Physics question - DMM Revolver

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Lasti

Trad climber

Budapest

Topic Author's Original Post - May 15, 2009 - 09:46am PT

My friends and I have a rather long ongoing discussion about the effect the introduction of a DMM Revolver carabiner into the belay system has on forces on anchors in the event of a fall.

Since we do not have easy access to testing equipment and do not know all the characteristics of the ropes and hardware involved, I thought that smart Tacoans could answer our question either by convincing theories and calculations or empirical tests.

So. One of the alleged pros of putting a DMM Revolver into protection and clipping the rope into that instead of a normal carabiner is that in the event of a fall the low rope drag at the DMM Revolver means that the bottom section (from top piece of protection to belayer) of the rope participates more actively in the dynamic process called in the wolrd of physics 'catching yo ass', distributing force more evenly throughout the rope, leading to a softer fall.

An argument against this was that though a longer section of rope participates fully in the elongation, introducing a pullley into the system also means that forces will be higher in the lower section of rope, ultimately making the forces on the anchor higher than with a normal carabiner, where some of the force in the top section (pro to falling leader) is dissipated through friction in the top carabiner.

Any thoughts?

Thanks,
Lasti

p.s.: So Locker doesn't have to:

No matter what carabiner you use,

"Yer all gonna die!!!!!"

rgold

Trad climber

Poughkeepsie, NY

May 15, 2009 - 10:16am PT

Well, without cranking up the calculation machine, I'd note that there is also an effect contrary to the one you described that increases anchor load without the revolver, which is that the inabilility of the lower section of rope to participate fully in energy absorbtion leads to a "stiffening" of the system, since a shorter initial section has to stretch more than it would have to, if there had been less friction, in order to absorb all the fall energy.

So the use of the revolver reduces the stiffness of the system, lowering the peak anchor load, but also reduces carabiner friction, increasing the peak anchor load. I'd guess that in many situations, the reducing effects from having the rope participate more fully in energy absorbtion would be more decisive than the increasing effects from less carabiner friction. I'd also guess that in many cases, the differences are not substantial and might well be completely overshadowed by differing belayer behaviors anyway.

By the way, if you get a lot of friction out of the system, the belayer may have to cope with higher loads than they are used to, slippage may happen at the belay, and then anchor loads go down, at the expense of longer falls and possibly singed belayer flesh.

Personally, those revolvers look too scary and weigh too much for me to carry, but as a fairly dedicated half-rope user, I've got less friction in my systems anyway.

MisterE

Trad climber

One Step Beyond!

May 15, 2009 - 10:21am PT

Just use them on anchors for toproping and lowering - it saves sheath wear and rope drag. Then you don't have to go through all of the....oh, wait - you want to, I forgot.

Carry on.

Dr.Sprock

Boulder climber

Sprocketville

May 15, 2009 - 11:13am PT

more rope stretch means more time during your acceleration to a stop.
this means less shock to the system I would think.

so more time and distance to stop, seems like a no brainer.

but, are you talking about after the fall is over and the guy is hanging?

ok, then, yes, there will be a slight difference, but this is a static situation, static friction has locked the rope to the biner hopefully so your hands do not get sore.

but as soon as the rope slips, you will have dynamic friction, which means less friction, more pulley effect.
so just don't let the rope slide.
here is a good link on pulley effect in climbing:

http://www.southeastclimbing.com/faq/faq_pulley.htm

Studly

Trad climber

May 15, 2009 - 12:09pm PT

The Revolvers lessen carabiner friction thus bringing more rope into play on a fall and you get a softer catch. I have taken lead falls on them(up to 25 feet). They sustained no damamge to the pulley, hold up great, and I love em! I use a few on quickdraws for places where normally you might use a runner. Get with the program, they make a huge difference when leading and don't weight much more then a normal biner.

