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.
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. The smaller motorcycle was believed to be a more suitable companion product for their Henderson four-cylinder motorcycle.
In 1929, the Excelsior-Henderson concern restyled both its motorcycle offerings, the Excelsior Super X and the Henderson Four, for a more contemporary look. 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..
That year, Harley-Davidson released their forty-five cubic inch motorcycles, the D and the DL.
The Super X competed with the 101 Scout, the D, and the DL until 1931, when the 101 Scout was replaced by a Scout model based on the heavier Chief frame, the D and DL were joined by the sport solo DLD, and the Excelsior-Henderson concern ceased production of motorcycles upon the order of its proprietor, Ignaz Schwinn.
Although the Super X came to an abrupt end, its competitors from Indian and Harley-Davidson would continue for at least a decade. The Indian Scout continued as a forty-five cubic inch motorcycle until it was discontinued at the end of World War II. The Harley-Davidson D series would evolve into the R series and the W series, including the military WLA and WLC motorcycles. These were replaced by the K series motorcycle, which used a revised flathead forty-five inch engine until 1953, after which the displacement was increased to fifty-five cubic inches for the '54-'56 KH models. However, the Harley-Davidson Servi-Car, which used the contemporary D, R, and W engines (but not the K engine), would continue to be manufactured by Harley-Davidson until 1973.