Ski Builders Forum

Nice write-up!
Q#1: If the same glass was used in both skis, wouldn't the thickness of the core material determine the stiffness of the skis? IE spacing between the composite layers? I ask because reading around people talk about what wood to use and it seems the consensus is it is all personal preference. I understand they are two completely different materials. I had a pair of foam cored skis and they did not seem any thicker. But that was several years ago and I'm going from memory (getting old and the memory is too).
Q#2: Falls, what pressure are you pressing at? Just wondering because in your comment you state that the tape leaves a very shallow depression. I'm at approx 55 lbs and if I have anything in the cassette it will leave an impression, sometimes big.
Thanks for the writeup,
RA

For Q#1: Exactly. I tried to calculate what the thickness of the foam cores should be so that the skis would have the same stiffness as the bamboo skis. The bamboo alone is very stiff, while the foam alone has very little stiffness.

But, my method of measuring the relative stiffnesses of the foam and bamboo materials alone was pretty shotty, so that's why my end result wasn't very good (foam skis are 14% stiffer than the bamboo skis).

Yesterday was day 3 on the skis. No issues and I'm loving them. I had my GPS with me on day 2 to see how fast I was going - I only managed 52mph that day, but that's still faster than I was ever really comfortable on my older powder skis (the piggy sticks). That has nothing to do with the foam, just the shape/design of the skis. And I love it! Yesterday was pretty crummy with about a foot of untracked snow - that had been rained on for a solid day. It was HEAVY, and these powered right through it with ease, and then when I popped back out onto a groomer they were fast and stable.

Like rockaukum alluded to, the core material's contribution to stiffness is pretty minor...the added 20% in facing separation however is major (cubic function). If you want I can always run these numbers through a program I wrote (similar to Bambi's) for this very purpose.

I really like the topsheets, do you always use die subbed white 8210? Ever try the clear?

I'm an engineer, so I'm well-versed in the equations, and have a big spreadsheet I dump everything into that spits out my numbers

But I'll challenge the statement that core material's contribution to stiffness is minor. I agree that core thickness plays a much larger role (as you said, moment of inertia goes with the thickness cubed, bh^3/12). Especially where the core gets thin at the tips, an extra 2mm on a 2mm tip will give you 8 times the stiffness.

But the core material alone can have a lot, or very little, stiffness, and the composite is just adding to that. I don't have my spreadsheet in front of me, but I know I looked at the cores with the same thickness (and same composite of course), and the foam cores were on the order of HALF as stiff as the bamboo cores. They would have been unskiable noodles!

"and same composite of course" is exactly the part I missed when I read your first post. I was under the impression that you were saying you increased thickness to compensate for the core material's deflection WITHOUT facings. Thats why I thought you were overlooking the effects of compression and shear modulus. Didnt mean to question the size of your spreadsheet
Mines pretty big too

Ha, this foam would have to be about 6" thick to be as stiff as the 10mm of bamboo (actually if I had my spreadsheet here I could work it out - at least with my shotty youngs moduli I calculated)

There's nothing quite like a nice big meaty spreadsheet is there!

Cool thanks for the weights! I kept looking in the journal and was wonderin.

How do you think the foam would machine in the typical planer crib?

I think skis .8lbs lighter is a huge diff. They say 1lb on your feet is like 10lbs on your back.

And I only weigh 130lb on a heavy day, so 10lb off my back is a pretty big percentage

I planed the foam cores after attaching the sidewalls just to flatten one side. There were no issues at all, the stuff cuts VERY easy with normal wood-working tools, including the planer.

I would be very cautious profiling though, once it gets thin its pretty soft and if it came up even a little bit, it would be destroyed in no time. At $40 a core, that's a costly mistake.

If you have a good method to hold the core down (my method of transfer tape + spray glue worked great on the CNC), then it should be fine. One up shot is that since it cuts so easily, the planer blades won't put a lot of force on it.

mattman wrote: I really like the topsheets, do you always use die subbed white 8210? Ever try the clear?

It's subbed nylon from Miller Studios. I love this stuff.

Is it the white 6661 or have you ever tried the clear 8210?

You'd have to ask Vince (Miller Studios) what this stuff is, I have no idea.

I've never used dye-subbed clear material, just PBT and this stuff.

rockakaum:
i am using 55psi also.
the tape i used is about 80 micron thick (0.08mm) so the depression is only that deep. 0.1mm prob comes off in grinding i would say.

great job on the bases

Bamboo is quite heavy (785kg/m3) probably due to all the glue holding it together. To have only achieved a saving of 180grams per ski makes me think that the foam cores absorbed a lot of epoxy in the lay up.
We have changed from bamboo to a Pacific Kauri (540kg/m3) , achieved the same deflection properties, and reduced the weight by 200grams per ski.
Paulownia (300kg/m3) would be even lighter again.