Pneumatic tire with tread having cutaway portions formed in center rib

1. A pneumatic tire comprising:

a pair of first main grooves which are formed in the center region of a tread and extend in the tire circumferential direction;

a pair of second main grooves which are formed respectively outward of the corresponding first main grooves and extend in the tire circumferential direction;

a plurality of first lug grooves and a plurality of second lug grooves which are alternately arranged in the tire circumferential direction, the first lug grooves each extending from one of two shoulder ends of the tread inward in the tire-width direction and communicating with the corresponding first main groove, the second lug grooves each extending from one of the two shoulder ends of the tread inward in the tire-width direction but not communicating with the corresponding first main groove;

a center rib which is defined between the pair of first main grooves and which extends continuously in the tire circumferential direction;

a shoulder block row which is defined between each of the second main grooves and a corresponding one of the shoulder ends and includes a plurality of blocks; and

an intermediate block row which is defined between each of the first main grooves and a corresponding one of the second main grooves and includes a plurality of blocks,

the pneumatic tire characterized in that:

each intermediate block row includes long blocks each of which has a tire-circumferential-direction length equivalent to the total length of each two blocks included in the corresponding shoulder block row;

a plurality of cutaway portions are formed in the center rib at certain intervals in the tire circumferential direction,

each cutaway portion has a triangular shape formed on a ground-contacting surface of the center rib and having an apex and a pair of end points all of which are connected to one another, and

each cutaway portion has a three-dimensional structure such that a valley line is formed by a first contour line connecting the apex to a bottom point defined at a position located at the bottom of the groove and between the pair of end points.

2. The pneumatic tire according to claim 1 characterized in that each intermediate block row includes:

the plurality of long blocks each of which has a tire-circumferential-direction length equivalent to the total length of each two blocks included in the corresponding shoulder block row; and

blocks which are arranged at the same pitch as the pitch for the plurality of blocks included in the corresponding shoulder block row.

3. The pneumatic tire according to claim 1 characterized in that the tread has a ratio of groove area of 25% to 40%, within a tire contact width.

4. The pneumatic tire according to claim 1 characterized in that the center rib has a width of 5% to 25% of the tire contact width.

5. The pneumatic tire according to claim 1 characterized in that

a plurality of sipes each of which extends in the tire-width direction are formed in the center rib, in each block of the intermediate block rows, and in each block of the shoulder block rows, and

the plurality of sipes formed in the center rib are arranged in the tire circumferential direction at smaller intervals than the intervals at which the sipes formed in any of the intermediate block rows and the shoulder block rows are arranged in the tire circumferential direction.

6. The pneumatic tire according to claim 1 characterized in that

at each end point of each cutaway portion, the center rib forms an acute angle portion, and, on the ground-contacting surface, the acute angle portion of the center rib has an angle α of 20° to 90°, and

at the apex of each cutaway portion, the cutaway portion forms an acute angle portion, and, on the ground-contacting surface, the acute angle portion of the cutaway portion has an angle β of 20° to 90°.

7. The pneumatic tire according to claim 6 characterized in that

each cutaway portion forms, on the ground-contacting surface of the center rib, a second contour line that extends in the tire-width direction and a third contour line that extends in the tire circumferential direction, and

the second contour line is shorter than the third contour line, and has a length of 10% to 50% of the width of the center rib.

8. The pneumatic tire according to claim 7 characterized in that the angle γ of the second contour line of each cutaway portion with respect to the tire-width direction is 0° to 60°.

9. The pneumatic tire according to claim 1 characterized in that, in each cutaway portion, a tire-circumferential-direction distance from the bottom point to one of the pair of end points that is closer to the apex is 10% to 50% of a distance between the pair of end points.