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conjoined twins

or Siamese twins

Identical twins (see multiple birth) whose embryos did not separate completely. Conjoined twins are physically joined (typically along the trunk or at the front, side, or back of the head) and often share some organs. Symmetrical conjoined twins usually have no birth anomalies except at the areas of fusion and can sometimes be separated by surgery. In asymmetrical conjoined twins, one is fairly well developed, but the other is severely underdeveloped and dependent on the larger twin for nutrition. The underdeveloped twin may have to be surgically separated to save the larger twin. The term originally referred to Chang and Eng, born in 1811 in Siam, who were joined by a ligament from breastbone to navel. Widely exhibited, they married two sisters and fathered several children.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

V-twin

A V-twin is a two cylinder internal combustion engine where the cylinders are arranged in a V configuration.

Configurations

"True V-twin" vs V-2

There is some ambiguity regarding what defines a V-twin which arose from 180 degree "V" engines (see Flat engine). When the cylinders are arranged in a V pattern, it is generally accepted as being a V engine. However, some people differentiate using "V-twin" to mean the classic V design of a shared crank pin for each pair of cylinders and "V-2" to indicate a boxer-style crank with a separate crank pin for each cylinder. This convention is not common, and applies only to two cylinder engines. For example, while most V8 engines use shared crank pins for cylinder pairs, they are not referred to as "V-octuplets". Most engine manufacturers do not use this convention and the 1983 Honda Shadow 750 is known as being the first V-twin with an offset-dual-pin crankshaft.

V angles

The most obvious configuration for a V-twin is a 90°, in which counterweighting can balance the engine, in odd-firing 90 degree Vees. This is seen in the Moto Guzzi, Ducati and Suzuki SV650, but other angles can be seen like the 45° of the classic Harley-Davidson engine, the 75° Suzuki, the 52° Honda, the 80° Honda CX-500, the 47° Vincent, the 42° Indian, the 60° Aprilia, the 45° S&S, and the 56.25° S&S X-Wedge.

The signature Ducati engine, a transverse 90° twin with the front cylinder approximately parallel to the ground and the rear cylinder vertical, is sometimes referred to as an "L" twin.

Orientations

The terms longitudinal engine and transverse engine are used to refer to the crankshaft orientation. A common mistake with V-2 engines is to refer to the cylinder orientation. However referring to the crankshaft gives a correct method to engine orientations as it gives the same orientation for all V-engine types like V-2, V-4 (which would be difficult to describe with cylinder orientation) and V-8.

Transverse mounting

Both two-cylinder V engines are common on motorcycles. The engine can be mounted in transverse position like on Harley-Davidsons, Ducatis and many recent Japanese motorcycles. This transverse position gives the motorcycle a reduced frontal area. The main disadvantage of this configuration is that the rear cylinder and the front cylinder will receive different air-flows making air cooling somewhat problematic especially for the rear cylinder. Cooling problems are somewhat mitigated by having all "four" sides of each cylinder exposed to air flow. This differs from a parallel-twin cylinder engine which has a distinct front, back, and sides, but the inside of each cylinder is not exposed to airflow as the cylinders are typically joined together with a cam chain running up through the block in-between the cylinders.

Longitudinal mounting

The longitudinal two-cylinder V as seen on Moto-Guzzis and some Hondas is less common. This position is well adapted to transmission shafting. When used in motorcycles, this approach has the slight disadvantage of causing a torque reaction that tends to lean the motorcycle slightly to one side as the angular rate of the crankshaft increases or decreases. The faster the change, the larger the torque it produces. However, many motorcycle manufacturers have corrected for torque reaction by rotating the transmission input shafts and/or the balance and drive shafts opposite that of the crankshaft so that there is approximately equal mass turning clockwise and counterclockwise at any time, thereby physically canceling the effect.

Non-V twin configurations

Flat twin

The flat twin, or boxer configuration, has a longitudinal crankshaft with the cylinders protruding horizontally from either side of the engine. BMW is the best-known manufacturer of this configuration, which has two advantages: the cylinders are squarely in the airstream and run cooler; and in the event of an accident the cylinders will help protect the rider. Disadvantages of this design are the torque reaction mentioned above in the section on longitudinal mounting, and the engine must be mounted higher in the frame to keep the cylinders from scraping the road in sharp curves.

According to the American Motorcyclist Association's Motorcycle Hall of Fame Museum, regarding the World-War-II-vintage Harley-Davidson XA, which has an opposed twin engine — "Mechanically, the large cooling fins stuck straight out in the breeze, reportedly keeping the XA’s oil temperature 100 degrees cooler than a standard Harley 45." The latter was a transversely mounted V-twin, in which the airstream cannot reach the rear cylinder as efficiently as on a longitudinally mounted flat twin.

Douglas Motorcycles made a flat-twin with fore-and-aft cylinders, which allowed the engine to be mounted lower in the frame. The disadvantage was that the rear cylinder tended to overheat. Currently there are no motorcycles with this configuration in mass production.

Vertical twin

The vertical twin or parallel twin is another logical alternative to the V-twin. It was made famous by such British motorcycles as Triumph, Ariel, AJS, BSA, Matchless, Norton and Royal Enfield. The parallel twin has also been used by BMW , Benelli, Honda, Kawasaki, Laverda, Suzuki, and Yamaha.