Great Bikes? The Ariel Square Four

Elsewhere you will have read about the Ariel motorcycle company and here I intend to talk more about their most famous design, the incredible Square Four.

- Harry Fisher

Introduced in 1931, the design was the work of Edward Turner, who would later become chairman of Triumph. Motorcycles had gripped him since his first ride on one, aged 14. In the mid-1920s, he had designed and made himself a single-cylinder, OHC engine. It featured gears to drive the camshafts and later a bevel-drive shaft to do the same job.

He had opened a motorcycle dealership in South London Chepstow Motors - and his search for a company to put his design into production led him to the English Midlands, the heart of the British Motorcycle Industry.

By this time, he had sketched out plans for a four-cylinder engine on the back of a cigarette packet, with the cylinders arranged in a square.

He later had proper drawings of the design made and touted them around various manufacturers. When BSA rejected the design, Jack Sangster of Ariel saw potential in both it and Turner himself. He was taken on as a forward design engineer and, alongside Bert Hopwood and working under Val Page, they got the Square Four into production in very short time.

Essentially, the engine was two transverse parallel twins, complete with two crankshafts, mounted one behind the other, the crankshafts being geared together, topped by a one-piece cylinder head.

But why bother with the complexity not only of a four-cylinder engine at a time when such machines were few and far between but a square four, with the attendant complexity of joining two engines together?