Pneumatic tire

1. A pneumatic tire that includes a belt layer having a circumferential reinforcing layer and a pair of cross belts, comprising:

at least three or more circumferential main grooves having groove bottoms that are convex in the depth direction in a tread portion, wherein

in a cross-sectional view in the tire meridian direction, of the circumferential main grooves, a circumferential main groove having the circumferential reinforcing layer below the groove is a first circumferential main groove, and another circumferential main groove is a second circumferential main groove not having the circumferential reinforcing layer below the groove, and

the curvature radius RA of the groove bottom of an arbitrary first circumferential main groove and the curvature radius RB of the groove bottom of an arbitrary second circumferential main groove have the relationship RA<RB.

2. The pneumatic tire according to claim 1 , wherein the curvature radius RA of the groove bottom of an arbitrary first circumferential main groove and the curvature radius RB of the groove bottom of an arbitrary second circumferential main groove have the relationship 1.10≦RB/RA≦1.60.

3. The pneumatic tire according to claim 1 , wherein there is a plurality of second circumferential main grooves in a region bounded by the tire equatorial plane, and the curvature radius RB_out of the groove bottom of the second circumferential main groove that is in the outermost side in the tire width direction and the curvature radius RB of the groove bottom of the other second circumferential main grooves have the relationship RB<RB_out.

4. The pneumatic tire according to claim 1 , wherein the belt layer has a plurality of circumferential reinforcing layers laminated on each other, and there is a plurality of first circumferential main grooves in a region bounded by the tire equatorial plane, and at least one of the first circumferential main grooves has the plurality of circumferential reinforcing layers below the groove, and

the curvature radius RA_min of the groove bottom of the first circumferential main groove having the smallest number of circumferential reinforcing layers from among all the first circumferential main grooves and the curvature radius RA of the groove bottom of the other first circumferential main grooves have the relationship RA<RA_min.

5. The pneumatic tire according to claim 1 , wherein the circumferential reinforcing layer is disposed on the inner side in the tire width direction from the left and right edge portions of the cross belt with the narrower width from among the pair of cross belts, and the width W of the narrower cross belt and the distance S from the edge portion of the circumferential reinforcing layer to the edge portion of the narrower cross belt are within the range 0.03≦S/W.

6. The pneumatic tire according to claim 1 , wherein wire from which the circumferential reinforcing layer is made is steel wire, and the circumferential reinforcing layer has not fewer than 17 ends/50 mm and not more than 30 ends/50 mm ends.

7. The pneumatic tire according to claim 1 , wherein the diameter of the wire not less than 1.2 mm and not more than 2.2 mm.

8. The pneumatic tire according claim 1 , wherein the circumferential reinforcing layer is formed by winding a single steel wire in a spiral manner.

9. The pneumatic tire according to claim 1 , wherein the circumferential reinforcing layer is formed by winding a plurality of wires spirally adjacent to one another.

10. The pneumatic tire according to claim 9 , wherein the plurality of wires includes five or fewer wires.

11. The pneumatic tire according to claim 10 , wherein a width of winding per layer of the wires when the wires are would in multiple layers is 12 mm or less.

12. The pneumatic tire according to claim 9 , wherein the plurality of wires are would while inclined with respect to the tire circumferential direction in the range of ±5 degrees.

13. The pneumatic tire according to claim 9 , wherein a wire diameter is in the range of not less than 1.2 mm and not more than 2.2 mm.

14. The pneumatic tire according to claim 13 , wherein the wire diameter is measured as a diameter of a circle circumscribing the plurality of wires.

15. The pneumatic tire according to claim 1 , wherein elongation of belt cords of the circumferential reinforcing layer when subjected to a tensile load of 150 N to 200 N is not less than 2.0% and not more than 3.5%.

16. The pneumatic tire according to claim 1 , wherein:

the curvature radius RA of the groove bottom of an arbitrary first circumferential main groove and the curvature radius RB of the groove bottom of an arbitrary second circumferential main groove have the relationship 1.10≦RB/RA≦1.60; and

there is a plurality of second circumferential main grooves in a region bounded by the tire equatorial plane, and the curvature radius RB_out of the groove bottom of the second circumferential main groove that is in the outermost side in the tire width direction and the curvature radius RB of the groove bottom of the other second circumferential main grooves have the relationship RB<RB_out.

17. The pneumatic tire according to claim 1 , wherein:

the belt layer has a plurality of circumferential reinforcing layers laminated on each other, and there is a plurality of first circumferential main grooves in a region bounded by the tire equatorial plane, and at least one of the first circumferential main grooves has the plurality of circumferential reinforcing layers below the groove, and the curvature radius RA_min of the groove bottom of the first circumferential main groove having the smallest number of circumferential reinforcing layers from among all the first circumferential main grooves and the curvature radius RA of the groove bottom of the other first circumferential main grooves have the relationship RA<RA_min; and

the circumferential reinforcing layer is disposed on the inner side in the tire width direction from the left and right edge portions of the cross belt with the narrower width from among the pair of cross belts, and the width W of the narrower cross belt and the distance S from the edge portion of the circumferential reinforcing layer to the edge portion of the narrower cross belt are within the range 0.03≦S/W.

18. The pneumatic tire according to claim 1 , wherein:

wire from which the circumferential reinforcing layer is made is steel wire, and the circumferential reinforcing layer has not fewer than 17 ends/50 mm and not more than 30 ends/50 mm ends; and

the diameter of the wire not less than 1.2 mm and not more than 2.2 mm.