Joined: Tue Dec 23, 2003 6:30 pm Posts: 853 Location: Traverse City, MI usa
No, 2 layers of 1/8 in birch ply are not strong enough, ya have to glass the top and bottom and middle....you lose any wt advantage. 3 sandwiched 1/8ply layers are just strong enough with poly on the outside, perfect for mowing the lawn riding, but high jumppin flying, THEY WILL TEND TO BREAK. BUT, I do find if you use good 1 1/4in AC 3 ply sheet and 1 1/8 birch 3 ply sheet. Makes for a relative Boom proof LIGHT flexible board, go figure.
Joined: Wed Nov 20, 2002 1:00 am Posts: 963 Location: rhode island
lots of useful observations to take advantage of.
if still not sure, a bit of experimenting during your next visit to HD will add some hard numbers to all of the speculation. (or you could look them up from someone like 'plywood association' of wherever) compare all your potential candidate materials.
one by one: prop up any of your favorite choices between two carts. toss a 50lb bag of cement in the middle measure deflection, length width and thickness.
Deflection = wL^3/(48 E*I) google 'beam deflection formula' for 3-point bending
the resulting numbers will be a good way to compare and give you enough to predict deflection for any of your design iterations.
It may also convince you that the cheepest and lightest crap you can find will be tough to beat with 1 or 2 layes of 4oz glass. imo, -bill
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