carbon and balsa core: ultralight skis
Posted: Fri Sep 08, 2006 6:36 am
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.
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.