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.[15]
Harley-Davidson works in 1911

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.[16] The spring tension could be adjusted to suit the rider's weight, and more than 3 inches (76 mm) of travel was available.[17] Harley-Davidson used seats of this type until 1958.[18]

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.