Ariel Square Four

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The Ariel Square Four, affectionately referred to by aficionados as the "Squariel", was created in 1928. The designer was Edward Turner, one of Britain's greatest motorcycle engineers and stylists.

It was a solitary project that began while Turner was operating from Chepstow Motors in Peckham, South London.

Chepstow Motors was a Velocette dealership that Edward Turner had bought soon after leaving the merchant navy. He was a struggling designer with a head full of ideas and a portfolio of sketches and drawings, some conventional, and some radical. What he needed was the right company with vision, ambition and the industrial expertise to campaign his novel ideas and take them to an already crowded market.

That company, following a rejection by BSA, was Ariel Motorcycles of Selly Oak, Birmingham which was then owned by Jack Sangster (1896 - 1977), aka "Mr Jack". Four years later, Sangster would acquire, and rescue, Triumph.

Square Four designer

Edward Turner (image immediately above) is perhaps more famous for bringing the world the seminal 500cc Triumph 5T Speed Twin. But today, it's hard to decide which is actually the greater motorcycle.

On the one hand, the Speed Twin has certainly enjoyed a more profound legacy inasmuch as the basic design of the 5T lasted into the 1960s and beyond. In fact, there's no fundamental difference between the original 1937 pre-unit construction 500cc pushrod Triumph twin as revealed at the Olympia Show in Kensington, London, and the final (1983 -1988) unit-construction 750cc T140 Bonneville twins as produced by the Meriden Workers Cooperative, or those built by Les Harris in Newton Abbott, Devon.

Additionally, the Speed Twin's parallel twin architecture inspired numerous imitators, not least from the likes of AMC, BSA and Norton, and the Triumph twin sold not merely in the thousands but tens of thousands.

But on the other hand, the 500cc Ariel Square Four unquestionably delivered more impact when it debuted in 1930 (also at Olympia) and went on sale the following year. And unlike the Speed Twin, the Square Four can rightly be called a superbike; not necessarily the first, but certainly one of the greatest.

Ariel 4F Square Four

The production 1928 500cc Square Four (actually 498cc) was designated the 4F. The design began as a twin-crankshaft, four-cylinder, air-cooled engine with the cylinders grouped in a single block.

Within this block, the pistons are arranged two-per-crank. Those pistons rise and fall vertically, one up and one down on each crank journal (i.e phased at 180 degrees like bicycle pedals).

The crankshafts are transversely arranged with one at the front and one immediately behind. These helically-geared cranks contra-rotate with the front crank spinning “backward” and the rear crank spinning “forward”, and it's the rear crank that handles the primary (chain) drive to the clutch and gearbox.

A single camshaft, indirectly chain-driven from the right side of the crankshaft, is located in the cylinder head between the front and rear cylinders. Via another chain, that camshaft also drives the generator and magneto. The oil pump is a gear type.

The valves are opened with rocker arms actuated by the camshaft lobes. Conventional valve springs keep those poppets under control in the usual way.

A single carburettor feeds all four cylinders and is located between the exhaust header pipes (see immediately above) which no doubt heavily compromises gas flow.

These early two-pipe 4F engines, like other Fours in the range, are arranged with exhaust ports exiting each side of the engine, whereby each exhaust serves two cylinders; one front, and one rear. The exhaust manifold are cast integrally with the cylinder head. Therefore, they're no detachable.

The bore and stroke is, respectively, 56mm x 61mm. The power output is around 23 - 25hp @ 5,600rpm.

Overall, it's a beautifully compact and technically smooth engine design, but with a tendency towards overheating the rear cylinders.

The clutch is a conventional wet multi-plate unit that transfers the power to a four-speed Burman gearbox. Gear-changing is by hand. Final drive is via the usual single row chain.

The frame is rigid. The front fork is a girder. The rest of the cycle parts are typical of the era, including the well-equipped and attractive "panel" tank.

The 7-inch drum brakes were never anything other than typical of the age, but by modern standards they're poor. The wheels are 3.25 x 19-inches front and rear.

Prototype

The prototype Square Four engine, note, differed markedly from the production version (see the image immediately above).

This 498cc unit featured overhanging cranks, meaning that there are no "outside" main bearings (think bicycle pedals again). The cranks, it appears, were connected via straight-cut gears which would have significantly increased engine noise.

It was all a good starting point, but the concept was soon revised (largely by the hugely competent Val Page, image immediately above, who did so much for Ariel, BSA and Triumph) and never saw production. Page's contribution to the Ariel Square Four is generally underrated.

