Ariel Square Four Motorcycles

The Square Four is a motorcycle built by Ariel and designed by Edward Turner, who devised the Square Four engine in 1928. At this time he was looking for work, showing drawings of his engine design to motorcycle manufacturers.
The engine was essentially a pair of 'across frame' OHC parallel twins joined by their geared central flywheels, with a four-cylinder block (or Monobloc) and single head. The idea for the engine was rejected by BSA, but adopted by Ariel. Thus it became the Ariel Square Four.


Ariel Square Four 4F (1931–1936)
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, specifically to accommodate owners who wanted a sidecar.
This model was used for the Maudes Trophy test, covering 700 miles (1,127 km) in 700 minutes, followed by a timed lap of 87.4 mph (140.7 km/h). (In 1923 a Mr George Pettyt, of Maudes Motor Mart, had donated a "challenge trophy" for the ACU to award each year for the most meritorious, observed endurance test for motorcycles, known as the Maudes Trophy.)
Ariel Square Four 4G 1938 (1936–1949)
The “Cammy” engine gained a reputation for overheating the rear cylinder heads, so in 1936 the engine was completely redesigned, emerging as the 1937 OHV 995 cc model 4G. In 1939 Ariel's patented Anstey-link plunger rear suspension became an option. In 1946, the plunger rear was available again, and oil damped telescopic front forks replaced the previous girder type.
Ariel Square Four Mark I (1949–1953)
In 1949, the Ariel Square Four Mark I saw the cast-iron cylinder head and barrel replaced by alloy head and barrel. This saved about 30 pounds (14 kg) in weight. The 1949 machine weighed around 435 lb (197 kg) dry, produced 35 bhp (26 kW) at 5,500 rpm. The Mark I was capable of 90 mph-plus.
Ariel Square Four Mark II (1953–1959)
In 1953, the ‘four pipe’ 997 cc Ariel Square Four Mk II was released, with separate barrels and a re-designed cylinder head, with four separate exhaust pipes. The Square Four was now a genuine 100 mph (160 km/h) motorcycle, due to 40 hp (30 kW), and weighing 425 lb (193 kg) and costing £336.16.6.
In 1954, Ariel built prototypes of a Mk3 with Earles forks, but the model was never put into production.
In 1959, Square Four production, and that of all other Ariel four-stroke models, ceased.
Ariel Square Four All Production Quantity

Model	Years produced	Production
4F-500	1931–1932	927
4F-600	1932–1940	2,674
4G-1000	1936–1948	4,288
Mk I	1949–1953	3,922
Mk II	1953–1958	3,828
All Models	1931–1958	15,639