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A flat-twin is a two cylinder internal combustion engine with the cylinders arranged on opposite sides of the crankshaft.
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BMW Motorrad manufactures a number of flat-twin engine motorcycles.[citation needed] The geometry gives (uniquely, among twin cylinder engines) virtually perfect horizontal and vertical balance.[citation needed] There remains a vibration which is noticeable at low engine revolutions, and a torque reaction when, for instance, changing gear.[citation needed]
In the very early days, flat-twins sometimes had their cylinders aligned along the frame. Douglas in the United Kingdom and Helios of Germany used this layout.[citation needed]
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Many motorcyclists appreciate the way that the cylinders of this "boxer twin" provide protection to the rider in the event of a collision or fall.[citation needed] The benefit of the natural low center of gravity is partly lost because the engine has to be high enough in the frame to provide the cylinder heads with banking clearance in turns.[citation needed]
Flat-twin engines were used in several economy cars, including the Citroën 2CV, the Panhard Dyna X and Dyna Z, BMW 600 and 700, several Jowett cars between World Wars I and II, and the Toyota Publica and Toyota Sport 800.
The layout lends itself particularly well to air-cooling, either natural or forced (common in motorcycles and cars respectively).[citation needed] The mechanical noise and poor provision for in-car heating were not considered serious disadvantages in European cars such as the 2CV (and flat-fours such as the Volkswagen Beetle} in the early 1950s.[citation needed]
Motorcycles
Automobiles
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