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What is Monocoque construction?

What is Monocoque construction?

Snapshot: A car's chassis requires a significant portion of development time. So what is a monocoque construction, we try to answer most commonly asked questions.

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A car perhaps falls in the most complex category of product that we use today. Its subsystems have evolved, it more feature-rich that ever and at the core what has really transformed is the way in which the cars are built to be efficient in every way. So what is monocoque construction? We answer it and more in this article.    

What is Monocoque construction?

A car employs an engine and transmission system to power it, while it rolls on wheels supported by suspension system. For the sake of supporting the engine and other systems, a support structure is used. One of these categories of support structure that’s very widely used is called monocoque construction. In case of the monocoque construction, the skin formed out of metal is employed to bear stresses. In principle, it’s much like an egg’s shell. The formed sheets are ultimately welded to strength and bear tensile, bending and torsional forces that act on it.

The sheet thickness that’s employed to form various sections and welding technology that’s used etc. are important decision as it can affect the car’s handling capability to safety that it provides to the passengers.

Most widely used form

Semi-monocoque is the most widely used construction form that modern cars employ these days. There are other body forms like ladder frame, backbone chassis etc. However still, none of them is as popular as the monocoque due to the inherent advantages that it offers.

Benefits that it offers

A monocoque construction is much lighter in comparison and the amount of material that it requires to provide significant strength to the car is much less, therefore from manufacturability point of view, it’s economical to produce.

All monocoque cars can be made lighter with the use of better materials and iterative computer analysis, so, another great benefit that it offers is the high fuel efficiency due to the low weight that it can achieve through inherent design that it has. 

How is it made?

In case of the body-on-frame construction, the frame is the load bearing member and body is bolted onto the frame separately. While in case of the monocoque construction the chassis as a whole is made by separately formed steel / aluminium panels which are welded together to form a load bearing structure. A monocoque made out of carbon composite (side side image) finds extensive use in sports cars.

So in principle the whole of the body acts as a load bearing member onto which suspension, engine and transmission is bolted. Reinforcements are provided at different sections like doors, bonnet etc. to avoid crash protection. 

Disadvantages

Development time that it requires is comparatively longer due to the complexity it involves and requires more rigorous testing and a more rigorous iterative design analysis. This form is not very well-suited for off-road vehicles as rigidity and torsional stiffness is not very easy to achieve without making the car top-heavy.

Due to the reason that there’s no separate structure that provides support, in case of accidental collision, chances of permanently damaging the car are quite high in case of a monocoque construction. While a body-on-frame construction is comparatively easy to repair.

Which cars use monocoque construction?

Almost no production car uses a complete monocoque construction, instead they use a unibody construction or a semi-monocoque construction where sheet metal only bears part of loads. Most modern cars use a unitary construction method, where strength is provide by a structure made by welding bulkheads, pipes and box sections with the sheetmetal.  

Read - How does disc brakes work in a car?

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