Carole Nash
Content Writer
Published: 19th December 2017

Some motorbikes are instant classics, while others take time to get going. The important thing is that they make a name for themselves, and that is the case for the Ariel Square Four. Created by legendary designer Edward Turner, the Square Four was a unique machine that came with a four-cylinder engine. Over a twenty-year period the Square Four was perfected until it reached its full potential. It’s one of the finest classic motorbikes of all time, and we’re looking into its history.



The story goes that Turner came up with the idea for the Square Four engine in 1928. He’d drawn it on the back of a cigarette pack and he was showing the design to various manufacturers. His concept involved two parallel twin-engines with their crankshafts geared together and turning in opposite directions. The design was well-balanced and compact.


BSA rejected Turner’s concept, but it was picked up by Ariel. The company launched the first 500 cc Ariel Square Four in 1930 and it appeared at the Olympia Motorcycle Show. Although Turner had intended his creation to be a fast, sporty bike, it didn’t have the power. Instead, it became known for its easy riding ability.


The early model had overheating problems and this was addressed in 1937 by Val Page. Not only did he add fins to the cylinder head, Page put a tunnel between the two front cylinders to direct cooling air to the back of the engine. The change came with the introduction of 600 cc version. The new engine were more powerful, making the Square Four better suited to pulling a sidecar. The 600 cc model was used for the Maudes Trophy Test and it covered 700 miles in 700 minutes.


Post WW2

Production stopped when World War Two broke out, but started up again soon after with the 1000 cc model. In 1949, the Square Four received an update, with the cast-iron cylinder head and barrel being replaced by an alloy head and barrel. This lightened the vehicle by 14 kg. The reduced weight helped to make the new Square Four faster, helping it exceed 90 mph.


1953 saw the introduction of the Square Four Mark II. It featured a high-mounted carburettor, rear-mounted distributor and redesigned frame. Ariel were finally able to say they’d produced a motorbike that could reach 100 mph.


Prototypes for an MK3 started in 1954, but the model never went into production. The Square Four stopped being made in 1959, but it remains a marvel of British engineering.


What do you think of the Ariel Square Four?