The Excelsior Super X was a motorcycle manufactured by the Excelsior Motor Manufacturing & Supply Company from 1925 to 1931.^[1] It was the most famous Excelsior motorcycle manufactured by that company and was the first American forty-five cubic inch motorcycle.^[2]

Arthur "Connie" Constantine, Assistant Chief Engineer at the Harley-Davidson Motor Company, drew up plans for a mid-sized V-Twin to compete against the Indian Scout. When he presented the unauthorized project to co-founder Walter Davidson, he was reprimanded for wasting the company's time.^[3]

Constantine resigned his position at Harley-Davidson and offered his services and his project to Excelsior. Both were accepted, leading to the introduction of the Excelsior Super X in 1925. The design proved to be competitive in motorsports in its first year despite competing against motorcycles with engines of greater capacity.^[3]

The Super X effectively replaced Excelsior's other mainstream model, a sixty-one cubic inch V-Twin, which was discontinued during the first year of Super X production.^[1] The smaller motorcycle was believed to be a more suitable companion product for their Henderson four-cylinder motorcycle.^[4]

The design of the Super X was a considerable departure from its predecessors at Excelsior. Where earlier Excelsiors had an enclosed primary chain transmitting power from the engine to a separate gearbox, the Super X had the engine and transmission together in a single crankcase, using a helical gear to power the transmission directly from the engine.^[2][5] The Super X also marked the return of leading-link forks on Excelsior motorcycles, which had earlier switched to trailing-link forks similar to those used by Indian but with coil springs instead of Indian's quarter-elliptic leaf springs.^[2][5]

The Super X had originally been envisioned as a competitor to the Indian Scout which was, at the time, powered by a thirty-seven cubic inch V-twin engine.^[3][6] Indian's initial response to the Super X came in 1927, when they enlarged the Scout engine to forty-five cubic inches. This defensive move was followed a year later with a more decisive attack, the introduction of the 101 Scout.^[6] The new Scout proved to be a formidable competitor both on the racetrack and in the marketplace.^[4][7]

In 1929, the Excelsior-Henderson concern restyled both its motorcycle offerings, the Excelsior Super X and the Henderson Four, for a more contemporary look.^[5] These "Streamline" models had tanks that hid the top tube of the frame and wide front fenders with holes for the forks to pass through.^[2]

That year, Harley-Davidson released their forty-five cubic inch motorcycles, the D and the DL.^[8]

The Super X competed with the 101 Scout, the D, and the DL, until 1931. During 1931, the Indian 101 Scout was replaced by a Scout model based on the heavier Chief frame, the sport solo DLD was added to the D and DL in the Harley-Davidson line, and the Excelsior-Henderson concern ceased production of motorcycles upon the order of its proprietor, Ignaz Schwinn.^[6][8][9]

The Super X was America's first forty-five cubic inch motorcycle, and the racing class for forty-five cubic inch motorcycles was started in the United States one year after the Super X's introduction.^[3] The forty-five cubic inch class became the premier class in dirt-track racing, in which such motorcycles as the Indian Sport Scout and Harley-Davidson WR, KR, and XR would compete.

Although the Super X came to an abrupt end, its competitors from Indian and Harley-Davidson would continue for at least a decade.^[8][10]