My last three pair of skis came out all twisted torsionally. Of course I didn't notice they were messed up until I skied a pair of them at the Bird yesterday. I checked the other pair I still had at my house and they are twisted even worse. I assumed the third pair was twisted as well, and sure enough, my friend Tim noticed they were twisted as he was feeding it through the tuning machine. I think these are pairs 42-44, and I've never noticed this happen before. Initially I assumed this was an epoxy curing issue, but I'm not sure anymore. All three pair were brought up to curing temperature at the same rate; it was the cooling rate that varied between all three. One pair I took out after only 10-15 minutes of cooling, and the pair that's the worst, I let cool overnight (under pressure) before I took them out. I usually take the skis out after 1.5-2 hours of cooling, but I have left them in much longer many times before.
The only common thread between these three pair that separate them from the rest are their cores. I made all three sets of cores at the same time from the same boards. All sidwalls were from one Maple board and the cores were all from 2 planks of poplar. They turned out to be some of my lightest, thinnest cores. They are 2.25mm tip, 11mm waist, and 3.25mm tail. I thought they would be a bit softer, but they are not noodles by any means. They are the first pair of poplar cores that I built for myself, and I'm beginning to think that poplar is stiffer than ash and maple. Anyway, is it possible there was too much moisture content in the wood? I've never had an issue with too much moisture before, but I'm all out guesses. I've noticed others have complained about this happening, but I'm not sure if anyone knows why. Any thoughts about why this happened? Any thoughts of how to fix these skis?
Why Twisted Skis. Why!!!
Moderators: Head Monkey, kelvin, bigKam, skidesmond, chrismp
-
MontuckyMadman
- Posts: 2395
- Joined: Fri Jun 20, 2008 9:41 pm
Re: Why Twisted Skis. Why!!!
attempted re-press? longer duration and soak and slightly more pressurepowderho wrote: Any thoughts of how to fix these skis?
What pressure?
What epoxy?
End of a glass roll?
Heat blanket malfunction?
that allot of skis and you have not seen this?
Post cure time before trim? or maybe you had them cure in a corner standing up?
Moisture does seem a viable culprit maybe.
I have no idea
-
skidesmond
- Posts: 2337
- Joined: Tue Apr 07, 2009 3:26 pm
- Location: Western Mass, USA
- Contact:
I'm thinking the wood may have had too much moisture or perhaps there was a heat malfunction. With everything else being consistent....
From past posts I've read, heat malfunction seems to effect camber most of the time.
Are the skis all warped in the same area? And were the core bottoms flat before you pressed them?
From past posts I've read, heat malfunction seems to effect camber most of the time.
Are the skis all warped in the same area? And were the core bottoms flat before you pressed them?
Here's a shot of one pair of twisted skis. You can see the edge is raised about 2mm off the table. Both skis are twisted in the same direction. The skis used the middle of a roll of some Owens Corning glass (22oz. top/bottom), QCM 43/32 epoxy, pressed with 50-55psi, post cure time of 12-36 hours. It's entirely possible the heat blanket malfunctioned. The PID controller didn't show anything unusual though. I've wanted to do a temperature map of my blanket from the start. The far edges never get as hot. It guess this could be a possible issue. I do post cure my skis standing up inside over a heat register. I have done this many times in the past, but with one difference. Usually when my skis are post curing they are still connected in the middle by the excess glass/epoxy. I think for all 3 of these pair in question I "broke" the skis into individual skis for post cure. Maybe keeping them together during post cure prevents them from twisting? At this point I think I'm going to start twisting on the skis trying to straighten them out until I hear some crunching/cracking.
Here's a couple more shots of my skis. The orange ones are twisted as well and the greens are the ones I have about 60 days on.
Here's a couple more shots of my skis. The orange ones are twisted as well and the greens are the ones I have about 60 days on.
-
MontuckyMadman
- Posts: 2395
- Joined: Fri Jun 20, 2008 9:41 pm
I think you figured right.
Heating too fast and bottom only and from the center out Iwould say causing and uneven cure and thus warpage.
Im building my own heat blanket.
I'm at $75 in materials, That's really cause I got some fancy rubber. Now I have to make it.
Heating too fast and bottom only and from the center out Iwould say causing and uneven cure and thus warpage.
Im building my own heat blanket.
I'm at $75 in materials, That's really cause I got some fancy rubber. Now I have to make it.
sammer wrote: I'm still a tang on top guy.
Twist, Twist...
Hoo boy.
I had this happen too.
I feel your pain.
I think you may be onto something about the heat.
A couple questions. Are all the skis twisted the same way? ie. Are all the left shovels/right hips touching, or are they switched Right tip/left hip with the ski's sibling left tip/right hip. Are you building the skis in pairs?
