1958 Harley-Davidson FLH

The FL was introduced to the Harley-Davidson model line in 1941. It used a 74 cu in (1,210 cc) version of the "Knucklehead" OHV engine that powered the EL in 61 cu in (1,000 cc) form. The FL shared its frame with the EL and with the U and UL, which used a 74 cubic inch flathead engine. The FL replaced the UH and ULH, which used the same frame with 80 cu in (1,300 cc) flathead engines.

The FL continued relatively unchanged until 1948, when it and the EL were given redesigned "Panhead" engines of the same capacities as before. These engines had several improvements over the earlier "Knuckleheads", including self-adjusting hydraulic lifters and aluminum cylinder heads to reduce weight and improve cooling. The U and UL flathead twins were discontinued in 1948, leaving the OHV EL and FL models as Harley-Davidson's large-frame motorcycles.

In 1949, a year after receiving the "Panhead" engine, the FL was given a new front suspension featuring hydraulically damped telescopic forks, replacing the leading link spring suspension of the time. These forks were standard on all big twin models for 1949, including the E, EL, F, and FL. Harley-Davidson offered the spring suspension units on these models and recommended their use on sidecar combinations, because the hydraulic forks did not have enough trail to allow the combination to be stable enough for safe operation.

During their debut model year of 1949, Harley-Davidson referred to their new suspension systems as the "hydraulic front ends". Harley-Davidson's marketing department promoted the new suspension systems by renaming the big twin models "Hydra Glide" for the 1950 model year. This was Harley-Davidson's first departure from its policy of using alphabetic lettering to identify its models. This name would change twice in the history of the basic large-framed E and F series models, each time signaling an improvement in the bike's technology. In addition, the Glide ending would be used on other models, based on both and FL and FX formats.

In 1952, the Hydra-Glide's transmission standard hand-shift/foot-clutch was supplemented by the optional foot-shift/hand-clutch. The original format continued to be offered as an option until 1978. 1952 was also the last year of the 61 cu in (1,000 cc) EL, making the FL the last remaining large-frame model.

Although the 1903 founding is now the basis for "Anniversary Models", Harley-Davidson's 50th Golden Anniversary was celebrated in 1954 with special paints and badges on the front fender. The first year of production was 1904.

A more highly tuned engine with high-compression heads, higher-lift cams, and polished ports, was offered with the FLH version of 1955. The FLH designation has continued up to the present.

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.