What is pop?

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endre
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What is pop?

Post by endre »

and I'm not talking about music.

what would the definition of "pop" in a ski be?

collin
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Post by collin »

Could be wrong since I'm no jibber... But isn't just how springy a ski is? Or being good at releasing stored energy, say when flexed when you hit a kicker.

Oh semantics... So would a ski that has a lot of pop be not damp?
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endre
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Post by endre »

I have seen definitions that claim poppy skis to retain their shape quicker. Would that effect on the skier? I don't see why a "poppy ski" would give the skier more kick (on a kicker) than a stiff, damp ski. (they retain their shape a lot quicker than the skier can jump anyway)

Or is pop the same as pretension in the ski? There is no doubt that a ski with a lot of camber (like a cross c. ski) feels very poppy.

Maby you have a point there collin, is pop the oposite of dampened?

o0norton0o
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Post by o0norton0o »

Dampening of force is what a soft ski does by being flexible. As your tip encounters a bump a soft ski will take longer to transmit the energy to the skier because the ski flexes. Stiff skis don't flex much so they instantly transmit the force to the skier.
This works in reverse too. When you initiate a turn, you are reversing the force and trying to push off on the hill through the ski to get lift to change your edging. This is the "pop" which you are reffering to. A certain amount of stiffness is needed to have this pop. Smooth ski techniques don't need as much stiffness in their design because their style doesn't utilize the dynamic turn as much as a someone who has a lot of snap in their style.
It seems that both designs have their place because snow conditions and skiing style can favor one or the other specification. If you feel you have a more dynamic style of skiing (more poppy, less smooth) then make a stiffer ski for yourself. If you have a more flowing smooth style make a softer more forgiving core.
There is such a wide variety of factors affecting ski design, you just have to see the big picture and try to chose what suits your style and location best
norton
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G-man
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Post by G-man »

Great ponderings here. It got me thinking, also. I really don't know much about physics, but I do enjoy trying to figure out why physical properties behave the way that they do. So, here goes.

Any object in motion represents a certain amount of energy. A pair of skis moving down a hill with a skier attached to them represents energy in motion. The amount of energy in motion is based on speed, weight, and maybe some other things. As long as the skier moves smoothly down the hill in the same direction and at the same speed, the energy stays relatively constant. But, if some other force (like a bump) comes along, the energy contained in the moving skis with skier attached is forced to change direction and/or speed. Energy is not lost or gained in this situation, a bit of it just changes direction from moving parallel to the slope to moving perpendicular to the slope. If the ski is soft enough to be deformed by this sudden perpendicular energy, the ski will absorb that energy with a flexing motion, then give it off as a small amount of heat (required energy to bend the ski) and give back to the slope a bit of energy in the form of regained speed down the slope. If the ski is flexible enough to redirect most of the perpendicular energy before it reaches the skier, the skier is affected very little by it. If, on the other hand, the ski is too rigid to give in to the perpendicular energy, this energy quickly reaches the skier. If the perpendicular energy originated from small bumps, then the skier is likely to feel these small frequency energy redirections in the form of teeth rattling. If the perpendicular energy originated from a larger bump, then some degree of air time may follow with much of the energy being given back to the slope upon landing. Anyway...

So, Collin, I, too, think you may be on to something when you question whether damp is just the opposite of pop. It's all just in the flow and transmission of energy and how the ski itself acts as a channel for energy redirection. And, Norton, thanks for pointing out that the energy redirections take place in both directions... so right you are. I like your suggestion that smooth style skiers stick with a more damp ski, and more aggressive skiers move towards a more stiff ski.

Like I said, I could be way off on the physics part of all this and I certainly welcome any critique of my possibly lame theory. I really do enjoy this discussion and the chance this forum gives us to bounce around ideas.

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endre
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Post by endre »

o0norton0o wrote:Dampening of force is what a soft ski does by being flexible. As your tip encounters a bump a soft ski will take longer to transmit the energy to the skier because the ski flexes.
norton
This is pretty much my definition of a ski's ability of energy absorbtion, which I don't think is mixable with dampening. (In "skiing mechanics" John Howe gives a quite thorough description of what a damp ski is, he allso describes a terrain absorbing ski.) I don't have the book here (so I can't quote him), but It is very clear that a ski don't have to be soft to be damp.

But can a ski be both damp and poppy?

