UKC

I frequently see people in the forums say that half ropes offer less impact force on gear than single ropes because they stretch more. But take these two ropes for example. The first two that I picked.

Cobra II 8.6 mm Half Rope (Impact Force 5.1 kN)
Booster III 9.7 mm Single Rope (Impact Force 7.3 kN)

My understanding is that half ropes are tested with a 55 kg weight and single ropes with an 80 kg weight. So of course the single will give higher impact forces in the tests. If you adjust this for the Cobra (I did this by dividing by 55 and multiplying by 80) then you get an impact force of 7.4 kN. This is almost exactly the same as the single rope.

So, is the statement that half ropes offer less impact force wrong, or have I got myself all confused? I'm sure someone on UKC knows the answer to this!

In reply to TonyB: Did you read the piece on Will Gadd's blog? Someone - Stirling or Blue Water did some tests and found pretty much what you suspect - not a big difference if all are tested with the same mass. I'll try and find the link later.

In reply to TonyB: It's my understanding that half ropes stretch more, therefore transmit lower forces to gear than single ropes, irrespective of how they're tested.

In reply to TonyB:

If you look at ropes which are both certified as double and singles ropes, then you will noticed different impact forces for the different certifications. Then you notice there really isn't that much difference in the real world, just with the test weights.

In reply to wayno265:

It was my understanding too, but I can't find any data to support this which makes me wonder if it's an urban myth.

I'm not really good at physics, but in answer to your point. As single ropes are thicker, more material would stretch and therefore they should absorb more energy by stretching less far.

In reply to TonyB:
from what i can remember about physics the impact force is almost entirely a factor of the time taken for a mass to stop moving because force is a factor of mass X acceleration and acceleration is a factor of change in speed over time.
So if a rope stretches more it takes longer for the climber to stop moving and therefore the force on the anchors is less

You would assume that a thinner rope stretches more than a fat rope but when you get these double/single dual rated ropes I've no idea how much they stretch.
some of the double ropes I've used have been really stretchy (Beal ice lines and zero G G-string eg)

In reply to TobyA:

This looks like the piece you were referring to, interesting reading, thanks for the suggestion:
http://gravsports.blogspot.com/2006/11/single-and-half-rope-impact-forces_3...

To summarise for the lazy... when tested on a 1.77 fall factor drop with an 80kg mass (rather than the half-rope standard 55kg) half ropes give broadly similar impact forces to single ropes.

As Will Gadd points out, this still leaves significant unanswered questions about how impact forces might vary in less severe falls.

In reply to gethin_allen:

That's a good point, I didn't think about how long it took to absorb the force, but now you mention it, it makes perfect sense. The question then would be does a thick rope and a thin rope extend at the same rate? I have absolutely no idea. Also thanks to Tobya and Luke90 for the Will Gadd link. It made for some pretty interesting reading.

In reply to Luke90:
I wonder if testing the ropes on a more realistic fall would result in different results; a 1.77 factor fall is a pretty massive fall that you shouldn't really experience if you do things correctly. If you have a look at this chart showing rope elongation under force

http://theeyegame.com/speleo/Pubs/rlenergy/Default.htm

It's not a straight line because rope is not a simple solid material, and the rope stretches more per kg on a lightly loaded rope than it does on a moderate or highly loaded rope.

So testing a rope with a high factor fall may be over-riding the stretch capacity of the ropes.

In reply to gethin_allen:

I agree entirely!

In reply to gethin_allen:

Fascinating link though, thanks. I do wonder whether it still makes too many simplifications to be truly informative about real ropes though. For example, I'm pretty certain the load-elongation graph will vary in shape depending on how quickly the load is applied and I didn't see any mention of the rate of loading.

In reply to TonyB:

The key thing in this case is impulse (the time it takes for a force to act). The shorter time, the bigger peak load on the gear. The issue is that in real life, a fall has several stages (free fall, slip through the belay plate, rope strech, belayer being moved etc), and some analysis would need to be done to find where the energy was disapated, as I'm sure its not regularly over the time of the fall. I would guess (a relatively educated guess, as a charter engineer), that the belayer has a greater effect than the rope. Get a good dynamic belayer, rather than spending money on a rope

Hi

Half ropes do offer more than just lower impact forces , the use of half ropes to reduce rope drag on long routes is invaluable even more so on long run out routes . Halfs do reduce the impact force on placed gear I wouldn't like to say by how much , but with lower impact force and the ability to reduce rope drag, combine that with being able to make longer absails of routes all speaks for it's self

Just my thoughts

In reply to fraserbarrett:

I'm inclined to think the same as you. In fact I found this on the Mammut website "Measurements by Mammut of typical sport climbing falls show, that with dynamic belaying the difference in impact force between two different ropes is barely discernable. It’s therefore important to provide a truly dynamic belay."

In reply to AndyE9:

I know that half ropes offer several advantages but in other situations single ropes are quite convenient.

In reply to AndyE9:


Please explain to us how you know this.

In reply to TobyA:

He just did? Half rope reduce drag, less drag is better for you when you're leading, provides more opportunity for gear placement *and* reduces the impact force on your gear in the event of a fall (less rope drag means that the shock absorbed is spread more evenly throughout the rope).

In reply to fraserbarrett:


I'm no engineer but I'd be fairly sure that there is going to be a pretty significant impact force on the the climber in the event of a fall no matter how good your belayer is if you haven't spent any money on ropes!!

Harry.

In reply to Wil Treasure:

I think he explained how in one situation half ropes can be an advantage and offer lower impact force. What I really meant was, compared with a single rope used on a suitable route with appropriately extended runners, do half ropes offer less impact force than a single rope? It seems that there may be reasons to suggest that this is the case, but so far no real evidence.

What he said ....

For me climbing trad half ropes , sport single .. Rope drag is less of a factor with sports routes just clip and go ...

In reply to TonyB:

I was just indicating that he had explained that half ropes offer more than just low impact force.

If you do the maths on elongation/impact force stats given by manufacturers the Young's modulus works out about the same for half ropes and single ropes. In other words, in a like for like scenario it won't make much odds to the impact force if you're on a half or a single.

In reply to TobyA: There is no simple answer to this as there are too many variables. I have read the Stirling rope chaps research and it goes as you would expect as they tested to his theory, however whilst they put hlf ropes through the single standard they did not put singles thorugh the half standard.

Two things come to mind here, firstly half ropes are manufactured for adventure climbing and will take a single rope standard loading if needs be but will not conform to the single standard i.e. they will not survive repeated loadings at this level.

If you load a single rope with a lower test load as per the half rope standard you can expect the impact force to be higher than if you load it with the single rope standard as the rope is designed to respond to the higher working load. So for most trad applications a single rope will still give higher impact forces unless you plan to take the 1.77, in which case Bon Chance!

Essentially the Blog you are referring to got hung up on climber weight, and the industry standard test, in reality the impact force on your runners will depend on many more factors and the test is just a test.

In reply to Wil Treasure:


Sorry - I quoted the wrong bit of Andy's post in haste. I meant this bit: "Halfs do reduce the impact force on placed gear".

That's the whole point. Do we really know this?

Tony's question wasn't what are the advantages of using double ropes.