Pneumatic tire

1. A pneumatic tire comprising:

a tread portion which is provided with tread grooves defining a tread pattern and which has an outboard tread edge and an inboard tread edge which are intended to be positioned away from the center of a vehicle body and close to the center of the vehicle body, respectively,

wherein the tread grooves include an outboard shoulder main groove and an outboard crown main groove which are disposed on the outboard tread edge side of the tire equator and extend zigzag continuously in the tire circumferential direction,

wherein the outboard shoulder main groove is composed of alternate long groove segments and short groove segments which are inclined with respect to the tire circumferential direction to the outboard tread edge toward one side in the tire circumferential direction,

wherein the long groove segments are inclined at an angle θa with respect to the tire circumferential direction,

wherein the short groove segments are inclined at an angle θb with respect to the tire circumferential direction which is larger than the angle θa of the first groove segments,

wherein groove widths of the long groove segments gradually increase toward the outboard tread edge,

wherein the outboard crown main groove is composed of alternate first groove segments and second groove segments, wherein the first groove segments are inclined with respect to the tire circumferential direction to the inboard tread edge toward one side in the tire circumferential direction,

wherein the second groove segments are inclined with respect to the tire circumferential direction to the outboard tread edge toward one side in the tire circumferential direction,

wherein the tread grooves include outboard middle lateral grooves extending from the outboard crown main groove to the outboard shoulder main groove so as to be linearly continued to the respective short groove segments of the outboard shoulder main groove,

wherein the tread grooves include secondary middle lateral grooves which are alternated with the outboard middle lateral grooves so as to not be linearly continued with the short groove segments of the outboard shoulder main groove and extend from the outboard crown main groove to the outboard shoulder main groove,

wherein groove widths of the secondary middle lateral grooves gradually increase toward the outboard tread edge,

wherein the tread grooves include outboard shoulder lateral grooves extending from the outboard shoulder main groove towards the outboard tread edge, and

wherein the outboard shoulder lateral grooves are linearly continued to the respective secondary middle lateral grooves through the outboard shoulder main groove.

2. The pneumatic tire according to claim 1 , wherein groove widths of the outboard middle lateral grooves are gradually increased toward the outboard tread edge.

3. The pneumatic tire according to claim 1 , wherein the outboard middle lateral grooves are connected to the outboard crown main groove so as to be linearly continued to the respective second groove segments of the outboard crown main groove.

4. The pneumatic tire according to claim 3 , wherein the tread grooves include

an inboard crown main groove which extends straight continuously in the tire circumferential direction and is disposed adjacently to the outboard crown main groove, and

crown lateral grooves extending from the outboard crown main groove to the inboard crown main groove so as to be connected to the respective second groove segments of the outboard crown main groove.

5. The pneumatic tire according to claim 4 , wherein the crown lateral grooves and the outboard middle lateral grooves are inclined with respect to the tire circumferential direction so that, toward the above-mentioned one side in the tire circumferential direction, the crown lateral grooves are inclined to one side in the tire axial direction, and the outboard middle lateral grooves are inclined to the other side in the tire axial direction.

6. The pneumatic tire according to claim 4 , wherein outboard middle blocks, which are divided by the outboard middle lateral grooves, the outboard shoulder main groove and the outboard crown main groove, are each provided with a notch located in a position where an extension of one of the crown lateral grooves intersects with an extension of one of the first groove segments of the outboard crown main groove.

7. The pneumatic tire of claim 1 , wherein groove widths of the outboard shoulder lateral grooves gradually increase toward the outboard tread edge.

8. The pneumatic tire of claim 7 , wherein in each of the secondary middle lateral grooves and one of the outboard shoulder lateral grooves which is linearly continued thereto, the groove widths thereof gradually increase from the outboard crown main groove to the outboard tread edge.