Duo-Glide
The FL model was given a new frame in 1958. This frame included a rear swingarm suspended by a pair of coil-over-shock suspension units. In honor of this fully suspended chassis, the FL's model name was changed from Hydra-Glide to Duo-Glide.
Unlike OHV configuration, aluminum heads, and telescopic-fork front suspension, however, this improvement in technology was applied to the small-frame bikes first, the K-series having received rear suspension in 1952.
Read more about this topic: Harley-Davidson FL