Ariel Motorcycles Square Four 4h 1939
Edward Turner conceived the Square Four engine in 1928.
The essence of the concept appeared so quickly he was required to write it down on the back of a cigarette packet. The engine was essentially a pair of ‘across frame’ OHC parallel twins joined by their geared central flywheels with one four – bloc (or monobloc) and one head. The idea was rejected by BSA but adopted by Ariel, thus it became the ‘Ariel Square Four’.
The idea of mounting an inline four into a performance motorcycle has fundamental problems. Mount it longitudinally and the wheel base is too long, transversely and the bike is too fat; both outcomes would result in poor handling. So anyone attempting to deliver the qualities that only a four cylinder motor might deliver to a sports bike required a new way of thinking. Turner’s vision was to provide “a four cylinder engine small enough to use as a solo motorcycle, yet it has to produce ample power for high performance without undue compression, racing cams or big choke carburettors’’. So essentially Turner’s design philosophy foreshadowed decades of modern design. He stated that he merely sought to ‘’deliver ultimate reliability and performance with minimum maintenance needs’’.