DESIGN ANALYSIS

of AC's post-war 2 Litre Saloon
Page 3

BODY CONSTRUCTION

Wood frame



The timber frame is worthy of separate mention because there are some very common misconceptions regarding wood as an engineering material. I suspect that watching too many cartoons may be at the root of the problem :) In fact, even professional design engineers have been known to possess the same misunderstandings, which shows what a specialised area of knowledge wooden structures has become.

The popular fallacies are:

1) "That wood may present a fire hazard to the car".
2) "That wood lacks strength".

The actual attributes of wood for body construction are:

1) High strength-to-weight ratio.
2) Even higher strength under impact loads (possibly 20-30% above nominal strength figures?).
3) High resilience (therefore absorbing impacts and reducing need for strength).
4) Does not 'yield' in the way that mild steel does.
5) Not prone to fatigue fractures.
6) Fastenings do not create stress-raisers (which also relates to fatigue)
7) Insulates against sound and heat.

Wood contains water, and so needs to be dried out sufficiently before it can readily combust. How easily it can be dried out depends partly on the type of wood, and largely on the size of the timber. Thin sections dry out quickly, so match sticks are easily kept dry enough to burn. Hefty 2 inch (50mm) thick wood frame members on the AC would be rather more difficult to dry out. The AC's outer body panelling was partly welded up in situ on the wood frame, which left nothing more than slight scorch-marks. It used to be a common sight to see professional decorators stripping paint from wooden window frames using a blow-torch, but without setting fire to the frames!

Far more hazardous were synthetic materials introduced for more modern transport, until legislation brought in restrictions. Wood has a major advantage in that it insulates heat effectively, offering some degree of protection if a fire breaks out. The thermal conductivity of steel is about 300 times that of hard-wood. The only *real* fire hazard for an AC owner to consider, is the fuel system.



As for strength of wood versus steel, that is interesting! Strength of materials is most commonly measured by a tensile test of a sample. Ultimate strength is the stress (force per unit cross-sectional area) needed to break the sample. For mild structural steel, the tensile strength is typically around 30 tons per square inch (tsi) - or 465 mega-Pascals (MPa) in modern units. This method of testing is less well suited to wood, because of the varying layout of the wood grain which causes big variations in the figures measured - which are not the best illustration of its strength for particular applications. As a rough guide, tensile strength figures for ash timber - with the grain - range from 2 to 7tsi (31 to 109MPa).

Next, we need to compare the density of ash and steel. Relative density of ash is around 0.6 and the figure for mild steel is 7.9, or 13 times the density of ash. Strength for a given weight of ash then works out at anything from a fraction less, to 3 times larger than mild steel. Careful selection of timber and design of frame members, may ensure that the strength will be towards the higher end of that range.

Actual strength in practice relates more to bending strength, which is easier to test on wood. We can compare a 2 inch (50mm) square ash pillar, say 30 inches (762mm) long, with a steel pillar of the same weight (hollow, made up of thin sheets), for bending strength. Even if we assumed that the steel version wouldn't buckle, it would still work out weaker than the ash version. In practice, buckling would reduce its effective bending strength, and would also make its crushing strength much lower too. For bending strength calculations, the modulus of rupture is used for wood. The equations for this assume that stress and strain are always proportional (Hooke's law) which is largely true for timbers.

Yielding

There is an important advantage over metal body construction that wood shares with modern carbon-fibre composites: It does not yield like metal. By "yield", I mean permanently deform (like soft clay) when placed under sufficient load, such as a major crash. Serious crashes often involve cutting trapped victims out of the wreckage.

Stiffness

I haven't mentioned stiffness so far, and I'm not trying to evade the issue :) Stiffness of wood is far lower than steel. However, while this makes it less suitable for contributing to chassis stiffness, it does help with crash resistance. A highly rigid body cannot absorb much of an impact, and thus increases the need for strength. Potentially a vicious circle depending on how ingenious the designers are. Carbon-fibre composites, in their most basic form, would appear to be worse still, since a big part of their benefit is extremely high stiffness for the car's structure. This problem is overcome by more complex laminate construction, often including aluminium honeycomb, to offer some resilience to crash loads. The bodies of coach built cars such as the AC, are not intended to contribute to chassis stiffness, so the timber's natural resilience can be considered a potential benefit. Having said that, large plywood components can provide a more significant addition to chassis rigidity, and I suspect that the AC's boot side panels may well assist the rear end of the chassis (until the wood rots away, that is!).

The main reason for the sheet steel/unitary construction of most modern cars, is for mass-production techniques. It is the cost-effective way to build cars in large volumes (and production volumes are about 10 times larger today!). This construction is often touted (especially in academic literature) as being the best for stiffness-to-weight ratio, but modern designs of separate chassis can achieve equally good results.

Sound Insulation

The AC 2 Litre Saloon is very quiet inside, and there is virtually no road/tyre noise evident. Old style tyre tread patterns contributes to this, as does the springing. Overall sound insulation has more to do with the wooden frame and the use of aluminium-alloy for panels and also engine castings. Again, academic literature sometimes contradicts this point, but it has been demonstrated that aluminium insulates against sound much better than steel and iron - even if component dimensions are identical. Harry (later Sir Harry) Ricardo (the great engine/fuel research pioneer) designed an engine for tanks in world war one. These had cast-iron cases originally, but later on, aluminium was substituted using the same moulds. The resulting engines were much quieter. With engines designed for aluminium in the first place, wall thickness of the metal are likely to be thicker than for iron and steel, and so resonant vibrations are less likely. In other words, less noise! I would expect that the alloy body panelling also helps to makes things quiet inside the car. The AC has hardly any added sound insulation, apart from a little behind the aluminium bulkhead.

Modern mass-produced car design is inherently noisy for occupants and has tested the engineers' ability to alleviate this for many years. If you've never travelled in a coach-built saloon car, then you will be pleasantly surprised if you do get such an opportunity in the future. Most of my passengers had high praise for the AC after their first ride - without any prompting from me.

Body Panelling



I believe that most of the outer body panelling was made of an aluminium alloy called Duralumin or one of its successors. Many thanks to the late John Fear for analysing a sample of body panel for me. Duralumin was the first aluminium alloy produced that could be heat treated for higher strength. It is an age-hardening alloy, so-called because after the hardening process, it takes a few days before it develops its full strength and hardness. Ultimate tensile strength is about 26tsi (400MPa) and its surface-hardness is similar to mild steel. It is, however, only just over a third of the density of steel, so weight-for-weight, it is well over twice as strong. In its annealed state, its strength reduces to around 16tsi (250MPa). A slight drawback is that its corrosion resistance is not as good as pure aluminium. Modern terminology labels this alloy as number 2017, but since superceded by others in the 2xxx series such as 2024. 18swg (1.2mm) panelling is used for the main bodyshell, and thicker 16swg (1.6mm) material for the wings. The wings have no framework - the compound curves are sufficient to keep them rigid, although the outer edges are reinforced with steel cord.

AC cleverly avoided a common problem of bodyshell cracks at the front. Coachbuilt cars used to crack near the bottom front corners, where the screen pillars met the sills, due to twisting of the car. A popular solution was to have a join in the bodyshell - in effect, a 'crack' placed by design. The AC 2 Litre has the door-step and screen/scuttle areas separated by the front wings, so that this crack issue does not arise.

The radiator-cowl is made from brass, as are many of the chromium-plated fittings such as window frames. Steel is used for the inner-wings, spare wheel compartment and rear foot-wells. Separate aluminium panels are employed under the rear seat and also for the metal portion of the bulkhead.

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Website started 29th December 2006