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 KR the new engine to save the company from its demise by 1953. Harley-Davidson chose to play to their own strengths, creating a pushrod OHV V-twin engine in 1936 retaining separate transmission, and the new, for Harley-Davidson, unit construction, introduced on the 1952 Model K (still side-valve). "Cycle World" road tested a 1963 KRTT (or KR-TT) alongside a Harley-Davidson Sprint TT racer. They recorded a top speed of , and acceleration of 5.8 seconds. The time was 14.1 seconds at . In 1956, every Class C race was won by a Harley-Davidson KR. From 1955 through 1969, 12 of the 15