Do Screamers Work?

Eric Moss wrote:That is the correct formula for kinetic energy, but we need to be looking at the potential energy formula in this case, Pe=F*d (weight times height). As others have pointed out, the (d) in this equation should be two times the extension of the screamer.

I can't remember much of the physics courses I took, so please excuse my ignorance...

Isn't that the formula for work, and not potential energy?

Doesn't F mean force and not weight? At rest, they'd be the same, right? But it seems that in this case the body isn't at rest: at the moment the screamer engages (the beginning of d), the body has already been moving.

I'm also curious how a screamer could add any energy at all. Would the total amount of energy in the system not be constant?

I can understand how it would allow more of the PE to change into KE, though, by allowing a longer fall.

Kent Richards wrote:I'm also curious how a screamer could add any energy at all. Would the total amount of energy in the system not be constant? I can understand how it would allow more of the PE to change into KE, though, by allowing a longer fall.

Technically it doesn't "add" energy. So yes, you are right, it allows more PE to be converted into KE, which is the only energy experienced by the rope, the climber, belayer, and top piece.

amarius wrote: Both Yates, and Black Diamond tested screamers, data is publicly available. There is also information from 3rd party sources available as well, a bit of googling Keep in mind - not everyone who cares about a specific climbing issue will participate in verbal exchanges that only increase entropy

Let he who increaseth not entropy cast the first stone. Uh, not so fast there Amarius...

The video isn't a test at all since there is no comparison of loads without a screamer. It only shows you can fully activate a screamer with very short falls, which isn't good news for their effectiveness in general.

The Yates "examples" aren't tests (you really think he went up in the middle of actual in-progress climbs with a dynamometer and replicated each situation with and without a screamer?), they are invented hypothetical scenarios. No one who has tested screamers has come anywhere near some of Yates preposterous numbers, like an 8kN reduction in a 70 foot factor 1 fall representing a 55% load reduction. You believe that and I've got a lovely bridge over the East River to sell you. (BD got at best an 18% load reduction in their 9 foot FF1 test.)

The BD engineers seem to misunderstand the physics by implying that the tests can be meaningfully parametrized by fall factor while ignoring the fall height, which is what actually matters. The fall factor is relevant to ideally elastic climbing ropes but irrelevant to screamer function. Sure, you vary the fall factor in a test as a way of varying the peak load to the anchor, but the fall height should be kept constant if you want to understand what the screamer is doing.

Moreover, by using a static belay setup, the BD tests cannot account for the more complex dynamics of an actual belay. In many cases, filtering out those extra sources of variation is a good thing, but in this case the more complicated dynamics appear to further diminish the small advantage screamers have for totally static belays---see the CCMT test results below, in which real belayers with either a tube-style device or a Munter hitch were used.

If you are rope-soloing, the BD tests might be more relevant.

Even if unrepresentative of what happens with real belays, the BD results are modest, except perhaps for that 26% reduction in peak load for the short fall test. But the percentage reductions tend to hide what is going on, because the screamer is absorbing a fixed amount of fall energy, as is reflected in the fact that in both tests, the screamer reduced the peak load by almost exactly the same amount, about 1.7 kN. Of course this looks dandy when the peak load without the screamer is all that big to start with.

Meanwhile, more reliable testers like Dave Custer at MIT, Jim Titt at Bolt Products, and the Commissione Centrale Materiali e Techniche of the Italian Alpine Club have verified what elementary theory predicts---which is that there isn't going to be much effect from screamers except for short falls. Moreover, the results suggest that long serious falls with a screamer present may result in higher peak loads if a belay device that allows for some slippage is used, perhaps, as Eric noted, because of a reduced dynamic belay from the belay device and/or, as Jim has mentioned, because of the variable behavior of stitch-ripping at different velocities.

The argument, first advanced by Yates (who seemed to think that you could absorb a force), that the extra loading time that results from the screamer extension should reduce the peak anchor force seems to be supported by incorrect arguments about the nature of impulse. However, it seems that loading time does have an effect on the holding power of ice screws, so that screamers on ice screws might have a more useful effect than they do on rock protection.

The relevant entries are the first two rows in each trial; the bottom two rows are tests of a "recyclable" load limiting device. The top row in blue in each category is the peak load without a screamer, the second row in red is the peak load with the screamer. The fall factors (either 1 or 1.5) are given in the fifth column.

Kent Richards wrote: I can't remember much of the physics courses I took, so please excuse my ignorance... Isn't that the formula for work, and not potential energy?

The change in potential energy equals the work done by the gravitational force.

Kent Richards wrote: Doesn't F mean force and not weight?

Weight is the gravitational force.

Kent Richards wrote: At rest, they'd be the same, right? But it seems that in this case the body isn't at rest: at the moment the screamer engages (the beginning of d), the body has already been moving.

