"gear to slow you down"

[layton]

this is for the engineers and physicists

 

Does putting a shitty piece in that will probably blow actually "slow you down" and exert less force on the next piece? Insignificant, a few kN, or a lot?

 

Same thing goes for, say, an unequalized anchor - like a horizontal clove hitched anchor. If piece one fails at, let's just say 5kN, and piece two and three are equally shitty, will your anchor rip?

 

The theory I heard from someone today is that there's no time for the rope to relax - but if the climber is a 10 kN force of accelerating mass, and the first piece can hold 5kN, wouldn't (rope stretch or not) the force be reduced to 5kN (10kN-5kN)

 

I've always assumed this, and I hope my assumption was right, but if I'm wrong, I'd sure like to know.

[Rad]

From a biologist who took college physics 24 years ago:

 

If the piece that fails slows you down then it has reduced the force on the lower piece. To see how, consider the simple math of F = MA:

 

Force = Mass times Acceleration

A = (Ending velocity - starting velocity)/time for change in velocity. Note that force, velocity and acceleration are all vectors, meaning that they have both magnitude and direction.

Mass won't change, so F is directly proportional to A.

 

Two factors affect A:

1 - Time. Longer stopping times result in lower forces. Milliseconds count. This is the theory behind the design of crumpling bumpers, air bags, and dynamic climbing ropes. It also explains why auto-locking devices create higher forces on gear than belay devices that slip and thus have longer stopping times.

 

2 - The difference between starting and ending velocities. Lower starting velocities result in lower forces generated. This explains why slab falls, where falling speeds are lower, generate less force on pieces than steeper falls. Final velocity should be zero (if the second piece stops you!). So if the first piece slows you down the force on the second piece is reduced due to the lower change in velocity during its attempt to stop the falling climber.

 

Another way to look at this is to look at the piece that failed. Ffailure = MA. If you know the breaking force of a piece (e.g. 5kN), and you know the time it will take to stop the climber (virtually impossible to measure), then one could, in theory, calculate the amount the first piece will slow the climber. Again, Ffailure = M x (Vat failure - Vstart)/Time from impact to piece failure. As noted above, if the climber is slowed, the force on the second piece is reduced.

 

So "psychological" pro may actually have value by reducing the impact force on lower pieces. One danger, though, is that when the rope comes taut on the top crappy piece, it might change the orientation of lower pieces, causing them to pop out or rotate to positions where they will fail at lower forces. Then when the top piece fails the piece below may no longer be bomber (e.g. a cam has rotated into a wider pod or a nut has moved out of a constriction).

 

Note that I haven't said that if the top piece fails at 5kN then the force on the lower piece is reduced by 5kn. It probably depends on how much velocity you 'recover' after the first piece fails and before the rope comes tight on the second piece.

 

I'm curious to see what real engineers have to say.

[rob]

this is basically the same principle as a screamer, isn't it?

 

You'd have to also factor in the distance between the failed piece and the piece that ends up stopping you -- the longer you fall after blowing a piece, the closer you get to achieving your original potential energy.

 

(from a physics dropout)

[Rad]

Edit to add that if your friend's assertion that the rope cannot relax between the first piece failing and the impact on the lower piece is true, then this will act to increase the force on the lower piece because the longer stopping time due to rope stretch has been eliminated. Then you come down to a question of whether the magnitude of this effect is large or small compared with the change in velocity effect as noted above.

 

Interesting question.

[Rad]

this is basically the same principle as a screamer, isn't it?

 

Yes, a screamer reduces force on a piece by increasing the stopping time.

[G-spotter]

I read in a British climbing magazine that if you place six or seven poor or bomber but weak nuts, like #1 RPs, in a row, and then fall on them, each one absorbs energy in breaking or failing and thus the first 6 may break or rip but the 7th one holds you, whereas if you had only placed the 7th it would fail. I think the point here is that it's best to place several poor pieces "in series" and close together. Two shitty pieces 10m apart aren't going to have much effect.

[lummox]

The extra friction of an additional gear placement whether it fails or not will lessen the load on everything.

Edited September 28, 2012 by lummox

[layton]

yeah, none of this is new - and I assume is correct and what I was hoping to hear. what i was wondering was if that our assumptions (based on pretty logical thinking) were wrong - as they sometimes are.

 

i think the only qualified response would be someone with a PhD in physics...sure we got one here somewhere? I'd ask skykilo but he majored in magic

[lummox]

i think the only qualified response would be someone with a PhD in physics...

Why do you assume you didn't get the "qualified" response you sought?

[mmeyers]

haha. a PhD in physics will mostly likely be able to explain to you the behavior of the wave function of particle colliding with your gear placement. no offense to any physics PhDs out there

 

I have a masters in mechanical engineering, and I approve the basics of what Rad is saying. pulling gear transfers energy from kinetic (falling) to other forms (thermal, material deformation, noise, etc). lower kinetic energy means lower velocity.

