Seat Post? With the three main tubes in place, it is time to add the seat tube (accurate but misleading name) and a false brake bridge to complete the basic frame. First I cut and shaped a piece of the tubing left over from the new chainstay tubes to fit between the chainstays about 3 inches forward of the end of the top tube. This will help maintain the proper spacing of the chainstays and provide a support for the new seat tube. I selected the position of of this tube to allow me to slope the seat tube to match the “old seat tube”.
To complete the basic frame, I replaced the seatstays, rotated to match the location of the new top tube and brazed them in place at each drop out and on the top tube.
The seat base is a “standard” industrial trike seat from the local bike shop. I paid about $25 for the seat and clamps. This seat has three mount points. The front point is a compression clamp that clamps to a single vertical tube (think department store kids bike seat clamp). The other two are simple single bolt pipe clamps for holding on a upside-down “U” shaped tube like the old banana seats.
The seat back is made of 1/2” EMT bent into a closed rectangle. The ends of the tube are brazed together and a brace is brazed about 1/3 of the way down from the top. A “U” shaped tube was then brazed in the bottom of the seat back to connect to the two remaining clamps on the seat base.
From the local canvas shop, I purchased several yards of “outdoor furniture mesh” fabric. I cut the fabric 2 inches wider then the seat back. Then I folded the ends of the fabric over twice and sewed the seams with dental floss. I then inserted a thin metal rod in each seam. Approximately every two inches around the perimeter of the seat fabric (inside of the rods) I melted lacing holes using a soldiering iron. Through the holes I passed loops of “parachute cord” to lace the seat fabric to the seat frame. The construction of the seat back only took one evening (after I had finally decide on a design).
The seat back is held in place by adjustable aluminum supports that run from the brace on the seat back down to the rear dropouts. These are simply a fitted rod and tube that have a number of holes drilled along the length that a hitch pin is pressed into. This feels quite solid, but there is some side to side play. The ends are made in the same way the steering linkage rod (below) using a steel sleeve and roll pins (omitting the “rod ends: fittings)…