CFK / Aramid - no clue what to take...

[sanchezz]

Hi, as I have no clue what kinda composites to use I wondered what you guys usually take and why?

The Skibuilder guys seem to favor triaxial 22oz (750g/mm²) GFK - what do you use?

Is there any "guideline" when to use triaxial, biax, atlas, etc? What's the difference and which breading is suidted best for skis?

If you use CFK, what kind do you use?

I planned something like this

CFK triax
Core
CFK triax
Aramid biax
base

Any suggestions? I guess you can see I'm a bit clueless

EDIT: and another question: If I wanted to make a Torsion Box, how would you do that? Because of the Sidecut I imagine it hard to just wrap some sheets of CFK around the core...

[ben_mtl]

I'm not sure you really want to use Aramid as it doesn't like humidity...
The standard construction most of us do (i guess) is only 1 triax on each side of the core (+ VDS rubber, edges and base of course)
Then you can add a topsheet, or a cloth +lightweight fiberglass for the finish.
You can also add some carbon here and there to play with the stiffness.

Some companies use a triax with a strip of UD carbon incorpored in the triax, I'd like to find it but don't know where they buy it.

About the torsion box I didn't really thought about it yet but it's something I want to try on my next skis, I guess you have to find a braid tube (hope it's the correct term as I'm not english...) and wrap it around your core, prior or during your layup. I'll try to make a quick sketch of want I would like to try.
The main concern for me is to keep the fiberglass tight around the core while it cures (vacuum bagging could help but how ?)

I'll follow this topic :p
Ben

[sanchezz]

Why do you usually use the same composite over and under your core?
Wouldn't it make more sense to use different materials? E.g. carbon under the core makes sense since carbon is really stiff and therefore is good under tension when the ski is flexed. But since it is so stiff carbon also tends to break under compression - so on top of the core it doesn't really make sense to me.
So I wonder if it wouldn't be better to use carbon under the core and fibreglass above the core?

Any thoughts?

[ben_mtl]

It would make sense to me.. or at least just add a carbon strip under the core.. just wanted to keep things simple for my first attempts.

[Kevin6q]

Hi Ben,
You can find braided tubes (sleeve) at Sollarcomposites.com . These guys have many different braids and sizes available. Keeping the braid tight around the core will be a PIA and vac bagging wont work too well as it tends to pucker and drag the sleeve to the corners. Wrap the core from one end to the other using electrical tape or heat shrink tubing. I think this will add moderate stiffness but a lot of weight from excess resin. A bias ply above and below the core will be simpler, lighter, and provide a lot of torsional stiffness. S-glass has better compressive strength than carbon and would be a nice addition to the top layer of the ski with uni-CF on the base side of the core.

[doughboyshredder]

Why do you usually use the same composite over and under your core?
Wouldn't it make more sense to use different materials? E.g. carbon under the core makes sense since carbon is really stiff and therefore is good under tension when the ski is flexed. But since it is so stiff carbon also tends to break under compression - so on top of the core it doesn't really make sense to me.
So I wonder if it wouldn't be better to use carbon under the core and fibreglass above the core?

Any thoughts?

I would agree 100%. Carbon should not be used above the core if you ask me. I wonder though, does this change if the board is full rocker?

[sanchezz]

Does anyone have any charts stating both tensile strength and compressive strength of common fibres such as carbon, aramid and fibreglass? I can't find anything good..

[bobbyrobie]

you normally want a balanced layup. or you will start 2 get a potato chip effect and get strange curvatures in the composite.

[sanchezz]

what do you mean by "balanced"? Symmetric to the core, so same stuff below and above??

[Kippax]

Hi, as I have no clue what kinda composites to use I wondered what you guys usually take and why?

> That would be a very lengthy discussion.

The Skibuilder guys seem to favor triaxial 22oz (750g/mm²) GFK - what do you use?

> Nothing wrong with 22oz.

Is there any "guideline" when to use triaxial, biax, atlas, etc? What's the difference and which breading is suidted best for skis?

> Triax is best for skies.

If you use CFK, what kind do you use?

I planned something like this

CFK triax
Core
CFK triax
Aramid biax - (don't waste your time with this layer)
base

Any suggestions? I guess you can see I'm a bit clueless Wink

EDIT: and another question: If I wanted to make a Torsion Box, how would you do that? Because of the Sidecut I imagine it hard to just wrap some sheets of CFK around the core...

> Maybe you should try this:

14oz Uni directional fiberglass
10oz fiberglass sock (has to be wide enough for the fibers to sit between 45 and 30 degrees when pulled tight onto the core)
Core inside the sock
10oz Uni directional fiberglass
Base

> That will make a nice light/snappy torsion box construction