TYPES OF CHASSIS FRAMES

LADDER FRAME The ladder type frame consists of two C section side members with welded-in tubular cross- members. The ladder frame is so called because it resembles a ladder with two side rails and a number of cross beams. Originally seen on almost all vehicles, the ladder frame was gradually phased out due to invention of better frames.

ADVANTAGES OF LADDER FRAME The greatest advantage of the ladder frame is its adaptability to accommodate a large variety of body shapes and types. Bodies ranging from flat platforms, box vans and tankers to detachable containers can all be easily attached to ladder frames. Ladder frames exhibit good bending strength and stiffness. The open channel section provides easy access for attaching brackets and components.

DISADVANTAGES OF LADDER FRAME The ladder frame exhibits poor resistance to torsion or warping if simple, perpendicular cross members are used. Also, the vehicle's overall height will be higher due to the floor pan sitting above the frame instead of inside it. If the open sections are replaced by closed box sections then the torsional stiffness is greatly improved. However, the strength of the joints becomes critical as the maximum bending on all members occurs at the joints and the attachment of brackets becomes more complex.

CRUCIFORM FRAMES The cruciform frame is made of two straight beams placed perpendicular to each other. The cruciform frame overcomes the poor torsional stiffness of the ladder frame. Here the torsional loads gets cancelled among the cross members and the side frame bears only bending loads.

ADVANTAGES It overcomes the weak torsional stiffness of ladder frame. When combined with a ladder frame, it can effectively take bending and torsional loads. The cross beams at the front and rear not only assist in carrying the torsion moment but also assist in carrying the lateral loads from the suspension mounting points.

DISADVANTAGES The maximum bending occurs at the joint hence joint design becomes critical. Since cruciform frame is used along with a ladder frame, it shares some of the disadvantages of ladder frame such as increased vehicle’s overall height.

SPACE FRAME The frames described earlier are all essentially 2-dimensional or at least their depth is very much less than their length and breadth. Adding depth to a frame considerably increases its bending strength and stiffness (i.e. truss type bridges). 3-dimensional Space frames have been used for specialist cars such as sports racing cars. This type of vehicle design can be used for low volume production with G.R.P. bodies.

ADVANTAGES Like a truss, a space frame is strong because of the inherent rigidity of the triangle. Space frames can be used to span large areas with few interior supports. In a spaceframe chassis, the suspension, engine, and body panels are attached to a skeletal frame of tubes, and the body panels have little or no structural function. Due to the depth of the frame, it is more stiffer to bending and torsional loads than the two dimensional frames. All the bending and torsional loads and the lateral loads are effectively resolved into tension and compression loads on the tubes.

DISADVANTAGES A drawback of the spaceframe chassis is that it encloses much of the working volume of the car and can make access for both the driver and to the engine difficult.  In this type of structure it is imperative to ensure all planes are fully triangulated so that the beam elements are essentially loaded in tension or compression. Due to the welded joints some bending and torsion restraints will occur at the joints, but to rely on these restraints will render the structure far less stiff.

INTEGRAL STRUCTURES The modern mass-produced passenger car is almost exclusively produced with sheet steel pressings spot welded together to form an integral structure. This is a structure where the component parts provide both structural and other functions. The depth of a structure such as a space frame, can improve the stiffness and in the integral structure the whole side frame with its depth and the roof are made to contribute to the vehicle bending and torsional stiffness.

ADVANTAGES It is stiffer in bending and torsion. It is of lower weight than when using a chassis and separate body. It can be produced with lower cost. It produces a quieter car for the passengers.

DISADVANTAGES Integral body is geometrically very complicated and the detailed stress distribution can only be determined by the use of Finite Element methods. The stress distribution within the structure is not only a function of the applied loads but also of the relative stiffness of the many components. During an impact the body will crumple completely beyond repair, whereas in a body-on-frame, only the body will be damaged and the chassis can be reused.

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