[layton]

found this on rockclimbing.com

http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?post=2589801

"A situation to watch out for with three-point anchors arranged horizontally is that a piece on one of the two outer arms is relatively weak. This should be avoided if possible. If an outer arm blows with the standard symmetrically rigged configuration, all the load will transfer to just a single piece, the middle piece, and the third arm will not be loaded unless that middle piece also blows, setting up the cascade failure scenario that seems the most likely way for a multi-point anchor to fail. (The fact that there is no momentary relaxing of tension in this scenario means that the extraction of anchor pieces will not reduce fall energy to any significant degree.) "

 

that defies common sense to me

[G-spotter]

Maybe your sense isn't common.

[mmeyers]

found this on rockclimbing.com

http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?post=2589801

"A situation to watch out for with three-point anchors arranged horizontally is that a piece on one of the two outer arms is relatively weak. This should be avoided if possible. If an outer arm blows with the standard symmetrically rigged configuration, all the load will transfer to just a single piece, the middle piece, and the third arm will not be loaded unless that middle piece also blows, setting up the cascade failure scenario that seems the most likely way for a multi-point anchor to fail. (The fact that there is no momentary relaxing of tension in this scenario means that the extraction of anchor pieces will not reduce fall energy to any significant degree.) "

 

that defies common sense to me

 

I think he's talking about 3 pieces placed horizontally, and tied together, such that the middle arm is the shortest. assume an ideal knot, and ideal arm lengths. if we assume good equalization, then we can reasonably assume that the middle arm already has a higher load than the outer ones. if, under load, one of the outside pieces failed, then yes, the middle piece would take most of the load because it still has the shortest arm. where he's wrong is that the the 3rd arm would still carry some load, assuming elasticity in the arms, and it would be more than it was carrying when all 3 pieces were intact.

 

the error in his logic is that he says the 3rd piece isn't carrying any load until the 2nd piece blows. if it was all "equalized", and the 3rd piece is not seeing any load, then the 1st piece would not see any load either, and therefore would not blow. all the load would be on the middle piece.

 

what he's describing would happen if there were 3 totally unequalized pieces in series. I think.

[layton]

This is the only sentence I care about "The fact that there is no momentary relaxing of tension in this scenario means that the extraction of anchor pieces will not reduce fall energy to any significant degree"

 

1st, is this true? how?

 

2nd, if you applied this in a lead situation (pieces are not equalized and are shock loaded), wouldn't that mean ripping gear would not lessen then force on each successive piece?

[mmeyers]

I'd say no, with one caveat.

 

if the piece that pulls is complete crap (ie, a piece of scotch tape or something), then yes, pulling that piece will not reduce the fall energy by any significant degree. you'd basically have a 2-piece anchor with a bit extra slack in the system. however, duct tape is a different story.

 

IMO, "momentary relaxing of tension" is meaningless. If you assume that the rope and slings are perfectly inelastic, then pulling a piece will still result in energy loss, and will reduce fall energy.

 

the measure of his use of the word "significant" is proportional to the quality of the placement of the piece that fails.

 

I expect tvashtarkatena to weigh in on this.

Edited September 28, 2012 by mmeyers

[boadman]

If the rope becomes static due to the impact from the first piece failing, and there is several feet between the two pieces, the added force on the second piece from the now-static rope could be enough to cause it to fail.

 

I think the only way to decide conclusively would be to do a bunch of testing with some load cells and find out what actually happens in the real world.

[alpine et]

I can't imagine that any two pieces would be put so close together that the rope wouldn't be able to unload after the top one blows. The rope is a really long spring and once stretched will recoil very quickly. The pieces would have to be almost equalized to keep the rope from relaxing... the second piece would have to be recieveing load before the first one failed.

[alpine et]

found this on rockclimbing.com

http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?post=2589801

"A situation to watch out for with three-point anchors arranged horizontally is that a piece on one of the two outer arms is relatively weak. This should be avoided if possible. If an outer arm blows with the standard symmetrically rigged configuration, all the load will transfer to just a single piece, the middle piece, and the third arm will not be loaded unless that middle piece also blows, setting up the cascade failure scenario that seems the most likely way for a multi-point anchor to fail. (The fact that there is no momentary relaxing of tension in this scenario means that the extraction of anchor pieces will not reduce fall energy to any significant degree.) "

 

that defies common sense to me

 

 

This scenario is quite different than climbing on lead. If the pieces are equalized, then the coredelette will not relax inbetween the first piece failing and the next one ripping out. That's not to say that the first piece didn't decerase the load, though[i mean load decreased via rope stretch, not cordalette, while this piece rips, not much energy absorbed by the actual piece blowing]. There is presumably a rope in the system that is stretching and taking load as that fisrt piece of the anchor fails. So the load can be lower on the next piece, but not beacuse of any energy absorbed by the cordelette. However I think this gets a bit too academic, as when I build my 3 point anchors, I make damn sure the 3 pieces are bomber.

