Structure of underrun protection

1. A structure of an underrun protection bound to a plurality of brackets disposed at a front or a rear portion of a frame of a vehicle, comprising:

a hollow having a closed section defined by a distal face in a longitudinal direction of the vehicle, a proximal face in the longitudinal direction, an upper face, and a lower face; and

a lightening hole formed on at least one of the upper face and the lower face of the underrun protection, wherein the lightening hole is not disposed at a point on the at least one of the upper face and the lower face of the underrun protection where a load on the underrun protection is a maximum load.

2. The structure according to claim 1 , wherein the lightening hole is formed in a part covering a middle portion of the underrun protection along the longitudinal direction of the vehicle and including an equidistance portion from the distal face and the proximal face.

3. The structure according to claim 1 , further comprising a plurality of lightening holes and wherein respective hole diameters of the plurality of lightening holes along the longitudinal direction of the vehicle are a same diameter.

4. The structure according to claim 1 , wherein the lightening hole is formed on both the upper face and the lower face.

5. The structure according to claim 4 , wherein the lightening holes are formed at a same horizontal position on the upper face and the lower face.

6. The structure according to claim 1 , wherein each of the distal face, the proximal face, the upper face, and the lower face is a flat plate and wherein the closed section has a rectangular shape.

7. The structure according to claim 1 , wherein the underrun protection comprises a distal panel disposed on a distal side in the longitudinal direction of the vehicle and a proximal panel disposed on a proximal side in the longitudinal direction of the vehicle and wherein the distal panel and the proximal panel are engaged with each other to form the hollow having the closed section.

8. The structure according to claim 7 , wherein:

the distal panel has a channel-shaped section;

the proximal panel has a channel-shaped section or a straight line section; and

the distal face in the longitudinal direction of the vehicle is formed of a web of the distal panel, the proximal face in the longitudinal direction of the vehicle is formed of a web of the proximal panel, and the upper face and the lower face are each formed of flanges of the distal panel or the proximal panel.

9. The structure according to claim 7 , wherein:

an edge of a flange of the distal panel extends to a position including a part where the lightening hole is formed; and

an inner face of the edge of the flange of the distal panel is in contact with and is engaged with an outer face of an edge of a corresponding flange of the proximal panel by welding.

10. The structure according to claim 7 , wherein the distal panel comprises edge bents formed by bending the distal panel toward the proximal panel at both edge portions in an extending direction of the underrun protection and wherein the proximal panel is formed to have a uniform cross section along the extending direction.