A kingpin assembly includes an aluminum frame member (16) formed of two identical extrusions. Each extrusion includes a box beam (18, 20) having a top (40) three vertical walls (42, 44, 46) and a bottom (47) including forward and rearward flanges. The forward extrusion includes a box beam (18), a rearward bottom flange (28) and a forward bottom flange (30). The rearward extrusion includes a box beam (20) a rearward bottom flange (24) and a forward bottom flange (26). Each box beam extrusion is positioned end-for-end from the other box beam extrusion. A steel kingpin (KP) is connected to an aluminum kingpin mounting block (36) which is positioned at the center of the frame member (16) between the box beams (18, 20). The box beam extrusions are welded together by a single transverse weld (W). The forward bottom flange (30) on the forward extrusion is connected to a forward panel (60) by a single longitudinal weld (W). The aluminum kingpin mounting block (36) is flanked by a pair of longitudinal channel beams (32, 34) which at their ends are connected to the box beams (18, 20).