In classical mechanics mass is independent of speed and near the Earth the gravitational field doesn't change much. So, the weight of a falling climber is effectively constant.

Eric Moss wrote:I'm trying to learn more about climbing safety (more, apparently, than the climbing community knows at this point).

This is the first problem with your thesis - the idea that somehow knowing the minutiae of idealized gear technical specs and testing has something fundamental to do with 'climbing safety' when in fact it has little to do with it which is why climbing has gotten along fine without it all these decades. In fact, the assertions begs the point you don't really understand what constitutes climbing safety and how it is achieved when actually climbing. In reality, the principal role all those fun and interesting tech specs play is more in standards setting, manufacturing QC and point-of-purchase 'consumer safety' and very little per se to do with safety on rock.

It's again that gear vs skills / experience / judgement thing - gear doesn't provide for 'climbing safety' and ANAM speaks to that point in volumes every year.

Eric Moss wrote:Sometimes that requires challenging conventional wisdom and I know from experience that can be quite upsetting.

New climbers attempting to 'innovate', 'improve upon' and 'challenge' conventional wisdom in climbing is fraught with peril and it simply cannot be overstated how ill-advised this kind of thinking is. Conventional wisdom in climbing is hard earned and it's boundaries are learned and set with a significant annual toll of injury and death.

Do not mistake a paucity of 'real world' technical testing for a lack of knowledge or wisdom in climbing. Yours is again a forest-for-the-trees myopia / obsession which has little to do with and next to nothing to contribute 'climbing safety'. If anything, it is misplaced and actually raises a yellow flag of concern about your physical well-being going forward if this remains your perception of climbing safety.

amarius wrote: Both Yates, and Black Diamond tested screamers, data is publicly available. There is also information from 3rd party sources available as well, a bit of googling Keep in mind - not everyone who cares about a specific climbing issue will participate in verbal exchanges that only increase entropy

The testing Yates has on their website is extremely questionable. It's not peer-reviewed, and most experts will refute the accuracy of their claims. Further, they have motivation to tell you a story that follows the path they want you to hear since they sell screamers. Black Diamond did conduct some testing, which does tell part of the story; however, they used a drop tower with a static belay and steel weights which is not analogous of real climbing. As far as the video goes, that's not a test of anything. They just took a top rope fall on a screamer, they dident even use a load cell to determine the peak force.

My offer still stands. If someone wants to send me a few so I have a reasonable sample size, I'll go to the crag on whip on them and compare them to a control using my load cell.

20 kN wrote:Maybe someone has tested screamers in the real world...

I've taken about thirty dives on them, but think finding 'real world' scenarios which would be useful or of any value would be difficult and finding such scenarios that have broad applicability would be even harder. What do you think would constitute such a scenario from a testing perspective?

This pull test video
is not about screamers, but shows a change in "activation force" with pull speed.

Healyje wrote:What do you think would constitute such a scenario from a testing perspective?

Replicating the most common type of fall found at the crag, which is pealing off at the crux. Our crag has a wide range of cruxes ranging from stuff that's hard at the ground to crux moves a fair bit past the last piece that's well off the ground. I would likely choose three different scenarios: a smaller fall close to the ground, a moderate fall in the middle of the route, and a large fall toward the top of a route. This would probably cover the most common types of lead falls one would expect at a single pitch crag. Obviously that's far from complete, but at least it would show some limited insight.

brenta wrote:In classical mechanics mass is independent of speed and near the Earth the gravitational field doesn't change much. So, the weight of a falling climber is effectively constant.

Mass is independent of motion but weight isn't. Weight is Mass*Acceleration so the weight of a falling climber is more than that of a stationary climber. And a climber falling for 1 second will weigh less than a climber who is falling for 3 seconds.

This is easily demonstrated with your typical bathroom scale. Quickly drop into a squat from a standing position and you will see the weight changes when you are in motion.

eli poss wrote: Mass is independent of motion but weight isn't. Weight is Mass*Acceleration so the weight of a falling climber is more than that of a stationary climber. And a climber falling for 1 second will weigh less than a climber who is falling for 3 seconds. This is easily demonstrated with your typical bathroom scale. Quickly drop into a squat from a standing position and you will see the weight changes when you are in motion.

You get an F.

eli poss wrote: Mass is independent of motion but weight isn't. Weight is Mass*Acceleration so the weight of a falling climber is more than that of a stationary climber. And a climber falling for 1 second will weigh less than a climber who is falling for 3 seconds. This is easily demonstrated with your typical bathroom scale. Quickly drop into a squat from a standing position and you will see the weight changes when you are in motion.

I did not realize that the force exerted by the moving mass was still called "weight". I thought "weight" was always the gravitational force (Fgrav), I.e the force exerted by the mass at rest.

eli poss wrote: Quickly drop into a squat from a standing position and you will see the weight changes when you are in motion.

