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Although the carrying capacity of truck side rails is directly related to their cross sectional properties, and limited thereby, they must be adequately positioned and stabilized by “attachments” if their full potential is to be realized. Furthermore, while the capacity of a truck frame cannot exceed the capability of its rails, such value is sometimes not achieved. Frame rails do fail at lesser values, at points other than maximum bending stress. Why this happens and how it affects designers is discussed in detail with special emphasis on the structural nature of frame parts; primary input forces; stress concentration; action of flat surfaces; and side load generation and influences. Illustrations graphically depict the small but destructive distortions that can occur at attachment points.

THE eventual solution to the question: “Shall it be of unit design or shall it have a separate frame?” will be known only when a complete understanding is reached of the ride problem, including car feel and quietness, and a better understanding is obtained of the structure's influence on these performance phases, Mr. Sherman believes. Such knowledge, he predicts, will result eventually in the lightest, least-expensive and best performing automobile, be it unitary in structure, conventional, or something as yet unconsidered. His paper brings out some of these considerations which have proved to be of paramount importance in research work conducted by his company. Pointing out that it is impossible to predict ahead of time just how a new car design will perform, he contends that it will be much easier to adjust the job to the best rigidity range with the separate-frame construction than with the unitary construction.

ALONG with larger tires, independent wheel suspensions, higher speeds, and the dynamic and vibrational problems associated with these innovations, came the need for chassis frames having a high resistance to torsional movements. The X-member-type frame has been the most generally adopted means for obtaining increases in this direction. However, the past few years have seen the need for torsional rigidity in the chassis frame to be intensified. Although considerable gains have been made, in general, these gains have been accomplished not by major improvements in the design of the structure but by the addition of material. Consequently, the weight of the chassis frame has become a serious problem, so much so that in many cases special heavy frames are being used for the open-body types where the need for a stiff frame is acute.