Noted for a style of customization that gave rise to the chopper motorcycle style, Harley-Davidson traditionally marketed heavyweight, air-cooled cruiser motorcycles with engine displacements greater than 700 cm³ and has broadened its offerings to include its more contemporary VRSC (2002) and middle-weight Street (2015) platforms.
In 1901, 20-year-old William S. Harley drew up plans for a small engine with a displacement of 7.07 cubic inches (116 cc³) and four-inch (102 mm) flywheels. The engine was designed for use in a regular pedal-bicycle frame. Over the next two years, Harley 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. They quickly wrote off their first motor-bicycle as a valuable learning experiment.
Work immediately began on a new and improved second-generation machine. This first "real" Harley-Davidson motorcycle had a bigger engine of 24.74 cubic inches (405 cc³) with 9.75 inches (25 cm) flywheels weighing 28 lb (13 kg). The machine's 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. The boys 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.
According to wikipedia
