Ariel Square Four

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Edward Turner conceived the Square Four engine in 1928. At this time he was looking for work, showing drawings of his engine design to motorcycle manufacturers. ^[1] The engine was essentially a pair of 'across frame' OHC parallel twins joined by their geared central flywheels, ^[2] with one fourcylinder block (or Monobloc) and one head. ^[3] The idea for the engine was rejected by BSA, but adopted by Ariel. Thus it became the Ariel Square Four.

Contents 1 Ariel Square Four 4F (1931 - 1936) 2 Ariel Square Four 4G (1936 - 1949) 3 Ariel Square Four Mark I (1949 - 1953) 4 Ariel Square Four Mark II (1953 - 1959) 5 Figures 5.1 1955 Square Four Specifications 5.2 Square Four Production 5.3 Ariel Square Four specifications (1958) 5.4 Pre-War Ariel Square Four Model Designations 6 Sources

[edit] Ariel Square Four 4F (1931 - 1936)

The first Ariel Square Four 4F ^[4] was shown at the Olympia Motorcycle Show in 1930 in chain driven overhead-camshaft 500 cc form. ^[5] Early Square Fours used a hand-change, four-speed Burman gearbox. ^[3]

In 1932 the cylinder bores were enlarged by 5 mm to give a capacity of 601 cc. This model was used for the Maudes Trophy test, covering 700 miles in 700 minutes, followed by a timed lap of 87.4 mph. ^[2] (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 motor cycles, known as the Maudes Trophy.)^[6]

[edit] Ariel Square Four 4G (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. ^[1] In 1939 Ariel's patented Anstey-link plunger rear suspension became an option. ^[5]

In 1946 the plunger rear was available again, and oil damped telescopic front forks replaced the previous girder type. ^[5]

[edit] 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. ^[5] This saved about 30 lb in weight. The 1949 machine weighed around 435 lb dry, produced 35 bhp at 5500 rpm. ^[3] The Mark I was capable of 90 mph-plus.

[edit] Ariel Square Four Mark II (1953 - 1959)

In 1953, the ‘four pipe’ 997 cc Ariel Square Four Mk II was released, with a separate barrels and a re-designed cylinder head, with four separate exhaust pipes. ^[4] The Square Four was now a genuine 100 mph motorcycle. ^[5]

In 1954 Ariel built prototypes of a Mk3 with Earles forks, but the model was never put into production. ^[3]

In 1959 Square Four production, and that of all other Ariel four-stroke models, ceased. ^[5]

[edit] Figures

[edit] 1955 Square Four Specifications

• Make : Ariel
• Model : 4G MK II
• Year : 1955
• Engine : Square Four
• Capacity : 997 cc
• Power : 42 bhp
• Transmission : 4 speed
• Brakes : Drum
• ength : 82"
• Width : 32"
• Height : 44"
• Weight : 460 lb

^[1]

[edit] Square Four Production

Model	Years Produced	Production
4F-500	1931-32	927
4F-600	1932-40	2674
4G-1000	1936-48	4288
Mk I	1949-53	3922
Mk II	1953-58	3828
All Models	1931-58	15,641

^[4]

[edit] Ariel Square Four specifications (1958)

• Engine: Aircooled 8-valve OHV pushrod square four
• Capacity: 997 cc (65 x 75 mm)
• Power: 45 bhp @ 5500 rpm Weight: 211 kg(465 lb)
• Top speed: 105 mph (168 km/h) ^[7]

[edit] Pre-War Ariel Square Four Model Designations

• 1931-1932: 498 cc 4F/31 Square Four.
• 1932: 498 cc 4F Square Four.
• 1932-1936: 601 cc 4F/600 Square Four.
• 1937-1948: 995 cc 4G Square Four.
• 1939: 599 cc 4F/600 Square Four.

[edit] Sources