Foam cores from Blankslateskis

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twizzstyle
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Foam cores from Blankslateskis

Post by twizzstyle »

I finally had a chance to ski on the foam core skis I made recently, so I have some data and results to share for anyone interested in trying this foam.

Disclaimer: I tried to do my best in my design and data collection to do some real engineering, however some of my measurement methods were quite crude, so take my numbers with a grain of salt - as usual, your mileage may vary!

The foam cores in question can be purchased here:

http://www.blankslateskis.com/foam-core ... -x-1854mm/

I made two pairs of skis - one pair using bamboo for the cores, the other using the foam material from Kelvin (Blankslate). One sheet of the foam per ski. I tried my best do make everything else identical between the skis for a direct comparison. The goal here was to make the foam core skis the same stiffness as the bamboo core, and see how much lighter they would be.

Ski specs (both pairs):

Length: 160cm
Widths: 155-120-140
Camber (mold): 10mm at the center
~30cm early-rise tips and tails

Materials (from top to bottom)

Sublimated nylon topsheet (from Miller Studios)
3" wide ~9oz uni carbon
20oz triax (longitudinal fibers up)
Core/UHMW sidewalls/6" tip spacers/inserts
20oz triax (longitudinal fibers down)
3" wide ~9oz uni carbon
VDS/Base/edges

On both pairs, the core blanks were first cut to the planform on my CNC router, then sidewalls were epoxied on, and then they were profiled on the CNC router.

To attempt at having the same stiffness between the skis, I had to make the foam cores thicker than the bamboo cores. To do this, I first setup a simple cantilever beam bending test with some sample pieces of bamboo and foam. I cut pieces of foam and bamboo the same size, clamped them to my work table, and then hung weights on the end and measured the deflection. Using this I was able to calculate the Young's modulus. It was crude - very crude - but I tried to be consistent so that at least relative values had some meaning. I did it multiple times and had fairly well matched data.

Using this data, and good old beam bending theory, I came up with a profile for the foam core that I thought should match the stiffness of the bamboo-core skis, all else the same. The bamboo cores are 10mm at their thickest point, while the foam core skis are 12mm at their thickest. Both of them taper to 2mm at the tip/tail to match the tip spacer.

Both pairs were pressed (one ski at a time) at 60psi, 150 deg F, for 2 hours using Entropy SuperSap CPM. (I press at a lower temperature and longer time (it's supposed to be 180 deg F for 30 minutes) to help with base flatness issues I had on previous pairs). The skis were heated from the top only.

There was one difference between the skis worth noting: The bamboo skis have a solid color base, while the foam core skis have two different colors with "STUMPYSTICKS" cut out (the name of my pretend ski company). I held all of the pieces together with quite a bit of medipore tape on top of the base material, which is now inside the skis.

The bamboo skis are the black/green robot skis, the foam core skis are the old maps.

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The results!

Weight

Bamboo skis

Left ski: 1780g
Right ski: 1808g
Average: 1794g

Foam skis

Left ski: 1593g
Right ski: 1634g
Average: 1613.5g

This is a weight savings of 180.5g per ski, or about 0.8lb for the pair - 10.06%. Is that a lot? I don't know.

Camber

Bamboo skis

Average camber: 6.47mm

Foam skis

Average camber: 10.93mm

Stiffness

To measure the stiffness of the skis I setup a very crude 3 point beam bending test. I placed each ski on my ski waxing stands, with them located at the contact points (roughly 30cm from the tip and tail). I placed a heavy object (the old motor from my ski grinder, 42.5lb or 19278g) in the middle, and measured the deflection. I then used some simple beam bending math to find the bending stiffness of the whole ski, EI, and compared the values between skis. The actual values of EI don't matter as much as the relative values between the skis, especially given the crude methods of measuring (crude, yet consistent!)

The end result is that the foam skis are 14.52% stiffer than the bamboo skis. Ok, so my core thickness design was off a little bit, oops! I would say the bamboo skis are on the soft side of medium stiffness, and the foam skis are just on the stiff side of medium stiffness. Both feel great.

Foam + heat
Something that needs to be noted with the foam is that it transfers heat faster than bamboo or wood, and it doesn't have as much "spring-back" as the bamboo. I heated only from the top, to reduce the camber in the mold. You can see with the bamboo, this worked as-planned. The foam core skis came out with the exact same shape as the mold. This needs to be considered if you are over- or under-cambering your mold to account for one-sided heating.

On the snow

Both skis ski powder great. With this much surface area, it's pretty effortless, and pretty much an obvious conclusion. It's everywhere else that is interesting (groomers, chunky crud, etc).

The short answer is that both skis are excellent. The short sidecut makes them turn hard on groomers. They aren't grabby at all, and today I had the foam skis on some very steep very nicely groomed runs, and took them up to probably 60-70mph. Not something I would have been able to do with my old powder skis that were all reverse camber. These ski like any good groomer ski, it's awesome, I love this shape!!!

