A rail frame assembly for a motor vehicle comprises a rearward frame module having a pair of generally parallel rearward frame rail members interconnected by a rearward cross-member assembly. A central frame module includes a pair of generally parallel central frame rail members interconnected by a central cross-member assembly, the central frame members each having a generally U-shaped transverse cross-sectional configuration including a vertical base wall portion and a pair of outwardly extending generally parallel horizontal wall portions extending from opposite ends of the base wall portion. The U-shaped central frame members are each bent between two spaced positions thereof, and one of the horizontal wall portions extending from one end of the base wall portion has metallic grains therein substantially stretched at a location between the spaced positions. In addition, an opposite one of the horizontal wall portions extending from an opposite end of the base wall portion has metallic grains therein substantially compressed at the location between the spaced positions. In addition, a forward frame module comprises a pair of generally parallel tubular forward frame rail members interconnected by a forward cross-member assembly. The tubular forward frame rail members each having a generally rectangular transverse cross-section of irregular shape and size.