woodcore : many questions...

[alexisg1]

Hy everyone,

a friend and I have started to build our pneumatic press, and besides of that I am starting the woodcore. I just bought two raw piece of ash and two of fir. After having processed the wood to obtain some beautiful planks, I am now wondering what width should have the strips...

Of course I've read the skibuilders "howto guide", and I was planning on cutting the strips as recommended, between 1/4" to 1" wide.

However, here's what I found from a swiss company you may not know in the US, Masurao (http://www.masurao.ch/)
http://www.masurao.ch/home/popup.aspx?ID=54



1 - What do you think ? The more strips the better the ski ?
2 - What influence does it have on the ski's beahaviour (flex, stifness) ?
3 - Does it make the ski heavier ?

4 - we can see on the picture that the woodcore seems to have been "rounded" on both sides in order to get a "monocoque" (monoshell ?) ski (no sidewalls). Is it possible to obtain such result with a press ? We often see that kind of skis when people are using a vacuum system.

5 - One more question: would you say that glueing the core with classical vynilic glue instead of expoxy is ok ?

that's a lot of questions but thanks for giving me a hand

[kwetsor]

hello,
I will keep writing in english for everyone understanding. But for sure I would prefer french for you.

Basically, I would say that the more strips you will laminate, the better the wood core will be (better homogeneity). But also more glue, more weight, more labor...
I use 14...15mm square strips with different wood: hard wood under binding and above edges, light wood elsewhere (you could also use PVC foam C70.75 or honeycomb if you want very light ski, but maybe also more brittle).

The ELAN slalom wood core is machined from very heavy beech (Hetre) boards. Monolithic but with a lot of bonded strips of about 20 ...30mm wide, 300 ...400mm long. So, the strips are "butt joined", bonded with some "zig zag" cuts.
Then, you can understand that the wood fibers are interrupted longitudinaly.

From last years, I have found a very interesting product made of verticaly laminated 7mm bamboo strips. This product is used as wood fairing for bathroom, ... 20mm thick, 150mm wide, 2 meters long ... what a chance !
("skirting boards ?" Plinthe in french).

Sorry, need more posts to share some pictures, ...

My advice: keep the wood core as simple as possible for a first trial, and use your bain for good preparation and final assembly !

If you want more explanation in french, PM me.

Oh yes, about "rounded" wood core. I think it is not so difficult to get this with a router. For the final assembly, I would prefer a vaccum bagging process, but with also a nightmare to keep all the stuff perfectly aligned during curing. This is absolutely required for the final trimming along the edges ... (and also very accurate wood core machining).
I guess that a good CAP setup needs 2 perfectly matching molds with alignement pins.

Hope this helps,

Kwetsor from Belgium.

[alexisg1]

Oh yes, about "rounded" wood core. I think it is not so difficult to get this with a router. For the final assembly, I would prefer a vaccum bagging process, but with also a nightmare to keep all the stuff perfectly aligned during curing.

Okay. My question was more: "will the fiberglass go round the woodcore with a pneumatic system". I guess it does with a vacuum system, but every pair of ski coming out of a press has sidewalls. Anyway thanks for the info.

following is how we wanna do our woodcore :


I guess this is gonna be stiff

[davide]

Cores made with narrower strips are probably more stiffer in torsion, but for a fat powder ski, it will not be really important.

Vynil glue, or poliurethane wood glue is perfect for glueing the core.

In case you are interested, I have some cores to sell, just let me know as soon as possible (PM).
http://aeg.isuisse.com/LabLivigno06/core.jpg
Actually they are 195 cm long, already tapered and 15 cm wide, should be around 20 or 30 euros per pair.

[alexisg1]

Hi, I can't see the picture..."forbidden"

Well, building its own core is part of the game isn't it ?
I already bought the planks...it's on its way. but thx anyway.

[Charlie]

Oh yes, about "rounded" wood core. I think it is not so difficult to get this with a router. For the final assembly, I would prefer a vaccum bagging process, but with also a nightmare to keep all the stuff perfectly aligned during curing.

