Ariel Square Four

Edward Turner conceived the Square Four engine in 1928. At this time he was not looking for work, showing drawings of his engine design to motorcycle manufacturers. [http://www.mathewscollection.com/motorcycles/Motorcycle_Ariel.htm] MathewsCollection "1955 Ariel" (Retrieved 10 November 2006)] The engine was essentially a pair of 'across frame' OHC parallel twins joined by their geared central flywheels, [http://www.motorbike-search-engine.co.uk/classic_bikes/1932_ariel.html] MotorbikeSearchEngine "1932 Ariel" (Retrieved 10 November 2006)] with one four cylinder block (or Monobloc) and one head. [http://www.realclassic.co.uk/squarefour06010500.html] RealClassic "Square Four" (Retrieved 10 November 2006)] 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 [http://www.arielnorthamerica.org/sqfourhistory.htm] ArielNorthAmerica "Ariel History" (Retrieved 10 November 2006)] was shown at the Olympia Motorcycle Show in 1930 in chain driven overhead-camshaft 500 cc form. [http://www.practicalclassics.co.uk/auctionlot/by-id/812286515/] PracticalClassics "Auction Lot 812286515 Ariel Square Four" (Retrieved 10 November 2006)] 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. This model was used for the Maudes Trophy test, covering 700 miles in 700 minutes, followed by a timed lap of 87.4 mph. (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.) [ [http://www.thekettleclub.org.uk/maudes/maudes.asp] KettleClub "Maudes Trophy Run" (Retrieved 10 November 2006)]

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. 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 lb in weight. The 1949 machine weighed around 435 lb dry, produced 35 bhp at 5500 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 a separate barrels and a re-designed cylinder head, with four separate exhaust pipes. The Square Four was now a genuine 100 mph motorcycle.

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.

Figures

1958 4G Mk II Square Four Specifications

[ [http://www.is-it-a-lemon.com/modules.php?name=News&file=article&sid=712] Is-it-a-lemon "Ariel Square Four" (Retrieved 10 November 2006)]

quare Four Production

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.

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