Best wide ski core thickness profile

[El Jimador]

Hi there. I just joined this fabulous site. I am working with John Hadley and his after school kids shop club helping them make skies and snowboards.

I’m learning as I go on my first pair of skies. They are to be somewhat likened to the BD Megawatt, that is 188 overall, 153-125-130, a consistent tip rocker arc starting back about 16 inches from the tip. Minor tail twin-tippage. Hopefully fairly stiff in the body and tail with soft rocker tip. I weigh in at ~200. I intend to mount alpine binders (some rental tyrolias with lots of fore and aft adjustment) and only use them for in-bounds powder.

As John has done previously done, the layup will consist of:

Base and edges
Rubber tape dampeners
22 oz. triax
Core (poplar and ash) w/ small 3mm tip and tail spacers
22 oz. triax
Some laser printed graphics on rice paper
No top sheet

Pressed in a heated vacuum bag.
Coat edges with epoxy.

My first questions is: what should the core profile look like? As this ski is so wide, I assume the thickness must be considerably less than that of a ski 88mm under the foot.

From reading on the site it seems like 2-2.5 mm in the tip and tail would be good. My tip will start about 16 inches back and slowly curve up to about 7-10 cm. Maybe a tiny kicker at the tip.

For the body (under boot flat area), I was previously thinking 10mm, but I saw teleman36 suggest to telexis that 6.5-7mm would be better for a ski that wide. That seems really thin, but then hey, I’ve never done this before.

So, given my preferences and beer gut, what profile would you folks recommend?

Thanks in advance,
Jim

[Grizzly Adams]

I built some real wide ones a short while ago and, thinking that the width would add a fair amount of stiffness, profiled the cores fairly thin. I made the cores out of poplar and pine and profiled them to 10mm underfoot and the standard 2mm tip and tail. They came out realllllly soft. I think the profile depends a lot on the wood you are using (in retrospect, DUH) Luckily they are only gonna be used in the softest of wasatch pow, so it shouldnt be a problem, but dont get tricked into profiling your cores down to nothingness. I think 10 is the thinnest I'd ever venture. That said, I'm kind of a large dude (210 during ski season). If you like a "fairly stiff body and tail" with the poplar and ash, I'd go 11-12mm underfoot.

Hope that helps, feel free to post differing opinions.

[MontuckyMadman]

I have made only one ski so far. 138mm at the waist and 2.5 mm tip continual grade to about 10.5 and the 3mm in the tail. I put in a long thing diamond shape of extra glass running from about 60cm from the tip to the tail I am really impressed with the burliness/stiffness of the ski. Solid poplar core. West system epoxy and pneumatic press with a cheap heat blanket at 50 psi. I also used some aluminum door screen as a binding retention device lightly stapled to the core and I think it has made a noticeable different in the stiffness of the ski in that area but still has a nice even flex.
I hope I can keep the next one similar to what I have.;

[trod]

I haven`t built any skis yet but have something to say about stiffnes that I have learnd in school. Stiffnes comes down to E*I where E modul (Youngs modul) depends on what material you use. so the E will be the same if you use the same material but change the width ang hight. So what changes the stiffnes is the I and I is given with : (width*hight^3)/12
So the hight has much more to say than the width of the ski. as an example a ski 120mm*2mm is almost twice as stiff as a ski that is 150*1,5mm.
Hope that could help you to get the right dimensions or your skis.

[camhard]

^ this is exactly what I was going to say. the a difference in height changes the stiffness (and strength) significantly more than a change in width.

[jono]

If you end up with a ski that is too soft you can rough up your no top sheet ski and run your ski through the vacuum bag process again and do the diamond of fiberglass montucky madman is talking about. The thick part of the diamond goes under foot and the points end where you want the ski to resume its previous stiffness profile. I've used the diamond method and have ended up with a nice progressive flex. Unidirectional glass works well for the diamond as does triaxial fiberglass.
If you end up with too stiff a ski you can just pull out the belt sander or planer and take off the glass and part of the core and then try again.
I usually err towards too stiff on my skis but I would say 10-11mm should be pretty good and give you a softer flex.
If you go too thick on the tips and tails you'll end up with a ski that is soft in the middle and stiff on the ends. This would make for a less than ideal ski. I would not go much over 2mm on the ends especially the tip.
good luck on your first pair!!!

[mattman]

simple solution to problems like these...stiffness is a factor of this relationship: width * thickness^3...so obviously the cubic part of the function makes a huge difference. so a good place to start is with a ski you know you like...say you know you like 80mm wide with a 12mm thick core and you want to know how thick to make a 120mm wide ski. solve this:

(80)*(12)^3 = (120)*(new height)^3

your new thickness is 10.5mm. i have always had great luck with this method!