Torturing Plywood

[alexlebrit]

First off I should apologise, this isn't really about boats at all. But one of my other hobbies is recumbent riding, and I'm thinking of making a ply seat, and where better to ask than in a place full of ply torturers.

Now I know usually you can't get a compound curve on plywood, and that to make a decent seat you dont slide out of it needs a bit of a bucket shape. So the two shouldn't be compatible. But I was thinking.

If I made cutouts like on this fibreglass seat.



Then would I by splaying the two sides out, be avle to get a compound curve? Or would the ply just ping?

[CDBarry]

The limiting issue is rolling sheer in the material. Inducing a compound curve (or warp, also) produces rolling shear. If this exceeds the allowable (see plywood construction codes) the material will fail internally. Compute the shear for the subject curve and check it.

[alexlebrit]

Not knowing much about ply, I wouldn't know how to computer the shear. In fact I'm only just keeping up with the word "shear" to be honest.

But being a practical sort of a person, perhaps trial and error is better for me. After all the worst that can happen is splinters in my backside.

[PAR]

That shape looks sufficiently complex, that it may be best to mold it from thin veneers, rather then trying to twist plywood into shapes it doesn't like.

[mobjack68]

Why plywood???? are you after the look of wood or will it be structural??
mobjack68

[PAR]

I wouldn't want to place my butt in a seat that wasn't structural, would you Mobjack68?

[mobjack68]

Hey Par, not a tongue-in-cheek question....what I was thinking was to create a fglass mold, if the LOOK was to be wood, they make a "woodgrain paper" that is used in conjuction with clear gel coat. Final product looks (kinda) like wood with the strength versatility of an fglass product. If a wood look is not required, fglass part covered with foam, and some sort of vinyl, etc...like a motorcycle seat...Structural??? it is just a bicycle....not like he's gonna be flying a hull on a C class cat...

mobjack68

[PAR]

Agreed, the bike will stay together without the seat (it would seem). I think the original poster was interested in an easier, possibly cheaper way to make his seat. I think veneers would give him both, though a foam core (or other low density product) in sandwich construction, maybe with a very thin veneer (for the wood look if desired) could get him lighter, then both the tortured ply or cold molded would.