Pneumatic tire with tread having sipes with expanded diameter sections

1. A pneumatic tire including a tread section provided with circumferential grooves extending in a tire circumferential direction, lateral grooves extending in a tread width direction, and a plurality of land blocks defined by the circumferential grooves and the lateral grooves, each of the land blocks provided with a plurality of sipes formed to extend to a predetermined depth from a tread surface inward in a tire radial direction, each of the sipes having an end portion communicating with at least any one of the circumferential grooves and the lateral grooves, wherein

at least one of the plurality of sipes includes an expanded diameter section on an inner end of the sipe in the tire radial direction,

the expanded diameter section is smallest in diameter in a center portion thereof and largest in diameter in side portions thereof,

the center portion of the expanded diameter section is formed to increase in diameter gradually and smoothly toward each of sidewalls of the land block, and

each side portion of the expanded diameter section has a substantially constant diameter along a length of 10% to 35% of the entire length of the expanded diameter section from both ends of the expanded diameter section in the longitudinal direction thereof.

2. The pneumatic tire according to claim 1 , wherein each sipe extends in the tread width direction.

3. The pneumatic tire according to claim 1 , wherein the sipe extends in the tire circumferential direction.

4. A pneumatic tire including a plurality of land blocks defined by circumferential grooves extending in a tire circumferential direction and lateral grooves extending in a tread width direction, each of the land blocks provided with a plurality of sipes formed to extend to a predetermined depth from a tread surface inward in a tire radial direction, each of the sipes having an end portion communicating with at least any one of the circumferential grooves and the lateral grooves, at least one of the plurality of sipes including an expanded diameter section on an inner end of the sipe in the tire radial direction, the expanded diameter section formed by increasing a width of the sipe, wherein

in a bottom portion of the sipe, a shallow bottom portion where the depth of the sipe is smallest is formed in a center portion of the sipe in a longitudinal direction thereof,

the expanded diameter section is smallest in diameter in a center portion thereof and largest in diameter in side portions thereof,

in the center portion of the expanded diameter section, the expanded diameter section curves and extends from the shallow bottom portion toward each of sidewalls of the land block, and increases gradually in size from the shallow bottom portion toward each of the sidewalls of the land block, and

each side portion of the expanded diameter section has a substantially constant diameter along a length of 10% to 35% of the entire length of the expanded diameter section from both ends of the expanded diameter section in the longitudinal direction thereof.

5. The pneumatic tire according to claim 4 , wherein as the expanded diameter section extends from the shallow bottom portion toward the sidewall of the land block, the sipe depth gradually becomes larger.

6. The pneumatic tire according to claim 4 , wherein the sipe extends in the tread width direction.

7. The pneumatic tire according to claim 4 , wherein the sipe extends in the tire circumferential direction.

8. The pneumatic tire according to claim 4 , wherein the expanded diameter section gradually increases in diameter extending from the shallow bottom portion toward each of the sidewalls of the land block.

9. The pneumatic tire according to claim 4 , wherein the expanded diameter section is shaped like a circle in a cross section intersecting at right angles a longitudinal direction of the sipe in a plan view of the tread surface.