DEVELOPMENT
and
HISTORY

of AC's post-war 2 Litre Saloon.
Page 1

Design Brief

From the mid 1930s, AC appeared to be looking at ways to make their fine sporting cars more practical and comfortable, in the face of increasing competition. Subtle improvements were made to provide more room inside their sleek 4 seater tourers, and also the more rare saloons produced. Unfortunately, these were still not much more than 2 plus 2 seaters. Therefore, AC tried a new chassis, over-slung at the rear, unlike the chassis that they had been using since 1933. It had a longer wheelbase, and the engine and bulkhead were much further forward. Suspension was still beam axles and leaf springs, although the springing was softer. The body for this car - known as the "Flying Sixteen" - was taller and had helmet style wings. Both a saloon and open tourer versions were built, and these achieved the main objective of roominess, although not very sleek in appearance, and reports that the sporting handling of previous cars had been lost. Production was only into single figures when the war intervened in 1939. After the war, the concept of a more practical, but still sporting, vehicle was pursued, although the emphasis had shifted to creating a saloon car.

Early Design Work

A design for a new box section chassis with independent front suspension (IFS), supporting a full width body, was proposed after WW2, drawn up by AC's engineering director E. H. Sidney. Had this been a well executed design (I don't know enough details to comment) then it might have been a great car. However, hindsight shows that many other manufacturers took time to iron out the snags of similar chassis-suspension combinations. The benefits of IFS at that time are doubtful, and it took perhaps another 15 years before independent suspension designing made it passed the steep end of the learning curve. The functioning of IFS is also heavily dependent on the chassis frame design (more so than for beam axles), so there was plenty of scope for early troubles. No doubt development costs and time would have influenced AC's decision not to follow that route.

Instead, a much improved version of the pre-war design was employed. It was based upon the improved SS Jaguar chassis developed by that company in 1935, with alterations to take the AC engine, and the postwar AC body style. Although still criticised for being "archaic", this chassis proved excellent for a real driver's car. The improvements over the old Standard chassis frame seen before the war, may look trivial, but would have been significant in practice.

Minor improvements to springing, damping, and parts of the chassis frame ensured that this was about the ultimate development possible for the conventional beam axle/leaf-spring arrangement. None of the serious problems associated with cars suspended this way, and none of the new problems being encountered with IFS. AC's new design engineer Z. T. Marczewski takes much of the credit for AC developing the old technology rather than following the route of most other manufacturers.

A real benefit of IFS for other car makers, was for a more forward mounting of the engine, which was mainly of value to smaller cars. The AC's engine block was still mounted entirely rearward of the front axle, to help maintain weight distribution for the desired handling. Rear seat passengers were also within the wheelbase, while the fuel tank was mounted above the rear axle. The brakes were improved by use of very large 12 inch (30cm) diameter drums, with hydraulic actuation at the front, and mechanical at the rear. Disc wheels with deep central wells, made sure that the drums were exposed to cooling air flow.

AC's wonderful straight 6 alloy block engine was retained, with some modifications. To achieve a lower bonnet level, the water-pump was moved to the left-hand side of the block, and pumped coolant from the radiator into the engine. The triple S.U. carburettors now featured a thermostatically controlled cold start device. The specification for the aluminium alloy castings was improved, to reduce the rate of corrosion.

Transmission was by a Moss 4 speed gearbox with synchromesh on 2nd, 3rd and 4th, plus a Moss back axle with hypoid-bevel final drive for keeping the propeller-shaft fairly low.

An experimental chassis, number L800, was built and fitted with an open tourer body and helmet wings. This permitted testing of the mechanical set up, before the body styling was finalised. L800 took to the roads in 1946.

"The Autocar" magazine carried an advert as early as 27th April 1945. This showed a tantalising glimpse of what the new car might look like. Although not obvious from that partial image, it was in fact from one of Connolly's alternative ideas for the body style, quite unlike what appeared in production.

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