Ariel Square Four
Ariel Square Four Background & History
The story of the Ariel Square Four is as much a story about its designer, Edward Turner. Turner would go on to achieve fame and fortune in the British motorcycle industry with his game-changing designs. He designed not only Ariel’s most famous bike, the Square Four, he went on to design one of, if not the most influential motorcycle engine designs of all time, the vertical twin. It was introduced to the world as the 1938 Triumph Speed Twin, but that’s another story. Turner’s stint at Ariel occurred before his work at Triumph.
This 600cc 1932 Ariel Square Four shows how complex the OHC engine was, and how hard it was to move enough air through it to keep it cool. Note that each side of the engine had two cylinders sharing one exhaust pipe. Also notice how the carburetor is on the front of the engine.
Ariel Square Four Cancels Vibration
In around 1927 Turner conceived an idea to deal with the biggest problem then facing the British motorcycle industry: engine vibration. Many things had been tried, but nothing seemed to help. Turner’s brainstorm was to mate two twin cylinder engines into one 4-cylinder machine in a square pattern, linking the two crankshafts such that they rotated in opposite directions, and timed the rising and falling of all those pistons to cancel each other out, vibration-wise. And it worked. While heavy and complicated, and not very fast, the Ariel Square Four was above all smooth, smoother than any other heavyweight motorcycle at the time.
The 1932 Ariel Square Four (this one with 600cc) used a rigid frame and a girder front end. Don't you just love that 'fishtail exhaust?
Ariel Square Four Engine Design
Turner shopped his design around, BSA turned him down, but Ariel listened. In fact, they hired him in 1929 in their design department, under the skilled tutelage of genius motorcycle designer Val Page. Work was begun in earnest on the new Ariel Square Four. Turners initial design displaced 500cc and used a chain-driven single overhead cam (SOHC) running all those valves up top. The total package, which included a 3-speed gearbox bolted directly to the engine (an early example of unit-construction, or at least semi-unit) was so compact that it fit into the frame of a 250cc Ariel Colt, which was a lightweight single-cylinder bike. The gearbox was driven off the rear crankshaft.
1932 Ariel Square Four in 600cc.
Ariel Square Four has Cooling Problems
Then the cost-cutters came in (aka: ‘Bean Counters’) and nixed the unitary construction. They also reduced the number and size of the cooling fins as a cost-cutting measure. This is a move that would come back to haunt them later. Ariel Square Fours, of every vintage, have always suffered from cooling problems, or perhaps they should be called ‘overheating problems’. The rearmost cylinders didn’t get nearly the airflow as the fronts and so always ran hot. Early OHC Ariel Square Fours used a cast iron cylinder block and one-piece head which made things worse. Later 1000cc OHV Ariel Square Fours used all alloy top ends, and it helped, but never truly solved the problem entirely.
This 1931 Ariel SF31 shows how similar it is, other than engine, to the Ariel Square Four. It ought to, they share frames and running gear.
Launch of the Ariel Square Four
The first Ariel Square Four was introduced as the Model 4F and was first revealed to the public at the Olympia Motorcycle Show in 1930. It debuted with a 4-speed Burman gearbox with a foot-operated clutch and a hand shifter mounted on the right side of the tank. By the time it reached production, the 1931 Ariel Square Four used a hand clutch and foot shifting. The whole thing ran on a rigid frame sourced from the Ariel SF31 499cc Sloper, with a girder front fork. In fact, the Ariel Square Four shared almost everything with the SF31, including running gear and bodywork. Both were 500cc bikes, one a single and one a 4-cylinder. Done purely for cost-cutting purposes, the matchup worked great.
Ariel Square Four Accomplishments
In fact, the new Ariel Square Four racked up quite a trail of successes:
- 7-hour endurance run at Brooklands;
- Consumption test: 700 miles on 7 shillings worth of petrol and oil;
- Head removed and decarbonized in 4 min 19 secs using only the bike’s toolkit;
- One-hour speed run at Brooklands: more than 80 miles covered;
- Run for 70 minutes in each of four gears on ordinary roads;
- Seven non-stop ascents and descents of seven of England’s most famous test hills;
- Traveled 700 miles in less than 670 minutes
In 1937 Ariel Square Four was a completely new engine. Gone was the OHC in favor of a simpler more robust OHV setup. Available in 600 and 1000cc, these early OHV Square Fours had only one pipe per side and so were called "two-pipers".
