Pneumatic tire

1. A pneumatic tire comprising

a tread portion which is provided with zigzag circumferential grooves, axial grooves extending between the zigzag circumferential grooves, and axial grooves extending between the zigzag circumferential grooves and tread edges, so that a row of circumferentially arranged blocks is formed on each side of each said zigzag circumferential groove, wherein

each of the zigzag circumferential grooves is composed of axially inner circumferential segments, axially outer circumferential segments and oblique segments,

the axially inner circumferential segments and the axially outer circumferential segments are arranged alternately in the tire circumferential direction,

the oblique segments connect between the axially inner circumferential segments and the axially outer circumferential segments, whereby the zigzag circumferential groove extends in the form of a trapezoidal wave,

the zigzag amplitude measured axially between the widthwise center line of the inner circumferential segments and the widthwise center line of the outer circumferential segments is in a range of from 4 to 15% of a half tread width (TW/2) measured axially between the tire equator and the tread edge,

the width of the zigzag circumferential groove is in a range of from 7 to 13% of said half tread width (TW/2),

the number of the axial grooves connected to each said zigzag circumferential groove on both sides thereof is in a range of from 8 to 12 when counted in a footprint of the tire,

the axial grooves are connected to the circumferential segments at the respective intermediate positions of the lengths of the circumferential segments and not connected to the oblique segments,

the axial grooves each have a groove width WY in a range of from 8 to 25% of a pitch length Py between the axial groove and the circumferentially adjacent axial groove, and

each of the axial grooves has a U-shaped cross-sectional shape along the length thereof wherein the groove bottom is a semicircle.

2. The pneumatic tire according to claim 1 , wherein

in each said zigzag circumferential groove, angles θ between the circumferential segments and the oblique segments are in a range of from 110 to 150 degrees.

3. The pneumatic tire according to claim 1 or 2 , wherein

each said the axial groove has a straight configuration or alternatively a smoothly curved configuration.

4. The pneumatic tire according to claim 1 or 2 , wherein

said zigzag circumferential grooves are

an axially inner zigzag circumferential groove and

an axially outer zigzag circumferential groove

which are disposed on each side of the tire equator, and

the zigzag amplitude of the axially inner zigzag circumferential groove is less than the zigzag amplitude of the axially outer zigzag circumferential groove.

5. The pneumatic tire according to claim 4 , wherein

the groove width of the axially inner zigzag circumferential groove is less than the groove width of the axially outer zigzag circumferential groove.

6. The pneumatic tire according to claim 1 , wherein

said zigzag circumferential grooves are an axially inner zigzag circumferential groove and an axially outer zigzag circumferential groove which are disposed on each side of the tire equator, and

the axially inner zigzag circumferential groove and the axially outer zigzag circumferential groove are substantially parallel with each other.

7. The pneumatic tire according to claim 6 , wherein

the axially inner zigzag circumferential grooves and the axially outer zigzag circumferential grooves are arranged symmetrically about the tire equator.

8. The pneumatic tire according to claim 7 , wherein

the zigzag amplitude of the axially inner zigzag circumferential groove is less than the zigzag amplitude of the axially outer zigzag circumferential groove.

9. The pneumatic tire according to claim 1 , wherein

the axial grooves are parallel with the tire axial direction.