Yvergenhauf

Gym climber

May 15, 2009 - 12:18pm PT

It seems to me like the only difference between the DMM and a regular 'biner is a little bit of friction effecting the tension on either side of said 'biner. I would agree with previous opinions that the main benefit is reducing friction when you have a piece way off to the side.
If you are interested in reducing forces my two suggestions would be a dynamic belay and Yates screamers.

dustonian

climber

Foresta

May 15, 2009 - 12:18pm PT

Less force on the pieces with a Revolver, more force on the ones without.

Josh Higgins

Trad climber

San Diego

May 15, 2009 - 01:56pm PT

I read somewhere recently that the pulley is meant to reduce rope drag only, and that under relatively mild loads it just locks up and acts like a normal carabiner? Is this true? Is this discussion a moot point? If so, how long was the OP arguing/discussing this with his friends? :)

Josh

Lasti

Trad climber

Budapest

Topic Author's Reply - May 16, 2009 - 06:47am PT

Thanks for all of your contributions.

Rgold: I described the effect you mention firstly as the alleged pro of the Revolver, but you described it quite nicely, thanks.

MisterE: Yes I want to, lots of beeers to be had. (Though many have been consumed during the discussion so far...)

Dr. Sprock: Neat site, some good data and thoughts there. Petzl says carabiner-rope friction force is about 66%. Interesting, that's a LOT of friction.

New thoughts:

That 66% Petzl data is obviously only true when the rope breaks at a high angle over the carabiner. (Is it static or kinetic friction - we'll never know) When falling the rope goes from (let's say) zero degrees to 180 degrees. Peak force would probably be around 180° since that's when you are under the rope on a vertical wall (overhanging wall: you never get to 180°). During the fall itself how much energy is transferred to the lower section of rope before peak load (and peak friction)?

I agree with del cross that different fall factors would mean that the advantage of more rope absoring energy would differ greatly. It seems to me that you gain the most rope when the lower section of rope is long relative to the top section. This is a low fall factor scenario, where loads are not that high anyway. Of course going from e.g. 'friction-induced' 1,5 to 'real' 1,2 is a substantial gain, but would that be true? I don't see me falling 30% farther because of the Revolver...

Full closure pending on this one. Someone will have to chuck himself of a cliff with a dynamic force gauge attached like a screamer to the top piece. Till then - no one knows.

Lasti

JohnRoe

Trad climber

State College, PA

May 16, 2009 - 07:34am PT

I posted a model in the "should the leader clip the belay anchor" thread which can be used to investigate this. Here's the link: http://www.supertopo.com/climbing/thread.html?topic_id=731822&msg=732669#msg732669

To model a Revolver as the top piece, you want to increase the number "Carabiner Coeff of Friction". This is the proportion of force that the top carabiner passes through, so =1 for a totally frictionless biner at the top. (Yes, I know that is a non-standard use of 'coefficient of friction' - I'm sorry.)

In the simulations, "anchor force" = force felt by the top piece, "belayer force" = force on the belayer.

Depending on the rope lengths out, reducing the friction at the top carabiner may reduce or increase the maximum force felt by the top piece (this is the "pulley effect", debated at length in the earlier thread!)

Standard disclaimer: These models apply to a mathematically idealized situation which will not reflect all the variables experienced in actual climbing.

JohnR

Lasti

Trad climber

Budapest

Topic Author's Reply - May 16, 2009 - 09:43am PT

JohnRoe!

Great stuff, didn't go through the calculations (yet) but didn't see any major criticism of your model in the other thread, so let's presume it's OK. (A common internet mistake.)

Playing around with it I found this:

(All calculations with g=9,8 (getting lazy are we? no air resistance?, no latitude-altitude correction?), 80-80 kg of mass (almost wrote weight) for fly-boy and belayer alike and rope modulus of 11200 N (given by JohnRoe))

Friction: 0,66 1

Rope out 60
lower section 55 4253 4235
lower section 50 5174 5173

Been wimpy up till now...
lower section 45 5858 5912
lower section 40 6416 6543
lower section 35 6891 7102

Now we're talking major air time...
lower section 30 7307 7610
lower section 25 7678 8078

Big-ass whipper territory (not big ass-whipper mind you)
lower section 20 8013 8515
lower section 15 8320 8926
lower section 10 8603 9315

Camp 4 campfire lore...
lower section 5 8865 9629

If JohnRoe's model takes into account all the effects described previously, it seems that 'pulley effect' is stronger than 'more elongating rope in play effect' except at low factor falls where forces are relatively low as is. (Though involuntarily falling 30 feet is never much fun.)

