1 DESCRIPTION CAB BACK PANEL 5 Technical Field [0001] The present invention relates to a cab back panel that serves as a partition provided at the rear of a cabin of a vehicle. Background Art 10 [0002] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was, in Australia, known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. 15 [0002A] Throughout the description of this specification the word "comprise" and variations of that word, such as "comprises" and "comprising", are not intended to exclude other additives or components or integers. [0003] 20 JP-A-11-34931 discloses a vibration isolation structure having plural reinforcing beads formed on a cab back panel of a flat-plate shape, and arranged parallel to each other in the vehicle-width direction. In the structure, the reinforcing beads protrude toward the front side of the vehicle while extending in the up-and down direction. According to this vibration isolation structure, while the increase in the thickness of 25 the cab back panel itself is suppressed, the in-plane stiffness thereof can be increased. For this reason, the cab back panel can have a lower level of the vibration in the low-frequency range and the abnormal noise generation can be reduced without causing any increase in cost or weight. [0004] 30 Patent Document 1: JP-A-1 1-34931 Disclosure of Invention Problems to be Solved by the Invention [0005] SPEC-844089.doc 2 In the above-described structure, however, the reinforcing beads are the only means for increasing the in-plane stiffness of the cab back panel. For this reason, there is a limit to reduce the weight of the cab back panel by way of the reduction in the plate thickness. 5 [0006] The present invention is made in view of the above-described circumstances. An object of the present invention is providing a cab back panel capable of securing enough in-plane stiffness to reduce the abnormal noise generation and of reducing the weight of the cab back plate by further reducing the plate thickness. 10 Means for Solving the Problems [0007] A cab back panel according to the present invention is disposed along a vehicle-width direction to stand upright, and which serves as a partition at the rear of 15 a cabin, the cab back panel comprising: a base portion having first regions and a second region; and a plurality of beads formed by bending the base portion, wherein the first regions are located on the respective outer sides of the base portion in the vehicle width direction, 20 the second region is located at a central portion of the base portion in the vehicle-width direction, each the beads extends in the up-and-down direction, the beads are arranged along the vehicle-width direction, the base portion is curved so that the second region bulges to one side of the 25 front side of the vehicle and the rear side of the vehicle rather than the first regions, the beads include first beads formed by bending the first regions and second beads formed by bending the second regions, each the first beads protrudes to the one side of the front side and rear side of the vehicle, and 30 each the second bead protrudes to the another one of the front side and the rear side of the vehicle. SPEC-844089 doc 2A [0008] In the above-described configuration, the base portion of the cab back panel has a shape bulging towards one side. For this reason, the cab back panel of the above-described configuration has higher in-plane stiffness than the in-panel stiffness 5 of a case where the base portion is formed in a flat plate shape. That is, in addition to the first and the second beads, the curved shape of the base portion can contribute to the increase in the in-plane stiffness of the cab back panel. Accordingly, while enough in-plane stiffness to reduce the abnormal noise generation can be secured, the weight of the cab SPEC-844089.doc 418000100AU specification back plate can be reduced by reducing the plate thickness. [0009] In addition, in the first regions located on the two outer sides of the base portion in the vehicle-width direction, the first beads protrude in the same direction as the direction in which the base portion bulges. In the second region located at the central portion of the base portion in the vehicle-width direction, the second bead protrudes in the opposite direction to the direction in which the base portion bulges. Accordingly, the area occupied by the cab back panel in the front-to-rear direction of the vehicle can be kept smaller than a case where the first and the second beads protrude in the same direction. As a consequence, the limited vehicle space can be effectively utilized. Effects of the Invention [0010] According to the present invention, while enough in-plane stiffness to reduce the abnormal noise generation is secured, the weight of the cab back plate can be reduced by reducing the plate thickness. In addition, the area occupied by the cab back panel in the front-to-rear direction of the vehicle can be kept small. Brief Description of the Drawings [0011] [Fig. 1] Fig. 1 is a rear view of a cab back panel seen from the rear side of the vehicle according to an embodiment of the present invention. [Fig. 2] Fig. 2 is a sectional view taken along the line II-II of Fig. 1. [Fig. 3] Fig. 3 is an exploded perspective view of Fig. 1. [Fig. 4] Fig. 4 is a perspective view of a cab over truck equipped with the cab back panel of Fig. 1. Description of Reference Numerals 3 418000100AUspecification [0012] 1. vehicle (cab over truck) 2. cabin 3. cab back panel 4. base portion 5. first beads 6. second bead 7. rear window panel 8. rear window opening 9. reinforcing member 10. joint portions 11. bed Best Mode for Carrying out the Invention [0013] An embodiment of the present invention will be described below with reference to the drawings. [0014] Fig. 1 is a rear view of a cab back panel seen from the rear side of the vehicle according to this embodiment. Fig. 2 is a sectional view taken along the line II-II of Fig. 1. Fig. 3 is an exploded perspective view of Fig. 1. Fig. 4 is a perspective view of a cab over truck equipped with the cab back panel of Fig. 1. Note that "FR" in the drawings indicates the front side of the vehicle, and "UP" indicates the upside of the vehicle. [0015] As Fig. 4 shows, a vehicle 1 includes a cabin 2 and a bed 11, and the bed 11 is disposed at the rear of the cabin 2. A cab back panel 3 is disposed along the vehicle-width direction and stands substantially upright, and serves as a partition at the rear of the cabin 2. Note