Basically you need something to be pulling unevenly somewhere in the laminate. My issues came because I was cutting a 0/90 cloth at 45 degrees to get my +/- 45 degee fabric. Eventually i forgot to pay attention to the actual angle of the fibers in the cloth. My cuts were closer 60 degrees when i noticed the twist. The rest is the best I could come up with in a thought experiment. The (admittedly) misaligned fabrics are getting warmed up during the cure cycle. The resin goes to "b" stage and everything gets "set" at some position, relative to each other. Everything is still pretty warm. As the skis come off the press things start to cool. And shrink. As the fibers contract they start to load the structure unevenly in torsion (in my case, at least) as the 60 degree fibers tend to pull more down the length of the ski, and the 30 degree fibers pull more across. I kind of noticed pulling the skis off the press when they didn't seem to lay flat on the bench. I really notice when they came out of the came out of the car at minus 18 celsius. Things looked like drill bits, well not that bad but, easily out 1 cm from shovel to hip.
Nothing in my process had changed, but suddenly I had warped skis. It took a while to figure out that it was the fiber angles that had changed because i was making a mistake. I did not realize that i had changed my cut angle. Dooh.
In your case, yeah, it could be heat, but if it is heat, and you are pressing 2 skis at once, and you are getting more heat in the middle than at the edges, I would expect to see bookmatched skis, ie. the tips are twisted different ways in sibling skis (by sibling i mean individual skis laid up at the same time).
If the skis are twisted/wound the same way i would be tempted to look at your fiber orientation. Are you ply flipping the glass (the glass is a 22 oz triax, right?)? That is, are you ensuring that the layers of zero axis glass are both against the core, and that the angle plies are matched in direction/position. This can induce significant twist if you get it wrong and are dealing with funny geometries (skis are definitely funny! by this i mean that ski's axial dimensions are radically different 1.8 m long, .02 m thick, .13 m wide, with curves in the length axis). I don't know. The fibers were definitely my issue, When i switched to a fabric with accurate +/- 45's everything went dead straight again. Problem gone.
In my opinion, you might as well start with heat. You want to get this working properly anyhow. Then when you make your next pair, make certain all stuff is lined up and you should be ok again.
Good luck. I sure remember the crushing heartbreak at seeing the twist, and the incredible frustration at trying to figure what the F%^&! is going on here? On the upside, you can ski on them. They just tend to want to hook up fast one way and wash the tails, and not initiate too well the other way, with killer tail grip. I sure would not sell them, but I used them. That said, they are way, way better flat/not twisted.
Later, carlito
I had this happen too.
I feel your pain.
I think you may be onto something about the heat.
A couple questions. Are all the skis twisted the same way? ie. Are all the left shovels/right hips touching, or are they switched Right tip/left hip with the ski's sibling left tip/right hip. Are you building the skis in pairs?
Basically you need something to be pulling unevenly somewhere in the laminate. My issues came because I was cutting a 0/90 cloth at 45 degrees to get my +/- 45 degee fabric. Eventually i forgot to pay attention to the actual angle of the fibers in the cloth. My cuts were closer 60 degrees when i noticed the twist. The rest is the best I could come up with in a thought experiment. The (admittedly) misaligned fabrics are getting warmed up during the cure cycle. The resin goes to "b" stage and everything gets "set" at some position, relative to each other. Everything is still pretty warm. As the skis come off the press things start to cool. And shrink. As the fibers contract they start to load the structure unevenly in torsion (in my case, at least) as the 60 degree fibers tend to pull more down the length of the ski, and the 30 degree fibers pull more across. I kind of noticed pulling the skis off the press when they didn't seem to lay flat on the bench. I really notice when they came out of the came out of the car at minus 18 celsius. Things looked like drill bits, well not that bad but, easily out 1 cm from shovel to hip.
Nothing in my process had changed, but suddenly I had warped skis. It took a while to figure out that it was the fiber angles that had changed because i was making a mistake. I did not realize that i had changed my cut angle. Dooh.
In your case, yeah, it could be heat, but if it is heat, and you are pressing 2 skis at once, and you are getting more heat in the middle than at the edges, I would expect to see bookmatched skis, ie. the tips are twisted different ways in sibling skis (by sibling i mean individual skis laid up at the same time).
If the skis are twisted/wound the same way i would be tempted to look at your fiber orientation. Are you ply flipping the glass (the glass is a 22 oz triax, right?)? That is, are you ensuring that the layers of zero axis glass are both against the core, and that the angle plies are matched in direction/position. This can induce significant twist if you get it wrong and are dealing with funny geometries (skis are definitely funny! by this i mean that ski's axial dimensions are radically different 1.8 m long, .02 m thick, .13 m wide, with curves in the length axis). I don't know. The fibers were definitely my issue, When i switched to a fabric with accurate +/- 45's everything went dead straight again. Problem gone.
In my opinion, you might as well start with heat. You want to get this working properly anyhow. Then when you make your next pair, make certain all stuff is lined up and you should be ok again.
Good luck. I sure remember the crushing heartbreak at seeing the twist, and the incredible frustration at trying to figure what the F%^&! is going on here? On the upside, you can ski on them. They just tend to want to hook up fast one way and wash the tails, and not initiate too well the other way, with killer tail grip. I sure would not sell them, but I used them. That said, they are way, way better flat/not twisted.
Later, carlito