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1957 Ariel Square Four Mark 4 Prototype1957 Ariel Square Four Mark 4 Prototype

Motorcyclists familiar with the 'Squariel' may be surprised to see a model fitted with swinging arm rear suspension. The machine was to have been the Mark 4 but it never came to the market, and only one prototype was ever built. This is it!

The first of the long line of Square Fours was shown to the public at the Olympia Show towards the end of 1930 and featured an engine with a chain-driven overhead camshaft and overhung cranks. As originally designed, the crankcase was split horizontally and the main bearings were clamped to the top half. Development over the succeeding years saw the introduction of 600cc ohc abd pushrod engines, and finally in 1937, the 995cc engine (an all cast iron unit). This remained in production until 1948, being succeeded by the Mark 1 and later still, in 1953, by the Mark 2, both of these being all-alloy with pressed-in iron cylinder liners.

In 1939, the Ariel spring frame, designed by Frank Anstey was introduced. It was undamped, very prone to wear and in fact only provided a constant chain tension over a limited arc of wheel movement. It lasted far longer than it deserved to, being still fitted to the last of the production Square Fours in 1959. The Earles forked Mark 3 did not reach production, and the Mark 4 was not to be very different from the Mark 2, except in the matter of suspension. On all other Ariel models except the Four, swinging arm suspension had been available since 1953, and experiments were in hand with both spring frame and trailing link front forks in acticipation of the Mark 4. However, Ariel had been under the control of BSA for some years now and a main board instruction cancelled the development work. A few years later another main board order ceased the production of all four-stroke Ariels and the Square Four story came to an end.


  • Engine - Ariel Square Four Mark 3 engine - 'the Four Piper'. All light-alloy construction with iron bars. 995cc capacity giving 40 bhp @ 5600 rpm.
  • Transmission - Ferodo lined clutch. Burman 4-speed foot-change gearbox.
  • Suspension - Swinging arm rear with combined spring/damper units. Late type Ariel front forks with cowled headlamp.
  • Manufacturer - Ariel Motors Ltd. Selly Oak, Birmingham.