A Look at the History of Ariel Square Four

1948 Ariel Square Four Mk 1

With Ian Fallon

Although four-cylinder motorcycles are commonplace these days, it was not always the case. Before World War II, four-legs in line, with engines running vertically, were considered unwieldy, and four-legs across the frame were too wide.

Then one day at the end of 1928, a London motorcycle dealer named Edward Turner sketched the idea of ​​two identical twins on the back of a cigarette pack.

Turner took a trip to the Midlands to try to sell the idea, and while the BSA nearly bought it, Ariel provided Turner with a small drawing office and resources to turn his idea into reality.

Turner continues to be arguably the most influential person in the British motorcycle industry, and his square bike, aka the Squaiel, became a legend during its lifetime.

The first production square four appeared in 1931 and was a cast-iron 500 with a single overhead camshaft driven by a chain. The basic layout of the cylinders at the corners of a square with two 180-degree crankshafts engaged and rotating in opposite directions would set the template for the future.

To make the engine more suitable for sidecar use, a 600 cc version was added in 1932, but there were many problems with the original design.

The early square quads needed no tuning, and even though Ben Bickell passed Brooklands at over 175 km/h, his turbocharged 500 kept blasting the cylinder head gaskets. and he never finished a race.

Similar problems occurred with Somerville Sykes in 1931 Senior TT. His blown 500 cc 4-cylinder made 40 hp but he retired with a blown cylinder head gasket.

To overcome these problems, the square four-cylinder engine was completely updated for 1936. Although the new engine retained the two-by-two cylinder format, in most other respects, it has been completely revised.

Instead of the previous overhead camshaft, the valves are now operated by push rods. The crankshafts are coupled by outboard gears on the left side and the large head bearings change from roller to white metal in light alloy connecting rods.

Capacity is increased to 1000 cc and weight is reduced. Apart from a sprung center that became an option in 1939, the square of four continued until the outbreak of war virtually unchanged.

Revived after World War II, the original square bike retains the 65 x 75 mm (997 cc) cast iron engine of the pre-war Squariel. But the addition of telescopic front forks and a compensating rear-link piston spring make the first post-war models now weigh more than 225 kg.

Ariel figured it was time to shed some weight and for the 1949 Mk 1 the old cast iron cylinder block and head were scrapped and replaced with alloy castings. The claimed weight savings is an optimistic “half a hundred pounds” (25 kg), but the actual weight saved is 15 kg.

The redesigned alloy cylinder head now includes 20 fixed points instead of 12 and incorporates shakers as well as intake and exhaust manifolds. Alloy wheels also improve cooling and with a compression ratio of 6:1, to deal with 72-octane “pool” gasoline and car-style two-way starter Solex carburetors, power is 34.5 hp at 5400 rpm.

Coil ignition, with an automotive-style distributor and a large 70 Watt separate generator replaced Lucas Magdyno. As on the pre-war examples, the lubricant was a dry sump and a four-speed Burman transmission.

The new engine also improves on Squaiel’s legendary character; its acceleration. The rotary torsion handle provides unusually refined throttle control, which combined with the relatively low flywheel weight and four-cylinder delivers instant response. The lighter engine also contributes to a slight improvement in handling and liveliness but the engine is always too large for the frame.

The Mk 1 frame is part rack and with an oil-damped telescopic front fork and with the option of a non-damped piston-link spring, the weight is 197 kg.

The rigid frame versions weigh 187 kg. Rolling on a 1422 mm wheelbase, the Mk 1 Squaiel is a surprisingly compact machine for its capacity.

The tires are also quite large for the day, measuring 3.25 x 19 inches up front and 4.00 x 18 inches behind. The four-tube Mark II Square-Four appeared in 1954, which lasted until 1958.

All later Square Fours had rear piston springs, although two prototype swingarm forks were built that did not enter production.

With a production run spanning 27 years, Ariel’s square-four has never been a mainstream motorcycle. Respected for its smoothness, comfort and acceleration, the Squaiel has always been expensive and seen as more of a status symbol than a regular means of transportation.

After production officially ended, George and Tim Healey began making their own parts and in 1973 released the square four Healey with a frame designed by Egli.

Only 20 Healeys were produced, the last in 1977. Ariel Square-Four engines were also manufactured under license in Canada and were used in pairs to power helicopters. In the end, a lack of development led to the square’s demise but it is now recognized as one of Britain’s finest classic motorcycles.