Ariel Square Four
Undoubtedly, the most famous bike produced under the Ariel marquee was the Square Four. It was produced from 1931 till 1958. Its powerplant, effectively a geared together set of parallel twins, was designed by Edward Turner shortly after the future Triumph boss joined Ariel in 1928. The four's engine size started off at 500cc and soon was increased to 600cc and then 997cc. Often called the "Squariel", the powerplant was supremely smooth but suffered from overheating of the rear cylinders. Although the biggest model was capable of more than 100mph (160kph) it's performance was severely handicapped by it's excessive weight. They commanded the same respect as a Vincent, with a lot of myths regarding their performance. One of the boasts was that they would accelerate from 30 to 100 mph in high gear.
After WWII the Square Four was updated dramatically. First with a lighter aluminum engine, then in 1954, with a new cylinder head and a breathtaking four-pipe exhaust system. By this time Ariel had added telescopic front forks and plunger rear suspension. Despite all these marked improvements, the heavy Four was still a cumbersome handler. Even in its final sophisticated guise, the engine was prone to overheating. But its redeeming factors of smoothness, comfort, and looks made the bike much treasured by those who could afford them.
Reception
When "The Motor Cycle" tested an example of the Ariel Square Four Mark II in April 1953 they found that the machine was capable of just shy of 100 mph describing it as being "a unique high-performance mount with gentle manners and outstanding acceleration" resulting from twenty years of development. The Square Four had been conceived by Edward Turner during his time at Selly Oak and was introduced as a 500cc, soon to be increased to 600cc over head cam design in 1931.
However the early engine suffered from a weak and insufficiently cooled cylinder head prone to blowing its head gasket. Edward Turner consequently developed a new 997cc over head valve engine retaining the name and layout, but little else for the 1936 season. The new engine, typed the 4G provided the basis for all Square Four production until 1948 when it was joined by the Mark II "alloy engine", which featured an alloy barrel and head, fitted with four separate exhaust pipes, in place of the iron units previously employed.
Equipped with telescopic forks and "Anstey" link rear suspension, the Square Four provided the discerning rider with a luxurious mount capable of covering long distances at high speed in comfort in either solo form, or, attached to a sidecar, surpassed only by the twins emanating from Stevenage.
1948
1954
1958
