The Ariel Square Four stands alongside the Brough Superior and Vincent HRD as one of the most distinguished large British motorcycles. It was built for long-range touring – a task, with or without sidecar, that it undertook with great success.
The Square Four was the brainchild of Edward Turner (1901-1973), later famous for his creation of the 1937 Triumph Speed Twin motorcycle engine and bike, and the 1959 Daimler ‘Hemi’ V8 car engines. He produced his first motorcycle in 1927, the “Turner Special”, with an OHC 350cc single-cylinder engine and a Sturmey-Archer three-speed gearbox. He also liked parallel twins (as his Triumph Speed Twin, Thunderbird and Bonneville models would later show), and in 1928 had the idea of putting two parallel twins together to make a square four-cylinder engine.
This apparent complexity was based on sound logic. Two parallel twin engines would have two crankshafts, turning in opposite directions and thus cancelling out each other’s gyroscopic effect. Diagonally opposite pistons could also be in top dead centre and bottom dead centre, to balance out each other’s motion. The result would be a smooth, perfectly balanced engine.
Turner was by now managing Chepstow Motors in Peckham Road, London. He offered his new square four, complete with a chain-driven overhead camshaft, to various manufacturers. He was rejected by BSA, but in 1929 Ariel were sufficiently captivated to hire Turner as an engineer. His square four, in unit with a three-speed gearbox driven from the rear camshaft, was compact enough to fit into an existing Ariel 250cc single-cylinder Colt frame and made prototype development easy and rapid. But the unitary gearbox and engine cooling fins were considered too expensive, and a cheaper construction was used in the first production model, the 498cc 4F.31, introduced at the 1930 Olympia Motorcycle Show. More power was eventually found to be needed, so from 1932 a 601cc version was also offered. The example in our Snapshot dates from around 1934 and could therefore be of either capacity.
The Ariel Square Four, despite being designed as a touring bike, had success in competition and endurance tests, and in 1933 Ben Bickell took his supercharged modified Ariel Square Four to Brooklands, set a lap speed of 110 mph, and only just failed to make it the first British 500cc motorcycle to achieve one hundred miles in one hour.
The OHC engine in the 4F acquired a reputation for overheating at the rear of the cylinder head, so in 1936 Turner, now General Manager and Chief Designer of Ariel, tasked the engineer Val Page, formerly the designer of the J.A. Prestwich V-twin engine of the Brough Superior SS100, with addressing the overheating problem. Page provided a cooling air channel between the front cylinders, added fins to the cylinder head and moved from a chain-driven driven overhead camshaft to an overhead valve system.
For 1937 both the 601cc 4F and the new 998cc 4G models were made to the revised Val Page OHV design. Both models were made with girder forks and a rigid frame. In 1938 Ariel dropped the 4F and listed two versions of the 998cc bike, a “De Luxe” model 4G, and a “Standard” model 4H. Then in 1939 the 4F returned alongside the 4G and 4H. During World War II Ariel manufactured simple single-cylinder military motorcycles but returned to the Square Four after the war. It remained on the market in various versions until 1959.