DPS S.S.- the next level

[rabbit]

Overdamped: The system returns to equilibrium without oscillating.
Critically damped: The system returns to equilibrium as quickly as possible without oscillating.
Underdamped: The system oscillates with the amplitude gradually decreasing to zero.

Disclaimer: Please ignore this post if it's not making any sense.

The above quote, together with your earlier "tuning-fork-explanation", proposes that the "width" between the "forks" will be critical.
Granted, I have no idea how this works in a macro-system such as a ski but, when two frequences have a numerical ratio the amplitude increases, right?
So; Is the amplitude the "dampening-effect" or the oscillation? How do you decide the width between the "forks"? Am I just tripping and, for all practical purposes, speaking gibberish?

/r

[marshalolson]

hmm.... dude, i don't know that i have an answer to that.

my explanations are more along the lines of describing the sensations that i felt while riding them than the math behind the science controlling it.

[SiSt]

I´ll keep bumping this until someone comes up with a good explanation on how metal influences the ski´s behaviour. (I know the feel, but I don´t understand the physics of it.)

Not directly relevant to the S.S. discussion, but I won´t bother starting a new thread.

[steved]

i had definitely better start saving

...Need Another Stimulus...to us!......

$.01

[SiSt]

Back on top! Still an optimist.

[tsproul]

I´m all ears.

I'm no enginerd but I liken a ski to a leaf spring in a truck.

If the spring isn't beefy (thick) enough, it bounces like mad once you hit a bump. You can see this when you follow behind an '88 Toyota pickup carrying a ton of concrete bags. Once they hit a bump, the rear end of the truck bounces 4-5-6-10 times before the suspension stabilizes. If the spring is too beefy, you get every little bump and pothole transmitted through, as if there was no 'spring' effect. You see this when the truck hits a bump and the rear wheels go airborne. At a certain range of beefiness, the spring dampens (flexes) when bumps/holes are encountered but doesn't continue to bounce (too much) afterward.

What is that perfect range of beefiness? Well, that depends pretty much on what the leaf spring (ski) has to hold up (the weight of a skier for a ski), the surface the skier travels over (groomed, crud, powder, bumps) and the speed at which the traveling occurs. And of course, the material of the spring.

The above is an oversimplification but I think a good starting point.

Thoughts on the metal alignment in these SS skis. Perhaps the metal aligned so the strips are tall but thin (across the width of the ski) makes most of the vibration oscillate in the same plane as the ski. Rather than the metal strips trying to vibrate up and down (tip rising up and down), they prefer to try and vibrate in line with their thinnest cross-section. That is, the ski wants to vibrate left/right (tip moving left/right instead of up/down) when there is vibration. This, we don't feel.

Take a thin strip of metal and hold it only at one end, leaving the other end free to move. Strike the metal on the thin edge to make it vibrate. I bet most of the vibration is 90 degrees to the vector which you struck the metal at. That is the metal strip will vibrate most where it is has the least resistance to vibrate....the thinnest cross-section regardless of the vector of the force applied. Probably need some enginerd equipment to 'see' this.

[marshalolson]

interesting analogy. it makes sense to me.

dps is traveling today, but plans to reply back tomorrow....

[TeleThor]

I'm no enginerd but I liken a ski to a leaf spring in a truck.

If the spring isn't beefy (thick) enough, it bounces like mad once you hit a bump. You can see this when you follow behind an '88 Toyota pickup carrying a ton of concrete bags. Once they hit a bump, the rear end of the truck bounces 4-5-6-10 times before the suspension stabilizes. If the spring is too beefy, you get every little bump and pothole transmitted through, as if there was no 'spring' effect. You see this when the truck hits a bump and the rear wheels go airborne. At a certain range of beefiness, the spring dampens (flexes) when bumps/holes are encountered but doesn't continue to bounce (too much) afterward.

What is that perfect range of beefiness? Well, that depends pretty much on what the leaf spring (ski) has to hold up (the weight of a skier for a ski), the surface the skier travels over (groomed, crud, powder, bumps) and the speed at which the traveling occurs. And of course, the material of the spring.

The above is an oversimplification but I think a good starting point.

Thoughts on the metal alignment in these SS skis. Perhaps the metal aligned so the strips are tall but thin (across the width of the ski) makes most of the vibration oscillate in the same plane as the ski. Rather than the metal strips trying to vibrate up and down (tip rising up and down), they prefer to try and vibrate in line with their thinnest cross-section. That is, the ski wants to vibrate left/right (tip moving left/right instead of up/down) when there is vibration. This, we don't feel.

Take a thin strip of metal and hold it only at one end, leaving the other end free to move. Strike the metal on the thin edge to make it vibrate. I bet most of the vibration is 90 degrees to the vector which you struck the metal at. That is the metal strip will vibrate most where it is has the least resistance to vibrate....the thinnest cross-section regardless of the vector of the force applied. Probably need some enginerd equipment to 'see' this.

Makes sense to me. Sounds like some sweet riding planks.