Does the model include everything described here? Is it correct? Did we miss anything?

I'm still all for jumping off cliffs with a force gauge as a top piece.

Lasti

EDIT: The numbers looked much better in the editing window, they were further apart and readable. Sorry. Summing up the numbers:

Frictionless pulley gave 0,1-0,5% less anchor force than 0,66 carabiner at wimpy falls and up to 8,6% higher anchor force on A5-approved death-falls. Not much difference in my book either way, a slightly dynamic belay or even holding a small umbrella or wearing a Tarbuster look-alike hat would make a bigger difference.

Dr.Sprock

Boulder climber

Sprocketville

May 16, 2009 - 11:26am PT

just get two identical ropes, two different biners, two identiacal concrete or steel weights.

tie the weights onto the ropes.
pass the rope thru the biner.
lift both weights with your arms.
put a fish scale between the biner and anchor.
not a golden trout fish scale, a cabo tuna scale.
notice the weird gyrations as the friction goes from static (your car tyres during a normal stop) and dynamic (your car tyres sliding during a panic stop-less friction)
this is no man's land. even the physics phreaks don't understand this in between stage completely.
this is where all the weird forces and impulses will jerk the piece right out of the rock.
drop both weights and feel for differentials.
then go have a beer and watch sports final.

Lasti

Trad climber

Budapest

Topic Author's Reply - May 16, 2009 - 11:56am PT

Ah yes Dr. Sprock, the definite empirical proof. That's what I'm after, just my friend at the University of Technology who did a few tests of this and that in the past has finished his thesis and moved on to the real world (poor fellow, at least he got a good 15 years out of it).

Fish scales? Tuna? I don't know. I think 3-4 kN is not exactly a high anchor force for a real-life fall and many different fall factors and fall lengths should be tested. But the beer bit has and will be done repetitively for good measure.

Lasti

Studly

Trad climber

May 17, 2009 - 12:25am PT

You guys need to get out and do some climbing and try the Revolver. Thats the main reason to use it is to reduce rope drag, both on lead and if you set a top rope anchor. It makes leading a joy where the rope snakes or zig zags around when used selectively. I have 5 of them and for long free routes that move all over the place, the Revolver rocks.
On top rope, it makes it much easier on the belayer tending the rope, the rope slides generally much easier.
As far as allowing more rope stretch or more fall factor, etc., I think that the difference is negligible, but take a few whippers and find out.
Hey, Lasti, are you the Hungarian guy that was with the CMC gals in Red Rocks a few months ago and wanted a shot at TR'ing Pale Ale 11a, but it got dark before your turn?

Lasti

Trad climber

Budapest

Topic Author's Reply - May 17, 2009 - 05:43pm PT

Studly, we all know reducing rope drag is one of the advantages of having a Revolver on your rack, but we just need to have discussions with numbers and clashing theories to whet our geekly appetites.

Red Rocks? Nope wasn't me, might have been Oszi et al. They were at Red Rocks fresh off Tribal Rite last year.

Lasti

dustonian

climber

Foresta

May 18, 2009 - 12:23am PT

Indeed, the difference in forces in fall is negligible, but the friction is greatly reduced. This is mostly an academic discussion, that's part of its charm I guess, really.

Jaybro

Social climber

Wolf City, Wyoming

Jul 15, 2009 - 12:10pm PT

Seems like a bad idea for belaying, the rope will go through that much faster and the belayer will have to be extra head's up. Not that there aren't places to use it.

Paul_in_Van

Trad climber

Near Squampton

Jul 16, 2009 - 02:01am PT

Aren't they supposed to pay attention? This is life and death after all, not just a theoretical argument...

Oh, wait. Nevermind. Forget I mentioned it.

P