What is a monocoque body?

When you think about how a car is constructed, you’ll most likely cast your mind back to pictures of Model T Fords on a production line.

Without its body fitted the Model T could move along the production line as a rolling chassis. A vehicle with a full chassis is self-supporting even without the body fitted, due to the frame that contiguously runs the entire length of the vehicle from the front wheels to the rear.

Most people have never heard of monocoque construction (also known as ‘unibody’ construction), despite the idea being put into practice many decades ago by the automotive pioneers.

Monocoques account for the vast majority of passenger cars, plus many SUVs and even light commercial vehicles on the road today.

Unlike a full-chassis (or ‘body-on-frame’) vehicle – such as virtually all of the popular mid-size utes sold in Australia today – a monocoque uses the car’s major body parts to supply strength and support to the whole unit, hence the ‘unibody’ synonym.

The major body parts are ‘load bearing’ and work together to keep the car from twisting and bending with the application of force at different points of the structure.

In the case of a vehicle with a monocoque body, the roof, the roof pillars, the floor pan, the firewall, rear bulkhead and other internal reinforcing members all resist the torsional (twisting) stresses imposed.

Even the windscreen can be a load-bearing member in a monocoque vehicle, as was the case with the Mazda 323 from the early 1980s.

If you’ve ever wondered why front-wheel drives have a transmission tunnel passing underneath the rear seat, it’s because the tunnel is like a sort of open box section, resisting bending forces, much as a backbone chassis does.

Think of a monocoque like this: you have a plastic ruler and a rigid square plastic tube of the same length and width. It’s very easy to twist and warp the ruler using just your hands, whereas it takes significantly greater force to twist the tube. The ruler is a chassis, the tube is a monocoque.

According to Wikipedia, monocoque construction in the automotive engineering context is not truly representative of the type. A proper monocoque relies entirely on the stressed metal skin alone for its strength, but most cars feature box sections and sub-frames to lend additional strength to the structure, which the online encyclopedia insists is more accurately a ‘unibody’ or ‘unitary-construction’ vehicle.

Unless you happen to own a Ford Crown Victoria, such as you see in American police shows, or some other large American car from years past, your passenger car is most likely to be a monocoque – provided it has been built during the years since 1945.

Passenger vehicles designed before the Second World War were mainly built on ladder-frame chassis, and unitary-body construction was quite an innovation.

Many all-terrain wagons – the forerunners to the modern SUV – were built on a full chassis, but the Mitsubishi Pajero since 2000 has been a monocoque, and many smaller SUVs developed from a passenger-car platform are also monocoques.

Quite a few larger SUVs are developed from passenger-car architecture too, so these ‘soft-roaders’ are also monocoques. And the Range Rover and Range Rover Sport are both monocoques, using aluminium body members bonded and rivetted together.

There are exceptions to the rule, however. The Land Rover Discovery 3 and 4 combined monocoque construction with chassis rails – not quite one thing or the other. This was why the Disco 3 was so heavy, and why reducing weight was such a priority for the fifth-generation Discovery.

Most light commercial vehicles – utes or pick-ups – are built on a full chassis, but there are some exceptions. Car-based utes like the Holden Ute (underpinned by the Commodore platform) and the Ford Falcon Ute were generally monocoques, but from 1999 Ford adopted a hybrid structure for the Falcon, with chassis rails for the rear wheels and a monocoque cabin up front. Holden later used this same construction method for its Commodore-based One Tonner.

In the main, one-tonne vans such as the Toyota HiAce and the Hyundai iLoad are monocoque vehicles. But there are some exceptions among vans. The larger Iveco Daily, like the Land Rover Discovery 3, is a monocoque body mounted on a full chassis.

Without a separate chassis the monocoque car is quieter than a vehicle with a full chassis. There are no fastening points where the body meets the chassis to rub and squeak or transmit vibration.

The monocoque is lighter, so fuel economy is better, and it’s often more spacious inside. Since the monocoque is better at resisting bending and twisting forces, it delivers a more stable platform for ‘chassis’ tuning, pound for pound. The monocoque car will handle and hold the road better than a vehicle with a separate chassis.

By its nature, a monocoque body provides better secondary safety (crash safety). A full chassis cannot as readily incorporate effective crumple zones for crash safety, and the chassis and the body are more prone to metal fatigue over a longer period.

Monocoques can be specifically engineered with different tensile-strength metals to channel load forces around the occupants in the cabin during a crash. Engineers design a ‘safety cell’ for the cabin, with harder steels for the roof pillars and door frames, but specify milder steels around the front and rear of the vehicle to absorb more of the impact force and even deflect some of the force to one or the other side of the car.

A properly designed monocoque can be much safer than any vehicle underpinned by a full chassis.

Unless weakened by a crash or long-term corrosion, a monocoque can be exceptionally strong and resistant to bending, making it an ideal platform to support a wider range of suspension systems.

With the exception of the Suzuki Grand Vitara, a full-chassis vehicle cannot generally run the cheap-but-effective MacPherson strut-type suspension system, since the strut top is reliant on the load-bearing strut towers inside the engine bay for fixed location.

If there’s a disadvantage to monocoque construction, it rests with the manufacturer. Once the ‘hard points’ of a monocoque are decided, and the dyes to stamp out the floor pan have been produced, it’s an expensive process that allows relatively little flexibility for change and cost management.

A full chassis, by comparison, can be tweaked easily for series production and can support any number of different ‘top hats’ (bodies). That’s why the Mercedes-Benz MFA platform (a modular monocoque architecture) used the same wheelbase for A-Class, GLA and CLA model lines.

In an industry that still draws upon concepts dating back to the horse and buggy era – concepts such as the Ackermann linkage, for example – the monocoque structure has been a remarkable innovation. Today’s global automotive industry without it would be unthinkable.

And yet hardly anyone in the street gives the monocoque body a second thought.