Ladder frame for internal combustion engine

A ladder frame for an internal combustion engine that supports a crankshaft between the ladder frame and a cylinder block, the ladder frame comprising:

• a first lateral wall and a second lateral wall;
• metallic crank caps;
• and
• first joining portions and second joining portions joining the crank caps respectively to the first lateral wall and the second lateral wall, wherein
• the first lateral wall and the second lateral wall are made of a metallic material having a lower rigidity than a rigidity of the crank caps,
• each of the crank caps includes an arc-shaped center portion, a first lateral portion and a second lateral portion that are located at positions where the center portion is interposed between the first lateral portion and the second lateral portion,
• each first lateral portion is joined to the first lateral wall via each first joining portion, and each second lateral portion is joined to the second lateral wall via each second joining portion,
• each center portion includes a supporting portion that rotatably supports the crankshaft, each center portion having a recess on an opposite side to the supporting portion,
• a residual portion is joined to each recess in such a manner as to be discontinuous from the first joining portion and the second joining portion, the residual portion being made of the same material as a material of the first joining portion and the second joining portion,
• respective thicknesses of each first lateral portion and each second lateral portion are the same as a thickness of the center portion,
• each recess is provided with a projection embedded in the residual portion so as to suppress the residual portion from coming off from the recess,
• each projection is provided at a position set back from a predetermined line segment, and
• the predetermined line segment is a line segment that passes through a center of a rotation axis of the crankshaft and is parallel to a reciprocating direction of a piston in synchronization with the crankshaft.