Monocoque or Unibody

Monocoque (French for “single shell”) or unibody is a construction technique that uses the external skin of an object to support some or most of the load on the structure. This stands in contrast with using an internal framework (or truss) that is then covered with a non-load-bearing skin. Monocoque construction was first widely used in aircraft, starting in the 1930s, and is the predominant automobile construction technology today.

Monocoque or Unibody Automobile Construction

The first automotive application of the monocoque technique was 1923’s Lancia Lambda. Chrysler and Citroën built the first mass-produced monocoque vehicles, both in 1934, with the innovative Chrysler Airflow and the Traction Avant, respectively. The popular Volkswagen Beetle also used a semi-monocoque body (its frame required the body for support) in 1938.

NSU (NSU Prinz) also built monocoque automobiles.

In the post-war period the technique became more widely used. The Alec Issigonis Morris Minor of 1948 featured a monocoque body. The Ford Consul introduced an evolution called unit body or unibody. In this system, separate body panels are still used but are bolted to a monocoque body-shell. Spot welded unibody construction is now the dominant technique in automobiles, though some vehicles (particularly trucks) still use the older body-on-frame technique.

Some American automobiles, such as the 1967 Chevrolet Camaro, used a compromise design with a partial monocoque combined with a subframe carrying the front end and powertrain. The intention was to provide some of the rigidity and strength of a unibody while easing manufacture, although the results were mixed, in large part because the powertrain subframe contained the greatest single portion of the vehicle’s overall mass, and thus movement of the subframe relative to the rest of the body could cause distortion and vibration. Subframes or partial subframes are still sometimes employed in otherwise monocoque construction, typically as a way of isolating the vibration and noise of powertrain or suspension components from the rest of the vehicle.

In automobiles, it is common to see true monocoque frames, where the structural members around the window and door frames are built by folding the skin material several times. In these situations the main concerns are spreading the load evenly, having no holes for corrosion to start, and reducing the overall workload. Compared to older techniques, in which a body is bolted to a frame, monocoque cars are less expensive and stronger.

Monocoque design is so sophisticated that windshield and rear window glass now often make an important contribution to the designed structural strength of automobiles.