17th August 2020

Motorcycling Landmarks - The Ariel Square Four

Perhaps only in England, in the late 1920s, would a highly successful company entrust an unqualified amateur to design a radical new motorcycle as their flagship model. The amateur was a certain Edward Turner, later to become managing director of Triumph then chief executive of the entire BSA Group. The company was Ariel and the motorcycle in question was the Square Four.

Turner’s concept was not a vee or an in-line four but effectively a pair of parallel twins, with their crankshafts geared together, mounted in a common crankcase. Because the cranks were contra-rotating they eliminated much of the vibration inherent in a parallel twin, while the overall dimensions were extremely compact for such a seemingly complex engine.

After some redesign by the company’s chief designer, Val Page, to make the design sufficiently practical and economic for manufacture, the 500cc Square Four was launched in 1930 using the chassis from Ariel’s 500 Sloper single. By 1932, a 600cc version was also available but the big change came in 1937 when the engine grew to 1,000cc with 38bhp. Ariel proclaimed proudly that top gear was good from 10mph to 100mph and backed this up with a high speed demonstration at Brooklands, lapping the banking repeatedly at 99-100mph in very windy conditions.

During WWII, Ariel concentrated on single cylinder military bikes. When Square Four production resumed, the big news was the arrival of telescopic front forks, replacing the previous vintage-style girders. Combined with Ariel’s effective Anstey link rear suspension, the Square Four was now at the cutting edge for ride comfort and roadholding as well as top-gear performance.

In 1949 the huge one-piece cylinder head that covered all four cylinders was redesigned in aluminium, to give better cooling than its iron predecessor. At the same time, the iron block was replaced by aluminium with iron just used for the cylinder liners, these changes together saving 32lb in weight. One reason for the massive construction of the head was that it included an integrated inlet manifold for the single carb and integral exhaust manifolds on either side, allowing the use of just two exhaust pipes.

The two pipes became four when Ariel announced the Mark II model late in 1952. With less interference between the exhaust pulses, the engine breathed better and claimed power went up to as much as 42bhp with high compression pistons. The fuel tank was increased in size to 5 gallons on the Mk II and a dual seat was made available as an optional replacement for the traditional sprung saddle.

Only detail changes were made to the machine during the remainder of the 1950s and when manufacture finally ceased in 1959 the ‘Squariel’, as it was nicknamed, was beginning to look decidedly past its prime after almost 30 years in production. Despite this, many enthusiasts lamented its passing; little else on the market gave the same combination of effortless yet gentle power, combined with easy kick starting. It’s been called the ‘Gold Wing’ of its day but maybe the Honda is just the Square Four reinvented for a more modern age?