Last updated on November 26, 2025
Today’s feature isn’t a car at all but one of the most fascinating British motorcycles ever built, the 1958 Ariel Square Four MK II owned by Fred Eldridge. I photographed it at the Boots & Bonnets All British Car Show hosted by the Southern California MG Club held at the Autry Museum in Griffith Park, located in Los Angeles, CA.
It is finished in black with gold pinstriping and a black seat. Power comes from a 42 hp, 997 cc, OHV four-cylinder “square” engine with SU carburetor, mated to a 4-speed manual Burman gearbox with a final drive ratio of 4.89:1. Riders used to brag that the 4-speed was hardly necessary, or should have been an option, since the bike could cruise comfortably at 70 mph in third gear. Slowing this beauty down are cable-operated drum brakes, while the front suspension is an Ariel-designed telescopic hydraulic fork and the rear uses a swingarm with twin Girling shock absorbers.
The Square Four was the brainchild of Edward Turner, the young engineer who would later become famous for his work at Triumph. First shown in 1930 and entering production in 1931, the Ariel Square Four was unlike anything else on the road. Turner designed a compact four-cylinder engine by placing two parallel twins side by side in a “square” configuration within a single crankcase. This unusual layout kept the engine relatively narrow and lightweight, while offering the smoothness of a four-cylinder.
Production of the Ariel Square Four spanned from 1931 to 1959, with roughly 15,000 units built across all versions. The earliest model, the 4F, displaced 498 cc and featured an overhead camshaft. In 1937 Ariel enlarged the engine to 601 cc with the 4G, partly to offset the heavier materials then being used due to the war effort. After World War II, displacement grew again, this time to 997 cc in the 1949 Mk I. In 1953 the Mk II arrived, retaining the 997 cc capacity but adding a revised alloy cylinder head and an overhead-valve layout. Output rose to about 42 horsepower, giving the Square Four a genuine 100 mph top speed and placing it among the fastest touring motorcycles of its era. Though never a mass-market machine, it earned a loyal following for its refinement, long-distance comfort, and distinctive engineering. The exact number of surviving examples, including those still regularly ridden remains unknown.
Today the Ariel Square Four is prized for its unique engine design, Edward Turner’s engineering pedigree, and its relative rarity. Well-sorted examples are admired not just as display pieces but also for their surprisingly relaxed and capable road manners. For vintage motorcycle enthusiasts, the Square Four stands as one of the most distinctive British bikes of its era.
The Ariel Square Four captures the look, sound, and feel of a truly unique British motorcycle. It still draws attention at British and European car and bike shows, Cars & Coffee events, and delivers real riding enjoyment every time it’s taken out on the road, especially on those coastal runs up PCH with a stop at Neptune’s Net for some nourishment with like minded riders.
Thanks for coming along for the ride.
Frank
