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A British made bike in production from 1931 to 1959, the Ariel Square Four started with a chain driven over head camshaft 497 cc engine. It has a top speed of 85 mph (137 km/h).
Like so many motorcycle makers, Ariel evolved from a bicycle business at the turn of the Century. Combining French engines with cycle-style chassis, Ariel's early specialty was the motor tricycle. The Square Four was designed by Edward Turner, who devised the Square Four engine in 1928. Looking for work, Turner showed drawings of his engine design to several motorcycle manufacturers. The engine design was a pair of across frame OHC parallel twins joined by geared central flywheels, with a four- cylinder block (Monobloc) and single head. The idea for the engine was rejected by BSA, but adopted by Ariel. Thus it became the Ariel Square Four 4F.
The first Ariel Square Four 4F was shown at the Olympia Motorcycle Show in 1930, in chain driven overhead-camshaft 500 cc form. Early Square Fours used a hand-change, four-speed Burman gearbox. In 1932, the cylinder bores were enlarged by 5 mm to give a capacity of 601 cc and the engine was redesigned with conventional pushrods and rocker arms. The gearbox was a built-in unit with the engine rather than mounted separately as was usual at the time. Because the four-cylinder engine had smaller, lighter flywheels than a single, it accelerated rapidly, giving a sporty feel. Later versions of this machine became the favored choice for sidecars, since the engine's ability to pull from lower rpm, was excellent.
Ariel only produced 927, 500cc Square Fours in 1931; however this increased to 2,674, 600cc Square Fours from 1932-1940.
Later on in its production run, the Ariel Four Square had increased engine capacities of 995cc from 1936-1949 and 997cc from 1953-1959. Ariel joined BSA in 1958 (forming BSA Ariel) and switched to making mass-market, two-stroke- powered bikes. During its entire production run from 1931 to 1959, a total of 15,639 Ariel Square Fours was made. Meanwhile, the evolving Square Four continued in production until 1959. BSA Ariel ended its enterprise in 1965.
Commercializing the four-cylinder engine was only partly successful, because the British industry remained craft-based, depending on experienced fitters rather than mass production. Only in 1969, with Honda's CB750, did the four- cylinder achieve mainstream success. Despite its limitations, the Square Four incorporates adventuresome features and remains an outstanding example of individual design.