something stiffer than bamboo...

[JackJack]

I have been using vertically laminated bamboo and have found that I can make a pretty thick core that is still not stiff enough. What woods are light but stiffer than bamboo?

[amidnightproject]

I have been using vertically laminated bamboo and have found that I can make a pretty thick core that is still not stiff enough. What woods are light but stiffer than bamboo?

I use bamboo in all my cores. May I ask what your layup consists of? What your using for glass?

[JackJack]

yep sure, i am using 1 sheet of 600gram triaxial fibre glass on top and bottom. West systems 105/205 epoxy holds it all together.

I dont really want to add any more fibreglass because this will make it pretty heavy.

[Richuk]

You need to look at the Young modulus of particular materials. There is plenty of material within the forum, you just need to hit the search button.

[falls]

Add some carbon fibre?
I have found you do need a fair bit of bamboo to make a stiffer ski.

[MontuckyMadman]

its the west if you are compression molding then it presses it all out and it has very little stiffness IMO.

Poplar will have the lightest weight to strength ratio of any abundant wood.
Pie are good to but screw retention is poor.
Aspen is softer but still very light and good wit the right epoxy.
We use poplar/maple or poplar/beach.

[twizzstyle]

Add carbon, or use pre-cured pre-tensioned fiberglass/carbon.

[JackJack]

thanks for the info, I dont really want to use any fabrics that do not go transparent as I want to see the wooden core at the end. is it a good idea to sandwhich uni directional fibreglass between the edges of the core and the sidewalls and let this cure before lay up?

I thought that this might make the whole thing much stiffer without having to make it really thick

[twizzstyle]

is it a good idea to sandwhich uni directional fibreglass between the edges of the core and the sidewalls and let this cure before lay up?

I thought that this might make the whole thing much stiffer without having to make it really thick

That will add very little stiffness unfortunately. You'd be better off using a bi-directional weave at +/-45 deg, since that layer between the core and sidewalls would be acting as a sheer layer.

[JackJack]

ok, do you think that I could achieve a better stiffness/weight ratio by using biaxial tape around the edges or just to make a thicker core without any edge reinforcement?

[MontuckyMadman]

Thicker core made of a lighter wood not boo.

[twizzstyle]

Yep, a thicker core is going to be your best bet for raising stiffness. All else the same (same wood/composite), bending stiffness goes with the thickness cubed. Double the core thickness, and you've made it 8 times stiffer.

[JackJack]

ah ok, so is there a formula I can use to work out how each milimeter of thickness will affect the stiffness of the ski?

[twizzstyle]

Ignoring the composite, the bending stiffness of a uniform rectangular beam is EI, where E is young's modulus (look it up), and I is the moment of inertia.

For a rectangular cross section of width b and height h, I = bh^3/12.

Of course it's more complicated than that because h (core thickness) isn't constant along the length, neither is b (if you have sidecut), and then you have the addition EI from the composite layers. But very generally speaking assuming (and therefore ignoring) consistent composite layers, ignoring core profile and sidecut, the stiffness goes with the thickness (h) cubed.