Core construction

[ruudsjoukes]

Dear forumreaders,

Located in Vancouver, BC, I'm about to build a skipress. The skiis will follow of course and I'm designing the core constructions right now.

I know a lot about the materials and the way that it should flex. Though I'd like to discuss with you guys on several types of cores. Back in the Netherlands I used to build longboards which were made up of bamboo because of it's perfect flex pattern. Now I started wondering how to build this up in a ski.

What I'd like to know and discuss are the types of wood and cores.

Basically I know these cores:
- Bamboo;
- Poplar;
- Maple.

With 2 different patterns;
- Skateboard layup;
- Laminated layup.

Along that comes the flex pattern by fibers or carbon;
- X-layup;
- Vertical layup;
- Only tip/tail flex by fibers.

Now we got some to discuss here:
Which core would be the best?
Which pattern would be the best?
Which flexpattern layup would be the best?

Thank you for your help!

[Alex13]

The most simple answer is "depends".

There's no ideal flex pattern, flex changes depending on what you want from the ski. Is it a race ski, freestyle ski, freeride ski? What length is it, and what weight is the rider?

Most people here use a vertical laminated layup for skis and boards. If you have another idea though it's always worth looking at.

There are tons of different woods you can use for the core, and again depends on what you're after.

Perhaps if you have more specific questions we can help more?

[ruudsjoukes]

Basically I don't really wanted to have a certain shape or statics but more of all create an overview of materials and options.

The things I'm gonna design are freestyle skiis and powder skiis.
Within this I'm interesting in 2 things:

- Bamboo cores
- Cross-layup (several different patterns at different areas, glued together)

So.. Let me get back to that then:
- What are the experiences with bamboo cores and why or why not should you use it?

- Why would you layup cross sections like K2 does in it's snowboards and would it be worth a shot in skiis?

And then what would you use as a thickness for bamboo in freestyle or powder skiis?

Thanks so far and I sitll hope we can build sort of a database on the materials and types etc (A)

[skidesmond]

Well, it still depends.... Lots of guys in the forum use bamboo but there's no reason not to use other woods such as maple, polar, ash, etc.

People have done vertical and horizontal layups w/ success.

When you say "cross section layup" do you mean horizontal and vertical wood laid on top of each other like plywood OR like Burton does in some of their boards where the center of the cores are typical glue up with perpendicular wood glued along side the core. If you mean the latter, IMO, I don't think it would make that much of a difference in a ski since it's much more narrow than a snowboard. And there are simpler was to achieve torsional rigidity than a complex glue up like that. Just my 2 cents.

There used to be a EXCEL spreadsheet that could predict flex but I don't know if it still exists in the forum. I can send you a copy of it. Hope this helps.

[twizzstyle]

My opinion - for your first pair, don't over-think it. There are plenty of other challenges in the ski making process that you should do the simplest and most tried and true method for the core (vertically laminated) first. Then start to experiment on subsequent pairs. You can theorize as much as you want about the different core-designs, but with the amount of variability inherent in hand-making skis, you just have to build the damn things to see the results.

[ruudsjoukes]

Have been building a lot before so I know stuff and already tried some.

What I basically just want to know is what another good method should be like bamboo.
I don't like to stick to the 'same old' and therefor will try a few different ones with bamboo, carbon etc. Thought you guys might had some ideas about it..

[skidesmond]

My opinion - for your first pair, don't over-think it. There are plenty of other challenges in the ski making process that you should do the simplest and most tried and true method for the core (vertically laminated) first. Then start to experiment on subsequent pairs. You can theorize as much as you want about the different core-designs, but with the amount of variability inherent in hand-making skis, you just have to build the damn things to see the results.

I agree. The variables and challenges seem endless. But you don't know what challenges you'll encounter until you start building. There's also different ways to achieve similar results. It can be a very subjective.

[SHIF]

- What are the experiences with bamboo cores and why or why not should you use it? ...

I only use vertically laminated bamboo flooring planks as a raw core material. I rip them to width and glue them together to make really wide cores. Using a template and router table, I first shape their side cut and then glue sidewall blocks in place. I used to use UHMW blocks but now prefer Amazon forest hardwoods, like IPE or Macaranbuba because it bonds much better and it looks cool. It’s just as tough as plastic yet it adds to the cores flexural properties, unlike plastic which is just dead weight. There is also a greatly reduced risk of sidewall failure during the core profiling process.

Profiling bamboo is more difficult than profiling wood cores. (bamboo is grass, not wood). The material saws and sands nicely but it really works my planer, and it eats planer knives quickly.

Bamboo cores exhibit natural damping properties too, which is good.

My typical ski construction has a layer of triaxial glass above and below the core. I also add a 3 inch wide strip of unidirectional carbon fiber above and below. I can vary the stiffness of the ski by two methods, core thickness and/or carbon fiber weight.

My ski designs are between 103mm and 117mm wide underfoot. My most successful ski cores are between 12.2mm and 13.4mm thick. They all taper down to 2mm at the ends to blend with the tip fillers. The length and location of the “flat” zone, where the cores are maximum thickness, influences the ski flex pattern too.

There are so many variables; Core materials, core profile, ski design width, composite fabric schedule, etc, etc. You just have to build a few pairs and measure and make notes about everything you do. Mount them up and ride ‘em. Eventually you’ll figure out what works for you.

-S

[ben_mtl]

That's interesting,

I'm making my cores out of ash and poplar + maple sidewalls.
Last pair I made last year was 98 underfoot, core thickness was 2-11.7-2mm (flat section is 400ish mm long, centered around boot center) with 1 stripe of 2" uni-carbon top and bottom + the usual 22Oz triax...
The ski is ways too stiff for me. Though, I have to admit it's nice and when charging on hardpack and "loading" the skis you're like on rails, it rocks on ice !
Anyway next pairs will only have 1" of carbon uni or I'll try different core thicknesses to have better flex, as you can see already 2 ways of tuning the flex ! I also want to play with the flat section, I suspect it's too long...

[SHIF]

My core profile “flat” section has evolved down to about 110mm long, centered underfoot. This yields nice long tapers resulting is a very rounded ski flex shape.

My triaxial glass is only 19oz fabric. I've used either 4.5oz or 9oz carbon in various skis. The carbon dramatically stiffens the ski. The 9 oz stuff is off the chart and best to use in much narrower strips than I did, or on cores that get over-worked and come out thin.

I like the snappiness that carbon fiber adds to the ski, lots of energy.

My skis are a little on the stiff side, I’d say medium-stiff, but I weigh 220 without gear and I prefer stiffer skis.

-S