CAR CHASSIS DESIGN

Car Chassis Design

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Car Chassis Design

Definition of a Chassis

The chassis is the framework to which everything is attached in a vehicle. In a modern vehicle, it is expected to fulfil the following functions:

Provide mounting points for the suspensions, the steering mechanism, the engine and gearbox, the final drive, the fuel tank and the seating for the occupants;

While fulfilling these functions, the chassis should be light enough to reduce inertia and offer satisfactory performance. It should also be tough enough to resist fatigue loads that are produced due to the interaction between the driver, the engine and power transmission and the road.

Overview of Chassis Types

Ladder frame

The history of the ladder frame chassis dates back to the times of the horse drawn carriage. It was used for the construction of 'body on chassis' vehicles, which meant a separately constructed body was mounted on a rolling chassis. The chassis consisted of two parallel beams mounted down each side of the car where the front and rear axles were leaf sprung beam axles. The beams were mainly channel sections with lateral cross members, hence the name. The main factor influencing the design was resistance to bending but there was no consideration of torsional stiffness. [Ref. 1]

A ladder frame acts as a grillage structure with the beams resisting the shear forces and bending loads. To increase the torsional stiffness of the ladder chassis cruciform bracing was added in the 1930's. The torque in the chassis is reacted by placing the cruciform members in bending, although the connections between the beams and the cruciform must be rigid. Ladder frames were used in car construction until the 1950's but in racing only until the mid 1930's [Ref. 2]. A typical ladder frame is shown below in Fig. 1.

Fig. 1 [Ref. 2]

Twin tube

The ladder frame chassis became obsolete in the mid 1930's with the advent of all-round independent suspension, pioneered by Mercedes Benz and Auto Union. The suspension was unable to operate effectively due to the lack of torsional stiffness. The ladder frame was modified to overcome these failings by making the side rails deeper and boxing them. A closed section has approximately one thousand times the torsional stiffness of an open section. Mercedes initially chose rectangular section, later switching to oval section, which has high torsional stiffness and high bending stiffness due to increased section depth, while Auto Union used tubular section. The original Mercedes design was further improved by mounting the cross members through the side rails and welding on both sides. The efficiency of twin tube chassis' is usually low due to the weight of the large tubes. They were still in use into the 1950's, the 1958 Lister-Jaguar being an example of this type [Ref. 1]. A typical twin-tube chassis is shown in Fig. 2 opposite.

Fig. 2 [Ref. 2]

A significant increase in bending stiffness was realised but there is little increase in the torsional stiffness due to the lack of triangulation causing lozenging of the bays.

Fig. 3 Lotus 21 ...