DESIGN ANALYSIS
of AC's post-war 2 Litre Saloon.
DESIGN ANALYSIS
of AC's post-war 2 Litre Saloon.
Ever since the AC 2 Litre was old enough to be classed as a collector's car, it has frequently received very poor write-ups from classic car author's and journalists. There are a number of reasons for this:
1) The 2 Litre Saloon tends to live in the shadows of AC's high performance cars.
2) Many motoring journalists (by no means all) fail to grasp one of the most basic requirements of assessing a car's design: That is, starting by finding out what the car was designed to do. Obviously, one would not assess a single-seater racing car's usefulness for taking a family on a shopping trip. It follows that the 2 Litre Saloon should not be assessed as a hot-rod.
3) Many writers fail to understand the engineering theory of chassis and suspension design, and base their criticisms upon what the trends of the day were - rather than actual merit.
4) A proportion of journalists appear to think that harsh and abusive criticism shows that they are being honest and impartial, regardless of whether or not they have actually done any small details like, say... research! I don't know how they get their egos through the door (turn sideways, maybe?).
Perhaps the most common summing up of the car I've read, is to describe the poor thing as "staid" or "conservative". Conservative compared to what? Not its direct competitors. Simply failing to follow current trends in design does not mean "conservative" (trends usually have little to do with the cutting-edge of technology). "Common sense" in the face of ill-informed criticism would be more appropriate? Not that it got much criticism in its day because it was such an effective design. Coming up with new, adventurous designs, that turn out to be third rate, would not be common sense, but would always impress some commentators (not AC owners though!).
So here is the definitive article on the 2 Litre Saloon design.
CHASSIS FRAME
Firstly, what was this chassis designed to do? Like any chassis, its primary purpose is to hold all the other components in their correct relative positions. Of particular interest to this analysis, is that it had to cope with beam-axles, front and rear, and no independent suspension. This is very significant. Had independent suspension been fitted, then the chassis would have required a much higher level of torsional stiffness. If the chassis twists easily, then independent suspension behaves more like a beam axle, but without the beam-axle advantages. The other important requirement of this chassis was to allow for much more passenger space than was found in pre-war ACs.
Overall stiffness is still required up to a point, otherwise the frame will behave like a spring, with resonant frequencies of vibration. The beam axle/leaf-spring suspension on the AC brings its own special demands on the chassis. Nasty side-effects such as wheel-wobble could be eliminated partly by adding transverse stiffness to the chassis at each end. The final main consideration is the efficiency of the design. That is, how light the chassis is relative to its stiffness and strength.
Modern books and articles often dismiss the AC's chassis as nothing more than the pre-war chassis re-used. Contemporary articles referred to the chassis as an entirely new design. In reality, it was a new design developed from the pre-war chassis. It has a family resemblance to AC's pre-war chassis, with its under-slung rear end, but it was longer and wider and improved significantly. Note that design changes that may look quite trivial, can in fact make a huge difference to the performance or efficiency of a frame.
One of the design trends of the day, was towards ladder-frame chassis (i.e chassis cross-members at 90 degrees to the side members) made up from fully enclosed box sections (or tubes). This was a more sound practise for overall torsional stiffness, than crusiform chassis that were common pre-war. While crusiform chassis could use open girders, such as channel sections, ladder frames must be made up from box sections or tubes to provide torsional stiffness for the chassis as a whole. The larger the cross-section of the chassis members, the higher the stiffness. In fact, a small increase in section size makes a very large increase in stiffness. This permits thinner sheet material to be employed and thus weight is saved. The main drawbacks are that corrosion can easily penetrate through the thin material, and it is easily damaged in a crash.
The post-war AC chassis, although it retained a crusiform cross-member near its centre, three-quarters of the chassis was in fact a ladder frame. The side members were boxed (by inserting a slightly smaller channel girder) and the cross-members were comprised of two box-sections and two round tubes. The small crusiform region of the chassis retained channel-section members. To look after the lateral stiffness required, an underpan was included at the rear. At the front end, the engine restricted what could be done, and so the main front cross-member was made very wide (effectively gusset plates added) where it met the side-members. Like the pre-war chassis, there were additional channel members extending from the crusiform towards the front end, acting as struts/ties.
The rear of the chassis was much wider than the pre-war version, with the springs moved inboard. The only major criticism of this chassis design (applying to pre and post-war versions) is the way it passes under the rear axle. Rather than sweep the chassis downwards for axle clearance (like the Y-series MGs), the side-members taper to a very shallow section. This leaves the rear end more flexible than desired, yet still leaves only about 3/4" (19mm) clearance between axle and chassis.
Overall, the chassis (combined with the suspension layout) helps to eliminate all the traditional problems associated with beam axle/leaf-spring suspended cars. The chassis permitted a huge increase in passenger space with only a 2 inch increase in wheelbase. A more modern design could have achieved lower weight (for the same stiffness qualities), but would have raised the centre of gravity, been more prone to rusting through, and more fragile in a crash. The very low centre of gravity is also helped by the light weight of the bodywork.