Technology – Monocoque Chassis (Unibody)

11 05 2009

AutoZine Technical School – Chassis

Monocoque, from Greek for single (mono) and French for shell (coque)

Is a construction technique that supports structural load by using an object’s external skin as opposed to using an internal frame or truss that is then covered with a non-load-bearing skin.

Today, 99% of conventional cars produced in a planet are made of steel monocoque chassis. The most common form of chassis construction for passenger cars and, ever more so, 4WD cars. thanks to its low production cost and suitability to robotised production.Monocoque is a one-piece structure which defines the overall shape of the car. While ladder, tubular space frame and backbone chassis provides only the stress members and need to build the body around them,  monoque chassis is already incoporated with the body in a single piece.

In fact, the “one-piece” chassis is actually made by welding several pieces together. The floorpan, which is the largest piece, and other pieces are press-made by big stamping machines. They are spot welded together by robot arms (some even use laser welding) in a stream production line. The whole process just takes minutes. After that, some accessories like doors, bonnet, boot lid, side panels and roof are added.Monocoque chassis also benefit crash protection. Because it uses a lot of metal, crumple zone can be built into the structure.

Another advantage is space efficiency. The whole structure is actually an outer shell, unlike other kinds of chassis, therefore there is no large transmission tunnel, high door sills, large roll over bar etc. Obviously, this is very attractive to mass production cars.

There are many disadvantages as well. It’s very heavy, thanks to the amount of metal used. As the shell is shaped to benefit space efficiency rather than strength, and the pressed sheet metal is not as strong as metal tubes or extruded metal, the rigidity-to-weight ratio is also the lowest among all kinds of chassis bar the ancient ladder chassis. Moreover, as the whole monocoque is made of steel, unlike some other chassis which combine steel chassis and a body made of aluminium or glass-fiber, monocoque is hopelessly heavier than others.

Advantage: Cheap for mass production. Inherently good crash protection. Space efficient.

Heavy. Impossible for small-volume production.

Who use it ? Nearly all mass production vehicles.




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