Concave Bases on early rise

[skidesmond]

Sammer, yes I get some of that, but it continues back through the early rise (20cm or 40cm). Did you have more concavity further back on the ski and ground it off or was it pretty flat after the tip? I actually made some skis with a extra patch of glass cloth in the early rise portion to take up volume and keep the skis from being railed. it worked, but made the tips stiffer than I wanted and it is not really solving the problem.

Twizz and others who have been pre-bending your tips, what prompted you to start doing this? Were you having any concavity problem like mine? Did pre-bending solve them?

My problem was edge fall. Pre-bending help solve that problem along with increasing pressure to 45-50psi. I'm doing 3/4 wrap. I haven't had concave problems, one of the few I haven't run into, yet.

[skidesmond]

I am getting concave pockets in my bases in the early rise portion of the ski. I have a 20cm early rise tip block and a 40cm one and it happens on both. Bases are flat through the middle of the ski. I have top and bottom heat and top cat track. I used to let skis cool in the press but following Black Sheep's advice stopped doing that. No improvement. I cut a cross section of the ski at the concave portion and found that the glass is far more compressed in the concave area than elsewhere. I have tried pressure every where between 30 and 90 psi with the same result. Many skis are too concave to grind flat. Very Frustrating. Has anyone had this problem and solved it?

Thanks,
Morgan

Have you checked your molds for any slight bump in that area? Is your top mold adjustable, does it slide front to back for different size skis? Maybe it needs to be adjusted for a better fit into the bottom mold.

[monkeyshowpictures]

I think I solved this. Pre-bent tip of base/edge and reduced the bottom heat in relation to the top heat. Pressed skis at 55 psi, top heat heat at 150F and bottom at 115F. Removed after 30 minutes of cook time. No cooling in press. Flat bases throughout. I had been pressing at between 175f and 200F, I think the high heat could have contributed to the warping. I think cooling in press is also a contributor to the problem as the top, with the cattrack, cools much faster than the bottom that is insulated by mdf. This in effect cooks the bottom hotter and longer than the top. Not sure how much the pre-bending in the slip roll helped, did not seem to much different from when I do it by hand. I will probably build a prebender anyhow just to make sure.

Thanks for the Help everyone.

[chrismp]

nice one on solving this! but doesn't that big of a difference in top and bottom heat cause a lot of change in camber/rocker?

[iggyskier]

Iggy - feel free to jump in at this point

Sounds like the problem got resolved for the OP but just my brief experience....

I hate prebending. Absolutely hate it. I did it all through my home building in Tacoma, and the first hundred or so pairs of ON3Ps back in our first season of production.

Changing over to a flat layup was a hallelujah moment for me. Way better base flattness, way better fit in the mold, way easier during layup. No time wasted bending edged-bases. Just better all around. You couldn't pay me to go back.

Obviously YMMV.

When we changed our core shape so that it extended into the tipspacer, the hose itself wasn't strong enough to get the core/ski into shape correctly, which is why we added the pneumatic rams to force the cassette into place. It has taken a bit of time to get dialed, but I am very happy with the results we are now getting. And we are still developing and improving the system, so I expect things to get even more and more dialed.

Good pressure is obviously a huge deal, so getting a really good fit of all the parts of the mold + ski/cassette helps so much with base flatness.

We press at a higher pressure than I am sure most people do here, and have spent a lot of time and money making sure our molds, top and bottom mold aluminum, pneumatic rams, cassettes, etc all fit correctly and get the right pressure for the ski.

Anyway, lots of ways you can tweak want you have to help your base flatness, so you can go at it a number of different ways. Or as MM said, you can just grind the sucker down.

And I would never rabbet a tipsacer. That would be a HUGE bitch and isn't needed.

[monkeyshowpictures]

nice one on solving this! but doesn't that big of a difference in top and bottom heat cause a lot of change in camber/rocker?

It might, the skis I just made were flat under foot, rocker tip and tail. They still came out flat. I'll see when I make a camber skis if they end up with too little camber

[chrismp]

well, with your heat set-up for this ski you should get more rocker than your mold has.
i have a flat mold as well and if i put more heat on the bottom i get camber, so with more heat on top your skis should have more rocker than intended.

[monkeyshowpictures]

Another problem I was having that could have contributed to this was the Tuning of of my PID controllers. I had both my blankets controlled by one controller and my second controller as only a monitor (Happy monkey style). Because of this I tended to get high temps (200F+) on my bottom blanket. So in order to control the two separately I added a second SSR so my monitor PID could control my top heat. I also retuned both PIDs and shortened the cycle times so that I would no longer over shoot my temps. I changed a lot of things at once to solve my problems so I can't really be sure if it was one thing or all of them that are the solution.

I am going to slowly start using more heat as I do not like pressing at under 175 as the resin just never gets to full strength.

As for pre bending tips I have always done it, just a quick tweek with my hands to make the base sit in my cassette. My cassettes are too rigid to unbend between every pressing. I think using a roller of some kind to pre-bend will relieve more tension in the base/edge assembly and prevent non-compliance.

[monkeyshowpictures]

Here is my pre-bender. I am not sure it quite does what I want. It just raps the edges and base around the wheel. It does bend them but not in the same manner as a slip roll. Hard to explain but it just seems different.

[MontuckyMadman]

yeah my roller prebends the tips but with a 3/4 wrap it just ignores the very end of the edge and that's the most important part to avoid edge fall. Slip roller is still king. If I pressed at 90 psi wit rams it wouldn't matter but I cant.