So how thick are the metal strips? I'm guessing pretty f-ing thin to only add 10g per ski. What would happen if you didn't have long strips connecting the two carbon layers, but rather short pieces of metal in the vertical laminate every 10 or 20 cm? They wouldn't change the flex as much as a long metal strip would, but should still act as the "tuning" section of the tuning fork.

Sorry to go off topic here, but I was looking at patents and found the United States Patent Application 20080106068 which covers rocker, taper, and zero camber (I think. I just glanced at it quickly). Patent law is tricky stuff so I may be off base, but this looks like DPS owns the rights to these now very common ski characteristics. Anyways, carry on.

[Dudley]

TSPROUL - That made sense of this for me... much obliged.

[climberevan]

re: tsproul's analogy... cars/trucks/motos/etc also have shock absorbers, which are charged with keeping the spring from oscillating. the leaf spring on a truck, minus shocks (or with underdamped shocks) will just keep oscillating no matter how strong/stiff it is... anyone who rides motorcycles and who has blown a shock will tell you that without the shock portion it's nearly unrideable.

not to rain on the parade, but it's more complex than just the stiffness of the springs. in skis there is both a spring and a shock...built into one. it sounds like what dps is going for is a stiff ski (spring) that's still damp (adequate shock absorption/control of oscillation).

[SiSt]

climberevan is right

The only things resembling a real shock absorber I´ve seen on skis is the rubber thingies salomon used to put on their top sheets.

Active damping using piezoelectrical elements have also been tried. (I used to own a K2 electra snowboard, soft, but still pretty damn stable at speed) They were supposedly tuned to dampen the second mode vibrations in the board, which is possible to do as long as you know their frequency. I can´t see how they dissipated all the energy through their "heat sink" though. Imagining that there would be quite a lot of energy to dissipate, and never feeling or seeing anything heat up.

The problem with trying to build a spring + shock into the same thing, comes down to the fact that you (normally) can´t have it both ways.

Tsprouls analogy is very interesting, but if you see the entire ski as one, it will be extremely stiff across the width, thus making this type of vibration almost impossible. (If a ski is 1cm thick and 10cms wide (and anamorphic, which it is not) it will be 100 times stiffer across the width compared to across the thickness.) Also, this would involve changing the direction of the vibration by 90 degrees, which I still cannot see how would be done.

[Core Shot]

Is the S.S. damping because it ties top of ski to bottom? That sounds like more of a stiffening function.
(which explains why the S.S. is 30% lighter - needs less carbon laminate layers)


Or is it damping just because it is a change in layer along the vertical core.
e.g., a vibration tries to chatter across a ski core, it hits a metal layer, then loses some amplitude, travels through the next wood layer, loses some more energy, etc etc.

Just a thought from a non-enginerd
I am curious to hear the DPS explanation.

[SiSt]

Almost made it to page 2, now float back up.

[GeezerSteve]

The problem with trying to build a spring + shock into the same thing, comes down to the fact that you (normally) can´t have it both ways.

Doesn't an engine mount act as both a spring and a shock? Or a rubber torsion bar used on some trailers?

But, of course, while that may be sorta what's going on with the addition of an elastic dampening ply in a ski, that's not what's going on here.

I'm just a lawyer/bicycle framebuilder, not an engineer, but my intuition tells me that Marshall's and tsproul's analogies are apt. In a metal ski, as the ski decambers, it's a no-brainer that the bottom sheet acts as a tension member and the top sheet a compression member, and they manifest some sort of oscillating wave as they return to their rested cambered state. Tie those two members together with a series of vertical members and the oscillation wave and corresponding resonant frequencies are going to change big time. That the wave would be "dampened" sounds right to me, based on my understanding of the term.

The physics of harmonic vibrations is hairy stuff, way beyond my ability to understand it, let alone describe it, but I do recall that the failure of the Tacoma Narrows bridge (a.k.a., Galloping Gridie) was remedied by replacing the failed single-plane bridge deck with a deck comprised of two tiers that were tied together with vertical members. That greatly changed the resonance of the bridge deck, i.e., it dampened the deck, and the bridge has held up for decades notwithstanding the stiff winds howling through the Narrows.

End of drivel. Again, I'm just a stoopid lawyer/framebuilder.

[SiSt]

It´s a bada-bump!

[Imz]

Hey Stephan, sounds great.

How does that differ from the carbon stringer I-beam construction of my Watea 101's?

[SiSt]

Nobody actually knows?

(Not that I´m not open to speculation, I´m just surprised.)

[tsproul]

cars/trucks/motos/etc also have shock absorbers, which are charged with keeping the spring from oscillating.

not to rain on the parade, but it's more complex than just the stiffness of the springs. in skis there is both a spring and a shock...built into one.

Last I checked...and that was this past week....my legs did a pretty damn good job of taking the role of charged shock absorbers.

[MultiVerse]

Here’s an attempt at a general explanation with the disclaimer that a better explanation is probably either more technical and lengthier or simpler and more specific with insider DPS knowledge.


