My experience when I built my trike semi-elliptic carbon spring was that it was easy to make the spring plenty strong enough, but hard to make it stiff enough. My first two attempts were far too soft and the final version looked ridiculously beefy just to get it to be stiff enough. One thing's for sure, it is unlikely to break, as there's enough uni carbon in the thing to hold up a small road bridge!
I found the key to getting enough stiffness, but keeping the weight (and cost of carbon fibre) down was to use a balsa wood core, pre-laminated into a curve. I wrapped that with biaxial glass, then laid up the unidirectional carbon on the top and bottom. Were I to do it again for a "proper" suspension system, then I'd be inclined to use a slighter harder wood core, as this would provide a bit more damping. As it is, the balsa works pretty well at damping that spring down, I don't think additional dampers would be needed if the thickness of core and spring rate could be adjusted to work well together.
Wood core skis may well be pretty close to what's needed already and would certainly be cheaper and quicker to build with if they can be bought at the right sort of surplus price.
I may do some more digging around to see if there's an easy way to work out the spring rate and damping coefficient for this stuff, just to save all the trial and error (and cost of wasted carbon fibre). Luckily, I still have around 80 or 90 metres of 2" unidirectional carbon left, probably enough for another couple of springs...........
Jeremy