SQUARE FOUR MODELS (1931-1958)
Ariel ran into financial trouble in the late 1920's and closed shop for a short period while the founder's son, Jack Sangster, took over and restructured the company. With his life savings he bought all of the tools, re-hired the best of Ariel's staff, and then moved 500 yards down the road to a new factory. In 1928 Edward Turner proposed a very interesting new engine design, sketched on the back of a pack of Wild Woodbine cigarettes. He had shown it to the motorcycle factories to find a buyer for the innovative design, but only Sangster at Ariel was interested. He offered Turner space at the new Ariel factory as well as the services of a junior draftsman, Bert Hopwood. After 18 months of intense development (mid-1930) the Ariel Square Four was unveiled internally.
Edward Turner with a 1931 4F Early Years Square Four Engine Early Years Square Four Crankshafts
Four cylinder engines had been built in a variety of configurations for 25 years previously, and included the in-line four, the v-four, and the transverse four. Turner decided they were inefficient and too large. He proposed the Ariel Square Four, the only motorcycle to have four cylinders arranged vertically and equidistant to form a square. The front two cylinders drove one crankshaft, and the rear two drove another. A helically toothed flywheel gear was in between. When these gears meshed, the crankpins of one shaft were at top AND bottom dead centers while the crankpins of the other were at half- stroke. This resulted in perfect balance and no vibration at any speed, all in a very compact unit.
1931 4F 500 1932 4F 500 1932 Square Four 600
1932 Square Four
At its public unveiling at the Olympia Motorcycle Show in late-1930 the 1931 Ariel Square-Four (shown above) caused a huge sensation, even over the new Matchless Silver Hawk narrow-angle v-four OHC. The first Square Four production models were 497cc lightweight touring bikes but the public wanted more power so Ariel increased the capacity to 597cc in 1932 models (above right 3). Since the four air-cooled cylinders were set in a square formation (two 180º parallel twins placed back to back) and the OHV gear was operated by a chain-driven camshaft, the early Square Fours experienced inadequate cooling and, therefore, overheating.
But in 1931 Ariel won the much coveted Maudes Trophy, a test of durability, reliability, and ease of use. It also averaged 60mph for 700 miles! The famous Ariel 7 test involved seven different Ariel models attempting tests related to the number 7. The test for the Square Four involved seven schoolboys where each one kick-started the Square Four 7 times each. The Square Four started on 48 of 49 kicks.
Square Four “Seven Schoolboys” Kick-Start Test
In 1933 the speed potential of the Square Four was obvious but unproven until Ben Bickell rode a highly-modified 600 with a supercharger at the Brooklands track, lapping at over 110 mph and only narrowly missing becoming the first British 500cc to achieve 100 miles in an hour. The Model 4F Square Fours include the 500cc and 600cc engine layouts. They have overhead cam (OHC) engines although a small number of 4F's were manufactured in 1937 with 600cc overhead valve (OHV) engines. The OHC models are commonly referred to as "cammers" because of their camshaft and cylinder layout.
1933 Bickell Race Bike Ron Bickell in 1933 on an Ariel Square Four
In November 1935 the Square Four went through a complete redesign, which replaced the camshafts with pushrods and the crank and crankcase were completely changed, now producing 997cc. The new 4G reduced the 4F "cammy" tendency to overheat around the cylinder head.
1935 4F 600 1936 4F 600
1937 4G 1000 1938 4G 1000
To prove the new design & gain publicity, Edward Turner had Freddie Clarke ride a 1936 Square Four 1000cc to Brooklands track and ran "ten mph to a hundred in top". By 1937 the British rearmament program put people back to work and motorcycle sales skyrocketed and the Square Four became "monarch of the multi's".
1936 “10 mph to 100 in top” 1948 Mk I Mk I Engine
In 1949 the Mk I 1000cc was introduced (above), and the engine became an all alloy type (reduced weight by 33 lbs) with the overall handling and acceleration improved. This style was built through 1953.
1949 Mk I 1952 Square Four
The popular Mk II 1000cc was built from 1953 to 1958 and featured the 4-pipe exhaust system. Its revised cylinder & head layout featured separate cylinders vs. the Mk I monobloc casting to promote more efficient cooling.
1955 Mk II Square Four 1000 1957 Mk II Square Four 1000
Over its 27 years of production, the Square Four evolved through progressive advancements: a chain tensioner, swinging arm rockers versus pushrods, and a special ducting system to transfer cooling air to the rear cylinders. It was so well-known for its reliability that law enforcement agencies used it as their vehicle of choice.
Australian Police on Square Fours Canadian Police on Square Fours
From 1931 to 1958 Ariel produced 15,639 Square Four motorcycles. The breakdown by years, size and style is shown in the table at the right.