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