V-twin
A V-twin is a two cylinder internal combustion engine where the cylinders are arranged in a V configuration. The V-twin is unique among V engines in tnat while many engines have a single crank pin shared between the two cylinders, the normal expectation in a V engine, others have separate crank pins, see engine balance.
The V-twin arrangement is common on motorcycles. The engine can be mounted in transversal position like on Harley-Davidsons, Ducatis and many recent Japanese motorcycles. This transversal position give the motorcycle a reduced front surface. The main disadvantage of this configuration is that the rear cylinder and the front cylinder will receive different air-flow making air cooling somewhat problematic especially for the rear cylinder.
The longitudinal V-twin as seen on Moto-Guzzis and some Hondas is less common. This position is well adapted to transmission shafting, but has the disadvantage of causing a torque reaction that tends to lean the motorcycle on the side. The longitudinal V-twin motorcycle engine is the logical alternative to the flat-twin configuration. The flat-twin has better overall engine balance but is wider, heavier and less responsive than the V-twin. This requires the flat-twin to be mounted high on the motorcycle to avoid cylinder heads touching the ground in curve. On the other hand a V-twin could be mounted lower and could result in a lower center of gravity for the whole motorcycle.
The most obvious configuration for a V-twin is a 90° angle, but other angles can be seen like the 45° of the classic Harley-Davidson engine, the 75° Suzuki and the 60° Aprilia.