Edited September 28, 2012 by alpine et

[genepires]

This is the only sentence I care about "The fact that there is no momentary relaxing of tension in this scenario means that the extraction of anchor pieces will not reduce fall energy to any significant degree"

 

1st, is this true? how?

 

2nd, if you applied this in a lead situation (pieces are not equalized and are shock loaded), wouldn't that mean ripping gear would not lessen then force on each successive piece?

 

 

if the piece failed, how much stretch would there be in the rope? I would think there is some but not significant stretch in the lead rope to prestretch out the rope much before landing on the next ppiece.

[layton]

not much energy absorbed by the actual piece blowing

 

Why not?

If a 7kN force rips a piece rated to 6kN, wouldn't that piece have absorbed 6kN regardless of rope stretch, or any other dynamic elements?

This is the heart of my question.

[lummox]

 

Why not?

If a 7kN force rips a piece rated to 6kN, wouldn't that piece have absorbed 6kN regardless of rope stretch, or any other dynamic elements?

This is the heart of my question.

Son, you're lacking perspective. You asked a question regarding work, then limit your vocabulary to force. The answer to your question is probably no (the scenario you describe is too incomplete to say no definitively). The explanation can be found in previous posts.

[Fromage]

This thread is asking the right question but is taking a beeline to Angry Nerdville. If you want to talk about gear that slows you down we should be identifying the true culprits instead of arguing over real-world applications of high school level kinematic equations.

 

Here is a cursory list of gear that slows you down:

 

1. Gaiters. Why, in the year 2012, do people still insist on wearing gargantuan Gore-Tex monstrosities on their ankles? Just think of how much extra work you have to do with every footstep when you wear huge honking gaiters. That slows you down.

 

2. 6,000 cubic inch backpacks on day hikes, der Toof, etc. You know who you are. You are the ones gasping for breath and staggering along the trail as the smart people blow past you wearing appropriately-sized packs that only contain what they need. That slows you down.

 

3. Plastic boots. Unless you are climbing Rainier in winter, these things are unnecessary in the Cascades. In a market where choices of lightweight, leather or synthetic, insulated mountaineering boots abound, plastics only do one thing: slow you down.

 

4. Expedition tents. These can be luxuriously spacious and instill a false sense of security, but they also weigh 9+ pounds, take a while to set up, are damned hot in the summer, and guess what? They slow you down.

 

5. 2" thick ground pads. You don't need a pad with an R-value of 12 to have a comfortable night's sleep. Unless you are carrying it 20 feet from the car to the campsite, they slow you down.

 

6. 10.5mm+ ropes. Does anyone climb in the mountains with fatty singles anymore? If so, they missed the memo that they slow you down.

 

7. Not having a map and appropriate navigational tools. You think carrying superfluous gear slows you down? Try getting lost. When you miss the obvious gully that Beckey describes you could spend hours bumbling around trying to get back on the approach route. That slows you down.

 

8. I could go on, but you get the idea.

[alpine et]

 

Why not?

If a 7kN force rips a piece rated to 6kN, wouldn't that piece have absorbed 6kN regardless of rope stretch, or any other dynamic elements?

This is the heart of my question.

Son, you're lacking perspective. You asked a question regarding work, then limit your vocabulary to force. The answer to your question is probably no (the scenario you describe is too incomplete to say no definitively). The explanation can be found in previous posts.

 

 

I think the most revelant reason is that a piece is going to blow because it wasn't set well or the rock wasn't solid. I'm not usually too concerned with the kN rating on the gear itself. 6kN is a hell of a lot.

 

But if we're in the situation where a bomber piece blows beacuse of mechanical failure, energy would be absorbed while breaking the cable, but if your pieces are spaced in a normal manner, you're free falling between them, and quickly picking up energy to throw at that next piece...

[layton]

 

 

Why not?

If a 7kN force rips a piece rated to 6kN, wouldn't that piece have absorbed 6kN regardless of rope stretch, or any other dynamic elements?

This is the heart of my question.

Son, you're lacking perspective. You asked a question regarding work, then limit your vocabulary to force. The answer to your question is probably no (the scenario you describe is too incomplete to say no definitively). The explanation can be found in previous posts.

 

why?

wouldn't a piece failing absorb quite a bit of energy and lessen the force to the next piece?

 

none of you ever do this?

Edited September 29, 2012 by layton

[chirp]

 

Why not?

If a 7kN force rips a piece rated to 6kN, wouldn't that piece have absorbed 6kN regardless of rope stretch, or any other dynamic elements?

This is the heart of my question.

Son, you're lacking perspective. You asked a question regarding work, then limit your vocabulary to force. The answer to your question is probably no (the scenario you describe is too incomplete to say no definitively). The explanation can be found in previous posts.

 

why?

wouldn't a piece failing absorb quite a bit of energy and lessen the force to the next piece?

 

none of you ever do this?

 

I think we all do this instinctively...I know when I place a bad piece and I am sure of that fact, I try to place at least 2 more above as close as possible. It seems like a no brainer.