Harley-Davidson models. In the late summer of 1929, Harley-Davidson introduced its 45 cubic inches (737 cm³) flathead V-Twin to compete with the Indian 101 Scout and the Excelsior Super X. This was the "D" model, produced from 1929 to 1931. Riders of Indian motorcycles derisively referred to this model as the "three cylinder Harley" because the generator was upright and parallel to the front cylinder. The Great Depression began a few months after the introduction of their 45 cubic inch (737 cm³) model. Harley-Davidson's sales fell from 21,000 in 1929 to 3,703 in 1933. Despite this, Harley-Davidson unveiled a new lineup
Harley-Davidson FL 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