The Harley-Davidson FLH motorcycle, manufactured by the Milwaukee-based Harley-Davidson Motorcycle Company, originated in 1949 as the Hydra-Glide touring model, so named for its hydraulically damped telescopic forks. Since the introduction of the Hydra-Glide, the Duo-Glide and Electra-Glide models also were produced and carry the FLH designation. The FLH motorcycles continue in 2009 as Electra-Glides.
All Harley-Davidson touring motorcycles, equipped with the large Big-Twin engines and large telescopic forks, are FL models, with the letters "FL" a simple series designation introduced in 1941. These big bikes were powered by the motorcycle maker's famed 1936-47 "Knucklehead" engine, which got its name for the odd shape of its rocker covers. The "Panhead" engine was introduced in 1948, so named for the new shape of the rocker covers. The following year, the FLH model was produced.
The 1949 FLH Hydra-Glide was powered by a 74-cubic-inch two-cylinder pushrod V-Twin engine. Identified as the Hydra-Glide for its new front suspension system, it sat on a 59.5-inch wheelbase. The hydraulic technology gives the FL series its "H" designation. Through 1951, the Hydra-Glide was operated with a hand-shift and foot-clutch. The configuration was reversed for 1952 with a foot-shift and hand-clutch. The foot-shift versions are identified as FLHF.
The 1955-57 Harley FLH still sported a 74-cubic-inch engine and teamed with high-compression heads and polished ports to generate 55 horsepower. Its wheelbase was stretched slightly by a half inch. Buyers could still find the old-style hand-shift, foot-clutch models as an option through 1978.
The model year 1958 brought a new frame that featured a rear swing-arm that was suspended by two coil-over-shock suspension components. The new suspension earned the FL series the Duo-Glide moniker, replacing the Hydra-Glide. The Duo-Glide remained an FLH model.
The 1958-65 Duo-Glide remains one of Harley-Davidson's most beautiful bikes with a near perfect combination of the Panhead engine and frame flourishes. It featured an unusual speedometer that dropped the zeros to read 1 to 12 to eliminate clutter. It was equipped with a dual fishtail exhaust, chromed oil tank, kick-start pedal and handle grips. Although the bike now featured new rear suspension, it still came equipped with the superfluous sprang seat.
The "Shovelhead," again named for the shape of its rocker covers, replaced the Panhead in 1966 as a 1200cc engine that was upgraded in 1978 to 1340cc. The year before the Shovelhead's debut, the Electra-Glide was introduced with an electric starter, eliminating the need for the kick-start. Using the same 74-cubic-inch engine as previous models, the Shovelhead's output was boosted to 60 horsepower.
The FLH Electra-Glide replaced the Duo-Guide and is still produced in 2009. An unfaired version, which is a partially covered front of the bike (to reduce air drag), was produced from 1977 to 1982 as the FLHS Electra-Glide. The FLHR Road King debuted in 1994 and remains in production as well.