Ariel Square Four can be Started by a Schoolboy
All of these accomplishments were impressive in 1931 when reliability and durability were always in question. Another clever publicity stunt was to bring a new Ariel Square Four to a schoolyard where seven schoolboys would be invited to kickstart the bike seven times each. The test was called the "Ariel Seven" and the result was 48 starts out of 49 attempts, proving that if a schoolboy could do it, the Ariel Square Four was surely an easy bike to start. And in the days before motorcycles with electric starters, this was very important to buyer, especially on a bike this big. No one wanted to endure the embarrassment of kicking and kicking their bike and it still won’t start. Around this same time in 1933 Ben Bickell set a lap speed record at Brooklands of 110 mph on a supercharge Ariel Square Four, becoming the first 500cc British motorcycle to do so.
The 'Ariel Seven', seven schoolboys who each kicked the Ariel Square Four seven times. It started 48 out of 49 times. Not bad.
These early OHC engines, nicknamed “Cammies” performed well enough, despite constant overheating problems. More power was needed for sidecar duty, a big market in England in the 1930s. In 1932, the bores were increased by 5mm boosting displacement from 498cc to 601cc. Sidecar work was a good fit for the Ariel Square Four. By 1933, the 500 was phased out. It was never a fast bike in stock form, but what it offered was a smooth, torquey power delivery offering endless miles of easy riding.
This 1956 Ariel Square Four shows what a gorgeous bike they could be. It also shows the new alloy cylinder dead with two pipes on each side, hence the nickname "four piper".
All New Ariel Square Four
Edward Turner was promoted to General Manager and Chief Designer at Ariel Motorcycles in 1936 and he threw all his weight at solving the overheating problems facing the Ariel Square Four. He put his best man on it, genius designer Val Page. Rather than retrofit the existing design, Page completely redesigned the engine top-to-bottom. Gone was the overhead cam in favor of a far simpler cam-in-block between the two cranks, with pushrods and rocker arms actuating the valves. The cylinder and head now had an air channel between the front cylinders bringing more cool air to the backs. He made generous use of cooling fins including on the cylinder head. The new and improved Ariel Square Four came in two sizes, the 600cc 4F and the 1000cc 4G in 1937. By 1938, they’d dropped the 600 and now offered two trim levels for the 1000:The “De Luxe” model 4G, and the “Standard” model 4H. Both were mechanically identical, in fact differences were few: full-valanced fenders and a side stand on the De Luxe, and standard fenders and no side stand on the Standard. However, World War 2 changed all of that and in 1939, with petrol becoming in short supply, it was deemed prudent to return the 600cc 4F to production. In 1939, Ariel made the Anstey-link plunger rear suspension available as an option on the Square Four.
World War 2 ended all civilian production in 1940, but luckily for Ariel they spent the war building simple, tough, reliable single-cylinder motorcycles for the military. At war’s end, civilian production quickly ramped up again and Ariel resurrected the 1939 Square Four at their 1945 model with virtually no changes. By 1946 however, Ariel had made the plunger rear suspension standard on all models along with telescopic forks in the place of the ancient girders.
This is a great angle shot of the massive engine in
Ariel Square Four Mk I
The Ariel Square Four Mk I debuted in 1949 with an all aluminum top end (cylinder block and head), saving some 30 lbs of weight but more importantly vastly improving the cooling because of aluminum’s far better heat dissipation over iron. The Lucan magdyno was swapped out for an automotive-style distributor-and-coil ignition system along with a 70-watt dynamo/generator for lights. The Ariel Square Four Mk I weighed in at 435 lbs and made 35 hp at 5,500 rpm. It was capable of a top speed of 90 mph, and would cruise at 70 all day long, which is what it was designed to do.
Ariel Square Four Mk II
In 1953 Ariel introduced the Square Four Mk II with significant changes made to the top end to hopefully solve the cooling problems once and for all. The front cylinders not only block cooling air to the rears, but they also preheat the air that finally reaches the back cylinders. It was an unfair arrangement and the back cylinders didn’t like it. V-twins from Harley-Davidson, Indian and <A href='vincent-motorcycles.html'>Vincent</a> all suffered the same challenges, but it was much worse on the Square Four. The cylinder barrels were separated into four pieces that didn’t touch. The head was completely redesigned, now with exhaust manifolds with two pipes each, designed to get the heat out and away from the engine ASAP. The intake manifold and rocker cover were combined and the carburetor used was now an SU, which was taller than the Amal and so required a frame modification. During 1953, both Mk I and Mk II Ariel Square Fours were sold side-by-side at dealerships, but by 1954 only the Mk II was offered. Changes, improvements and updates continued to be made year-by-year as the Ariel Square Four matured. 1956 saw a bump in horsepower to 40, allowing for a 100 mph top speed. The Mk II remained in production until 1959.
Ariel Square Four Books
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"Ariel Square Four Super Profile" Reprints of motorcycle magazine articles, road tests, new product releases, tech info & more. |
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