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Post by o0norton0o »

endre, Howe has added an aluminum torsion box mounted on rubber to add dampening. He has added an additional system to control what most skis designs have to compromise stiffness to control... dampening.
In general, negating any "additional system" added to control dampening of forces, stiff skis transmit the energy to the skier more readily than a flexible ski. The reason stiff skis exist is to have a longer contact length of ski edge to the snow surface. Stiff skis hold their edge better in hard packed conditions, by virtue of their longer effective edge due to their camber distributing the force.
I still stand by my statement that dampening in most applications is controled by stiffess.
I read his website info and was amazed that he has signifigantly shortened the arc radius of his skis to change the way his skis carve to be more of a carving ski than a "skidding ski". In pure physics I thought this made sense, I also wondered if they took more energy to ski them, because you have to turn them so many more times to use the shorter radius correctly.... their was a lot of stuff to read about on his site www.clawskis.com

I will say this he has all kinds of drawings and equations and angles on his site. All very impressive and correct, I was a math geek myself. He actually says you have to change how you ski to use his skis! He states that, to ski true carving arcs edge to edge you might have to ski out of the fall line to control speed. Also, you should look up hill so you don't cross up the line of other skiers who have a long radius ski and are more on the fall line with their decent.
I would say at that point, you had better be very flexible to maintain your upper body COUNTER rotation or you are now skiing with "upper body rotation" again like when you were learning to snowplow.
In closing, I would say that we are not machines. We dont have unlimited energy to apply to the hill. If some dampening mechanism helps you ski better,.... great. If it takes you off the hill one or two hours earlier because you are tired from making short radius turns on his skis that have a heavy aluminum box on them controling dampening.... cool. I really think that it is all in how your ski style is and what the condition of the snow favors in the end. Being adaptive and understanding your style and how the conditions effect it is probably your best chance to ski well and make determinations of what engineering will serve you best given your personal mechanical style and snow conditions.
Norton
ps.skiing widely variing ski designs on the same day illustrates best what differences in charicteristics do differently.
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endre
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Post by endre »

Hmm. got his book right here, if you'r into math (and carving) you would love it. I would not say his ideas about hourglass skis are revolutionary, (closer to concervative) but it's a really informative book with lots of numbers and equations in it.

I was really more into discussing pop than damp, does anyone else have an oppinion on this?

o0norton0o
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Post by o0norton0o »

hey endre, my point was that "pop" is usually associated with stiffer skis since you need a stiffer platform to transmit the energy into the hill rapidly to achieve a dynamic turn.
Although stiffer skis have more pop and inherently less dampening, you are hard pressed to get both maximized. You have to chose what suits your skiing style. I don't think I can make it any clearer than that
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endre
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Post by endre »

I have allways ment the same thing as you norton, but all the talk about pop makes me wonder. If what you claim is true , that pop is the same thing as stiffness, why are we calling it pop?

And, if pop is the same as stiffness: Why does carbon add pop when a thicker core adds stiffness? (why the carbon ollie band)

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Post by davide »

I just finished two pairs of skis. One is made with poplar/ash woodcore and fiberglass. The other one is made with ash woodcore and carbon only. The carbon one is less stiff than the glass one, because I used less reinforcement and the core is slightly thinner.
Then I took a plastic hammer and I hit gently the skis: the carbon (softer) ski made a high pitch sound, while the glass (stiffer) ski made a lower sound. So, the soft carbon ski is less damp than the glass one.
I suppose than when talking about damping, people thinks mainly at the very high frequency damping, that I think is not really an issue in skiing. I suppose it would be more interesting to have damping at low frequency.

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Post by o0norton0o »

endre, If you have a dynamic skiing technique you might like a stiffer ski so you can take advantage of the way the ski releases when you change edges.
If you ski with a smoother type of style you might not need the "pop" a stiffer ski imparts. you might want a ski that is softer.

I believe that the use of carbon fiber adds "pop" because it is a material that is very strong. it is laminated between the core and the petex base to create the strong rebound when the ski releases one edge to go to the other.
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Alex
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Post by Alex »

I think pop is the way the stiffness of a ski changes when flexed. This depends on the materials use an the way they take the load. Most Skis are a mixture of materials with different properties (Young Modulus). Carbon bands for example take load very quickly at small bending angles creating a strong force making the ski return strong and quick to intital shape.

I experienced this myself when adding carbon fiber to my initial glass fiber contruction. When you flex the ski it first feels pretty soft but gains stiffness at higher angles.

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Post by davide »

Alex, I do not agree.
As long as the ski is in the elastic regime, the stiffness (overall Young modulus) does not depend on the deformation.
When using carbon and glass together, you still have a linear behaviour of the global composite: think of two spring (F=kx) in parallel.

Alex
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Post by Alex »

I'm not 100% shure about it but i think think it is possible to create a different spring characteristic (progressive/degressive) with the combination of materials with different mechanical properties. A ski might be compared to a leaf spring (single leaf).

But this definetly needs further analysis - right now it's just a experimental observation i can't prove.....

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