Somewhere to sit
After initially building a seat mock-up to check ergonomics, I designed the seat sub frame in two parts.
The upper seat frame sits on a lower frame which is height adjustable. Both are welded 1.5mm aluminium. The upper frame slides forward and back to accommodate leg length. It also allows the whole seat to be quickly removed to access the engine. The design supports a seat transport much like Royce Creasey's design to allow for a 2 up configuration if required.
As shown the bike weighs 155kg
The seat base started as the original seat base from the car seat. It is from a Fiat Punto. It is an all foam (no springs) base in a steel pan. I removed it from the pan and cut away what I needed to get my legs down.
Next I squeezed the sides together to make the bulge and held it like that with string. It order to permanently hold it, I stitched between the pieces of wire you can see running the length of the seat. They are there in the original configuration to hold the seat fabric in place.
I smoothed the rough cuts on the foam with a surform tool. It is a little slow on flexible PU foam (FPF) but gives a very uniform finish.