Whereas the F Frames, or classic Moulton bicycles, had a single main beam connecting the head tube to the seat tube. The main beam on the Y frame split was half as long, and toward the rear met a triangulated structure, composed of a short head tube, bottom tube and seat tube.
After Alex Moulton ceased to work with Raleigh, he continued to refine his bicycle concept. In the late 1970s, he fully intended to develop a production bicycle based on the Y frame. However, the Y Frame never made it into production, as Alex Moulton strived to develop a frame that was as light as a modern lightweight machine.
However, it offers an interesting link in the evolution of the Moulton bicycle design from the classic F-Frame to the modern spaceframes. It can be seen how the classic F frame, evolved into a Y frame, creating a stronger, more triangulated structure. The next step in the prototyping process, of course, was an X frame from which the spaceframe models are derived.