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.
[flickr]http://www.flickr.com/photos/moultonbuzz/3919305956[/flickr]
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.