fiberglass and carbon proportions O_O!

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kipi
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fiberglass and carbon proportions O_O!

Post by kipi »

hello,

in the previous board that i built there were 2 layers of fiibrglass which i ordered from skibuilders store. they are 744g/m^2 each, so for the whole board i had 1488g/m^2.

now, for my new boards i wanted the top composite layer to be Carbon (for strengh and to get a black color under the topsheet).

My question is: how do i know what weight the carbon should be? same as the 744fiberglass or lighter cos the carbon is stronger?
how will i know the board wont turn too stiff?

What are the proportions between Fiberglass and Carbon ?~!?~!

help!!
tnx(:

twizzstyle
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Post by twizzstyle »

There are too many variables to just give a quick answer.

It will completely depend on the weight of the carbon you get, the style of weave, the direction of weave, etc.

Also if you are doing heated cures, it might be a good idea to do the same layup on either side of your core (if you put one layer of carbon fiber on top, put it on the bottom as well). If you don't, when the board cools you could get some drastic camber changes (different thermal expansion between the carbon fiber and fiberglass, and core). But if you go overkill you may end up with a board that is too stiff or too poppy.

kipi
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Post by kipi »

im not doing heated cures.. is it ok to do the 'carbon at top fiberglass at bottom' layup??

i still need an answer about the weight of the carbon...
the fiberglass is 744g/m, 3 dierctions weave(o,45,90). and lets say the carbon will be also 3 directions.. what weight do i need?@!?~!?~!?!~?~!? helpppp

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MontuckyMadman
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Post by MontuckyMadman »

Latest field reports would suggest that a single layer of carbon above the core may not improve performance noticeably.
Better go with a layer below and above core.
Carbon has three times the strength to weight ratio as glass.
Thinner core profile for use with carbon. 8 oz uni carbon has been suggested to me. I will get more details from my sources.
I think it was uni, maybe biax.
Hope this helps to confuse.

kipi
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Post by kipi »

so you are saying that 744g/m fiberglass = 250~g/m carbon??

what is uni ?? some kind of a weaveO_o?

and how come it wont improve preformance, isnt carbon much more strong/tough/cool/durable/sexy?

twizzstyle
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Post by twizzstyle »

Ok, lets stop a minute and think about this technically.

Uni means unidirectional, i.e. fibers all running the same direction, no woven pattern. If you're going for looks, this isn't what you want, but if you're going for function, this is what you want. Having carbon fiber at 45 or 90 deg to the ski is overkill and a waste of CF, personal opinion.

When a ski flexes it is bending upwards. Carbon fiber is much stiffer than fiberglass (strength, and stiffness are two very different things, we don't care about strength in this case, stiffness is what matters). Carbon, or fiberglass, takes a lot of load in tension, but almost nothing in compression.

When you flex a ski upwards, the top is under compression, bottom under tension - that is why carbon on the top doesn't do a whole lot. It will change the stiffness when flexing the opposite direction, but that isn't often. Carbon on the bottom would make a much larger difference because then you are putting the carbon in tension. Putting it on both sides is really just to make sure you don't get cure-related camber changes due to a non-symmetric composite layup (non-symmetric about the core). But again, if you're not doing heated cures, that might not matter?

You really can't just say "this much carbon fiber gives the same stiffness as this much fiberglass" without having stiffness data for each material. You can probably find modulus of elasticity data online, but it could vary even between manufacturers of the material.

Really - you'll probably just have to experiment and make a few skis with different layups until you find what you like.

Also, side note - there are so many other variables with handmade skis that trying to come up with a perfect fiberglass/carbon combination might be moot anyways. Big one being core thickness. Stiffness of a beam (ski) is proportional to the cube of the thickness. So if you get your thickness off even a little, it will make HUGE differences in the stiffness of the ski. So if you are going to experiment, make you sure have very accurate and repeatable ways to make a good core, or you won't be able to attribute ski differences to material differences. ;)

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MontuckyMadman
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Post by MontuckyMadman »

this I have in an email from a bit back from a pro:
I'd suggest and eight ounce. However, 45/45 will only give you torsional rigidity and not stiffen tip to tail, You'll need to back it up with some 0/90 for that.
0/45/45 deft triax would be best.
This makes NO sense to me.

Try aspen core, 12mm on center/high point, even taper to 2.3mm tip/tail, or maybe 10.5mm poplar core on center even taper to 2mm and 8 once uni CF top and bottom?
Think you need another axis to provide more torsional stiffness.

What is your weight and height?

What are you ski dims?

What is your application? ie: BC light weight, or inbounds hard slammer or a % of each?

I ordered some 1" carbon tape a while ago and I was going to run stringers on top, like line, but I think I am finally convinced to do them under the core. Hope it will do something positive.

I will be using heat. Thanx KRP. Do you think expansion/contraction will pose a significant problem?

kipi
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Post by kipi »

Thanks a lot for the answers^_^!
btw im building snowboards not skis..

my conclusions and questions:
1. carbon at top only is useless bocs it will be under compression and not tesion. in that case the only good thing about it is the looks.
2. uni is best? that sounds odd.. are you sure it will be good for a snowboard?? cos i need torsional rigidity and if i got it right uni wont provide it..
3. why having 45 or 90 deg cf will be a waste of carbon? and what do you mean by overkill O:
4. the wieght should be aroung 225g/m.

