Concave, railed, edge high, whatever you want to call it

[Dr. Delam]

My last pair that I built came out with concave bases bad enough that they won't grind flat without losing most of the material. I put them on the stone grinder today and sparks flew and emotions ran high.

I did the usual prep work with rabbeting the core for the edge relief. This was my first pair with a cat track. However, my building buddy just built a pair with the cat track and they came out flat. My last pair without the cat track came out nice and flat.

I am pressing at 50 psi, 180 degrees, bottom heat, aluminum over the heat blanket and mold, one hour press time and pulling from the press 10 minutes after shutting off the heat.

A friend(three31) suggested that maybe epoxy got under the base during layup and this can cause it to push the base up. There was some that snuck under there but not so much that I think it would do this. Also, the base is very consistently concave throughout both skis tip to tail.

I don't want this to happen again as I hate railed skis. I am at a loss as to why this happened. Any words of wisdom?

[doughboyshredder]

maybe your core had the same convexity in it?

Do you know if it was convex immediately out of the press? Or did it cure in?

[bigKam]

how wide are the skis and what was the outside air temp? pulling them out too early may be a problem if there's a large temp gradient through the thickness of the ski. i'd suggest cooling them under pressure -- longer than 10 minutes.

i doubt the epoxy caused the problem.

[Dr. Delam]

Come to think of it the cores may have had some cupping going on. I didn't vertically laminate them, I just used some single poplar boards bought from Home Depot. I remember when I was profiling that I had the grain such that the apex of the growth rings were on top to help keep the edges from curling up. I used a vacuum table and when I had the core flipped over the other way the edges didn't want to stay nice and tight down to the table.

I would think that the pressing would make them flat but either way I plan on vertically laminating cores for the next pair.

The skis aren't super wide measuring 114-88-104.

The bases were concave right out of the press. The garage temp was about 50 degrees F.

I am also thinking that using bottom heat only might have something to do with it. Since the base expands tip to tail and camber is added, I assume that the base expands edge to edge as well and then contracts relative to the upper layers of the ski adding to the concavity.

Are most people letting the skis totally cool before removing them from the press? The press I use isn't at my house and I am usually in a rush to get them done. Otherwise I would just let them sit under pressure until total cool down. Plus I am impatient!

I would really like to get another heat blanket to do top and bottom heat but it's not in the budget right now. If anyone in North Tahoe/Reno would like to lend a heat blanket for an experimental pressing let me know!

[bigKam]

Come to think of it the cores may have had some cupping going on. I didn't vertically laminate them, I just used some single poplar boards bought from Home Depot. I remember when I was profiling that I had the grain such that the apex of the growth rings were on top to help keep the edges from curling up. I used a vacuum table and when I had the core flipped over the other way the edges didn't want to stay nice and tight down to the table.

I doubt this is a problem because the restoring force of a thin piece of wood that is bowed is probably not enough to bend/flex semi to fully cured fiberglass and epoxy. I've made a couple pair of skis using a single piece of wood and the bases were fairly flat.

I think the temp gradient through the thickness during cure is very very important and it can contribute to the concavity. I sometimes use a zero-camber mold, with bottom heater and then play with the temp gradient to get camber in my skis.

Have you also checked your mold?

PM me if you want to discuss over the phone about other possible causes...

[jdh]

Hi,

I recently finished my first pare of skies. Unfortunately I ended up with a concave base as well. I used the vacuum bag method. In retro I think this was the resean for ending up with a concave base.

I am curious to the results of others using the vacuum bag method. Are there people using fillings to prevent the base to became concave? I mean on to of the base between the "fingers" of the edges. I think it should have the same thickness as the fingers of the edges.

What kind of filling do they use? I think it should have a thikness of about

I'm looking forward to the responses.

By the way, the skies performed well in the deep snow. The behavior during sliped turns on the piste were terrible. I think this was caused by the concave base.

Regards,
Jack

[hugocacola]

hELLO
Are you using the same fibers on top and base??? because i had made shit on 2 boards because of using diferent fibers...

[thetradwoodboat]

jdh,
i think you have the most probable explanation. i've built ten pair in a vacuum bag and they have all come out with slightly concave bases and tops. although i haven't done it, it can be relieved by routing the core so that it steps down just inside the edge tangs taking up the space left between the edge tangs just above the base. you could also fill it with a separate thin piece of wood or another layer of glass or two. putting a flat core over the edge/base/glass layers will leave you with an hourglass shape in cross section after pressing, especially at tip and tail where the core is thin and easily bowed across the ski. I have had good luck just having my local ski tuner take most of it out on his grinder, a pretty minimal loss of material in my mind.
i know this has been addressed in other threads on this site.
I'd love to know how others have dealt with the issue.

nateski

[knightsofnii]

its the heat from underneath, for the fellow who's using the heated press.

Leave the board in the press under pressure until it gets cold.

This didn't start happening to me until i started heating from the bottom.
On a colder day, the gradient can be huge (difference in temp between bottom and top)

With our blanket at over 200 degrees on the bottom, the bottom portion of the cattrack bars are getting up to 145 degrees. So I'd have to guess and say the top layer is hitting 165. Anyway some came out with huge camber, and concavity in the base. Too much to grind away. To make the board enjoyable on snow we had to pretty much round off the edges.

Concave bases suck, means you're always on an edge, grabs way too hard when you tilt it, 10x harder than magnetraction. I'm not talking from a park rat perspective either, i did mostly freeriding on my boards, any time i hit a patch of hardpack/ice it would grab so hard the torque would force the board back down, so it was like touch and go over ice.

Leaving the board/ski in the press until cool will definitely help. Sucks to have to use that extra time, but its worth it. When pressed right, only thing recessing the core will help with is getting rid of edge teeth marks. I honestly feel that the teeth marks are no big deal, and that if the edge teeth are squeezed into the other layers more it can only help keep them where they belong. But its a cosmetic thing and I understand most people dont like it.

[knightsofnii]

another thing about edge teeth is that if the teeth marks are too pronounced and are actual lumps, when you base grind it you can actually grind the lumps too far and grind thru the base. So if you're getting teeth "lumps" then definitely address that with a recessed core.