Originally carpenters made horsedrawn carriages with wooden bodies. They carried this technology over to the horseless carriage.
Then it became clear that for large scale production, a saw, hammer and some nails would not be up to the task. Design involves choosing a balance between what the material needs to do and how it can be formed. The appearance emerges from this compromise – the required look can drive material selection and vice versa. Often the balance is not even something at the forefront of the designer’s mind if they are working out of habit or tradition.
Gradually, pressed steel parts began to become more pervasive elements of the horseless carriage as it dawned on manufacturers that the new ‘automobile’ needed new materials and designers asked for more than wood could do. Simple rolling operations could turn sheet steel into mud-guards. A form of rolled iron served for the bumpers and riveting could be used to join large flat pieces for the engine housing. Wood retreated gradually but still served for the main structure of the cabin. The real breakthrough came in the 20s when Budd & Co were able to extend the technology so it could be used for closed-bodied cars made for Dodge.
I imagine a very large amount of experimentation was required to make even simple panels from pressed steel, with a lot of effort expended on fiddly areas such as the rope flanges that formed the edges. The technology developed quickly and in Europe Citroen used the combination of pressed steel and monococque construction for the 1934 Traction Avant (check out the source, please, it’s a good site.). The resultant car had improved torsional rigidity and a lower weight than a body-on-frame vehicle.
The downside of pressed steel production is the enormous cost of tool-making. In a way, pressed-steel both answered the problem of how make cars in large volumes at the expense of requiring even larger volumes to break even. It is not a coincidence that around this time there was a mass die-off of smaller firms who could not compete on the prices offered by those who made pressed-steel cars. Those who could charge higher prices were able to retain hand-production and body-on-frame construction which is Rolls persisted with BOF for so long.
What is curious is the length of time it took for unibody construction to leave the mainstream of car production. BOF cars disappeared from the mainstream European market in the 1960s; in the US they were still far from unusual until about the middle 90s: GM sold their Caprice, Roadmaster and Fleetwood cars with BOF construction until about 1997. Ford soldiered on with the much-loved and much-reviled Panther platform until 2011, some 80-odd years after it was not strictly necessary.
In the interim, competitors for steel remained and emerged. Wood is still used by Morgan and many European and US estate cars had part-wood bodies until the 1960s. Bristol who made a big deal about their aerospace heritage insisted on the use of wood until 1963 (I am prepared to be corrected on that precise date); aluminium found favour among firms who valued lightness (Alvis, Bristol?) and were prepared to tolerate the high price of manufacture and the following list of demerits: it’s hard to paint, it doesn’t like being stamped, it has little tensile strength, it fatigues more rapidly and dents easily.
However, Zagato, Ferrari, Aston Martin and latterly Audi and Jaguar have made good progress with the material and note, they aren’t cheap cars; we touched on plastics last week and while there are few all-plastic cars, certain parts are often made of the stuff: bonnets, bootlids, wings and doors along with bumpers and virtually everything else but seldom all at once; Chrysler produced a nice concept car called the CCV made mostly of injection-moulded polyethelene and this went nowhere owing to the problems of moulding such large parts at the right cost and to the right quality; carbon fibre has been used for larger and larger parts by McLaren (the whole body tub) and the cost of the material is falling as it becomes easier to increase production volumes. Alas, it looks horrible and is frighteningly fragile when shunted. Let’s not forget glass-fibre, a favourite of Tabur, TVR, Lotus and Reliant.
Which leads us back to steel which, it turns out, can do a lot more than it was ever expected to do. Using different stamping, welding and hydroforming processes along with variable thicknesses of plate, it can be made to perform in or near the performance envelope of lots of its competitors at a much lower cost. I can’t recall the figures but a 1997 Buick Park Avenue weighed the same as an Audi A8 and cost rather less. Of course, the Audi had a bigger footprint and, in counterpoint, the Buick was already quite huge.
In summary, the lesson I draw from this is that the best material for a car is probably all of them, used with care in the right place with reference to the vehicle’s expected price and duties. For some applications a steel BOF is unsurpassed: durable, comfortable, inexpensive and cheap to tool and revise which is why the US market favoured this concept; an aluminium body is best suited to higher-priced performance cars and even then, I suspect its use is justified only by the apparent USP. Morgan get by with wood because it’s what they have always done and plastic will probably always only be a supporting character in the car’s construction yet an essential one.