Flat twin engine

A flat-twin is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. It is a flat engine with two cylinders. Used in motorcycles for more than a century, flat-twins have also been used in automobiles, light aircraft, stationary powerplants, and household appliances.

Early flat-twin motorcycles' engines were mounted with the cylinders in line with the frame. This caused uneven cooling of the cylinders and required the motorcycle to have a long wheelbase. Later flat-twin motorcycles' engines were mounted with their cylinders across the frame for better air cooling and a shorter wheelbase. Disadvantages of this layout include torque reaction in the motorcycle in turns and the potential to damage cylinder heads.

Crank configurations

The traditional crank configuration used with flat-twin engines is the boxer twin whose 180° crankshaft moves the pistons in phase with each other. In this configuration, the primary force generated by one piston counteracts that generated by the other at all times, resulting in excellent primary balance. Since the pistons are not in line with each other, the equal and opposing forces generate a rocking couple on the crankshaft. Unlike the ignition of a V-twin or a 180° parallel twin, a boxer engine has equally spaced firing intervals, adding to its smoothness.