of AC's post-war 2 Litre Saloon
Page 6


AC wanted the body to be roomy for 4 occupants, and to have a large window area for good all round visibility and a light interior. This is evident from the stylist's drawings done for alternative body shapes. These essential requirements placed tight restrictions on the styling of the roof and window areas. The windscreen had to be deep and wide and square to provide the best view. Hence, the roof had to be somewhat square at its front end.

Any good artist knows that there are solutions (or partial solutions) to any styling problems. To make the windscreen look smaller from the front view, the front wings need to be enlarged. On the AC, the wings are taller than on most contemporary cars, and also mark the widest part of the car (5' 7" / 170cm). From a close up front view, the wings totally dominate and the screen looks relatively small. As one moves further back to view the car, the effect diminishes and the screen eventually dominates from a more distant view. So, it wasn't a total solution, but it was pretty effective.

The rear window area was less restrictive to styling, since traditionally, rear windows were so tiny at that time. From a side elevation of the car, it is very easy to see how a series of flowing curves were drawn, that are closely related. The roof line, side window, side moulding, and the elongated helmet wings. The compound curve of the radiator grill also gave it an added modern feature that most other cars lacked.

Windows and Doors

The split rear windows and shallow v-screens, were similar to styles seen in the USA in the 1930s and fitted nicely into the complex curves of the body.
The doors are another excellent feature of this design. They are extremely wide, permitting easy access to the rear seats. The doors are incredibly heavy, and need attention over the years to ensure a good fit. A slight sagging, and they will scrape the door-steps. Apart from hinge wear, simply keeping the hinge screws and nuts tight will help. The doors are fitted with little rollers to try and prevent them from scraping the door-step plate.

Opinions vs Taste!

1940s articles and road tests showed great enthusiasm for its good looks. By the late 1950s, with second-hand car tests appearing, fashion had changed and its looks tended to fall out of favour. The classic car boom of the 1980s brought highly polarised opinions being voiced. Since then, curves have come back into fashion! As an artist, it often amuses me how individual tastes can change from one extreme to another, as fashions come and go. The AC is the same shape, but it is interpreted differently as time passes - good, bad, good, etc.!


This is one of the most mis-understood of technical topics related to cars. In my school days (1970s), enthusiasts were convinced that a sharp wedge shape was needed to "cut through the air"! Nothing could be further removed from the reality of fluid dynamics. There are really two criteria for cars here: Aero-dynamic stability, and aero-dynamic drag. Simply rounding the front end with compound curves can be a substantial improvement for drag, over alternative car styling. Actual air resistance depends on the frontal area as well as the shape, and the AC presents a fairly large object for the air to negotiate. Simple fluid dynamics of a moving object becomes modified because said "object" is so close to the stationary road surface. Limiting the amount of air passing underneath is a more modern practice that would improve this AC's performance.

I have done an approximate calculation for the AC's coefficient of drag (Cd). This came out at a respectable 0.38. The data and assumptions used in my calculation are as follows: Top speed 83mph (37m/s). Engine RPM at that speed = 4610. Power at that speed estimated at 73bhp. Engine torque would then be 83 lb-ft. Back axle ratio 4.55:1. Rolling radius of tyres = 1.17 ft. Final drive efficiency assumed to be 96%. Losses in transmission bearings estimated at 1%. Tractive force thus calculated in pounds, is then multiplied by 4.45 to convert to newtons. Next we need the resistance due to the tyres. I checked a few graphs found online for cross-ply (bias-ply) tyres travelling at 83mph, and found the coefficient of rolling resistance to be 0.021. Multiply that by the car's laden weight in newtons (14793), and one finds the net tractive effort battling against air resistance. Divide that force by the product of frontal area (2.03 square metres) and speed squared. Quite simple! If I've under estimated friction losses in the transmission or tyres, then the Cd figure will be lower.

For stability, it helps if the side area of the bodywork is concentrated towards the rear, behind the centre of gravity. This is the same principle as the tale-fin on an aircraft, racing car or an arrow. Air flowing alongside this rear-biased area, will tend to keep the car pointing in the direction it is travelling. The higher the speed, the greater the effect. The AC's long bonnet and high roof-line ensures that its shape is very stable in this respect. The 2 Litre Saloon carries about 52% of its unladen weight on its rear wheels, and up to about 58% if heavily laden. But the aero-dynamics are such that stability is maintained. A lecturer once argued with me that cars are more stable with front biased weight, because "'s the front wheels that steer"!!! Hmmm... correct me if I'm wrong, but the rear wheels have a say in the matter too!!! Or better still, check out Formula 1 car design. Rear weight bias, and rear wing supports that double as tail-fins.

As an aside from aero-dynamics, that slight rearward weight-bias is also a help for traction in slippery conditions. The next best thing to 4 wheel drive is a rear weight bias for rear wheel drive. I recall an AC owner in the 1960s writing that the last cars to get stuck in the snow were Minis and ACs. The rear drive also helps when weight is transferred to the rear when hill climbing or simply accelerating in moderately slippery conditions.

Over-bonnet view

One of the AC's greatest selling points is its beautiful over-bonnet view. Few classics can match the AC's looks in this respect.

Noise, vibration and harshness - NVH

This is a retrospective view on the AC's qualities versus more modern cars, and so may apply to a lot of cars contempory to the AC 2 Litre.

Design engineers have been tackling noise/vibration in cars for many years, to improve comfort and make long journeys less tiring. Unfortunately, they have over-looked important factors: Firstly, the masking effect of sounds. It might seem obvious that one sound would tend to mask other quieter sounds, but the actual effect is very pronounced, and one sound can make another one totally unnoticed. They also overlooked the fact that it is not just how loud a sound is, but also how irritating it is, that affects comfort. One of the first things engineers did was to make gears much quieter. Their whining noises disappeared and this revealed all sorts of far more irritating noises! That created new problems of reducing these other noises (that never bothered anybody before). By the 1990s, cars were being designed with exceptionally stiff body structures in an effort to remove yet more harsh frequencies.

The coming of unitary bodies (no chassis under-frame) was a very big factor in creating a challenge for noise reduction, as was independent suspension and modern tyre tread patterns. A more modern trend has also created a problem for ride harshness, and that is low unsprung mass. Light weight wheels and tyres, etc. helps roadholding on bumpy roads, but taken too far, allows the wheels to transmit high-frequency vibrations to the body. The fashion in the early 21st century seems to be decorative alloy wheels and low profile tyres, which increases this problem further. A carry-over from motor sport, this approach to wheels and tyres creates numerous problems in every day motoring, offsetting the general improvement in tyre technology over the years.

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Website started 29th December 2006