1955 Ariel Square Four MKII

Four cylinder bikes are nothing new today, nor were they new back in the 1930s, but Ariel's take on the four banger bike was one of the most interesting.

Edward Turner, the man who later went on to design many Triumph classics, came up with the 'square' concept as an answer to the overheating problems caused by inline fours and the excessive width of transverse fours. Turner believed that mounting the cylinders in pairs, with one pair set behind the other, would be the solution, as well as making a bike that was easier to kick over and operate than a mammoth single or twin of similar capacity. Turner's original design was extremely light and featured unit construction, but the need to accommodate existing Ariel componentry (to reduce cost) meant the production version that debuted in 1931 was compromised.

That aside, the 500cc (and later 600cc) design still worked, but the main issue it was created to address - overheating - proved to be the Achilles Heel of early Square Fours. At consistent high speeds, the cast iron head would warp and blow head gaskets with disturbing regularity. Turner went back to the drawing board and released an all-new design, still using the square layout, that made its first appearance in 1936. At 997cc, this new pushrod and overhead valve design was much more powerful and reliable than its predecessor. The bigger capacity proved more popular, too, especially as a sidecar hauler.

By the time of the release of the 'Mark II' version in 1953, the Square Four had gained plunger-style Anstey link rear suspension, telescopic front forks and a number of other changes and refinements that improved the overall package, but added weight, too. The redesign of the Square Four engine started back in 1948, replacing the cast iron unit with an all-alloy powerplant which went some way to dropping the kilos that the Square Four had stacked on over the years. The 4-into-2 exhaust redesign followed in 1953 and is the most obvious visual distinction between Mark I and Mark II models. The redesign featured separately-cast exhaust manifolds which allowed air to pass over the exhaust ports to alleviate the cooling issues, while inlet porting was enlarged. To mark the engine redesign, a new Wedgwood Blue colour was offered for 1953, but was never as popular as Ariel's traditional maroon.

Following the release of the Mark II, there was little further development of the Square Four, although there were experiments with a swingarm rear end to replace the Anstey link rear (which needs regular lubrication) near the end of its production life. Production of the Square Four, along with all other Ariel four strokes, ceased in 1959.

Irrespective of whether you choose one with or without a sidecar (with which many Mark I and Mark II models seem to have been fitted), you've got a classic tourer in the form of the Square Four, but good luck finding one. Many went to the USA, where the Square Four fitted into the whole "bigger is better" ethic. The good news is that with their classic status well and truly established in this country, most of the examples that do appear for sale have been restored to varying degrees.

The Mark II didn't entirely eradicate the problems of overheating, so a Square Four still needs to be ridden with a degree of caution and an eye on the temperature, especially in traffic and slow speed commuting. Get it out on the highway, though, and the Square Four is in its element, able to cruise easily at 100kph, and reach a top speed of over 160kph. The "old technology" rear suspension can take a little getting used to, as it's reported to be skittish at high speed. Front brakes, despite being drums, work well.

Replacing the factory Solex carb with an Amal or SU appears to be a common upgrade, while some owners have gone with 12 volt electrics, too. Cartridge type oil filters and the addition of an oil cooler can address the overheating issues, but the key to keeping the Square Four running well is regular oil changes and diligent maintenance.

Specifications - 1955 Ariel Square Four Mark II
Engine: 995cc vertical OHV 4 cylinder 4 stroke
Bore/Stroke: 65 x 75 mm
Compression: 7.2:1
Power/Torque: 40hp@5600rpm / 40lb/ft@5600rpm
Fuel system: 26 mm Solex carb
Cooling system: Air
Electrical system: 6 volt
Transmission/Drive: 4 speed manual/chain drive
Front Suspension: Telescopic forks
Rear Suspension: Plunger compensated linkage system
Front Brake: 7" (178 mm) drum
Rear Brake: 7" (178 mm) drum
Front Tyre: 3.25 x 19"
Rear Tyre: 4.00" x 18"
Weight: 412lb wet (187 kg)
Seat Height: 28" (711 mm)
Length: 86" (2184 mm)
Ground Clearance: 5" (127 mm)
Fuel Capacity: 5 gal (22.7 lt)
Top Speed: 168 kph (solo) approx

TRIVIA
Both Mark I and Mark II Square Fours proved popular in the USA and many were raced. The power and speed saw a number of Police forces adopt the Square Four as their chase/patrol motorcycles too, including here in Australia, where a number of the last Mk I models were sent.

Early attempts to get more power out of the Square Four's engine included supercharging. While this achieved the desired result, adding forced induction usually added to the Square Four's overheating problems and considerably shortened engine life.
In the late 1960s, Tim and George Healey (no relation to Donald Healey of Austin-Healey fame), created the 'Healey 1000/4', a revised and modernised version of the Square Four, featuring a redesigned frame and swingarm, oil cooler and larger oil filter, as well as Laverda brakes & hubs.

Source: JUST BIKES, November 2010, Issue #257