Timmm wrote: ↑
Tue Apr 24, 2018 9:12 pm
I want to end up with a medium flex and be able to ride the board with boots.
What layup would i use???
I haven't got much experience in controlling flex but I think I can still help.
The big shock loadings are going to be between your feet and extending forward and back from them.
When you land (the biggest shock to the board) the tips lift and the centre lifts. As they lift, the bow shape resists bending on toeside and heel side edges.
Just under your heels but on top of the board the forces are compression. These forces spread sideways, becoming weaker towards the ends.
Between your heels and on top of the board are tension forces. Under the board everything is reversed.
If you place strengthening with clear boundaries then the boundaries also become highly stressed regions.
Composites gain strength from the skin surfaces. Stiff fabrics sandwiching soft fabrics resist both puncture and deformation. Sandwiching a stiff fabric between soft fabrics is a waste unless you are trying to get flexibility.
Don't skin with an outer surface like kevlar, polyester or innegra, it will fray if damaged when sanded.
Zigzag your fabric boundaries if you are zoning the composites, this will merge an easy bending zone into a stiff area more softly and soften stresses.
Think about where you want flex, DON'T use carbon where you want flex except maybe on the inside of your intended curve.
Glass and s-glass, basalt are all more stretchy than carbon. Kevlar and Innegra too though they have very little compression strength.
kevlar/carbon and innegra/carbon weaves look pretty but aren't as effective as sandwiching innegra or kevlar between carbon.
For a 10mm thick board my gut feel is that a couple of layers of 200 or 300 oz glass top and bottom would be close to strong enough, but I'd probably run a strip of unidirectional 150mm wide carbon on top between the heels and extending another 150mm past each heel. To reduce the risk of cracking the underside i'd look at either a bit more glass or sandwiching some innegra cloth between underside layers of glass. And I'd zigzag the ends of the carbon on top.
Keep in mind for your flex response that further out to the tips you hardly need any strength as there is no wood past the ends to apply force near the edges, you only need material there for abrasion resistance.
A good reinforcement in line with your heels is most important, then of lesser importance is a skin to keep the rest of your paulonia aligned and free from buckling.