<P> A motorized bicycle is a bicycle with an attached motor or engine and transmission used either to power the vehicle unassisted, or to assist with pedalling . Since it always retains both pedals and a discrete connected drive for rider - powered propulsion, the motorized bicycle is in technical terms a true bicycle, albeit a power - assisted one . However, for purposes of governmental licensing and registration requirements, the type may be legally defined as a motor vehicle, motorcycle, moped, or a separate class of hybrid vehicle . Powered by a variety of engine types and designs, the motorized bicycle formed the prototype for what would later become the motorcycle . </P> <P> Motorized bicycles have utilized all variety of engines, from internal - combustion (IC) two - stroke and four - stroke gasoline engines to electric, diesel, or even steam propulsion . Most motorized bicycles are based or derived from standard general - purpose bicycle frame designs and technologies, although exceptions abound . In addition, modifications to a standard bicycle frame to support motorization may be extensive . </P> <P> The earliest motorized bicycles were ordinary utility bicycles fitted with an add - on motor and transmission to assist normal pedal propulsion, and it is this form that principally distinguishes the motorized bicycle from a moped or motorcycle . In a day when gasoline engine and transmission designs were in their infancy, and power - to - weight ratios were low, a dual - purpose propulsion system seemed particularly advantageous . As time went on, pedal propulsion was increasingly replaced by constant use of a two or four - stroke gasoline engine . Nevertheless, the concept of using motor assist for the ordinary bicycle has persisted, and the concept has periodically resurfaced over the years, particularly in times of austerity or fuel shortages . In countries where automobiles and / or fuels are prohibitively expensive, the motorized bicycle has enjoyed continued popularity as a primary mode of transportation . </P> <P> The design of the motorized bicycle or motorbike varies widely according to intended use . Some motorized bicycles are powerful enough to be self - propelled, without use of the pedals . A development of the motorized bicycle is the moped, which commonly has only a vestigial pedal drive fitted primarily to satisfy legal requirements, and suitable only for starting the engine or for emergency use . The alternate design philosophy to the moped is the so - called motor - assist or pedal - assist bicycle . These machines utilize the pedals as the dominant form of propulsion, with the motor used only to give extra assistance when needed for hills or long journeys . </P>

Is putting a motor on a bike illegal