stringers to optimize your torsional/long. stiffness ratio

[brianmwaters]

so this one's an idea/question for all you people out there.

i've been doing a lot of research lately trying to figure out how to calculate longitudinal stiffness curves, "tip/tail spring constants," etc. i'm not the first person to do this. it's good to do because then we can make skis with predictable properties, by comparing them to other skis with similar properties (endre made this possible).

i've also been trying to figure out how to predict a ski's torsional stiffness, but i've made little progress. but then i realized it's not necessary to predict this accurately at all, because we always want skis that are as torsionally stiff as possible. so all we have to do is maximize it, without worrying about exact numbers.

the challenge is to make skis with a high torsional stiffness/longitudinal stiffness ratio. triaxial glass is one solution. black diamond goes a step further and CNC's their cores, so that they're thicker near the sidewalls. i've twist-tested some of those skis in the store, and they're STIFF. what they've essentially done is to increase the skis' twisting resistance at points far from the centerline, and decrease it nearer the centerline. it's similar in concept to a steel I-beam, but for torsion instead of bending.

unfortunately, most of us don't have CNC or the fancy molds used to press such a ski. however, we can get a similar effect (probably not as pronounced though) by adding stringers of a stiffer wood close to the edges. or (and in my opinion, this would work better) we could add strips of glass/kevlar/carbon above (or below) the edges of the core.

i don't know if anyone's tried this, but if you have it would be cool if you spoke up. i might try it on a pair of skis, i'll get back on that.

[Yuki_otoko]

On snowboards like to put a stiffer stringer next to the sidewall. This is for two reasons. To add a little torsional stiffness, and to stiffen the sidewall incase of impact.

Its hard to tell how it went because I didnt have something similar to compare it too. In all it was stiff torsionally, over the length it felt nice for flex and it held well when on edge in hard pack.

I too wondered about putting the stiffer stringers to the outside and making up for the lack of pop in the nose and tail with flat carbon spars, or even an extra layer of lightweight fibreglass.

[plywood]

i work a lot with carbon strings, actually two sorts of carbon bands, but most of the time i use 25mm wide 220g/m2 unidirectional carbon bands. it`s a bit pricey - compared to regular fibreglass.

but it`s pretty cool to work with because it allows you to really modulate stiffness over the whole ski. most of the time i use those bands like the forks they use in snowboards, so basically starting from the binding with V odr X to tip and tail. really worked out for me and has a impact you truly can feel when skiing.

of course you also can use fibreglass bands. but for the torsional forks i mostly use carbon, longitudinally sometimes an extra band of unidirectional fibreglass following the edges...