Frame-Up: Chassis Construction In Detail

car frame

Few would argue about a car’s needing a strong structure. Many, on the other hand, aren’t totally sure about chassis construction. So here, a run through how car chassis have developed over the years, and how they now work, will enlighten many.

In the good old days, cars had a separate chassis, a structure usually made of steel. Ladder-frame chassis, which were configured exactly as their name suggests, were the norm. Such chassis could be found under such luminaries as the Model T Ford and the Le Mans Blower Bentleys. However, the snag with such chassis was that they lacked torsional stiffness. Which is? In short, resistance to twisting.

The early 1920s saw the beginnings of monocoque construction, a practice robbed from the aircraft industry. ‘Monocoque’ means ‘single shell’ and as time progressed, cars’ bodywork and roof structures became what’s known as ‘stressed’. That is, they contributed to the strength of the whole structure.

That said, ‘monocoque’ in this context isn’t strictly accurate. For an example of a true monocoque car, we only have to consider the post-war ‘tank racers’. These were cars made of the aluminium ‘drop tanks’, the additional fuel tanks used to add range to aircraft. Enthusiasts would take one of these, cut holes for a cockpit and for the transmission to emerge and fit the engine, gearbox, steering and brakes from a car with a chassis. The skin of the former fuel tank gave the tank racer its strength and they were, of course, very light in weight.

The cars we know have ‘unitary construction’. The car’s floorpan (usually reinforced with welded rails), the roof structure, the bulkheads and inner wings all contribute to the strength of the bodyshell. In fact, modern materials and methods mean that even the bonded-in glass of the windscreen and rear window add to the strength. So, unitary construction rules the roost. Many cars nowadays share a common floorpan, or platform, with their outer panels acting as styling elements. Unsurprisingly, many mechanical components are used across model ranges and sometimes, by different manufacturers.

The vast majority of modern cars use unitary construction but there are, of course, exceptions to every rule. Some low-volume production cars use a tubular steel chassis. Most have a spaceframe, which generally has its strength in the hollow section that runs down the centre of the car, i.e. the transmission tunnel. You will find a carbon fibre monocoque under some exclusive (and expensive) sports/GT cars, a la Formula One car construction.

There has been the occasional oddity along the way. Some Marcos cars had a plywood chassis, incorrectly called a monocoque. Further low volume production cars (and some kit cars) had a GRP (fibreglass) monocoque. But in the main, we have unitary construction bodyshells, whose upperworks and roof contribute to their strength. Now you know why, generally, convertible cars are heavier than their closed counterparts. The strength given by the roof’s being part of the structure has to be replaced, by further structural metalwork added to the platform.


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