VEHICLE BODY LOWER PART STRUCTURE

PROBLEM TO BE SOLVED: To provide a vehicle body lower part structure that is enhanced in rigidity by using a cross member having excellent torsional rigidity and shaft rigidity.SOLUTION: The vehicle body lower part structure comprises a vehicle body frame 1, and a first tunnel cross member 21 which is extended in a vehicle width direction and assembled to a lower part of the vehicle body frame. The first tunnel cross member 21 includes: a flat plate part 31 having a length enough to stride a tunnel part 13; a pair of fixed parts 32, arranged at both ends respectively of the flat plate part 31, which are fixed to the vehicle body frame 1; a pair of vertical wall parts 33 extending downward from both edges respectively in a vehicle body longitudinal direction of the flat plate part 31; and a rib 34 erected on a lower surface of the flat plate part 31 so as to connect the pair of vertical wall parts 33 to each other. The flat plate part 31 is offset toward a lower side of the vehicle body with respect to the pair of fixed parts 32. This can increase cross-section second polar moment of a cross section part constituted of the first tunnel cross member 21 and the tunnel part 13.SELECTED DRAWING: Figure 8 【課題】優れた捻り剛性を備えるクロスメンバを用いることで、剛性が高められた車両の下部車体構造を提供する。【解決手段】車両の下部車体構造は、車体フレーム１と、車幅方向に延び、前記車体フレームの下部に組み付けられる第１トンネルクロスメンバ２１とを備える。第１トンネルクロスメンバ２１は、トンネル部１３を跨ぐ長さを有する平板部３１と、平板部３１の両端に各々配置され、車体フレーム１に固定される一対の固定部３２と、平板部３１の車体前後方向の両端縁から各々下方に延びる一対の縦壁部３３と、一対の縦壁部３３同士を繋ぐように平板部３１の下面に立設されたリブ３４とを含む。平板部３１は、一対の固定部３２に対して、車体下方側に向けてオフセットしている。これにより、第１トンネルクロスメンバ２１とトンネル部１３とで構成される断面部分の断面二次極モーメントを大きくすることができる。【選択図】図８