DESIGN ANALYSIS

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

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. More modern technical articles and books tend to dismiss the traditional pre-war design of chassis as useless, because of its relatively low torsional stiffness. As my above explanation shows, they miss the point that the old style chassis has a very different criteria to modern ones. Lateral stiffness is required to prevent vibrations in sympathy with beam axles being set up. This is because the high roll-centres of beam axled cars, causes a sidways component to the body/chassis movement when a single wheel strikes a bump.

Overall torsional stiffness is still required up to a point, otherwise the frame will behave like a spring, with resonant frequencies of vibration that might be reached within the vehicle's speed range. The beam axle/leaf-spring suspension on the AC brings its own special demands on the chassis. Nasty side-effects such as wheel-wobble and shimmy could be eliminated by adding good hydraulic dampers, mounted at an angle to the vertical. Their effectiveness also relies upon high transverse stiffness of the chassis. The final main consideration is the efficiency of the design. That is, how light the chassis is relative to its stiffness and strength.

Articles tend to dismiss the AC's chassis as nothing better than conventional prewar design, but one needs to understand the old technology to see that it is a cut above most girder chassis. The design evolved from the Standard chassis supplied to both AC and SS (Jaguar) from about 1933. They were largely of channel section with a central cruciform, and thus of very conventional design for the 1930s. The chassis as used for the postwar AC, has the front and rear ends made of box sections and tubes. That might not sound like a big deal, but one has to understand that a ladder chassis made up of closed tubes or box sections, has a far greater torsional stiffness than a cruciform chassis, boxed or open channel. On the AC, the central portion remained as an open channel cruciform, but the front and rear ends followed the ladder principle.

This chassis was also wider at the rear than the original Standard design, with rear springs mounted inboard, and the front springs featured sliding shackles for a more rigid location.

Interestly, these improvements were made over at SS in 1935 by their new engineer William Heynes, who had to improve the old chassis for a faster model of car. The postwar AC's chassis does have some differences, including: Front main cross-member/engine mounts; square tube exensions at the rear to mount the longer tail of the body; some bends in a tubular cross-member to allow it to pass under the prop-shaft; AC's wheelbase was 9ft 9in versus 9ft 11in for the SS Jaguar.

Retaining the central cruciform was something of a compromise, as it probably provided way more lateral stiffness than was required, while having poorer torsional stiffness. SS introduced the full ladder design to their saloons in the late 1930s, and it became the conventional approach after the war when independent front suspension became the norm. Whether or not it was up to that new task, is another matter! The more efficient designs appearing on some postwar cars, featured larger section chassis members, since a small increase in section size, makes a substantial rise in stiffness, and permits thinner walls. The downside is that the thinner walls are prone to rust holes and are more easily damaged in a crash. Higher stiffness generally (torsion and bending), often produces a less rugged chassis, so there are pros and cons to every design approach.

My only major criticism of this chassis design 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 1.25" (31mm) clearance between axle and chassis.

Overall, the chassis (combined with the suspension layout) was the ultimate development of the old technology, being devoid of most of the old troubles normally associated with this approach. At the same time, it was rugged and had a low centre of gravity.

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Website started 29th December 2006