Updated: Apr 11, 2020
Ideally , the purpose of making a motor car chassis is to connect all four wheels with a structure which is rigid in bending and torsion - that is one which will neither sag or twist . It must be able to support all the all components and must absorb all the loads fed into it without deflecting unduly.
There are multiple types of chassis that have been used since the history of automobiles namely ( ladder type , space frame , monocoque ). Our main topic of discussion in this blog will be on the major two types of chassis that are and have been used throughout the history of automobiles.
There are multiple chassis types , but primarily they can be classified into two types :
1) Structures created by joining multiple tubes (space frame chassis).
2) Structures created by joining multiple panels creating a uni body chassis , which is mostly used in commercial industry (monocoque chassis).
The space frame chassis is manufactured by joining multiple tubes or pipes and are constructed like shown in the picture below.
Triangulation is basically the main principle used to manufacture such chassis for increased strength that they provide to the chassis .
As you can see an un-triangulated box is weak . This action is depicted above. As the hand pushes the box gets easily deformed .
Now, if an additional member is added to the box diagonally then the deformation of the box is restricted as the diagonal members shares the applied load or resists the deformation in simple language .
Behavior under different loads(compression and tension )
In the above diagram we can see , the nature of the force applied to the box is tension ( i.e. the diagonal member is being stretched) , the member actually resists the change of the square shaped box to a parallelogram , unlike the above case when there was no diagonal member present.
Apart from tensile forces, compression forces also act on the chassis whose nature is opposite to that of the tensile forces . The compression force tends to crush or compress the tube unlike tensile force that kind of tears it apart . Buckling of the pipe members can become a real issue if compression forces are not resisted properly .The diagram below depicts how compression forces are resisted by triangulation
Generally round or square shaped tubes are used for the manufacture of space frame chassis. Square tubes are easy when it comes to manufacturing phase of the chassis as compared to the round tubes . Notching is used frequently to connect round tubes properly .
Analysis of the loads in a space frame chassis design is the key aspect of designing a space frame chassis . Most of those are done by finite element analysis , to provide proper triangulation for the areas of compression and tensile over loads proper tube thickness is also decided by these analysis . For compression load a larger thickness is preferred and a smaller thickness for dealing with tensile forces comparatively . Which also affect the weight management of the chassis .
The box on the left uses a panel of material in place of a member , this same principle is used to manufacture monocoque chassis to use proper load distribution. In case of the panel shear stresses are developed on the panel , when it is pushed from the left or simply stating , when tensile forces act on it and panel is much more effective to deal with the forces than a single member , as shown below.
If the box is pushed from the other side as shown below , compression forces are developed and the member in space frame structure could collapse under huge loads compared to the panel used for the monocoques. The compression and tensile forces are dealt with more effectively in case of the panel compared to a member .
Both the types of chassis can be made equally strong if enough triangulation is added , as shown below . That strength is achieved in space frame chassis at the cost of huge chassis weight , as compared to monocoque, which are comparatively lighter .
In spite of being lighter and stronger than space frame , there are many downsides of the monocoque design compared to space frame.
Firstly designing the monocoque is much more difficult than designing a space frame chassis. The diagram shown below illustrates this to some extent. If one side of the monocoque box is missing then there is no path of load distribution on that side , as shown in the diagram below . In this case the design is flawed and unsafe.
If the box is pushed on the side where the three panels meet there is not much deformation compared to , when it is pushed from the end where there is one panel missing .
The primary objective in these types of chassis are to make sure that there are no unhanded load paths , that could possibly cause the structure to buckle . If there is a design flaw in a monocoque compared to a space frame , the space frame structure is much more likely to be safe for the driver , because steel material usually provides a more gradual material failure than a monocoque panels (depending on the material used )
This highlights another downside of monocoque - If it is damaged , it is difficult to repair a monocoque than a space frame chassis .