I decided to build the seat from fiberglass. I carefully weighed the options between carbon fiber, glass, and wood, and decided on glass for the simple reason that I already have about 3 yards of the stuff in the basement and thus won't need to order anything.
My general approach is to make a simple wooden frame and stretch fabric over it, then make a plaster mold of me sitting in the fabric seat. This should recreate a comfortable shape for me while allowing a lot of control over the position.
I started by using an adjustable seat mock up I made some time ago to decide on an approximate shape. I then pulled this shape off it by scribing a piece of plywood, and made a simple form from plywood to hold tensioned fabric. I added more lumbar curve to the form from what I had on the mock up because I expected that to collapse under weight more than other places on the seat. I also scribed a line about an inch smaller than the line I took from the hard seat to account for fabric stretch.
I covered the seat with several layers of scrap fabric until I got the right amount of strength and stiffness. Most of the fabric is either muslin or some other lightweight non-stretchy cotton fabric. A heavier fabric like canvas would be preferable and would require fewer layers, but this is all stuff I had around the house so it didn't cost me anything.
I had originally intended to saturate this fabric directly with plaster, but a test showed this wouldn't work well. So instead I covered it with plastic and saturated a few layers of burlap. Much furious mixing and spreading ensued, but with the help of my lovely wife, I was able to get everything in place, another layer of plastic over the plaster, and sit in it before the plaster set up too much. I hung out and watched some TV for an hour while sitting in my seat. And then carefully pulled myself up by using a sling I had pre-rigged from the ceiling (to avoid cracking the plaster) .
Tonight I'll add some stiffening ribs to the plaster and then tomorrow pull it from the "seat". After that I'll add some more layers of burlap to build up the thickness to account for a seat pad, and also give it more stability. Then pull the glass mold from it.
2 comments:
I'm looking forward to seeing the results. Are you going to reuse your current seat mounting system, or venture off into the world of the RANS pseudo-standard?
Also (although it's probably a little late for this!), have you though about a Euromesh or Hoagie-style seat, rather than the hardshell?
I'm going to stick with my mounting system for this bike. All the parts are already made so it's a lot easier than transitioning to a new system at this point. For the next bike... we'll have to see.
As for the mesh backed seats, I have thought about them. I decided to stick with the hardshell for this attempt in part because I can get it done a bit faster. If I can be comfortable on a hardshell I suspect they are a bit faster/more efficient/lighter. If I can't, my next step will be a hoagie seat copy.
My one concern with the mesh seat for this bike is I designed it for a hardshell which requires less of a gap between the seat base and the frame. The wooden seat is only about 1/8" above the frame. This would mean boosting the seat height at least 1" and probably a little more.
One possibility is using a hardshell seatpan and possibly even lumbar support and then transition (somehow) into a mesh upper. I find the hardshell very comfortable for my lower back, it's the upper back that gives me problems.
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