page_title: Straight-twin engine
section_title: History
hierarchical_section_title: Straight-twin engine / Motorcycle use / History
caption_attribution_description: The Quail Motorcycle Gathering 2015, Carmel by the Sea, CA
context_page_description: A straight-twin engine, also known as straight-two, inline-twin, vertical-twin, or parallel-twin is a two-cylinder piston engine which has its cylinders arranged side by side and its pistons connected to a common crankshaft. Compared to V-twins and flat-twins, straight-twins are more compact, simpler, and usually cheaper to make, but may generate more vibration during operation.
  Straight-twin engines have been primarily used in motorcycles, but are also used in automobiles and in powersports applications. Automobiles with straight-twin engines are usually very small and include city cars and kei cars. Recent examples of cars with straight-twin engines include the Tata Nano and Fiat Group automobiles using the TwinAir engine. Powersports applications include use in outboard motors, personal water craft, all-terrain vehicles, snowmobiles, and ultralight aircraft.
  Different crankshaft angles are used in four-stroke straight-twins to achieve different characteristics of firing intervals and engine balance, affecting vibrations and power delivery. The traditional British parallel twin had 360° crankshafts, while some larger Japanese twins of the 1960s adopted the 180° crankshaft.
context_section_description: The first production motorcycle using a straight-twin motor was the Hildebrand & Wolfmuller of 1894, which was also the first motorcycle to be serially produced. Its cylinders lay flat and forward-facing, its pistons connected directly to the rear wheel with a locomotive-style connecting rod.  Approximately 2000 were produced through 1897.
  The second production motorcycle to use a straight twin design, this time a 'parallel twin' with vertical cylinders akin to the later British type, was the Werner of 1903.  It used cast-iron cylinders with integral heads, and side valves, with a capacity of 500cc. In 1919 Louis Blériot ventured into motorcycle production with a 500cc vertical parallel twin which had 'unit' construction, i.e. the engine and gearbox were combined into a single engine casing.
  The most famous example of the straight-twin engine was produced by Triumph from 1937 onwards; the 5T Speed Twin.  Experiments with this engine type began as designer Edward Turner and his supervisor, Ariel chief engineer Val Page both saw potential in the straight-twin as a motorcycle engine. From the experiments, it was shown that a 360° crank angle was better suited to the use of a single carburettor than a 180° crank angle. After leaving Ariel for the Triumph Motor Company, Page designed the Triumph 6/1 with a 650 cc 360° twin as a sidecar hauler. A 6/1 hitched to a Triumph sidecar won an International Six Days Trial silver medal and the 1933 Maudes Trophy. The decline in the sidecar market caused the 6/1 to be discontinued in 1935.
  Edward Turner's 1937 Triumph Speed Twin started a trend, and up to the mid-1970s four-stroke 360° 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 of 2010, particularly for middleweight bikes.
  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 World Championship, the AJS E-90 Porcupine of 1949, which had nearly horizontal cylinders.
  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.
caption: Yamaha with its popular Two-stroke twin RD350
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