carbon and balsa core: ultralight skis

[davide]

Two weeks ago I got the balsa plate. I could choose between two types: Baltek and another company (I don't know the name), that was cheaper. I chose the cheap one, but when I received the panel I discovered that the blocks used to make the panel have not all the same density: some are really hard, some are very soft. Of course also the mechanical properties differs from one block to the other. Baltek instead, claims that the density variation is less than 10 %.
I machined the panel (tapering) and I cut a 5 cm wide, 1.8m long strip, and I laminated the bottom and top with UD carbon, 200g/m2. I did not do it carefully, for example the tear-off fabric was not flat, but it was folded in few points. These points turned out to be weak points and the ski-dummy break in several pieces when I loaded it.
Then I worked on the remnant part of the panel, but this time I did really a nice work. I laminated the core with UD carbon, 200g/m2; I put also quite a lot of resin, that was soaked up into the porous balsa core to make it stronger. Now it is more robust.
Then I decided to go on, but rather then using expensive carbon I switched to cheap biaxial glass. The structure of the finished ski is:

Biaxial glass, 400 g/m2
UD carbon 200g/m2
Balsa core (3mm at tip and tail, 13mm in the centre)
UD carbon 200g/m2
Biaxial glass, 400 g/m2
Base and edges

The sizes are 112-84-108, 178cm; the weight is 1780 g per pair. With biaxial carbon 250g/m2, they would be even lighter.
I measured the stiffness, as I do usually: put the ski on two block 70 cm apart and stand up. They are rather soft. When I tried to measure on 90 cm, the upper carbon layer collapsed, just under my foot. Now there is a crack across the width of the ski.
So, for carbon fibres, compressive strength is about 30% lower than tensile strength: I should have put more carbon on the upper layer. Probably I can fix the ski, so I can still try them on the snow.
Apart from this little problem, they are the lightest skis ever built, lighter than DB or race randonnè skis.

Probably I will try to build one more pair, using the more homogeneous (I hope) Baltek balsa core and more carbon on the upper layer. I think I will also use a thinner core.

[kelvin]

Nice! I'm working on a pair of lightweight skis as well. I'm using airex foam core and once profiled, it is pretty soft and flexible. I'll probably have to stiffen it with carbon fiber. It is light though, a full core with uhmw sidewalls is only 380 gms.

-kelvin

[collin]

2lbs a pair! Holy crap! Nice work man, but where are the pics?

Incase anyone else was wondering the 200 and 400 g/m^2 fabrics are ~6 and ~12 oz/yard^2, if I did the conversion right.

I think it was me and you discussing balsa in another thread, and you were trying to convince me that the end grain was better than the normal orientation. But I've forgotten why I was convinced you were right.

Do you think that there was any crushing of the core? And that's why the top layer cracked? And argument against end grain? Thoughts?

[Alex]

What about using carbon for the biax +/- 45° layer and glass for unidirectional? You can save a lot of weight compared to a glass ski, gain torsional stiffness an the ski is not as likely to fail due to torsional overloading.

@kelvin: I'm using an airex core on my current ski in the tip area with normal glass layers (390g/m2 0/90° and 400g/m2 +- 45°). It's very stiff and rugged.

[davide]

I did few considerations on weight: in a standard ski, the weights are distributed like this:
wood core 900g
base and edges 300g
glass (750g/m2) 250g
resin 150/200g

Balsa or Airex core would weight less than 250g, while carbon (triaxial 250g/m2) would be 90g, so only 150g can be saved when replacing carbon with glass.
Probably a balsa, glass ski would be very light but still cheap.

Kelvin, which kind of Airex foam are you using? Maybe C70.75?

Regarding the failure of my ski, it could be that the core collapsed and the upper carbon layer broke or the carbon failed, and then the core was squeezed. I think it is the second one. In sandwich construction when the core fails, it will break first, and most of the time the upper and lower skins do not break.

It seems that end grain panels can not tolerate high bending strain. Could be that they are used mainly for rigid parts.
Maybe I should consider also to try standard balsa panels. It should be interesting to make few tests.

[Mutombo]

Hi Davide!! Very interesting your investigations as allways.
I have something to say in this post. Our first pair we built have been the lightest skis I´ve ever known. With alpine bindings they are about 4500 gr, taken into account that alpine bindings are about 2500 gr/pair they weight less than 2000 gr/pair. They are really big, the sizes are about 130-97-120 (they are not the same in the two skis) and 1790 mm long.
We used diferent amount of fiberglass in the skis, the first one aout 700 gr/m² and the second one about 1200 gr/m². The first one is really soft but the second one is stiffer maybe a little bit less could be good for my purpose. We used samba wood (4 mm tip and tail and 14 mm waist), it´s really a soft and light wood I thought it was going to brake in the first try but everything held in position till now. I was doing easy rails/boxes and little jumps for 3 hours and everything ok.
Now I have to test it harder conditions. I´ll post you what happen but this wood can be an alternative material to make light skis.

[davide]

Here is the picture of the fractured upper carbon layer:

The white thing is the biaxial glass layer, which was pushed up a bit.

Now I have added a UD glass layer (400g/m2) on top. The skis are stiffer now. Weight is 1Kg per ski. Actually it is only 100g lighter than the ash/carbon skis I made in spring, http://www.skibuilders.com/phpBB2/viewtopic.php?t=322
I will also glue a thin wood plate at the ski center, to fix the broken ski, and for mouting the bindings.