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Harley-Davidson, Inc. (H-D, or simply Harley) is an American motorcycle manufacturer headquartered in Milwaukee, Wisconsin, United States. Founded in 1903, it is one of two major American motorcycle manufacturers to survive the Great Depression along with its historical rival, Indian Motorcycles. The company has survived numerous ownership arrangements, subsidiary arrangements, periods of poor economic health and product quality, and intense global competition to become one of the world's largest motorcycle manufacturers and an iconic brand widely known for its loyal following. There are owner clubs and events worldwide, as well as a company-sponsored, brand-focused museum.

Harley-Davidson is noted for a style of customization that gave rise to the chopper motorcycle style. The company traditionally marketed heavyweight, air-cooled cruiser motorcycles with engine displacements greater than 700 cc, but it has broadened its offerings to include more contemporary VRSC (2002) and middle-weight Street (2015) platforms.

Harley-Davidson manufactures its motorcycles at factories in York, Pennsylvania; Milwaukee, Wisconsin; Manaus, Brazil; Bawal, India; and Pluak Daeng, Thailand.The company markets its products worldwide, and also licenses and markets merchandise under the Harley-Davidson brand, among them apparel, home décor and ornaments, accessories, toys, scale models of its motorcycles, and video games based on its motorcycle line and the community.

In 1901, 20-year-old William S. Harley drew up plans for a small engine with a displacement of 7.07 cubic inches (116 cc) and four-inch (102 mm) flywheels designed for use in a regular pedal-bicycle frame. Over the next two years, he and his childhood friend Arthur Davidson worked on their motor-bicycle using the northside Milwaukee machine shop at the home of their friend Henry Melk. It was finished in 1903 with the help of Arthur's brother Walter Davidson. Upon testing their power-cycle, Harley and the Davidson brothers found it unable to climb the hills around Milwaukee without pedal assistance, and they wrote off their first motor-bicycle as a valuable learning experiment

The three began work on a new and improved machine with an engine of 24.74 cubic inches (405 cc) with 9.75 in (24.8 cm) flywheels weighing 28 lb (13 kg). Its advanced loop-frame pattern was similar to the 1903 Milwaukee Merkel motorcycle designed by Joseph Merkel, later of Flying Merkel fame. The bigger engine and loop-frame design took it out of the motorized bicycle category and marked the path to future motorcycle designs. They also received help with their bigger engine from outboard motor pioneer Ole Evinrude, who was then building gas engines of his own design for automotive use on Milwaukee's Lake Street.

The prototype of the new loop-frame Harley-Davidson was assembled in a 10 ft × 15 ft (3.0 m × 4.6 m) shed in the Davidson family backyard. Most of the major parts, however, were made elsewhere, including some probably fabricated at the West Milwaukee railshops where oldest brother William A. Davidson was toolroom foreman. This prototype machine was functional by September 8, 1904, when it competed in a Milwaukee motorcycle race held at State Fair Park. Edward Hildebrand rode it and placed fourth in the race.

In January 1905, the company placed small advertisements in the Automobile and Cycle Trade Journal offering bare Harley-Davidson engines to the do-it-yourself trade. By April, they were producing complete motorcycles on a very limited basis. That year, Harley-Davidson dealer Carl H. Lang of Chicago sold three bikes from the five built in the Davidson backyard shed. Years later, the company moved the original shed to the Juneau Avenue factory where it stood for many decades as a tribute.

In 1906, Harley and the Davidson brothers built their first factory on Chestnut Street (later Juneau Avenue), at the current location of Harley-Davidson's corporate headquarters. The first Juneau Avenue plant was a 40 ft × 60 ft (12 m × 18 m) single-story wooden structure. The company produced about 50 motorcycles that year.

In 1907, William S. Harley graduated from the University of Wisconsin–Madison with a degree in mechanical engineering. That year, they expanded the factory with a second floor and later with facings and additions of Milwaukee pale yellow ("cream") brick. With the new facilities, production increased to 150 motorcycles in 1907. The company was officially incorporated that September. They also began selling their motorcycles to police departments around this time, a market that has been important to them ever since. In 1907, William A. Davidson quit his job as tool foreman for the Milwaukee Road railroad and joined the Motor Company.

Production in 1905 and 1906 were all single-cylinder models with 26.84-cubic-inch (440 cc) engines. In February 1907, they displayed a prototype model at the Chicago Automobile Show with a 45-degree V-Twin engine. Very few V-Twin models were built between 1907 and 1910. These first V-Twins displaced 53.68 cubic inches (880 cc) and produced about 7 horsepower (5.2 kW). This gave about double the power of the first singles, and top speed was about 60 mph (100 km/h). Production jumped from 450 motorcycles in 1908 to 1,149 machines in 1909.

In 1911, the company introduced an improved V-Twin model with a displacement of 49.48 cubic inches (811 cc) and mechanically operated intake valves, as opposed to the "automatic" intake valves used on earlier V-Twins that opened by engine vacuum. It was smaller than earlier twins but gave better performance. After 1913, the majority of bikes produced by Harley-Davidson were V-Twin models.

In 1912, Harley-Davidson introduced their patented "Ful-Floteing Seat", which was suspended by a coil spring inside the seat tube. The spring tension could be adjusted to suit the rider's weight, and more than 3 inches (76 mm) of travel was available. Harley-Davidson used seats of this type until 1958.

By 1913, the yellow brick factory had been demolished and a new five-story structure had been built on the site which took up two blocks along Juneau Avenue and around the corner on 38th Street. Despite the competition, Harley-Davidson was already pulling ahead of Indian and dominated motorcycle racing after 1914. Production that year swelled to 16,284 machines.

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 that it was adopted for military issue, first with the British Model H produced by Triumph Motorcycles Ltd in 1915. The U.S. military purchased over 20,000 motorcycles from Harley-Davidson.

Harley-Davidson launched a line of bicycles in 1917 in hopes of recruiting more domestic customers for its motorcycles. Models included the traditional diamond frame men's bicycle, 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.