Pneumatic tire

1. A pneumatic tire comprising:

at least two crossed belt layers disposed on an outer circumferential side of a carcass layer in a tread portion;

at least one circumferential-direction reinforcing belt layer disposed between the crossed belt layers and having a width smaller than those of the crossed belt layers;

a stress relaxation layer disposed between the crossed belt layers while lying adjacent to an edge portion of and outside, in a width direction of, the circumferential-direction reinforcing belt layer, the stress relaxation layer being made of a rubber composition having a fixed thickness; and

an edge-portion buffering layer, made of a rubber composition, disposed between the crossed belt layers and at a position corresponding to edge portions of the crossed belt layers,

wherein the stress relaxation layer is disposed over an entire region between the circumferential-direction reinforcing belt layer and the edge-portion buffering layer.

2. The pneumatic tire according to claim 1 , wherein the rubber composition constituting the stress relaxation layer has a smaller modulus at 100% elongation than a modulus at 100% elongation of a rubber composition that covers cords of the crossed belt layers.

3. The pneumatic tire according to claim 1 , wherein a modulus Ea at 100% elongation of the rubber composition constituting the stress relaxation layer and a modulus Eco at 100% elongation of a rubber composition that covers cords of the crossed belt layers satisfy a relation of 0.6 ≦Ea/Eco ≦0.9.

4. The pneumatic tire according to any one of claims 1 to 3 , wherein the modulus at 100% elongation of the rubber composition constituting the stress relaxation layer is within a range from 4.0 MPa to 5.5 MPa.

5. The pneumatic tire according to any one of claims 1 to 3 , wherein:

the circumferential-direction reinforcing belt layer has a cord angle of 0 degrees to 5 degrees with respect to the tire circumferential direction, and

the circumferential-direction reinforcing belt layer has a width that is from 60% to 75% of a carcass section width.

6. The pneumatic tire according to claim 5 , wherein:

each of the crossed belt layers has a cord angle of 10 degrees to 45 degrees with respect to the tire circumferential direction, and

the crossed belt layer has a width greater than that of the circumferential-direction reinforcing belt layer by at least 10% of the carcass section width.

7. The pneumatic tire according to any one of claims 1 to 3 wherein an inter-cord rubber gauge t of a portion where the stress relaxation layer is interposed between the crossed belt layers satisfies a relation of 0.8D≦t≦1.7D where D represents a cord diameter of the circumferential-direction reinforcing belt layer.

8. The pneumatic tire according to any one of claims 1 - 3 , further comprising:

a high-angle belt layer disposed between the carcass layer and the crossed belt layers, the high-angle belt layer having a cord angle of 45 degrees to 90 degrees with respect to the tire circumferential direction and having a width of 60% to 85% of a carcass section width.

9. The pneumatic tire according to claim 8 , further comprising:

a protection belt layer disposed on the outer circumferential side of the crossed belt layers, the protection belt layer having a cord angle of 10 degrees to 45 degrees with respect to the tire circumferential direction, and having a width of 60% to 85% of the carcass section width.

10. The pneumatic tire according to any one of claims 1 to 3 wherein the pneumatic tire has an aspect ratio of 60% or less.