that a cab over truck is illustrated as the vehicle 1 in this embodiment, but the structure of the cab back panel 3 of the present invention is also applicable to various other types of vehicles. (0016] 4 418000100AU specification As Figs. 1 to 4 show, the cab back panel 3 is a metal panel pressed into shapes, and includes a base portion 4, first beads 5, and second beads 6. [0017] The base portion 4 is the part of the cab back panel 3 other than the first beads 5 and the second beads 6. The entire body of the base portion 4 is curved with the center portion thereof in the vehicle-width direction bulging towards the rear side of the vehicle. This curved shape increases the in-plane stiffness of the cab back panel 3. [0018] A rear window opening 8, to which a rear window panel 7 is fitted, is formed in an upper middle portion of the base portion 4. The base portion 4 includes joint portions 10 which are integrally formed in the periphery of the base portion 4. The joint portion 10 of the upper edge is joined to a ceiling panel (not illustrated) that forms the outer surface of the ceiling for the cabin 2. The joint portions 10 of the lateral side edges are joined respectively to side outer panels (not illustrated) that form the outer side surfaces of the cabin 2. The joint portion 10 of the lower edge is joined to a cross member (not illustrated) that is a framework member of the vehicle. [0019] In first regions located at the two outer sides of the base portion 4 in the vehicle-width direction, the plural (specifically, two in this embodiment) first beads 5 are aligned so as to be substantially parallel to one other. In this embodiment, the first regions are set at the outer sides of the rear window opening 8 in the vehicle-width direction. Each of the first beads 5 is formed by bending part of the base portion 4 into a substantially trapezoidal shape so as to protrude toward the rear side of the vehicle and to extend in the up-and-down direction from the vicinity of the upper edge of the base portion 4 to the vicinity of the lower edge thereof. (0020] In a second region located at the central portion of the 5 418000100AU specification base portion 4 in the vehicle-width direction, the plural (specifically, five in this embodiment) second beads 6 are aligned so as to be substantially parallel to one another. In this embodiment, the second region is set below the rear window opening 8. Each of the second beads 6 is formed by bending part of the base portion 4 into a substantially trapezoidal shape so as to protrude toward the front side of the vehicle and to extend in the up-and-down direction from the vicinity of the lower edge of the rear window opening 8 to the vicinity of the lower edge of the base portion 4. [0021] These plural first and second beads 5 and 6 are arranged almost equidistantly along the vehicle-width direction. The first and the second beads 5 and 6 increase the in-plane stiffness of the cab back panel 3. [00221 A reinforcing member 9 is joined to the front-side surface of the cab back panel 3. The reinforcing member 9 is disposed below the rear window opening 8, and extends in the vehicle-width direction. In addition, in portions of the reinforce member 9 which comes into contact with the cab back panel 3, recessed portions 9a are formed. The recessed portions 9a respectively accommodate the second beads 6 that protrude towards the front side of the vehicle. [0023] Each second bead 6 is set to protrude from the base portion 4 in such a manner that the upper-side portion, housed in the corresponding recessed portion 9a of the reinforcing member 9, of the second bead 6 is slightly lower than the lower-side portion thereof. Each of the first beads 5 protrudes from the base portion 4 with a height that is substantially equal to the protruding height of the lower-side portion of each of the second beads 6. [0024] In this embodiment, the base portion 4 of the cab back panel 3 has a shape that bulges toward one direction 6 418000100AU_specification (specifically, toward the rear side of the vehicle). Accordingly, the in-plane stiffness of the cab back panel 3 is higher than a case where the base 4 is formed into a flat plate shape. That is, in addition to the first and the second beads 5 and 6, the curved shape of the base portion 4 can contribute to the increase in the in-plane stiffness of the cab back panel 3. Accordingly, while enough in-plane stiffness to reduce the abnormal noise generation is secured, the weight of the cab back plate 3 can be reduced by reducing the plate thickness. [0025] In addition, in the first regions located at the two outer sides of the base portion 4 in the vehicle-width direction, the first beads 5 protrude in the same direction as the direction in which the base portion 4 bulges (i.e. towards the rear side of the vehicle) . In the second region located at the central portion of the base portion 4 in the vehicle-width direction, the second beads 6 protrude in the opposite direction to the direction in which the base portion 4 bulges (i.e., towards the front side of the vehicle) . Accordingly, the area occupied by the cab back panel 3 in the front-to-rear direction of the vehicle can be kept smaller than a case where the first and the second beads 5 and 6 protrude in the same direction. As a consequence, the limited vehicle space can be effectively utilized. [0026] When the base portion 4 is curved with the center portion thereof in the vehicle-width direction bulging towards the front side of the vehicle, in the first regions located on the two outer sides in the vehicle-width direction, the first beads 5 protrude toward the front side of the vehicle, and in the second region located at the central portion in the vehicle-width direction, the second beads 6 protrude toward the rear side of the vehicle. [0027] Incidentally, the cross-sectional shape of each of the first beads 5 and of each of the second beads 6 is not limited 7 418000100AU specification to the substantially trapezoidal shape as described above, and may have various cross-sectional shapes, such as a substantially arc shape and a substantially rectangular shape. [0028] In addition, the rear window opening 8 is formed in the cab back panel 3 in the embodiment described above, but the present invention is applicable to a cab back panel with no rear window opening 8 formed therein. Field of the Industrial Application [0029] The cab back panel structure of the present invention is applicable to various types of vehicles. 8