Straight-twin Engine - Motorcycle Use

Motorcycle Use

In motorcycles, as with cars and other vehicles, the terms parallel-twin, inline-twin, vertical-twin and straight-two are used. Sometimes, parallel-twin is used with the special meaning that the crankshaft is transverse across the frame, while inline-twin means the cylinders are arranged front to back, inline with the direction of travel. This special meaning for inline is used for either bikes with a longitudinal crankshaft, such as the Sunbeam S7, or for tandem twins, with a transverse pair of crankshafts, but the cylinders arranged longitudinally, one in front of the other. Other times, parallel-twin, inline-twin and the other variants are used interchangeably and treated as equivalent. In four-stroke designs, the parallel twin is usually vertical or near vertical. One exception is the only parallel-twin to win a 500cc Grand Prix, the AJS E-90 Porcupine of 1949, which had nearly horizontal cylinders.

Edward Turner's 1937 Triumph Speed Twin started a trend, and up to the mid-1970s four-stroke parallel-twins were the most common type of British motorcycles, being produced by Triumph, BSA, Norton, Ariel, Matchless and AJS. Italian and German manufacturers have also made parallel-twins, as had American manufacturer Indian, whose parallel-twins included the 1949 440 cc Indian Scout and the 1950 500 cc Indian Warrior. BMW and Japanese manufacturers still made them as at 2010, particularly for middleweight bikes.

Although the rise in popularity of the large V-twin motorcycle has seen the parallel-twin fall out of favour, the latter retains these advantages over the former:

• A parallel-twin is cheaper to make, having only one cylinder block and one cylinder head.
• Both cylinders can have the exhaust pipe exiting at the front, in the cool air stream.
• Siting of ancillaries (air-filter, carburetters, ignition, etc.) is simpler.
• This simpler layout can potentially make maintenance access easier.
• The parallel twin is both lighter and narrower, allowing a lighter frame and shorter wheelbase if placed across the frame, or a narrower frame if placed in line with the frame.
• The motorcycle's centre of gravity can be sited optimally, i.e. lower and further forward.
• Provided a 270° crank is used, a four-stroke parallel twin can simulate the "feel" of a four-stroke V-twin.

Straight-twin engines are used in large scooters such as the Yamaha TMAX and Honda Silver Wing. They are also used in motorcross sidecar racing.