fiberglass and carbon proportions O_O!

For discussions related to the type of materials to build skis/snowboards and where to get them.

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doughboyshredder
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Post by doughboyshredder »

twizzstyle wrote:
doughboyshredder wrote:Carbon also springs back to its molded shape quicker and with more force than fiberglass.
That = stiffness. ;)
Nope, rebound does not equal stiffness. For example a super stiff piece of steel will not rebound to its original shape once deformed.
The stiffness, k, of a body is a measure of the resistance offered by an elastic body to deformation (bending, stretching or compression).


where

P is a steady force applied on the body
δ is the displacement produced by the force (for instance, the deflection of a beam)

twizzstyle
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Post by twizzstyle »

Well you're talking about two different things there: plastic and elastic deformation. A piece of spring steel will spring back and won't deform, as long as you don't bend it too much.

The difference between carbon (or fiberglass) and metal, is that there is much less plastic region before it fails. Carbon will just take a load of force without much deformation, then break. Where as steel will bend quite a bit in the plastic region (depending on the kind of steel obviously) before it fails.

You said carbon springs back with more force, that's because its stiffer (stiffness is just a force per displacement). But we're off topic now :)

doughboyshredder
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Post by doughboyshredder »

I'm not an engineer, but I smoked with one the other day.............

anyways, somewheres I have seen a specific measurement for rebound of a material, and it definitely is not measured in stiffness.

I am of the understanding that a piece of carbon fiber with the same stiffness (same force required for deformation) as a piece of fiberglass will rebound faster than the fiberglass.

Are you saying this is not correct? (not arguing, just trying to understand)

So, if two items have the same stiffness they will rebound the same? I don't think so. I think there is another factor being left out of the equation. I have read about this somewhere, but haven't been able to find it again.

twizzstyle
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Post by twizzstyle »

I am an engineer - aeronautical, but I had to do a fair bit of composites and structures stuff back in school.

I suppose the only thing that could cause differences in rebound speed would be damping of the material. Where as stiffness gives you a force per displacement, damping gives you a force per speed (shock absorber on a car for instance... more force the faster the shock shaft moves).

But I think the difference in damping between carbon fiber and fiberglass might be negligible assuming they have the same amount of resin, but thats a total guess.

I still think that yes, if you have a carbon fiber beam and a fiberglass beam of the same stiffness, they will rebound the same. I could still be missing something though...

doughboyshredder
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Post by doughboyshredder »

Elastic Recovery is what I was looking for.

Which is directly related to modulus. I.E. stiffness.

So a cured piece of cf with the same stiffness as fg will absolutely rebound the same.

But, that piece of cf will be lighter than the piece of fg.

thanks twizz, you're a smart dude.

twizzstyle
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Post by twizzstyle »

doughboyshredder wrote: So a cured piece of cf with the same stiffness as fg will absolutely rebound the same.

But, that piece of cf will be lighter than the piece of fg.
Exactly, not only lighter from less fiber, but less resin too!

jono
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Post by jono »

twizzstyle wrote:K2's have carbon top and bottom, but its woven into the whole seamless fiberglass sleeve around the core,


Are you referring to the carbon stringers running down the top of the cores I see in lots of K2 skis from the past 10 years or so? Does this mean there is another stringer under these cores too?

If carbon is similar to fiberglass in compression but stronger in tension, wouldn't carbon only on top lead to a ski that feels like a fiberglass ski as you load the ski in a turn or in deep snow (hopefully both) and then rebounds or snaps back to its normal shape more quickly than a ski with just fiberglass after the forces from the turn are reduced? This seems like the kind of ski that would be fun to use.

doughboyshredder
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Post by doughboyshredder »

Jono, I think you're off a bit on your understanding. In a turn the ski is flexed in such a way that the top layer of the laminate is under compression. Cf is no stronger than fg in this regard. So, adding x amount of cf doesn't increase the rebound of the ski anymore than adding x amount of fg. I don't have the specs table in front of me, there may be a little bit of weight savings using cf, but you have to use much more cf under compression than under tension to match the strength of fg. Somewhere I was looking at a comparison chart of cf, kevlar, and fg, and was surprised that under compression glass is superior to both cf and kevlar.

twizzstyle
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Post by twizzstyle »

jono wrote:Are you referring to the carbon stringers running down the top of the cores I see in lots of K2 skis from the past 10 years or so? Does this mean there is another stringer under these cores too?
Thats right, K2 has their awesome seamless weaving machine, and it weaves the carbon right in there, top and bottom.

And remember guys - "strength" and "stiffness" are two very different things, you are talking about stiffness here, not strength.

feldybikes
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Post by feldybikes »

twizzstyle wrote:I still think that yes, if you have a carbon fiber beam and a fiberglass beam of the same stiffness, they will rebound the same. I could still be missing something though...
[physics geek]

Almost, for a spring,

F = -kx

where k is the spring constant (i.e. stiffness) and x is the displacement. So the carbon and fg beam will have the same rebound force. However,

F=ma

where m = mass and a = acceleration

So,

a=-kx/m

IOW, the 3 times (?) heavier fg beam will accelerate back 3 times less.

[/physics geek]

I mean, let's go shred the gnar, brah!

doughboyshredder
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Post by doughboyshredder »

Ahhhhh, welcome feldy. Thank you for the contribution. :D :D

doughboyshredder
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Post by doughboyshredder »

oh, and that means that the cf will probably feel "snappier". Right?

feldybikes
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Post by feldybikes »

I guess. I don't mean to sound flip, but I don't really know what it really means for a ski to be snappier or have more pop. I just haven't skied all that many different skis and very rarely have I ever skied two different skis back to back. But I gather these things are pretty much the same?

doughboyshredder
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Post by doughboyshredder »

I have been wondering for quite a while what would happen if you did a blind "taste test" with multiple different layups all with the same shape. I wonder if the average skier/rider can even feel any difference with additional reinforcements, dampening, sidewall bevel, etc.... I venture that it's only the top riders that can feel any significant difference, and I wonder how much of that is perceived and not actually real. And how much of it even matters in pow?

twizzstyle
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Post by twizzstyle »

Yeah I'm definitely with you on that doughboy. I mean, I can certainly tell a difference if I go from a super soft ski of mine to a super stiff ski. But I have two pairs of Line 1260's, very old ones that have no carbon, and slightly less old ones that have carbon stringers in them. I would bet if I did a blind test I couldn't tell the difference. A slight adjustment in my skiing maybe, but that would be subconscious.

And yeah... in powder nothing really matters. Lots of surface area, and softness just to keep the tips on top. :)

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