Machining core for improving torsion

[alexisg1]

Hi everyone,

we're a group of skibuilders and I am responsible of core making.

1 - Our cores seem pretty good, but still, we have some issues with torsion (bit too soft) and secondly we try everything to gain weight...

2 - if you look at most of skis in shops, they have different shape aiming at making them more rigid in torsion (at least that's what I think). For example the BD havoc :


3 - In my latest skis, the core is 1,1 kilos ( 2-12-2 thickness and 135-95-125 shape) and the finished ski is 1.8 kilos. so machining cores can brind interesting weight loss.

My question is :
1 - has anyone ever tried to machine a core in order to enhance rigidity in torsion ?
2 - if yes, and as we press only vertically, has this person try to fill out the vacuous with foam ? ( so that fibers don'tfold while pressing)

Thanks for the answer(s) !

[plywood]

i think it would be a pain to build such a core as for the havocs shown in your picture. witz our regular skibuilding methods we`d never get the accuracy needed for such things. i think such skis can`t be done without a cnc to cut the core. then the easiest would be to press such skis with vaccum - it`s really hard to build such a 3D-topmold that fits exactely to such a surface.

the easiest way to increase the torsional stiffness is to use torsion forks, as used on snowboards. on all of my skis i used either 220g/m2, 25mm wide carbon or fibreglass bands, alighned crosswise from binding to tip and binding to tail



normally i just use them on top of the core (combined with 2 layers of 308g/m2 45° biaxial fibreglass, one layer on top and one below the core). and i can`t complain about torsional stiffness, even though my skis are a little wider than yours and if you`re not happy with the result you still can build in an other pair of forks below the core.

for your second question: on my first pair of skis i did what you`re asking for (at least i think so). i routed out some gaps in the woodcore to reduce the weight. at the same time i placed these pockets in a way that i still was able to place the carbon forks mentioned above onto solid woodcore.



afterwards i filled up those pockets with honeycomb material. let`s just say it was a pain in the ass to do the frustrating part was, that i just gained 20g of weight on each ski. so i think in the end i didn`t gain any weight because during layup some honeycomb cells got filled with epoxy.

maybe you had to build larger pockets. but then i fear that the stability of the ski would decrease and suffer.

so if you want to gain weight, use lighter wood for the core. this will have a bigger impact than routing out some pockets. and it will be done quicker

[alexisg1]

Hi plywood and thanks for the answer.

I'm pretty happy with the torsion of my skis because I did just like you (I remembered that picture when I decided to order som carbon strings ).

We just wondered if we could get the same result in a cheaper way by routing the core, and thus also gain some weight.

We actually did it this afternoon and skis should be pressed next week.




We thought of filling the gap with foam, or maybe put the piece of foam over the fibers in order to press them down in the holes.
I'll tell you what it look slike next week

[Greg]

If you can get a vacuum mold going, you might be better off by not filling the empty space with foam, and instead letting the fiberglass shape itself to fill the gap.

This way, you get the extra strength of the fiberglass being in the vertical position (like a beam) instead of the horizontal direction (where it doesn't really affect stiffness very much). The fiberglass should be stronger than the wood, so it is advantageous to have it vertical as well. Having varying directions should also help with the torsional stiffness just by improving beam stiffness of the whole ski.

I was looking at some cross country skis the other day that had been cut apart, and they did this and were amazingly stiff for how little material they used. It is all about beam theory, and keeping the layers bonded together well.

I think this is something to experiment more with once I get my new press up and going.

[burny]

I know from experience: the thinner the woodstripes you use, the more torsionally stiffer the ski gets. I've tried a core with 2.5cm wide stripes and one with 0.5cm wide stripes. Identical wood (obachi). The cores I used the 0.5cm stipes came out noticable (a lot) stiffer, longitudional and torsional.
I don't think those pockets or any humpy shaped cores used by BlackDiamond or Salomon or Atomic or any other brand does anything positive to the ski. The surface those humps have is just way to small to deliver palpable differences, all that will happen is, you get a softer ski because you took millimeters of wood from the ski. The stongest thing providing stiffness is the distance between the upper and the bottom side of the core.
One more thing I learned: carbon forks work great, but heavy 45°-45° glass works better (adds wheight however).

[ben_mtl]

so ?
how did it work when pressing those "machined core" skis ?

I'm actually having a really close look on ski building and shaping a core like BD does is something I want to invistigate... The only difference with Alexis is I will vaccum-press my skis but I don't know how to do for the topsheet to follow the shape of the core (maybe by using only fiberglass with kevlar tape on the upper corners as topsheet...)

[alexisg1]

Hi Ben,

if you want to have a look at the skis, check out this link : viewtopic.php?t=1250&start=15

As you said and as we are pressing with a pneumatic press, we prefered to fil in the holes with foam and actually it looks perfectly flat. Of course you may want to make the shape appear on the skis (as on BD skis), but itseemed to us pretty difficult.

I can't say about using vacuum...but I'd be glad to see your result !

[Craig]

ben_mtl,

For pressing a 3D core, you would need to construct a matching top mold to go in your press - the difficult part, as other have mentioned.

Machining an accurate top mold by hand would be near impossible, I'd guess, and CNCing one would be very expensive, unless you had the CAD skills and the resources to do it yourself.

I think your best bet would be using a pourable moldmaking gel to pour over your piece to build an identical negative for use as a mold. The procedure would involve sealing your positive piece (in this case, your 3D core), constructing an enclosure to stop horizontal gel flow before curing is complete, and pouring your choice of moldmaking material over the core.

As far as material, moldmaking supply companies make all kinds of different medias for this purpose. I can't really say what to use, but I know there are plenty of options out there. I'd look for suppliers and see who will talk to you about the material specs for the project (there are plenty of suppliers in the US, at least).

The shortcomings of this for your purposes (it seems to me):

-The mold would only match the core you used to make the mold. Unless you were accurately reproducing the core, your could only use it for the ski that formed it.

-From what I understand, moldmaking epoxies are not cheap.

-and, if you mold does not release properly from your core when it cures, you are out of luck on the mold ($$), and might ruin your core.

Even with all that said, it sounds like a lot of fun to try. Good luck

[ben_mtl]

Great ! thanks for those answers.
Actually I was not thinking press-forming my first skis but using vacuum, I'm a lot more used to this method as I vacuum-made a couple kayaks.

I guess if I can make a jig to groove some shapes in the core (not complex grooves) I will be able to duplicate the same shape for both skis. My main concern is about the top sheet, a rigid topsheet will not follow a "3D" core. I'm not an expert skier so when skiing my skis often hit each other, a top sheet would prevent the skis from being used to fast. I have to find a solution to bend the topsheet or another option would be to find an alternative to topsheet (any suggestion ?)
CNC cuting is an interesting option though, I'm asking all around me to find someone who could help me... but i'll still have the topsheet problem.

I just started the mold yesterday... I'll post some pics as the "Goat-Ham-Ah Project" goes on (As you may have understood my first shape is based on Gotama dimensions)