Which motorcycle company produced the Ariel Square Four?

Ariel Motorcycles produced the Ariel Square Four.

Edward Turner designed the original Square Four engine while working at Ariel. The prototype appeared in the late 1920s, and Ariel introduced the production motorcycle in 1931. Its engine used four cylinders arranged in a square, with two crank throws and a compact layout that resembled two parallel twins joined together.

The model was built in several displacements, including 500 cc and later 600 cc and 997 cc versions. The design provided smooth power for touring while remaining more compact than many conventional inline-four arrangements.

The name refers to the engine’s cylinder arrangement, not to the motorcycle’s frame shape. Ariel was an established British manufacturer before and after the Square Four, and the model should not be attributed to BSA or Norton, despite those companies’ prominent roles in British motorcycle history. Production ended in the early 1950s.