Merlin already summed it up nicely, but let me add that weight is defined as gravitational force. When you start dropping into a squat, the sum of the forces acting on you is nonzero: the reaction applied by the scales to your soles is not equal to your weight. Said otherwise, if you don't stand still the scales don't reliably measure your weight.

brenta wrote: Merlin already summed it up nicely, but let me add that weight is defined as gravitational force. When you start dropping into a squat, the sum of the forces acting on you is nonzero: the reaction applied by the scales to your soles is not equal to your weight. Said otherwise, if you don't stand still the scales don't measure your weight.

How do you describe these additional forces? This is what I was trying to understand in my earlier posts: when the mass is moving, such as is the case with the falling climber hitting a screamer, there is more force involved than when the mass is not moving, right?

So in the work equation W = F * d, isn't that additional force also part of F?

20 kN wrote: Replicating the most common type of fall found at the crag, which is pealing off at the crux.

Screamer is an energy dissipation device, with a certain activation threshold. If a falling object has less energy at the moment of engaging screamer than activation value nothing will happen - this is why healyje mentioned strategic cutting of stitches. If the last stitch was ripped, and screamer bottomed out, there was too much energy for screamer to handle and the last piece of protection definitely got loaded more than activation threshold. The interval of usefulness is then defined between those parameters - 1st and last stitch. This is ignoring stretching of material that screamer is made from.

Variables I can think off right now -
- energy of object coming in, i.e. velocity of object, and mass of object. Velocity of fall is related to the height above the last piece of protection.
- peak force, i.e. mass of object, velocity, characteristics of rope - modulus, energy dissipation within the rope, length of rope. Energy dissipation in the rope system - carabiners, angles along the rope, length of rope, type of catch provided will complicate a subjective study. As rgold pointed out - fall factor is not really relevant. Video I linked to shows - even a short fall relatively close to protection with limited length of rope in the system will rip stitches - you will observe different outcomes if the crux is close to the ground, or 100ft up in the air on straight rope.

That said - any study should have somewhat defined goals - what are you planning to investigate?

I dunno why I'm adding fuel to this fire...

farside.ph.utexas.edu/teach…

Kent Richards wrote: How do you describe these additional forces? This is what I was trying to understand in my earlier posts: when the mass is moving, such as is the case with the falling climber hitting a screamer, there is more force involved than when the mass is not moving, right? So in the work equation W = F * d, isn't that additional force also part of F?

Let's ignore air drag, various forms of friction, variations in gravitational field, non-inertial frames of reference, relativity, and all that jazz.

During the free-fall portion of the fall, the only force acting on the climber is gravity. The body accelerates and potential energy is converted into kinetic energy.

When the rope comes taut, the free-body diagram changes, because now there's a spring pulling the climber up. Without a Screamer, the climber reaches the lowest point of the fall when the spring has stored all the potential energy the climber has lost. The spring has to overcome the weight of the climber, and decelerate its body at the same time. Hence, after an initial phase, it applies a force that is greater than the weight of the climber.

If energy were not dissipated in various ways (especially inside the rope itself) the climber would then be pulled up and would continue to go up and down forever.

The diffence with a Screamer, in first approximation, is that some work is done in ripping the stitches, and, at the same time, some extra length is added to the fall.

Does the above address your concern?

yukonjack wrote:I dunno why I'm adding fuel to this fire... farside.ph.utexas.edu/teach…

The change in weight is only measured in the non-inertial frame of reference of the elevator. Not what we have been talking about here and, I suspect, not what we want to talk about.

EDIT: On the other hand, thanks for bringing this up, so that we can properly lay non-inertial frames of reference to rest. (Please, do smile at the pun. Thank you.)

This thread is becoming a graveyard where physics goes to die. I don't know if Brenta can hold back the tide.

amarius wrote: Screamer is an energy dissipation device, with a certain activation threshold.

Ok so far---but that activation threshold is a force!

amarius wrote: If a falling object has less energy at the moment of engaging screamer than activation value nothing will happen

No, the activation threshold is not some (presumably kinetic) energy value. For example, if I hang giant boulder from the screamer, it will activate the stitch ripping, but (depending on what you consider the initial position of the boulder to be) at the moment of activation both the potential and kinetic energy of the boulder are zero.

What activates a screamer is that the falling climber stretches the rope until the tension in the rope (on both sides of the carabiner)---this is a force---attains the activation threshold and then stitch ripping occurs. In stretching the rope, work is done. If the work done is equal in magnitude to the climber's total change in potential energy, and if this happens before the rope tension from stretching reaches the screamer activation threshold, then the climber stops and the screamer doesn't activate, because all of the climber's potential energy change was absorbed by the rope.