MOTOR CYCLE, a bicycle, the frame of which supports an internal-combus tion engine which is used for propulsion. The early models, which followed the lines of the foot-powered bicycle, were uncomfortable and likely to tip because of their higher center of gravity. The first models had the fuel tank over the rear wheel and often the single-cylinder motor was made part of the frame rather than suspended from it. In both the early and modern models, the rear wheel is the drive wheel, and the machine is steered by the front wheel. Flexible belts, shafts, and chains have been used to transmit the power to the drive wheel, the two latter means being most favored at present.
Modern motor-cycle design, which has adapted for its own use almost every practice of automobile design, has evolved a machine with a low center of gravity. The twin-cylinder V-type air cooled motor with its axis located where the tread shaft of a bicycle would be, is the generally accepted type of design. The chain drive is most used, although there are very successful four-cylinder shaft drive models on the market. The
fuel tank—which as a rule has two com partments, one for oil and one for gas oline—is located above the motor, fas tened to the top bars of the machine, and feeds the carburetor and the lubricating system by gravity. The cycle is usually equipped with a magneto for ignition, though the motor-generator battery sys tem has a wide following, and some of the late models are provided with electric self-starters, and a light clutch is gener ally provided, and even a variable speed gear is not unusual.
While the motor cycle does not give the comfort and protection of the auto mobile, the advantages of lower first cost and smaller upkeep, the high speed which may be attained, and the ease with which it may be handled, are important factors. With the side car attachment it is much used commercially for parcel delivery, or for family touring, and for dispatch work and transporting light articles it played an important part in the World War.