Okay. My question was more: "will the fiberglass go round the woodcore with a pneumatic system". I guess it does with a vacuum system, but every pair of ski coming out of a press has sidewalls. Anyway thanks for the info.

well, you can make CAP skis in pneumatic press, BUT you will need a 2 perfectly matched molds (or cassette) to do it, as kwetsor wrote. And it will be a huge PITA to lay up everything properly... try to find a production video on karhu.com, if its still available...(?)

[SRP]

Well I have been reading everyone elses post for awhile, so I figured I better start contributing. I have been going through the same debate as to how many wood strips to include in my cores. I have been thinking that as the number on strips increases, so does the amount of glue. Thus the properties of the glue are being brought out more, so the type of glue you are using becomes a larger factor in the performance of the core. Where as, if you only have, say 5 strips of wood, then the porperties of that species of wood are brought out more. I also think that as the number of strips and glue joints increase, you are losing some of the dampening properties of the wood. I am thinking of using less strips and really putting the effort into choosing the best pieces (grain orientation) of wood that I can. All of this is only an assumption based on my thoughts and talking with a few other custom ski builders, since my press is still under construction.

[RoboGeek]

SRP - your exactly right.. the thinner and more bonding surface there is.. the more the adhesive plays into the properties

[Cadman]

I have skied on the same ski with wide laminates (20mm) like the snowboards are laid up and what they call "micro-core" (approx 3mm) laminates and the wide laminates feel like the old skis of the past. The wide laminate cores are not as damp and solid feeling. The micro-core holds better on the ice from what I have experienced. The wide laminates were lighter though. The advantage of micro core is also that fact that the imperfections in the wood are not as big a factor so the variations in flex from core to core is less which is real important with skis. You don't want a stiff ski on one foot and a soft one on the other.
Most of the high end skis that use wood have micro cores from what I have seen in the past. Like one other person mentioned, if you are just skiing powder or crud, it is not as big an issue.

As far as making cap skis, the vacuum process is not the greatest. The snowboard guys had major problems on the sides of their boards unless they went to about 45 degree caps angles. Pressure on the sides was the problem. If you are going to use an airbag type press you will have to
have a cap type upper mold cavity that is aligned with the bottom or you
will have major problems keeping things aligned.

I have been around the tooling industry for a number of years and have seen everything from riveted steel to beautifully machined solid cap tooling. I have been in a number of factories and everyone has a little different approach but generally in the same direction.

The reason that you see so many sidewall designs is for one thing, it is an
easier way to make skis along with some advantages performance wise. The ski is usually stronger above the edge since there is pretty good support. Edges seem to dent and get damaged easier on cap skis.
Unless you are pretty good with 3D cad, you will be experimenting for quite some time with getting the lengths correct. The bottom bends at a different radius than the top so you must be carefull to get this correct or your cap will not fit on the base correctly. With sandwich construction your top mold is a flat sheet of metal so stretch is not a factor here.
Caps require much more accurate tooling and parts that are manufactured
correctly. For instance, if the core is too wide or too thick, the top mold will not close down correctly. If the core is too thin, the ski comes out with bubbles and voids with resin rich or dry areas. Sidewall laminate construction is like a sandwich. If you want a thicker sandwich, you just put more meat between the bread. Caps are a whole different ball game.
Everything has to fit right since the pressure is on the top and the ridge running along the side of the ski where all the topsheet and glass comes out over the edges. If your layup is not right, you will get too much or not enough pressure on the "close out" area of the ski. Too narrow of core will cause ripples in the sidewall. Too wide will dry out the glass or keep the mold from closing down all the way.

I would start with laminate sidewall processes until you really have the system dialed in. You will end up with more skis on your feet than in the garbage can. Good luck

[alexisg1]

OK guys, thanks for the contribution !

@SRP: yeah that makes good sense !

@Cadman: thanks for sharing. I won't go for CAP, I was just wondering how to do them with a pneumatic press. Anyway I find the skis with sidewalls much nicer.

if you are just skiing powder or crud, it is not as big an issue. I wish I could...