The Pettiford springs were developed by a long distance rider, Ken Pettiford, at about the same time as the Series Ds were becoming more available and people realised that the comfort of the rear end could be improved. Ken's solution was to have longer springs made which provided about one inch of extra travel. I believe that they are still available and many of us have used them for years. More comfort is available if the rear of the dual seat is fully sprung. The appearance is not changed much. These days the AVO rear coil over damper is probably a better solution but does change the appearance.