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Saturday July 26th 2014

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Car chassis construction

Chassis have to be stiff enough so that they withstand the forces applied to them. This is point is really important in the suspension settings. If the chassis bends a little the car in not going to behave as expected (as linear) because the ride is being modified, in short, the suspension settings are modified. However, you can not make the chassis completely stiff. That would cause it to be brittle. There will start to appear weak points and it would end breaking throw the weakest. So you need to reach a point where it is neither too stiff nor too weak.

As said before, the car needs to withstand various forces, so which are these forces:

  • Lateral G (cornering speed)
  • Longitudinal G (determined by acceleration and braking
  • Load (passengers or goods)
  • Road irregularities (bumps, surfaces and cambers)

The main materials used to build chassis are steel alloys , aluminium alloys, titanium alloys composites… Everyone of those have different properties and applications. Prices vary vastly.


These materials are joined in various ways: riveted, bolted, welded, glued…

There are several types of chassis:

  • Ladder frame
  • Space frame
  • Torsion box
  • Monococque
  • Combination

In the next lines I am going to explain each of them in more depth.

—————————-

Ladder frame

This was the first type of chassis to be designed. Almost all car manufacturers used it until the early 60s. Nowadays some SUVs still use it. As its name connotes, ladder chassis resembles a shape of a ladder having two longitudinal rails inter linked by several lateral and cross braces. It uses few, large diameter tubes. Axels pay an important role in sustaining weight.

Advantages: Cheap to hand build. Can cope with heavy loads.

Disadvantages: Few torsional rigidity, that is because it is a 2D chassis.

Who uses it: Old cars, some SUVs (SsangYong,…), buses, truks

Space frame

Uses several small tubes to create a 3D chassis. Tubes are placed in several directions to cope with the forces they need to withstand. Normally, these chassis are designed for a purpose. These tubes are welded together. To make the welding simpler square-section tubes are also used (however, circular tubes provide the maximum strength). They are normally used in sport applications. Optimizing the design makes hard to put openings (like doors). That is why space frame chassis make acceding to the driving seat difficult (high doors). Take for example the  1950’s Mercedes-Benz 300SL Gullwing.

Advantages: Very accurate. High stiffness. Great torsional rigidity. You can select your materials depending on the purpose.

Disadvantages: Very expensive. Handmade. Tubes need to be cut, shaped, welded manually.

Who uses it: Some competition cars. Some sporty road cars use space frame design in  some parts of the chassis.

Monocoque

Monocoque is a one piece structure which defines the final shape of the car. Metal sheets are pressed with big stamping machines. These parts are welded (normally spot welding)  or riveted together to conform the chassis. The welding process is mostly robotized. These chassis are produced in a stream production line, so they are fast to make. Tolerances are tiny. Nowadays 99% of the production cars use this type of chassis. They are heavy chassis because they use a lot of metal. However, they have really good impact strength. Normally steel is used, steel has 3 times the modulus of Aluminum but only 2 times the weight, and is cheaper and easier to weld.


Every time more, aluminium is being used in body panels of these type of chassis to reduce weight. Also, some Monocoque chassis are fully made from aluminium. It is a complex subject. We will see where this leads to.

Advantages: Space efficiency, because the chassis is the outer-shell of the car. Good for mass production. Cheap to produce if in mass production.  Inherent crash protection.

Disadvantages: Much, much heavier than other types of chassis. Low rigidity since the pressed metal is not as stiff as tubular tubes. Impossible to produce if not in mass production.

Who uses it: Almost all mass production cars.

 

Combination

Combination of Monocoque and space frame. The safety cell is made through Monocoque chassis construction.  The rest of the chassis is made trough space frame design. It has some of the advantages of each one. Another advantages is that is simpler and cheaper to produce than Monocoque alone.


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11 Comments for “Car chassis construction”

  • claudio macri peña says:

    thank sr for describes this things please continue doing works including fotos and anithings that help me in researchs info atentamente desde venezuela
    claudio macri peña.

  • Sherif 1979 says:

    Tell Me Why In 2000′s From 2000 Till 2009 Why All Of Car Companies Donnot Manifacire cars Shapes Like Caprice Classic And Lincoln Model 1977 ?

  • Admin says:

    Hello there,

    Cars don’t have shapes like that anymore mainly because they are designed to consume less fuel. They design the cars to be aerodynamically efficient. This leads to that almost all modern cars have similar shapes… I know, makes them a bit boring!
    However, there is a brand which still designs cars like those, check out Morgan (http://www.morgan-motor.co.uk/). Some of them use a wooden chassis!!!

  • Maher says:

    Dear Sir,

    I am just designing my own car. having a special shape. and fitting a very powerful engine on it..

    Actually i decided to Make a Space Frame Chassis..

    But my engine is about 750 Hp…

    So can you please advice me!! which metal allow should i use to build my chassis??

    Awaiting your reply!!

    Thanks & Best Regards,

    Maher

  • Admin says:

    It depends on lots of factors…
    If it is for small numbers and hand made production I would suggest using some steel alloys. They are easy to work with (I have some experience in Formula Student) and weld. You can specify different grades of Steel alloys and different tubular sections depending on how many stresses and loads each part of the chassis needs to withstand.

    But before that an accurate knowledge of the loads, and stresses involved in the chassis need to be known… (you don’t want to design a floppy chassis, want you?)

  • ENVY K says:

    hi there,
    Well i am a mechancal engineer and hobby car designer willing to learn some new software for car body designing work interior and exterior both…I would like to know which cad softwares are be used by different car companys to design Monocoque type car chassis.

    Awaiting for an affirtmative response

  • Admin says:

    A post covering this subject will be posted soon. Thanks for your comment.

  • channdrannshu says:

    how can we find different stresses involved in the chassis and what is done after we find those forces which will bbe actinng onn the chassis.

  • gautham says:

    what is the approximate mass of the chassis???

  • kia Mahboub says:

    hi ,
    I am an industrial designer and I’m designing tricycle andprefer Space Frame Chassis and I would like to know which softwares ar be used for chassis design and primery calcullates.


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