Ariel Square Four

The Ariel Square Four was produced for 25 years in a range of 500cc to 1000cc models. The Square Four had been designed by Edward Turner who drew his original idea for the motorcycle on the back of a cigarette packet. After taking it to several manufacturers, Ariel was willing to develop Turner’s design and put it into production. Turner was hoping to install a four-cylinder engine that was small enough for use in a solo motorcycle, but could produce enough power for really high performance. When introduced in 1931, the Square Four 500cc model was remarkably compact. The original OHC engine is similar to two parallel twins, which share a common crankcase with the two crankshafts geared together at the middle pinions. The overhead cam gear was chain driven and early versions of the Square Four used a hand-change,

prototypes of a Mk3 version in 1954, the model was never put into production. The Mk2 was the last Square Four version to be produced before finally being discontinued in 1959. At this time a new Square Four would have cost £335, but today, collectors and enthusiasts will offer anything up to £4,000 for an original Square Four model.

four-speed Burman gearbox. The Square Four grew to 600cc in 1932, giving the motorcycle more power – this increase was intended to be used for a sidecar tug. This Square Four version produced a very smooth ride but the engine proved difficult to tune when looking to improve its performance. By the late 1930s the 997cc Square Four was introduced with an OHV pushrod, all-iron engine, alongside a similarly engineered 600cc version. The front end still used girder forks with the rear adopting a Frank Anstey’s sprung rear suspension system. After World War II, the Square Four was fitted with an alloy engine, saving around 30lbs in weight. In 1949 the Square Four had a dry weight of around 435lbs and produced 35bhp at 5500rpm. In 1953 the most renowned four-piper Mk2 Square Four, still with Anstey link rear suspension was introduced. Although Ariel began building


The Ariel Square Four was one of the most charismatic British bikes ever built after World War II.

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