Last year, my wooden seatback, which is about a foot wide by seven inches from top to bottom, started hurting my back after two and a half hours of pedalling my homebuilt LWB. At least, it became quite uncomfortable.
I was thinking about using a plastic-metal-foam seatback that came off one of those black made-in-China swiveling cushioned office chairs made of plastic and metal. It's a lot more comfortable. After carefully measuring, though, I discovered it would be about six inches thicker than my wooden seatback. I cannot afford to put my hips six inches closer to the pedals, without rebuilding the whole frame. The fit is just right as is.
Somewhere I read that a wooden seatback can be curved in one direction, but not in two orthogonal directions. And I prefer using wooden plywood to CF or fibreglass because I can get scrap pieces of it free from a local furniture maker. And my experience is with wood, but not with synthetic composites. So I thought about making a seatback that is curved so that the middle is farther aft of its left and right edges.
So, since Christmas I've been thinking about making a larger seatback out of a couple layers eighth-inch (4mm) plywood epoxied together and laminated with half-inch closed cell foam. This is the technique I used for the old seatback, which has held up well.
Last night, I thought maybe the most comfortable seatback might be one shaped like the back of an adirondack chair.
Like this: http://www.woodworkersworkshop.com/r...unge&andor=and
I don't have an adirondack chair, so I don't know if this idea would indeed work, seeing as my frame geometry doesn't allow for seatback force to be transmitted to the base of the chair. So, obviously it's not going to support my back from tailbone to shoulders the way a real adirondack chair would. (This area of my back measures 15 inches wide at torso by 28.5 inches from chairbottom to top of shoulders.)
So I'm considering making a real adirondack chair back and then using it as a mold against which to bend, laminate, and epoxy a couple of plies of thin plywood, which I would then trim to final size and cover with half inch foam. In other words, the final seatback would be a double-ply thickness cushioned hardshell in the shape of an adirondack chair seatback, but without the actual five slats of the adirondack chair back. And unlike a real adirondack chair, the back would not extend to the seat bottom. I could, however, add a lumbar cushion (like this: http://k2healthproducts.com/Products...hion-3-S_1.jpg)
if I felt like it.
Regarding the frame, the seatback attachment point is 11.5 inches above the top of the seatbottom. The top edge of the seatback can be much higher than this. (This is simply the point to which pressure on the seatback will be transmitted to the rest of the frame.) The side-to-side distance is also unlimited by the frame. And the frame geometry allows a seatback angle of 69 degrees between the seatback and the frame extension.
I haven't yet measured the seatback angle of an adirondack chair.
Good idea or bad idea? Your thoughts?