Edward Turner was among great British motorcycle designers and engineers. He came on the scene in the 1930’s with some interesting parallel twins. He also conceived and designed the very refined Ariel Square Four in 1928.
With the exception of Pierce Fours, the Hendersons and the American Ace, then the Indian Fours, up until about 1970 the world was used to riding mostly singles and parallel or V-twins. But though it adds complexity and expense, adding a couple of cylinders can make for a well balanced, smooth running engine.
Edward Turner’s approach to a Four was two parallel twins in tandem, one behind the other. The two crankshafts were geared together and a chain drove the overhead camshaft, later Square Fours were pushrod. A manifold brought fuel to each of the four inlets from a single carburetor. Unique for British machines of the time, the cases were split horizontally. Earliest, 1931 and 1932 machines were overhead cam and 500 cc’s, but went to 600cc’s for 1932 to 1937. By 1937 a less complex pushrod overhead valve setup was used with displacement bumped to 600 cc’s to give a bit more power for a pulling a sidecar, then popular in England. By 1937, displacement was up to 997 cc’s and the smaller engines were no longer produced. The cast iron top end was replaced by an alloy cylinder and head with attractive polished fins. With its all alloy engine and four graceful exhaust headers the final Mark II Square Four was a very sleek machine.
But the engine was plagued with cooling problems even the all-alloy top end could not cure. High temperatures in the rear cylinders due to lack of air flow there would cause the head to warp and the head gasket to fail. Crankshafts and connecting rods did not hold up to even moderate abuse; high revs mixed with low oil levels often meant a rod went through the cases.
The Square Four chassis also evolved from rigid to plunger rear suspension so seating went from a sprung solo seat to a rigid mounted dual seat. More expensive than most other British bikes, though with quality design details, the Square Four was like the Vincent or Brough, made for gentlemen riders seeking something finer that stood out from the crowd.
For comparison, also on display are other Square Fours through 1958, the final year of production, plus a great range of British bikes from early in the 20th Century up though the 1970’s, stock, custom and competition. This fine 1947 Ariel Square Four is on loan to the National Motorcycle Museum from Theresa Norris.
- Engine: Air-Cooled Square Four, Four-Stroke
- Type: Overhead Valve, Two Valves per Cylinder
- Bore & Stroke: 65mm x 75mm
- Displacement: 997cc’s
- Compression Ratio: 7.2 – 1
- Ignition: Distributor, Points, Coil, 6V Battery
- Lubrication: Dry Sump
- Carburetor: Solex
- Starting: Kick
- Horsepower: 42HP
- Primary: Chain Driven
- Clutch: Wet, Multi-Plate
- Final Drive: Chain
- Transmission: 4-Speed Burman
- Frame: Semi-Cradle/Brazed Lug
- Suspension: Girder Fork / Plunger Rear
- Brakes: 7″ Drum Front / 8″ Drum Rear
- Wheelbase: 57 Inches
- Wheels / Tires: 3.25 x 19 / 4.00 x 18
- Weight: 460 Pounds
- Top Speed: 97 MPH