History from 1915 to 1940

n 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.

Army personnel with bike

Almost half of all Harley-Davidson motorcycles produced are sold for use by the U.S. military in World War I. At War's end, it is estimated that the Army used some 20,000 motorcycles in their efforts, most of which were Harley-Davidsons. One day after the signing of the Armistice, Corporal Roy Holtz of Chippewa Falls, Wis., is the first American to enter Germany. He is riding a Harley-Davidson.
The 37 cubic inch opposed twin cylinder Sport model is introduced and gains great popularity overseas. Unique not only for the cylinder configuration, which was directly opposed and flat, the Sport quickly earns a reputation for being uncommonly quiet.

By 1920, Harley-Davidson was the largest motorcycle manufacturer in the world, with 28,189 machines produced, and dealers in 67 countries.

1922 model

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.

1925 model

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.

JDH two-cam motorcycle 

The first Harley-Davidson two cam engine is made available to the public on the JD series motorcycles. The bike is capable of top speeds between 85 - 100 mph.

GREAT DEPRESSION:

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.

Servi Car

In the mid-1930s, Alfred Rich Child opened a production line in Japan with the 74 cubic inches (1,210 cc) VL. The Japanese license-holder, Sankyo Seiyako Corporation, severed its business relations with Harley-Davidson in 1936 and continued manufacturing the VL under the Rikuo name.

Harley-Davidson dealer in Texas, ca. 1930-1945

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.