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.