Need help with kid snowboard

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SleepingAwake
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Need help with kid snowboard

Post by SleepingAwake » Thu Sep 05, 2019 5:20 am

Hey Guys

I'm a skier myself and built quite a few skis and know what i do there. But i would love to make a snowboard for my niece as a christmas gift and need some pointers as i have no clue how stiff a snowboard for a 8 year old girl should be. Anyone who can give me a starting point with a core thickness and and a basic layup so I have a base to work from for my calculations?

Thanks in advance

leahicmoradnuf
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Location: Connecticut

Re: Need help with kid snowboard

Post by leahicmoradnuf » Thu Sep 05, 2019 8:25 am

I would guess 4-5mm for core thickness. For reference I am 215lb and ride a board with a 7mm poplar core. I would also explore using a softer wood, too. I think some kids boards may even have partial foam cores.

SleepingAwake
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Re: Need help with kid snowboard

Post by SleepingAwake » Fri Sep 06, 2019 12:03 am

Yeah that was my gut feeling too. What do you think about the layup?

I might just end up making a board 1/3 of the stiffness of an adult board for 1/3 of the rider weight. Kind of makes sense i guess?
Even tho the surface area of a kids board is more than 1/3 of an adult one so not quite convinced it scales this easily...

gav wa
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Re: Need help with kid snowboard

Post by gav wa » Tue Sep 10, 2019 2:29 am

The only problem I see with really thin cores is the very small inserts you will need and they will only have three turns on each screw.
I would think about 450gsm glass with only 45/45 degree tows.
Then a 6mm core will still be plenty flexy.

leahicmoradnuf
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Location: Connecticut

Re: Need help with kid snowboard

Post by leahicmoradnuf » Tue Sep 10, 2019 10:37 am

1/3 the stiffness for 1/3 the rider makes sense to me. But scaling dimensions is not the only means to get there, plus it runs you into insert problems like mentioned.

If I may nerd out a little bit, allow me to reference beam theory. https://en.wikipedia.org/wiki/Bending_stiffness

Let's assume your niece is popping an ollie (rad), and applying a moment to the board. We're guessing that that is 1/3 the moment an adult rider would generate. In order to get a proportional amount of pop, you'd want to adjust stiffness such that you get the same amount of beam curvature (k).

M = EIk

The term "EI" is your board's bending stiffness. E is the modulus of elasticity, tricky to determine for composite materials (wood+fiberglass), but you can use it as a rough guide for selecting a softer wood or even a foam. I is bending moment of inertia, you can tweak this by changing up the board's cross-sectional area. https://www.engineeringtoolbox.com/area ... _1328.html. You're kinda limited here though by things like insert thickness and the size of your niece's feet.

So the goal would be to modify E and I such that the product of the two is 1/3 that of your adult board of reference. It can be achieved by a smaller cross section, softer wood, and a lighter weight, biaxial glass.

gav wa
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Location: Perth

Re: Need help with kid snowboard

Post by gav wa » Tue Sep 10, 2019 4:00 pm

Check out Divide Rides journal, in the last few pages he goes through the build of his kids board, has the whole layup in there I think.

SleepingAwake
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Re: Need help with kid snowboard

Post by SleepingAwake » Wed Sep 11, 2019 4:35 am

thank you guys for the inputs. I'm rather fit with the nerdy stuff and i ran some numbers and with the 1/3 stiffness approach and a light-ish glass layup and a Bcore balsa core that i use in my skis i still get a reasonable core thickness. otherwise i would increase the core thickness locally in the binding area. I used to do this on my super flexy carbon kiteboards.

My fear by using a pure biax layup is that it will be too stiff torsionally. i would like to keep it rather soft in torsion to get easier rider inputs.

Divide has the layup specified but not the core thickness.

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chrismp
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Re: Need help with kid snowboard

Post by chrismp » Thu Sep 12, 2019 12:57 am

In snowboards a biax layup usually refers to 0/90° fibers and not +45/-45°. That way you get a board that has some stiffness mainly along its length but is torsonally soft.

SleepingAwake
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Re: Need help with kid snowboard

Post by SleepingAwake » Thu Sep 12, 2019 1:08 am

Aha thanks chrismp for the clarification. this is what i had in mind actually using a unbalanced fabric with more fibers running in 0°
flex.PNG
flex.PNG (27.9 KiB) Viewed 933 times
this where i'm at right now. I'll keep you guys posted on the progress.

gav wa
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Re: Need help with kid snowboard

Post by gav wa » Thu Sep 12, 2019 6:46 am

I've done a couple of boards with 450gsm fibre top and bottom, 0' / 90'. 6.7mm core and they were really quite soft, even with some carbon fibre added.
You could grab a bit of 5mm wood that is about 230mm wide and press it with glass and Ptex and see how flexy it is. Even a 900mm length will give you the info you need.

Dtrain
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Re: Need help with kid snowboard

Post by Dtrain » Tue Sep 17, 2019 8:59 pm

I’ve built kids boards. Get some lightweight 0/90 glass and some light wood with some flex. You need to keep the core thick enough to hold the inserts. 5mm is the smallest insert I had. Put a decent sidecut in it. 6 meter

Dtrain
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Location: Prince Rupert/Terrace B.C.

Re: Need help with kid snowboard

Post by Dtrain » Tue Sep 17, 2019 9:03 pm

I used 18oz. Sound heavy but it worked

leahicmoradnuf
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Location: Connecticut

Re: Need help with kid snowboard

Post by leahicmoradnuf » Wed Sep 18, 2019 6:10 am

SleepingAwake, I'd be interested to poke around in that spreadsheet of yours if you wouldn't mind sharing.

SleepingAwake
Posts: 138
Joined: Mon Oct 11, 2010 11:06 pm

Re: Need help with kid snowboard

Post by SleepingAwake » Thu Sep 19, 2019 3:56 am

I would love to share the spreadsheet, but cannot because it is an adapted work thing. But it is just based on sandwich theory and basic mechanics and for sandwich panels we get within 1 to 2 percent of the calculated stiffness.

SleepingAwake
Posts: 138
Joined: Mon Oct 11, 2010 11:06 pm

Re: Need help with kid snowboard

Post by SleepingAwake » Tue Oct 15, 2019 11:52 am

Alright making some progress here. Balsa core with some weird wood at the inserts, bamboo sidewalls with cast tip and tail spacers. core thickness would be 5.6mm, so i'm going to locally increase the core thickness in the insert area to accommodate the 6mm inserts.
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