600cc Square Four

In 1932, the 500cc Squariel continued to be produced (927 were built) but it was joined by a new 600cc variant (actually 601cc). To achieve this, the cylinder bores of the engine were opened 5mm, but the stroke remained the same.

The extra power was broadly welcomed, not least by "sidecar men", but the engine ran slightly less smooth and slightly hotter. Nevertheless, it was still a great power unit, and although bike sales were still relatively low at 2,675 units, the 600 4F was eagerly received.

Gear-shifting, incidentally, was now by foot rather than hand. However, the big news that year was that Ariel had run into financial trouble which necessitated a major restructuring by Sangster. His strategy was a little brutal, but it worked and the company became more streamlined, albeit smaller.

4G Square Four

By 1937, a redesign saw the Square Four engine re-emerge, still as a two-piper, but now with 995cc on tap plus a few extra vibes. The overhead camshaft was lost, however. With a (nominally) 1,000cc engine, the aluminium housing for the right-side cam chain would have made the engine uncomfortably wide and impractical, so partly for this reason the decision was taken to move to pushrods.

With this change, some of the OHC mystique was lost, but given the technical limitations and materials technology of the era, it was a safer solution, and Ariel wanted nothing if not reliability from its product.

A 600cc version (actually 599cc) of this pushrod 4G engine (still listed as a 4F), was also available for a while. Most were exported.

Two years later, Ariel upgraded the rear suspension from rigid and introduced its optional "Anstey link" (see image immediately above). You can think of this system as something between swinging-arm suspension and plunger.

The idea was to maintain constant drive chain tension. But the penalty is numerous links and pivots that require regular maintenance for optimum performance, and the amount of suspension travel is, at the end of all that, just a couple of inches.

It works, but not all Ariel men and women are fans.

Post-war (1946) saw the end of the girder fork which was replaced by telescopic units.

4G Mk1 Square Four

In 1949, the era of the "iron" (head and barrel) 4G models came to an end and the confusingly designated 4G Mk1 appeared.

The most significant difference was the introduction of an all-aluminium cylinder head and barrel which replaced the cast iron components. Now around 30lbs lighter, the new model cooled a little better, was a little brisker, and certainly sounded different.

Coil ignition was also introduced, and performance was improved slightly at 90mph.

4G Mk2 Square Four

The 40hp "four pipe" Squariel appeared in the second half of 1953 (see images immediately above). To achieve this new look, the cylinder head was re-cast to accept two bolt-on cast-aluminium manifolds, one for each pair of header pipes, and the inlet manifold also became integral with the cylinder head. A single SU carburettor handled the induction mixture and helped minimise pre-ignition on the rear cylinders. And because fuel quality had improved around the world, the compression ratio was hiked to 7.2: 1.

Gas flow was significantly improved, and weight was kept down to around 425lbs taking the bike all the way to 100mph coupled with more sprightly acceleration.

The cylinders featured cast-iron liners. The crank ran on plain and roller bearings. The big ends were plain.

A new frame was also part of the package, but the Fourster never made the jump to swinging arm. However, a few have since been converted using an Ariel Huntmaster rolling chassis.

The seat height, as with the 4F, 4G and Mk1 was low at around 31-inches.

The price, incidentally, was around £322 including UK purchase tax. US Importer Johnson Motors advertised it as "The only four cylinder motorcycles in the USA!"

The Ariel Square Four soldiered along in this form until 1959. It had had a fair run over 29 years, and 15,639 bikes are said to have been manufactured (3,922 for the Mk1, and 3,828 for the Mk2).

Many of the bikes were exported for precious post-war dollars. But a large number remained in the UK, and many have been repatriated.

Through the 1970s to the 1990s, customisers took a liking to the engine, and a fair number of Squariels were converted into choppers and trikes.

But there are still hundreds around, and prices are, as of 2016, comparatively reasonable after some significant hikes from 2010 through to 2013.

Technically, it's a flawed masterpiece, if that makes any sense. It's smooth, refined, unique, purposeful, torquey, friendly and pleasant to ride.

But it overheats, has only average suspension on the girder models (and only one-way damping on the teles), has poor to average brakes, and is blighted by the maintenance-heavy and limited travel Anstey link (which, perversely, can work well when sorted).

So is the Squariel really a greater motorcycle than the Speed Twin?

You decide.

Prices

See Sump Classic Bike News March 2016 for more on Ariel Square Four prices.

Further information

Ariel Motorcycle Club

of North America

www.arielnorthamerica.org

Ariel Owners Club

www.arielownersmcc.co.uk

Draganfly Motorcycles

(Ariel spares)

www.draganfly.co.uk

Australian Ariel Register

www.australian-ariel-register.com