Most engineering materials are isotropic. This means that the properties are not a function of direction. All planes that pass through a point in the material are planes of material property symmetry. To define the material, only two elastic constants are needed to describe what happens when the material is distorted like when a ski is bent, twisted, and vibrated to carve a turn (distorted).

These two constants are called: Young’s modulus and Poisson’s ratio which for these purposes simple means that an axially loaded rectangular strip like a ski will remain rectangular as it is distorted.

Normally, ski material is assumed to be homogeneous and isotropic with ski designers combining different materials like metal, fiberglass, and carbon fiber sandwiched together around a wood core to create the desired ski characteristics.



In the case of a purely laminated composite like carbon fiber built up of sheets (lamina: parallel fibers embedded into the matrix material ) of uniform thickness, each lamina may be isotropic, orthotropic, or anisotropic with either homogeneous or heterogeneous characteristics. Once the carbon laminates are joined to each other, the most general case is what is called: coupled anisotropic.

All of which is enginerd speak for saying that carbon fiber has three planes of material symmetry requiring four material constants to describe what happens when the material is distorted.

These four constants are: 1) Modulus of elasticity of fiber + matrix, 2) Poisson’s ratio of fiber + matrix, 3) Volume fraction of fiber + matrix 4) shear modulus of fiber + matrix. Plus, there are also boundary conditions associated with anisotropic materials. All of which means that an axially loaded rectangular strip like a ski is much more mathematically complicated to model as it is distorted.

Unlike the isotropic material in a typical ski where the stress resultants are in general also a function of the bending strains giving it neutral characteristics or a zero matrix (mathematically speaking), an anisotropic carbon fiber ski does not have a neutral surface but instead has a much more complicated relationship between strains and the force and moment resultants as the ski is distorted.

Which is probably an overly complicated way of saying that the natural frequencies (see Marshal’s post) of carbon fiber are different in different directions and under different loads. Which is itself an oversimplification.

This is where the minimal amount of metal comes into play. The metal acts as a sandwich plate with more of the edges supported, allowing the ski to perform more like a homogeneous and isotropic material responding to distortion (vibrations etc.) with natural frequencies similar to a conventional ski.

[SiSt]

Bump in time for the weekend.

[lobstahmeatwad]

jeesus saltmind that is pretty well put but still I am confused.
The real question is how thick is that metal?
.2mm .3mm?
Its interesting that it is only near the outermost sections of the laminate core.

[Beyond]

Normally, ski material is assumed to be homogeneous and isotropic with ski designers combining different materials like metal, fiberglass, and carbon fiber sandwiched together around a wood core to create the desired ski characteristics.

Great explanation, followed most of it far enough, but question about this premise above: Realize you're differentiating laminates on the basis of composition. I can see metal or plastic being assumed homogeneous and isotropic, but seems to me that fiberglass - and even wood if you think about what it looks like under a microscope - are more like carbon fiber, eg, matrices with embedded fibers vaguely parallel on a micro level but actually running in various directions as it's made into a laminate or sheet or whatnot. Glass especially.

Then the macro level: Unless you buy a Ogasaka, believe wood cores are laminates tied together with resin; a few play around with the direction of the grain, so more axes. Finally, believe that in a torsion box, not uncommon, each glass sheet is wrapped at angles relative to previous sheets. So my problem is seeing how this is different than carbon, either micro or macro.

Obviously missing something rudimentary here. Is it that orientation of glass or wood fibers can be assumed to be along one plane, no matter the actual orientation? While carbon's end up in three dimensions because lamina run at angles to each other, eg, "quad axial" etc.? Just having trouble fitting my rudimentary knowledge of what this stuff looks like to the formal assumptions of carbon being so different.

Ironically, get the idea about the metal, which was what's been driving the bumps. Appreciate the teaching.

[SiSt]

^
Exactly. Glass as a material is isotropic, but fiberglass mats are not. Biax, triax, quadax +++
Wood is not isotropic at all, but really a natural laminate, which is one of the reasons it works so well for keeping trees upright and all the other stuff. (Such as bend easily, but still have a high compression strength.) Grain orientation has been given a lot of attention in snowboard design (specifically Burton high end boards), but not so much in skis. (with Line afterbangs, and Endre Hals´ skis being an exeption (look closely at the pics of the proto Renegades, and you´ll see what I mean))

And I´d be really happy for an explanation of how the metal works in a normal ski when it´s used as a sheet. Is it the modulus? Is it the viscosity? (for lack of a better word, resistance against quick deformations while letting slow deformations happen?) Is it the added weight? Or a combination of the above? Or as Saltmind suggested, that it unifies all the vibrations of the fiber/resin matrix, resulting in a simpler vibration model?

I appreciate all the speculation up to this point, but somebody has to actually know.

[SiSt]

Another bump.

I´ll give up soon.

[Beyond]

I'll take a turn at the bump. Given general level of interest, would be helpful if DPS jumped in with the gory details.