I got a stiffy!!

[telehead]

Well it's that time of year. I have a core question for you all. All my past skiis have been a little on the stiff side, ( some real stiff). I am using bamboo for the core, with bamboo sidewalls. all vertical laminate. My cores are from 10 mm to 12mm center (20cm flat underfoot) taper to 2mm to 2.5mm at tip and tail. My question is that I bought some 17 oz. triaxial fiberglass that I want to try on my next pair, (instead of the 22 oz that i have been using) should I use it on top, and bottom, or top only, or bottom only or maybe no at all? I am a little concerned about going any thinner on the cores for fear i will compromise screw retention. Layup as follows: base, vds, fiberglass, core, fiberglass, Wood veneer (approx. .050) resin topsheet. Thanks alot....KT

[KevyWevy]

I don't know about the 17oz fiberglass, but why don't you try some non-linear taper on the core. keep the center ~12mm and the tips ~2mm.

my first pair of powder skis aren't linear taper and aren't stiff at all with 12mm center and 2mm tips. granted my cores are made of white wood with poplar stringers. my next pair will likely be bamboo.

[jono]

I too tend to make my skis too stiff.
On my next pair I'm using 20oz on the bottom and then 11 oz biax (45/45)on top. I'm trying to use the same core profile as I dial in what happens with different glass weights.
My last pair used 39oz on the bottom with 20 oz on top and they ended up being as stiff as my super G skis. I tried to use a thinner core profile but the glass over rode that. This is why I'm now using a standard core and only treating the glass as a variable.
Another thing I've done is to use an additional layer of glass, uni works well for this, under the binding area that tapers, like a long triangle, towards the tips/tails. This way you can stiffen/soften the ski in a controlled way.
I would make sure to use glass on both sides of the core.

[RoboGeek]

There is no rule that says the core has to be perfectly level all the way across - experiment with some grooves cut lengthwise with a router, fore and aft of the mounting area. You can do a lot of tweaking that way without losing too much strength. I've been playing with diagonal cuts - right ski cut one direction, left the other. Even the angle has a lot of effect

Layup is harder and it seems better suited for vacuum bags than presses (especially with cat tracks) but you have a bit more control over all kinds of parameters.

You can also layer other materials into the grooves if you need to do a flat top ski - little foam for filler, etc

[twizzstyle]

Are you guys doing heated cures? If you are, you want to keep the layup symmetric about the core top and bottom. If you don't (different weight fiberglass top and bottom, or biax only on top, etc) when the ski cools you will get camber changes due to different thermal retraction on top and bottom.

When you say your cores are "10mm to 12mm center) do you mean you taper it from 12mm down to 10mm? Or is that your range of accuracy? If your accuracy is only within 2mm you need to try to perfect your profiling method to get that better. Since bending stiffness goes with the thickness cubed, a 2mm increase at a 10mm thick core will make it about 75% stiffer... that is huge.

I personally think core thickness is a more controlled way to vary stiffness than fiberglass weight, but you need to have good tight tolerances on your profiling.

[telehead]

Thanks for the comments, and info. When I say the cores are 10mm to 12mm that is the thickness from one core to another, depending on length of ski. I just pulled out my most recent pair 143/106/130 @ 156cm. Short and fat (just like my wife) JK. The skis are still very stiff even with the 17oz. fiberglass both on top and at bottom. It's Hard to tell any diffrence from the 22 oz. My core was 20mm flat underfoor with 9.65 mm to 2mm. (.380 to .080) I am thinking that I will try getting rid of the 20cm flat under foot and just taper to the ends???? My tolerences are very close on the profile, within .005 for sure. Maybe put some Poplar in to soften the flex??? Or maybe just go buy some skis. Thanks to all...KT

[twizzstyle]

Ah ok, .005" is certainly good enough

Have you only used bamboo for your cores? Try an identical core (exact same profile) to a pair you've already made with a soft wood like poplar or spruce, and see the difference. Then with that you can gauge how much of each wood to use to get the perfect stiffness you're after.

[twizzstyle]

Or maybe just go buy some skis.

Psh, don't give in!

[telehead]

Thanks for the thoughts Twizz. I have done poplar cores, and have found a good flex for some of my skis. Most of the poplar cores are from .400 to .480 center to .080 to .100 at tip and tail with the same 20 cm flat underfoot. With that said, that is why I am concerned about going thinner on the bamboo cores for fear of srew retention. After thinking alot about this, I think I will try 30% of the core (center part) with Poplar, and the other 60% bamboo outer part of core, and sidewalls. Any thoughts???

[doughboyshredder]

go thinner and add a layer of reinforcement where your screws mount.