I started with the seat from Cafe-Bike-Connection, an eBay seller. Looks like he only sells the pan now, but mine came with the pan, foam and cover. Looks like he offers another one in fiberglass, but it's kind of spendy.
Once I got the seat, I determined that the stock frame was about 6" too long. Happily, the extra frame was not load-bearing, as it only supported the fender and tail-light, and perhaps a passenger. People don't like or trust me, passengers will never be an issue. So I lopped off the back ends of the frame, ditched the old tail-light, signals and fender. We were through the looking glass now.
The seat is made from ABS, so I had to fabricate a means to connect the seat pan to the frame.
I did that with a piece of treated 2x12 lumber that I cut to fit within the empty space under the seat. Then I cut and planed the board to thin it out. Once it was the right size and shape, I screwed the seat onto the top of the board, and then attached the old seat hinges to the bottom. Now the whole seat flips on the frame hinges.
To level it out, however, I needed to close a roughly 3/8" gap on the non-hinge side. The original seat pan has rubber pads that deal with this. However, I also needed new mounting points for my license plate and turn signals, since the fender and frame were no longer.
I took a piece of 1/2" or 3/8" (don't recall) birch plywood, and trimmed it to fit the oval shape of the seat pan. To leave the bolt-holes on the frame in place, I notched out part of the board.
I fastened that to the frame with a pair of pipe/conduit clamps that secure it to the crossbar behind the battery. They are attached with carriage bolts and star washers with nylock nuts. The whole platform is painted black to hide the edges.
*say look at that rust on the frame. Gonna need to sort that out over the winter.
The tail-light is a standard cateye from eBay. The seat is designed so that you can cut an oval and "french" in the light, but that requires more hardware to bolt in the tail-light itself. Instead, I drilled holes for the bolts and for the wiring, and left the light outside the seat pan. I like the bit of chrome on top of the light anyway.
The turn signals and plate mount on a standard bracket found on eBay. The signals are LED bullets -- again pretty standard eBay stuff. I did have to bend the bracket some to angle the plate out, as I worried that it would brush the wheel on a hearty pot-hole bump.
I'll come back to this thread later with some pictures. Maybe even check in with a retrospective on how I butchered the front end to match.
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