Ralph Hepburn on his Harley racing bike in this 1919 photo.
World War I
In 1917, the United States entered World War I and the military demanded motorcycles for the war effort. Harleys had already been used by the military in the Pancho Villa Expedition but World War I was the first time the motorcycle had been adopted for combat service. Harley-Davidson provided about 15,000 machines to the military forces during World War I.
Harley-Davidson launched a line of bicycles in 1917 in hopes of recruiting customers for its motorcycles. Besides the traditional diamond frame men's bicycle, models included a step-through frame 3-18 "Ladies Standard" and a 5-17 "Boy Scout" for youth. The effort was discontinued in 1923 because of disappointing sales.
The bicycles were built for Harley-Davidson in Dayton, Ohio, by the Davis Machine Company from 1917 to 1921, when Davis stopped manufacturing bicycles.
Harley-Davidson 1000 cc HT 1923
By 1920, Harley-Davidson was the largest motorcycle manufacturer in the world, with 28,189 machines produced, and dealers in 67 countries.
In 1921, a Harley-Davidson, ridden by Otto Walker, was the first motorcycle ever to win a race at an average speed greater than 100 mph (160 km/h).
During the 1920s, several improvements were put in place, such as a new 74 cubic inch (1,212.6 cc) V-Twin, introduced in 1922, and the "Teardrop" gas tank in 1925. A front brake was added in 1928 although notably only on the J/JD models.
In the late summer of 1929, Harley-Davidson introduced its 45 cubic inches (737 cc) 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 2.745 in (69.7 mm) bore and 3.8125 in (96.8 mm) stroke would continue in most versions of the 750 engine; exceptions include the XA and the XR-750.
Harley-Davidson 1,200 cc SV 1931
The Great Depression began a few months after the introduction of their 45 cubic inch model. Harley-Davidson's sales fell from 21,000 in 1929 to 3,703 in 1933. Despite this, Harley-Davidson unveiled a new lineup for 1934, which included a flathead with Art Deco styling.
In order to survive the remainder of the Depression, the company manufactured industrial powerplants based on their motorcycle engines. They also designed and built a three-wheeled delivery vehicle called the Servi-Car, which remained in production until 1973.
An 80 cubic inches (1,300 cc) flathead engine was added to the line in 1935, by which time the single-cylinder motorcycles had been discontinued.
In 1936, the 61E and 61EL models with the "Knucklehead" OHV engines was introduced.Valvetrain problems in early Knucklehead engines required a redesign halfway through its first year of production and retrofitting of the new valvetrain on earlier engines.
By 1937, all Harley-Davidson's flathead engines were equipped with dry-sump oil recirculation systems similar to the one introduced in the "Knucklehead" OHV engine. The revised 74 cubic inches (1,210 cc) V and VL models were renamed U and UL, the 80 cubic inches (1,300 cc) VH and VLH to be renamed UH and ULH, and the 45 cubic inches (740 cc) R to be renamed W.
In 1941, the 74 cubic inches (1,210 cc) "Knucklehead" was introduced as the F and the FL. The 80 cubic inches (1,300 cc) flathead UH and ULH models were discontinued after 1941, while the 74" U & UL flathead models were produced up to 1948.
To be continued...