An automobile undercarriage component comprising: a web; and a rib; wherein the rib includes a first section and a second section,, the first section being the widest section, the second section being narrower than the first section; and wherein the second section is narrower than the first section as an inner wall surface formed continuously from the web is disposed in proximity to an opposing outer wall surface; wherein the first section includes a first inner wall surface formed continuously from the web and a first outer wall surface opposing the first inner wall surface, the first inner wall surface having an inclination angle θ of θa (°), an angular portion formed on the leading end of the first inner wall surface having a curvature radius R of Ra (mm), the first outer wall surface having an inclination angle θ of θb (°), an angular portion formed on the leading end of the first outer wall surface having a curvature radius R of Rb (mm), a planar portion having a width of ws (mm)and formed between the two angular portions (42,44) having widths wa and wb respectively, so that the width w of the first section of the rib is expressed by the equation w = wa + ws + wb; wherein the second section includes a second inner wall surface being the inner wall surface and a second outer wall surface a being the outer wall surface, the second inner wall surface having an inclination angle θ of θa' (°), an angular portion formed on the leading end of the second inner wall surface having a curvature radius R of Ra' (mm), the second outer wall surface having an inclination angle θ of θb'(°), an angular portion formed on the leading end of the second outer wall surface having a curvature radius R of Rb' (mm), a planar portion having a width of ws' (mm)and formed between the two angular portions (42',44') having widths wa' and wb' respectively, so that the width w' of the second section of the rib is expressed by the equation w' = wa' + ws' + wb'; and wherein wa, wb, wa', and wb' are defined to satisfy the equations wa = Ra × cosθa ⋅ Ra − Ra × sinθa × tanθa wb = Rb × cosθb ⋅ Rb − Rb × sinθb × tanθb wa' = Ra' × cosθa' ⋅ Ra' − Ra' × sinθa' × tanθa' wb' = Rb' × cosθb' ⋅ Rb' − Rb' × sinθb' × tanθb'; and wherein the difference L (mm) in width between the first section and the second section is expressed by the equation L = {(wa - wa') + (wb - wb') + (ws - ws')} and L > 0. a web; and a rib; wherein the rib includes a first section and a second section,, the first section being the widest section, the second section being narrower than the first section; and wherein the second section is narrower than the first section as an inner wall surface formed continuously from the web is disposed in proximity to an opposing outer wall surface; wherein the first section includes a first inner wall surface formed continuously from the web and a first outer wall surface opposing the first inner wall surface, the first inner wall surface having an inclination angle θ of θa (°), an angular portion formed on the leading end of the first inner wall surface having a curvature radius R of Ra (mm), the first outer wall surface having an inclination angle θ of θb (°), an angular portion formed on the leading end of the first outer wall surface having a curvature radius R of Rb (mm), a planar portion having a width of ws (mm)and formed between the two angular portions (42,44) having widths wa and wb respectively, so that the width w of the first section of the rib is expressed by the equation w = wa + ws + wb; wherein the second section includes a second inner wall surface being the inner wall surface and a second outer wall surface a being the outer wall surface, the second inner wall surface having an inclination angle θ of θa' (°), an angular portion formed on the leading end of the second inner wall surface having a curvature radius R of Ra' (mm), the second outer wall surface having an inclination angle θ of θb'(°), an angular portion formed on the leading end of the second outer wall surface having a curvature radius R of Rb' (mm), a planar portion having a width of ws' (mm)and formed between the two angular portions (42',44') having widths wa' and wb' respectively, so that the width w' of the second section of the rib is expressed by the equation w' = wa' + ws' + wb'; and wherein wa, wb, wa', and wb' are defined to satisfy the equations wa = Ra × cosθa ⋅ Ra − Ra × sinθa × tanθa wb = Rb × cosθb ⋅ Rb − Rb × sinθb × tanθb wa' = Ra' × cosθa' ⋅ Ra' − Ra' × sinθa' × tanθa' wb' = Rb' × cosθb' ⋅ Rb' − Rb' × sinθb' × tanθb'; and - wb') + (ws - ws')} and L > 0.