Torsion box + tip layers?

[justplanesteve]

Hoping/planning to build skis for self/fun/challenge. I've been reading/searching here off and on over the past year, have not seen these 2 personal puzzlements addressed.

1.) The flat layers of glass method of torsion control/modification bothers me, but it seems industry standard.
Does no one use 45+/- fiberglass socks (full ski wrap tubes) for torsion? Is it because the ski would be overly stiff, torsionally? It would seem that lighter weight could be used to attain a given torsional flex? I would like to try this, has it been proven poor practice? I do understand that it either complicates or obviates side-wall construction, that is not the question.

My thought is the 45 sock, with a layer of unidirectional carbon top and bottom. Weights to be determined. I plan to use a wood top sheet for a concept ski, and for damping.

2.) Most ski builds show cores stopping short of the bend area for tip and tail. Either a plastic piece is keyed and butted to the core at these positions, or the extensions consist of several layers of heavy fiberglass butted to the core thickness. In either case, the fabric layers then go over the hinged on parts. Is there a technical (ski performance) reason, rather than materials cost/convenience to not continue the thinned core to become the tip and tail as well?

Thank you!
smt

[TimW]

1. I only have experience building snowboards but there is actually not that much to be gained here. I have build boards where I let ±45° layers pass behind the sidewall to connect the top and bottom laminate, thinking it would increase torsional stiffness.
More recently I also included torsion in my modelling, based on this paper:
https://www.fpl.fs.usda.gov/documnts/fplrp/fplrp156.pdf
Doing the modelling I noticed that the shear stiffness of the core has very little effect on the overall torsional stiffness. This makes sense when you think of it for a wide section like a snowboard. The shear flow from the top laminates passes on the sides of the board to the bottom laminate. The core may have a low shear stiffness, but there is a lot of width (material). So when your glass laminate is 0.5mm with a shear modulus of 20 GPA, this stiffness is already matched by the outer 10mm of sidewall / core with a shear modulus of 1GPa. And then we ignore the rest of the core. So for a snowboard it is easy to see that the core/sidewall is not the limiting factor for torsional stiffness.
Now for a ski the width is of course less, and for a narrow ski there may be more to be gained here. But the torsional moment on a narrow ski will also be a lot less. So I doubt there is much performance to be gained from the additional stiffness. You would lose some torsional damping, which may be a negative.
If you want to try the effect of high torsional stiffness, I would just go for a cap construction with ±45° carbon layers, easy to do and you cannot get torsionally much stiffer than that. (now I have been considering aluminium sidewalls with titanal topsheet, but that is more for the looks of it.)

2. you can do a continuous core if you make it thin enough and do not want a too steep rising tip. It is a bit more difficult for layup as a ptex tip conforms much easier to your mold. For longer tips I prefer a continuous woodcore over having a ptex core in the tip.

[justplanesteve]

Tim - i really appreciate your considered reply - it gives a lot of insight.
Keeping in mind that i don't really know what i'm doing design-wise, there are several clues for me in your response.

The design ignorance - wanting to build a ski for old school noodling through the bumps (not the modern hi-energy, knee bashing comp model at all) Slow line...well, in my 70th year this season, maybe not even so fast. Fluid, is what i'm hoping to further develop as a skier.

So thinking 75-ish to maybe 80 underfoot. Somewhat long - 180 - 185.
Learning how little a complete wrap affects torsion actually makes me want to try it, since the corollary is that it probably won't be too stiff, which was my prime worry.

While i don't know how to design a ski, my business was woodworking & i am a machinist. There is a lot of experience making tooling; & sawing & bending wood almost any way possible (except never used ammonia) for architectural millwork and some furniture. Given my further understanding from your notes, i will plan to make continuous wooden cores. I'm thinking of 2 flat layers over a shaped middle layer. laminated up with the carbon uni-directional fiber to basic shape. Clean that up to shape and rout the bottom perimeter for edge relief. "figure out" what and where to add tip, tail, possibly side-wall protection. With a second lay-up, include the 45 +/- sock over the wood core, on top of the base with edges, and the wooden top-sheet. I may regret it, but with a first effort, i see adding protective side-walls to this lay-up as unnecessary complication until there is some use to verify the design works or not.

BTW, there is no consideration that the skis will make me a better skier. It's not about that. Most of the build initiative is about a visual concept/joke while learning some new skills. But it falls flat if the skis don't have some performance, or strongly inform the next pair.

Thank you!
smt