 Using grade A mild steel, the size of this engine determined the space between the top and bottom rails of the full duplex cradle.In 1960 the top rails were installed at the rear of the tank.Riders complained that these "wideline" Featherbed frames were uncomfortably wide at but it was not until 1960 that the top runs of the frame were narrowed towards the front of the seat, with corresponding overall styling changes including tank and seat to create the "slimline" frame.