Field of the Invention
The present invention relates to pneumatic tires, and in particular relates to a pneumatic tire being provided with a middle lug groove including a sipe portion formed between a protrusion on a first groove wall and a second groove wall.
Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2011-31773 discloses a pneumatic tire capable of improving wet performance and noise performance in good balance while ensuring steering stability.
The tire includes a tread portion provided with a crown main groove and a shoulder main groove to form a crown rib, a middle rib and a shoulder rib. The middle rib is provided with lug grooves extending from the shoulder main groove to improve drainage property of the middle rib. Furthermore, the middle rib has sufficient rigidity to ensure steering stability in high level since the lug grooves include axially inner ends that terminate within the middle rib. In addition, since the tire prevents that an air compressed between a lug groove and the ground flows into the crown main groove, the pipe resonance generated from the crown main groove, which is a main cause of road noise, can be reduced.
However, since the above mentioned lug grooves include outer ends having a large width, it has been insufficient to prevent reduction of the rigidity of the middle rib. Furthermore, the air compressed by the lug grooves flows into the shoulder main groove to promote the pipe resonance. Therefore, noise reduction effect has also been insufficient.
As illustrated in FIGS. 9A and 9B, Japanese Unexamined Patent Application Publication No. 2013-233822 discloses a tire tread provided with a shoulder main groove (a) and a lug groove (b), wherein the lug groove includes a tie-bar (c) provided with a sipe (d). Unfortunately, when a depth (h) of the tie-bar (c) is shallow, sufficient drainage performance cannot be ensured since water would be blocked by the tie-bar. On the other hand, when the depth of the tie-bar (c) is deep, it may be difficult to ensure steering stability and noise performance of the tire. Accordingly, it has been difficult to improve steering stability, wet performance and noise performance at a high level.