MountuckyMadman were those questions aim to me? your msg is kinda confusing

twizzstyle
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Post by twizzstyle »

kipi wrote: 2. uni is best? that sounds odd.. are you sure it will be good for a snowboard?? cos i need torsional rigidity and if i got it right uni wont provide it..
3. why having 45 or 90 deg cf will be a waste of carbon? and what do you mean by overkill O:
Having it at 45/-45 is goodness, that will provide torsional stiffness as you say. Imagine twisting a ski (or snowboard). What you are doing is putting it in tension at 45 deg, and compression at -45 degrees. Twist it the other way, and that reverses. So again, its all about tension/compression.

Having fibers at 90 (going across the ski/board laterally) gives it lateral stiffness. The width-to-thickness ratio of a ski or snowboard is such that you really don't get any bending sideways. Grab a snowboard by its edges on the side, and try to see if you can bend it - you can't.

So this is why 0/45/45 is popular (thats what tri-axial weave is) for skis/snowboards because you get longitudinal stiffness as well as torsional, but no lateral. I have never seen a triax woven carbon, so you're stuck with uni or biax (which comes in a variety of weaves, twill being the most popular for looks).

twizzstyle
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Post by twizzstyle »

MontuckyMadman wrote: I will be using heat. Thanx KRP. Do you think expansion/contraction will pose a significant problem?
Depending on how much CF you use, yes it could. If you do it symmetrically about the core (same layers of material on both sides of the core, in the same order in the same direction) you will have no problems. If you do things differently on top than bottom, you might get camber changes.

Two examples:

Symmetric layup (top to bottom)

Topsheet
0 deg fiberglass
45\-45 deg fiberglass
0 deg carbon fiber
Core
0 deg carbon fiber
45/-45 deg fiberglass
0 deg fiberglass
Base

Asymmetric layup

Topsheet
0 deg carbon fiber
45\-45 deg fiberglass
0 deg fiberglass
core
0 deg carbon fiber
45/-45 deg fiberglass
0 deg fiberglass
Base

You'll see there isn't much difference, but its enough that when cured with heat, you could get hefty camber changes.

Also remember that where you place the various fibers in relationship to the core can have big effects on the stiffness. (closer to, or farther away from the core). Having a composite layer further away from the core will give you a stiffer resultant ski/board. Remember that stiffness = E*I, E is a constant property of the material (modulus of elasticity) I is a function of the geometry (a rectangle in the case of skis/snowboards) and layup. The further you are from the center of the core, the higher I is, and the stiffer the ski is.

iggyskier
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Post by iggyskier »

I will just say that bamboo core vs bamboo core with carbon stringer on top = very different. I definitely want to try to adding more carbon to our skis to help drop the weight down. It is something my engineer and I have been talking about for awhile with a new BC version of one of our models, and if we like the layup, we might swap everything over to it. But I would say at least in my experience, carbon on top of the core adds noticeable pop and responsiveness.

twizzstyle
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Post by twizzstyle »

iggyskier wrote:I will just say that bamboo core vs bamboo core with carbon stringer on top = very different. I definitely want to try to adding more carbon to our skis to help drop the weight down. It is something my engineer and I have been talking about for awhile with a new BC version of one of our models, and if we like the layup, we might swap everything over to it. But I would say at least in my experience, carbon on top of the core adds noticeable pop and responsiveness.
I suppose I should add a caveat to my remarks about carbon on top doing nothing in that some ski companies do only put carbon on top just like you guys are doing, so obviously it works - I just struggle from an engineering standpoint to understand why. I know Line skis (at least a few years ago) only had carbon on top. K2's have carbon top and bottom, but its woven into the whole seamless fiberglass sleeve around the core, and now that K2 owns Line maybe they do it differently.

When you say you guys want to use more carbon to bring weight down, are you planning on cutting down core thickness and making that back up with carbon, or cutting back on fiberglass and making that back up with the carbon? Maybe a combo of both I suppose.

davide
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Post by davide »

You need about three time less carbon to get the same stiffness.
Carbon under compression is just a bit stiffer than glass, while in tension is much stiffer, so it is better to put a layer under the core than above the core.
I dos not really matter what the big companies are doing.

doughboyshredder
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Post by doughboyshredder »

Carbon also springs back to its molded shape quicker and with more force than fiberglass. This is what we refer to as snap. That is why adding carbon above the core can effect the feel of the ski. CF is not strong enough under compression to replace x amount of fg with 1/3x of cf, but you can use lighter glass and supplement it with cf, with no increase in weight, but an increase in snappiness.

At least that's my theory.

twizzstyle
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Post by twizzstyle »

doughboyshredder wrote:Carbon also springs back to its molded shape quicker and with more force than fiberglass.
That = stiffness. ;)

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