Ariel Square Four: history, specs, pictures

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1955 Ariel Square Four
1958 Ariel Square Four

The Ariel 4G MK II was a square four cylinder allround motorcycle produced between 1936 and 1967. It could reach a top speed of 99 mph (160 km/h). Claimed horsepower was 43.99 HP (32.8 KW) @ 5500 RPM.


Undoubtedly, the most famous bike produced under the Ariel marquee was the Square Four. It was produced from 1931 till 1958. Its power-plant, effectively a geared together set of parallel twins, was designed by Edward Turner shortly after the future Triumph boss joined Ariel in 1928. The four's engine size started off at 500cc and soon was increased to 600cc and then 997cc. Often called the "Squariel", the power-plant was supremely smooth but suffered from overheating of the rear cylinders. Although the biggest model was capable of more than 100mph (160kph) it's performance was severely handicapped by it's excessive weight. They commanded the same respect as a Vincent, with a lot of myths regarding their performance. One of the boasts was that they would accelerate from 30 to 100 mph in high gear.


After WWII the Square Four was updated dramatically. First with a lighter aluminum engine, then in 1954, with a new cylinder head and a breathtaking four-pipe exhaust system. By this time Ariel had added telescopic front forks and plunger rear suspension. Despite all these marked improvements, the heavy Four was still a cumbersome handler. Even in its final sophisticated guise, the engine was prone to overheating. But its redeeming factors of smoothness, comfort, and looks made the bike much treasured by those who could afford them.

Reception[edit]

When "The Motor Cycle" tested an example of the Ariel Square Four Mark II in April 1953 they found that the machine was capable of just shy of 100 mph describing it as being "a unique high-performance mount with gentle manners and outstanding acceleration" resulting from twenty years of development. The Square Four had been conceived by Edward Turner during his time at Selly Oak and was introduced as a 500cc, soon to be increased to 600cc over head cam design in 1931.

However the early engine suffered from a weak and insufficiently cooled cylinder head prone to blowing its head gasket. Edward Turner consequently developed a new 997cc over head valve engine retaining the name and layout, but little else for the 1936 season. The new engine, typed the 4G provided the basis for all Square Four production until 1948 when it was joined by the Mark II "alloy engine", which featured an alloy barrel and head, fitted with four separate exhaust pipes, in place of the iron units previously employed.

Equipped with telescopic forks and "Anstey" link rear suspension, the Square Four provided the discerning rider with a luxurious mount capable of covering long distances at high speed in comfort in either solo form, or, attached to a sidecar, surpassed only by the twins emanating from Stevenage.

Engine[edit]

The engine was a air cooled square four cylinder. A 65.0mm bore x 78.0mm stroke result in a displacement of just 1000.0 cubic centimeters. The engine featured a 6.0:1 compression ratio. Fuel was supplied via a overhead valves (ohv).

Drive[edit]

The bike has a 4-speed transmission. The final drive was via chain. Power was moderated via the dry, 3-plate.

Chassis[edit]

It came with a 3.25-19 front tire and a 4-18 rear tire. Stopping was achieved via expanding brake (drum brake) in the front and a expanding brake (drum brake) in the rear. The front suspension was a telescopic while the rear was equiped with a plungertype with cil-springs. The 4G MK II was fitted with a 6.08 Gallon (23.00 Liters) fuel tank. The bike weighed just 455.92 pounds (206.8 Kg). The wheelbase was 55.98 inches (1422 mm) long.



1948[edit]

1948 Ariel Square Four in Black
1948 Ariel Square Four in Black
1948 Ariel Square Four in Black
1948 Ariel Square Four in Black
1948 Ariel Square Four in Black
1948 Ariel Square Four in Black
1948 Ariel Square Four in Black
1948 Ariel Square Four in Black



1954[edit]

1954 Ariel Square Four in Black
1954 Ariel Square Four in Black
1954 Ariel Square Four in Black



1958[edit]

1958 Ariel Square Four in Red
1958 Ariel Square Four in Red
1958 Ariel Square Four in Red
1958 Ariel Square Four in Red
1958 Ariel Square Four in Red
1958 Ariel Square Four in Red
1958 Ariel Square Four in Red


4G Mk II=[edit]

1958 Ariel Square Four 4G Mk II with fully enclosed sidecar attached

In Media[edit]

Ariel Square Four 1000
Manufacturer Ariel
Also called 4G Square Four 1000, 4G Square Four Mark I, MK1 Square Four, 4G Square Four Mark II, MK2 Square Four, 4G MK II
Production 1936 - 1967
Class Road
Engine square four cylinder
Bore / Stroke 65.0mm x 78.0mm
Top speed 99 mph (160 km/h)
Horse Power 43.99 HP (32.8 KW) @ 5500RPM
Fuel system carburetor. su
Battery 6.0:1
Transmission Gear box: 4-speed

Final Drive: chain

Clutch: dry, 3-plate
Suspension Front: telescopic
Rear: plungertype with cil-springs
Brakes Front: expanding brake (drum brake)
Rear: expanding brake (drum brake)
Front Tire 3.25-19
Rear Tire 4-18
Wheelbase 55.98 inches (1422 mm)
Dimensions L 82.68 inches (2100 mm)
W 27.01 inches (686 mm)
Weight 455.92 pounds (206.8 Kg) (dry), 197.3 kg (wet)
Fuel capacity 6.08 Gallon (23.00 Liters)
Oil capacity 0.9 Gallon (3.40 Liters)
Fuel consumption 3.04 liters/100 km (32.9 km/l or 77.38 mpg)
Manuals Service Manual
Tech Specs · Brochures · Reviews · Ads · Videos

The Ariel 4G MK II was a square four cylinder Road motorcycle produced by Ariel between 1936 and 1967. It could reach a top speed of 99 mph (160 km/h). Claimed horsepower was 43.99 HP (32.8 KW) @ 5500 RPM.

Engine[edit]

The engine was a air cooled square four cylinder. A 65.0mm bore x 78.0mm stroke result in a displacement of just 1000.0 cubic centimeters. Fuel was supplied via a overhead valves (ohv).

Drive[edit]

The bike has a 4-speed transmission. Power was moderated via the dry, 3-plate.

Chassis[edit]

It came with a 3.25-19 front tire and a 4-18 rear tire. Stopping was achieved via expanding brake (drum brake) in the front and a expanding brake (drum brake) in the rear. The front suspension was a telescopic while the rear was equipped with a plungertype with cil-springs. The 4G MK II was fitted with a 6.08 Gallon (23.00 Liters) fuel tank. The bike weighed just 455.92 pounds (206.8 Kg). The wheelbase was 55.98 inches (1422 mm) long.

In Media[edit]