Twisted Skis

[bullion mine]

Has anyone had problems with skis having a slight twist to them? It's hard to tell if my mold has issues, if it twists while cooling down, or if my wood core is slightly twisted, causing it to twist after pressing.
Also hase anyone had their bases change shape after cutting. I had 2 sets of bases match perfectly after cut-out. Then I checked them a few days after I attached the edges and they were 2mm out in the sidecut. No gaps between edge & base or anything like that. Could the edges be pulling the bases out of wack? Could slight tempurature fluctuations be pulling things in diff. directions?

[SHIF]

Is your press frame twisted? Any twist in your lower steel I-beam will transfer into your ski. I know this from (bad) experience. My beam is slightly tweaked so now I force it true by jacking one of the four wheels up off the floor.

-S

[petemorgan(pmoskico)]

How thin are your cores? And what kind of composites are you using?

My initial thought was that you aren't doing a balanced and symmetrical layup with relation to the fiber direction of the composites.

With thick cores, it usually isnt noticeable. I have done experiments seeing how warping and twisting ect occurs when a layup isn't balanced and symmetrical. I recall a vocab word called Isotropic, but I don't think that applicalable here.

A balanced and symmetrical layup would look like this:

Base then zero, 45, -45 core -45, 45, zero, top sheet
To achieve this you have to turn one composite layer 180 degrees.

I haven't seen any twisting on my skis because the thicker core prevents it but if your tips and tails were thin then maybe

[vinman]

Twisting can come from a few sources.

asym layup, meaning the way you put your glass down or adding unequal amounts of carbon, your mold could be off or you could have put them in the mold crooked.

For the lay up I always make sure that I put the same glass layer up. Meaning 45s facing the base and top sheet or longitudinal fibers facing the bases and the top sheet.

If you use carbon, be sure you lay it nice and straight or if you put it on an angle be sure you do it symmetrically.

For the mold, go around and take measurements, sand and adjust as needed

for putting things in squarely, make some marks on your cassette to indicate where your tip should be in relation to the end of the cassette and so that it sits in line longitudinally on the cassette. Also make a mark on you mold to show where the cassette needs to sit so your tip and tail or rockers/camber come out right.

For cooling the laminate, I usually let it sit in the mold until it cools to ~140F, then I pull it out and let it rest on its side to cool to room temp. This was a tip from a friend that works for a global snowboard biz.

For bases: yes temp variance can warp bases. I try to cut my base material off the roll, split it in half and let it rest far a day under a stack of hardwood. This decreases base warpage for me. When cutting keep it out of the sun and away from temp changes. Definitely let it sit a day or so flat under some weight before cutting to shape. I think this helps the most.

For edges, yes edge tension can warp bases, bend them to shape better so there is less chance of this.

[powderho]

Oh no.......twisted skis! Literally my biggest fear when I pull my skis from the press. I had 3 pair in a row come out twisted a couple of years ago and I couldn't figure out why. I ended up decided it was the fiberglass I used because nothing else made any since. The glass I used on those three pair was some Owens Corning stuff. Almost every other ski I've built I used Vectoryply. Other than the different glass, I have built almost-exact copies of those skis without any issue. I don't think this is caused by an asymetrical laminate. I think an asymetrical laminate will effect camber and tip/tail rise, but it won't twist it. If you ran a bunch of carbon tow along one edge of a ski, only below the core, they maybe it would twist. Another thing I started doing was separating my skis after I pull them from the press to cool. I usually end up with a bunch of excess epoxy holding the two skis together. I brake them apart right away. My thought is that the extra expoxy is pulling on the skis while it's finishing its cure, or post-curing, or doing whatever the epoxy does from when you pull it from the press and it is fully cured.

[twizzstyle]

I had 2 sets of bases match perfectly after cut-out. Then I checked them a few days after I attached the edges

There's your problem. Don't cut out your bases until you are ready to lay them up. The base material can change shape a lot over time.

[barnboy]

All have mentioned good stuff to look out for, I'd add making sure your hoses are not over inflating, and that they lay directly over your skis. I know I've seen some twist stemming from off lined bladders in the past.

[MontuckyMadman]

Ho, I thought you said you had hardened epoxy under your layups accidently?

[bullion mine]

Thanks everyone for the input. I lay my fiberglass 45 toward core top and bottom and I use a carbon strip on top of a wood core and thought I laid it up pretty strait and square but really did not pay that much attention to it. I also will check my press for square and my mold. The mold is where I think the problem might be but to tweak/fix any twist or irregularities in the mold seems a bit daunting. I do have a cat track so I am not sure how much the fire hose scenario plays a part but I also will check that out to make sure.
Thanks again

[bullion mine]

Thanks everyone for the input. I lay my fiberglass 45 toward core top and bottom and I use a carbon strip on top of a wood core and thought I laid it up pretty strait and square but really did not pay that much attention to it. I also will check my press for square and my mold. The mold is where I think the problem might be but to tweak/fix any twist or irregularities in the mold seems a bit daunting. I do have a cat track so I am not sure how much the fire hose scenario plays a part but I also will check that out to make sure.
Thanks again