, however, many/ most of the half ropes would survive exactly 1 drop with the heavier mass and as a tool used to compare different products having them all show up as "1" does not really help so they ended up lowering the mass to get a spread of results. However the initial idea was to make sure that even a single half rope could survive at least 1 of the "worst scenario" test drop and thus your real world fall.

NickMartel wrote: Actually my understanding is that they use the lower mass not assuming the other strand takes the weight. Instead they originally required surviving 1 of the same 80kg test as single ropes, however, many/ most of the half ropes would survive exactly 1 drop with the heavier mass and as a tool used to compare different products having them all show up as "1" does not really help so they ended up lowering the mass to get a spread of results. However the initial idea was to make sure that even a single half rope could survive at least 1 of the "worst scenario" test drop and thus your real world fall.

Well if you look at the UIAA drop test youll see that if you were to have a second half rope attached to a belay device in their scenario, it would take some of the force ... Theres a gap of 300m from the "belay device" to the first runner

alpy4000.cz/soubory/UIAA101…

The problem is that unless we are falling off the belay or from closely spaced pieces on each rope, its likely that a single strand will take the majority of the load in a "normal" fall ... Especially on wandering or traversing pitches which is where half ropes have the advantage

;)

old post by Erik W with a bit of history from primary sources.

What? 7% elongation.