Even though 10% weight savings didn't sound like much to me, it took a few runs to get used to. It's almost like there wasn't a lot of feedback? I'm not sure how to describe the feeling, but I didn't feel as "connected" to the skis. Oddly enough, the faster I went, the stiffness and camber took over, and they felt rock solid.

I think the best part about the foam skis will be getting my way through tight spots, like the woods. I was in some very thick woods with some very steep sections, finding my way through fallen logs, and big rocks. Jump turns are effortless on these skis. There is no swing weight at all. I did a few 360s off of some small jumps and small drops, and it felt like I didn't have skis on my feet at all. Almost a disconcerting feeling, until you land and realize you still have all the support of a normal ski.

So overall - they pretty much ski the same - the light skis are noticeable at low speeds, but pay for themselves in tight spots, and when we are exploring.

The big question is going to be durability. I am going to plan on skiing the foam core skis exclusively this season, at least when we're skiing powder (we ski park maybe 10% of the time), until I have an issue. I will update this as the season goes to share how they're holding up.

Worth the cost? My bamboo cores cost something like $30 for a pair, while this was nearly $80 for the pair. I can't really answer this question until I get more days on the skis and see how they hold up, but at first glance, I'd say yes.
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vinman
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Post by vinman »

nice write up twizz and great looking skis.

I see you used inserts for mounting. What do you think you'd need to do to mount the foam cores without using pre set inserts?
Fighting gravity on a daily basis
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twizzstyle
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Post by twizzstyle »

That's a great question, I'm not really sure without doing some tests.

The foam is very hard stuff, it might be sufficient to hold normal binding screws as-is - you just don't have the benefit of the wood grain distributing the load.

I have some left over scraps of the stuff, maybe I'll do some pull-out and stripping tests to see how it compares to bamboo. Or if anybody wants to do their own tests, I can send you a piece.
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Post by Guest »

Nice write up. Would love to see some shots of the skis togather to see the camber and tip rise not that it has anything to do with the experiment but nice to see. Unfortunatly this is the most scientific experiment we have seen on here in quite a while. Thanks twizz
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More
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Post by More »

Great write up Twizz, thanks. looking forward to hearing if the stiffness degrades.
Toddh77
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Post by Toddh77 »

Great info. Not to get off subject, but I really like the bases. How are you doing them?
twizzstyle
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Post by twizzstyle »

Anonymous wrote:Nice write up. Would love to see some shots of the skis togather to see the camber and tip rise not that it has anything to do with the experiment but nice to see.
It's all part of the experiment! :)

I'll try to get a side-by-side profile shot of the skis so you can see a full length comparison of the camber and tip shapes. (who are you? stupid guest posting :( I wish we didn't have that feature)

More, you've given me an idea just now about stiffness degradation. Perhaps at the end of the season (or heck every few days) I will measure the stiffness again, and plot stiffness vs time to see if there are any trends. Since I won't be skiing the bamboo skis as much, I won't have anything to baseline it against, but I'll just go on the assumption that the bamboo has no degradation.
Jekul
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Post by Jekul »

Nice detailed writeup. Great to see engineering work still being done by the true industry innovators. Now hurry up and apply for a patent before big brother steals your ideas.
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MontuckyMadman
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Post by MontuckyMadman »

Guest was me. Dont kno what happened.
sammer wrote: I'm still a tang on top guy.
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Post by Guest »

It's easy to do by mistake. It's a new feature that showed up a couple of months ago. Just invites more spammers :(

(posted by twizz)
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Post by Guest »

MontuckyMadman wrote:Guest was me. Dont kno what happened.
You don't have to sign-in to reply to a posts any longer. You show up as Guest (skidesmond)
skidesmond
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Post by skidesmond »

Nice write-up Twizz. I like it when a certain amount of scientific info can be captured. I'm surprised there is only 10.6% weight difference. Doesn't seem like that much to me.

I'll be interested in seeing how they hold up through out the season.
twizzstyle
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Post by twizzstyle »

Toddh77 wrote:Great info. Not to get off subject, but I really like the bases. How are you doing them?
Homemade drag knife on my CNC machine ($8 carbide blank from McMaster-Carr that I ground into a blade shape, and stick in the router with the router turned off).

For cutting out the actual base shape, it works amazing. This was my first time doing multiple color cutouts, and it turned out so-so. It was difficult getting everything together right, and I used a lot of medipore tape to hold it all together for the layup.

I put together a nice little video of the making of these skis, but I'm waiting to get some good footage skiing on them to finish off the video. Of course I'll be posting that on here once it's done!
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chrismp
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Post by chrismp »

have you tried putting the base pieces together using packing tape or transfer paper? i think i've seen this in some production videos...
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falls
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Post by falls »

yeah chris is right. Usually they place transfer tape on the base side outside the ski when holding the die cuts together. I used contact paper (for covering school books) to join a very simple butt joint die cut when I was using up left over base material. It peeled off after pressing leaving a very very shallow depression that ground out fine.
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