Some of the earliest experimental motorcycles were powered by steam engines, which were attached to modified bicycles. Nikolaus A. Otto is credited with laying the groundwork for the modern motorcycle. He developed the concept of the four-stroke engine. Gottlieb Damlier developed the single-cylinder air-cooled engine. Before what we now see as a motorcycle evolved, several major problems had to be overcome: leather belts used to transfer power from the engine to the wheels slipped when wet and broke often; the clutch had not been invented; and the only way to stop the cycle was to turn off the engine. By the beginning of the 20th century, steel chains replaced leather belts for power transmission. The friction clutch was introduced in 1904.