Ariel Square Four

In the days before Italy, and then Japan proved otherwise, mounting an in-line four cylinder engine in anything but a touring motorcycle posed an insurmountable problem: mount it longitudinally, and the bike was too long; transversely, and it was unacceptably wide. So any four with sporting pretensions required a different layout.

This was the thinking of a young engineer in a small machine shop in Dulwich, South London. His response? A 497cc overhead cam square four. His name? Edward Turner, later to find fame as the designer of Triumph’s immortal Speed Twin.

Turner hawked his engine around several manufacturers until Jack Sangster of Ariel decided to give it a chance. T he resulting prototype, housed (with room to spare) in a 500 ‘sloper’ frame, was the sensation of the Olympia motor cycle show. T he year was just 1930 .







top Speed

Years in Production

air-cooled 997cc OHV square-four

42 bhp @ 5800 rpm


tubular single loop

drum / drum

100 mph

1931 – 1959

The 497cc engine, essentially two vertical twins on a common crankcase , comprised paired transverse crankshafts, geared together at their centres. A chain on the right side of the engine drove both a magdyno and the single overhead camshaft. T he wet-sump crankcases split horizontally. It was an astonishingly light and compact design which delivered most of what Turner had se t out to achieve. The whole machine weighed only 330lbs.

Throughout its life, however, the ‘Squariel’ had one major weakness. The cylinder head was prone to distortion, as the rear cylinders sat in the heat shadow of the front ones, and the provision of cooling air around the head was always marginal. On early examples this was exacerbated by inadequate lubrication. Early attempts to race the four, albeit in supercharged form, were plagued by problems of heads warping. Although a normally¬≠ aspirated version managed to take the coveted Maudes Trophy by covering 700 miles in 668 minutes, overheating and the inefficient ‘cruciform’ inlet tract would always impose a limit on the Squariel’s performance.

When the machine went on sale it cost £70. A year later, a bored 597cc version was displayed at Olympia, but shortly afterwards the Depression hit the British economy. In 1937 a less ambitious range of fours was launched. Available in both 597 (the 4F) and 997cc (4G) form, these featured push-rod valve actuation, vertically-split crankcases and, partly to suit sidecar use, very much heavier flywheels. The 4G produced 38bhp at 5500rpm. In 1939, a clever form of plunger rear suspension was added.

After the war the 600cc ‘Squariel’ was dropped, and by 1948 the 1000 had developed telescopic forks. Thanks largely to a new light alloy cylinder head and block, weigh t was down by some 33lb, but this was still a 500lb machine – and it still ran very hot.

ariel square four

1954 brought the final Square Four, the MkII ‘four-piper’ . Power had risen to 42bhp, but plunger rear suspension was retained in an age when swinging forks were becoming commonplace on machines of this price. In 1958, the last Squariel rolled off the Selly Oak production line. To remain competitive the four would have needed expensive and radical surgery, and the money was needed for a new generation of mould-breaking Ariels, the Leader range of two-stroke twins.

Ariel’s Square Four, like the Sunbeam twin produced down the road at Redditch, was another case of ‘Nice try, but no cigar’. Both were clever and advanced designs, yet both were fatally flawed. Of the two, perhaps the Ariel best deserves the epithet ‘ superbike’. It was smooth , effortless and, above all, imposing. It attracted a certain mystique (so much so that a much revised Square Four , the Healy 1000, was produced in limited number s in the mid-seventies). Yet in the end, far from liberating
British motorcycle manufacturers from the yoke of the vertical twin, it probably deterred them from departing from what they knew best. It is surely the ultimate irony that one man, Edward Turner, was responsible for both.

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