The best rope? ;-)

[Scott McCrea]

yes, it is that simple.

So, then where does friction fit into the equation? There are variables. What if the rappeller descended without a rope? I doubt 400+ degree (F) temps would be generated then.

[David_Campen]

As to the "bounce=lost energy" equation, it makes sense to me, tho like Scott I didn't do so well in HS Physics.

The solution to that is to learn how to frog properly so that there isn't all that bouncing. Again, if someone is bouncing wildly while frogging on any normal brand of caving rope they should blame their technique not complain that if they just had PMI rope everything would be OK; there is just not that much difference between the ropes.

[David_Campen]

What if the rappeller descended without a rope?

If someone tried rapelling without a rope I would expect them to crater. Are you talking about freeclimbing down a cliff?

[RescueMan]

As to the "bounce=lost energy" equation, it makes sense to me

It depends on technique and timing.

If you can time your ascending cycles to match the frequency of the bounce, then you can actually use the rebound energy to help lift you upwards.

If your "bounce" and the rope's bounce are out of phase, then you're not going to be a happy climber.

Think about manually pushing a stuck car out of a muddy road. If you time your pushing with the rocking motion of the car, you often find you can move the 6,000 lb. beast - if you're timing is off, then not.

- Robert

[cob]

yes, it is that simple.

So, then where does friction fit into the equation? There are variables. What if the rappeller descended without a rope? I doubt 400+ degree (F) temps would be generated then.

I am not real sure how the physics describe this Scott, but I wonder how much heat is generated (and dissipated) when one craters? Compression does generate heat. As I understand it, the energy is the same, the only difference is how it is expressed (more or less)

Dave, is that right(more or less)?

Dave, on the diff between BW and PMI, in my experience BW does have more stretch to it. This is not a slam on BW. I like it and use it. In fact I have found that it affects my climbing none at all (like you, up to depths of 300'). Technique is the main thing, and with my RW, once I hit my rythm, it is like riding a bike: my first step takes out the stretch and the rest just hold it (clear as mud, eh).

I will bow down to your greater experience with the frog, unless someone comes to my rescue?

tom

[RescueMan]

The solution to that is to learn how to frog properly ; there is just not that much difference between the ropes.

Actually, there's quite a bit of difference in the static elongation of the various ropes.

With a 300 lb load, the BWII+ 11.6mm has about 2.3% elongation, Sterling HTP 12mm 1.4%, PMI Classic 11mm 1.2%, (all of these ropes are labelled 7/16").

The static stretch of the BW is double that of the PMI. That makes a difference.

- Robert

[hank moon]

...I heard their pleas for help. Neither knew how to down climb, ...tom

God must love American cavers...bouncing Fantastic without ability to d/c? Wow. Wonder if they knew how to change over?

[lostgravity]

With a 300 lb load, the BWII+ 11.6mm has about 2.3% elongation, Sterling HTP 12mm 1.4%, PMI Classic 11mm 1.2%, (all of these ropes are labelled 7/16").

Is there any other source for such data in addition to the manufacturer's website? Just asking because, when I initially started my research, I wasn't able to find this information for some of them...

Thanks,
David

[Tim White]

Actually, there's quite a bit of difference in the static elongation of the various ropes.

With a 300 lb load, the BWII+ 11.6mm has about 2.3% elongation, Sterling HTP 12mm 1.4%, PMI Classic 11mm 1.2%, (all of these ropes are labelled 7/16").

The static stretch of the BW is double that of the PMI. That makes a difference.

- Robert

Robert!

[Scott McCrea]

Alright, I just finished reading this book:

How to Impersonate an Engineer. minimum grade 10 required.

Now, I can accept that there is a specific amount of energy associated with a descent. And no matter what, a specific amount of energy needs to be released to go down. So, how do you explain the variation in temps recorded?

[David_Campen]

I suppose a good bit of energy could be going into friction of the rope fibers rubbing against each other as the rope flexes around the bars of the rack so that a significant amount of heat is deposited inside the rope and all along its length. Also that different ropes could exhibit different ratios of internal friction due to flexing vs friction from the rope rubbing against the rack bars.

Hmm, is this what you were suggesting in your earlier post?

[Scott McCrea]

I suppose a good bit of energy could be going into friction of the rope fibers rubbing against each other as the rope flexes around the bars of the rack so that a significant amount of heat is deposited inside the rope and all along its length. Also that different ropes could exhibit different ratios of internal friction due to flexing vs friction from the rope rubbing against the rack bars.

Hmm, is this what you were suggesting in your earlier post?

There is certainly friction happening when a rope is bent and the internal fibers rub against each other. But, wouldn't the friction between the sheath and the bars have a much greater influence on the temp readings? If not, then I suppose I could replace the bars on my rack with rollers.

[David_Campen]

But, wouldn't the friction between the sheath and the bars have a much greater influence on the temp readings?

I would sort of think so. Still, it remains that if a person of weight W rappels distance D (and assuming gravity is some invariant value) then W times D amount of energy has to be converted to heat. The type of rope, bars, rack, speed etc does not affect the value W times D that has to be converted to heat if the rappeller is to be traveling at near zero speed when they touch the deck.

[hunter]

Amazing where this discussion went while I was busy...

Scott, I think there are a couple of things that could change surface temps:

1) Time, Because heat transport happens per unit time the faster you go the less time energy has to dissipate and hence the hotter things get. I don't think this really comes into play unless people consistently go faster on a given type of rope (due to diameter or something).

2)The amount of heat absorbed by your device changes. If you put a certain amount of energy (heat) into 1 pound of aluminum it will rise to a higher temperature than if you put the same amount of energy into 2lbs of aluminum. This means larger devices of the same material will have a lower surface temperature (unless you go very fast).
At the same time different materials have a different heat capacity(aka specific heat). To raise 1Lb of aluminum 1 degree C it takes roughly twice as much energy as for 1Lb of iron (I got this from looking at the specific heat of different metals on the web). In the end this means if you don't go really fast and you have a 1Lb aluminum device vs a 1Lb steel device the steel one will be hotter.

3)The amount of energy (heat) dissipated changes. I can see three parts to this and I'm not really sure how much they come into play. It seems to me that the rope can dissipate some heat (as discussed), the device can dissipate some heat (like heat going to the frame of a rack from the bars) and some heat can be dissipated into the air.
The first part might change depending on rope type but I'm a bit sceptical. In practice I think this mainly comes into play when you have a wet rope. Water is very good at dissipating heat and has a high heat capacity so a wet rope will give you a much lower device temperature.
The second one is tough to tell, A smaller device always gets much hotter but I'm not sure how much is due to heat capacity and how much is due to dissipation (I suspect it's mostly heat capacity but I don't know).
The third one is also tough, I think the times involved mean this is a minimal factor but I'm not certain. Certainly cooler air will give you a cooler device at the bottom but I don't know how much cooler.

Anyway, that ended up being more of a ramble than I expected. The one other thing that might change the behavior you see with the sheath melting is just different melting points for different materials.

Hunter

[Scott McCrea]

The type of rope, bars, rack, speed etc does not affect the value W times D that has to be converted to heat if the rappeller is to be traveling at near zero speed when they touch the deck.

Ok. So, let me restate my original conclusion... I believe the 48 carrier sheath of the Sterling rope causes more heat to gather in the bars than other ropes with lower carrier counts.