Need help with kid snowboard

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

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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Joined: Mon Oct 11, 2010 11:06 pm

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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Joined: Mon Oct 11, 2010 11:06 pm

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 35 times
this where i'm at right now. I'll keep you guys posted on the progress.

gav wa
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Location: Perth

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.

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