1957 Ariel Square Four Mark
4 Prototype
Motorcyclists familiar with the 'Squariel'
may be surprised to see a model fitted with
swinging arm rear suspension. The machine was
to have been the Mark 4 but it never came to
the market, and only one prototype was ever
built. This is it!
The first of the long line of Square Fours
was shown to the public at the Olympia Show
towards the end of 1930 and featured an engine
with a chain-driven overhead camshaft and overhung
cranks. As originally designed, the crankcase
was split horizontally and the main bearings
were clamped to the top half. Development over
the succeeding years saw the introduction of
600cc ohc abd pushrod engines, and finally in
1937, the 995cc engine (an all cast iron unit).
This remained in production until 1948, being
succeeded by the Mark 1 and later still, in
1953, by the Mark 2, both of these being all-alloy
with pressed-in iron cylinder liners.
In 1939, the Ariel spring frame, designed by
Frank Anstey was introduced. It was undamped,
very prone to wear and in fact only provided
a constant chain tension over a limited arc
of wheel movement. It lasted far longer than
it deserved to, being still fitted to the last
of the production Square Fours in 1959. The
Earles forked Mark 3 did not reach production,
and the Mark 4 was not to be very different
from the Mark 2, except in the matter of suspension.
On all other Ariel models except the Four, swinging
arm suspension had been available since 1953,
and experiments were in hand with both spring
frame and trailing link front forks in acticipation
of the Mark 4. However, Ariel had been under
the control of BSA for some years now and a
main board instruction cancelled the development
work. A few years later another main board order
ceased the production of all four-stroke Ariels
and the Square Four story came to an end.
Specification;
Engine - Ariel Square Four Mark 3 engine
- 'the Four Piper'. All light-alloy construction
with iron bars. 995cc capacity giving 40 bhp
@ 5600 rpm.
Transmission - Ferodo lined clutch. Burman
4-speed foot-change gearbox.
Suspension - Swinging arm rear with combined
spring/damper units. Late type Ariel front
forks with cowled headlamp.
Manufacturer - Ariel Motors Ltd. Selly Oak,
Birmingham.
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