Abstract:
Stepped sections ( 110 ) whereby the height of a block ( 100 ) gradually decreases towards the outside of the tire peripheral direction from the center of the block ( 100 ) are formed in a peripheral direction end section ( 160 ) of the block ( 100 ). The stepped sections ( 110 ) have a first stepped surface ( 111   a ) and a second stepped surface ( 121   a ). A stepped surface ( 110   a ) extends from an outside end section ( 140 ) towards an inside end section ( 150 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2012/065621, filed on Jun. 19, 2012, which claims priority from Japanese Patent Application No. 2011-140437, filed on Jun. 24, 2011, the contents of all of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a tire including a block partitioned by a groove portion that includes a circumferential groove positioned at the outer side in a tread width direction from a tire equator line and extending in a tire circumferential direction and a lug groove extending in the tread width direction, in a tread planar view. 
     BACKGROUND ART 
     Conventionally, as a tire used for a snow-covered road surface, there has been known a tire including a block partitioned by a groove portion that includes a circumferential groove positioned at the outer side in a tread width direction from a tire equator line and extending in a tire circumferential direction and a lug groove extending in the tread width direction (see Patent Literature 1, for example). For example, in the tire of the Patent Literature 1, two block arrays in which a plurality of blocks are arranged in the tire circumferential direction are formed. 
     As described above, the block partitioned by the groove portion includes an edge portion biting into a snow surface to rub against the snow, and thus, the aforementioned tire has a snow traction performance. Furthermore, snow having entered the groove portion is trodden down by loads of the tire and a vehicle to form snow columns. The aforementioned tire is imparted with a snow traction performance by a snow shearing force effect obtained by kicking out the snow columns. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Patent Application Publication No. 2010-132217 
     SUMMARY OF INVENTION 
     In the aforementioned tire, two block arrays in which the plurality of blocks are arranged in the tire circumferential direction are formed, and thus, a tread width direction outer end of the block is distanced from the tire equator line more than a tread width direction inner end of the block and the tread width direction outer end of the block is adjacent to the circumferential groove. In such a tire, there has been a tendency that grounding pressures at the tread width direction inner end and tread width direction outer end of each block differ from each other. Specifically, the tread width direction outer end has a lower grounding pressure than that of the tread width direction inner end. For this reason, an edge pressure at the tread width direction outer end becomes low, and as a result, it is not possible to obtain sufficient edge effects of the block as a whole. Therefore, there is a room for improvement in the snow traction performance. 
     Furthermore, the amount of snow having entered the groove portion is reduced if the block is worn down by use, and as a result, the snow shearing force effect is reduced. Thus, there is a problem that the snow traction performance is deteriorated in a tire with worn blocks. 
     Therefore, the present invention has been achieved in view of the above situations, and an object thereof is to provide a tire configured to include a block partitioned by a groove portion that includes a circumferential groove extending in a tire circumferential direction and a lug groove extending in a tread width direction, and configured so that tread width direction outer end of the block is adjacent to the circumferential groove and the tread width direction outer end is distanced from a tire equator line more than a tread width direction inner end of the block, with which it is possible to improve snow traction performance and suppress the deterioration of the snow traction performance due to worn blocks. 
     The present invention has following features to solve the above problem. The feature of the present invention is summarized as a tire comprising: a block (block  100 ) partitioned by a groove portion that includes a circumferential groove (circumferential groove  20 ) positioned at an outer side in a tread width direction from a tire equator line (tire equator line CL) and extending in a tire circumferential direction and a lug groove (lug groove  50 ) extending in the tread width direction, in a tread planar view, wherein a tread width direction outer end (outer end  140 ) of the block is adjacent to the circumferential groove, the tread width direction outer end is distanced from the tire equator line more than a tread width direction inner end (inner end  150 ) of the block, the block includes a step portion (step portion  110 ) formed at a tire circumferential end (circumferential end  160 ), in which an height of the block gradually reduces as it goes towards an outer side in the tire circumferential direction from the center of the block, the step portion includes a step surface (step surface  110   a ) that is a surface facing an outer side in a tire radial direction, and that includes at least a first step surface (first step surface  111   a ) facing the outer side in the tire radial direction and lower by one step from a tread surface and a second step surface (second step surface  121   a ) facing the outer side in the tire radial direction and lower by two steps from the tread surface, and the step surface extends from the tread width direction outer end towards the tread width direction inner end. 
     When a first direction is defined as a direction extending from the tread width direction outer end towards the tread width direction inner end in the step surface, a first direction length of the second step surface may be longer than a first direction length of the first step surface. 
     When a first direction length of a side surface of the block is defined as a length L, the first direction length of the first step surface is defined as a length L 1 , and the first direction length of the second step surface is defined as a length L 2 , at the tire circumferential end including the step portion, the length L 1  and the length L 2  may satisfy 0.1L≦L 1 ≦0.3L, and 2L 1 ≦L 2 . 
     When a tire radial direction length from the tread surface to the first step surface is defined as a length D 1  and a tire radial direction length from the first step surface to the second step surface is defined as a length D 2 , the length D 1  and the length D 2  may satisfy D 2 &lt;D 1 . 
     When the height of the block is defined as a height H, the length D 1  and the length D 2  may satisfy 0.1H≦D 2 &lt;D 1 ≦0.3H. 
     When a first direction is defined as a direction extending from the tread width direction outer end towards the tread width direction inner end, and a second direction is defined as a direction perpendicular to the first direction and the tire radial direction, in the step surface, and when an average length of the block in the second direction is defined as a width W, a length of the first step surface in the second direction is defined as a width W 1 , and a length of the second step surface in the second direction is defined as a width W 2 , the width W 1  and the width W 2  may satisfy 0.02W≦W 1 ≦0.05W and 0.02W≦W 2 ≦0.05W. 
     The groove portion may include an intersectional groove (intersectional groove  80 ) extending to intersect with the lug groove, the intersectional groove may communicate to the block, and an extension direction end (extension direction end  88 ) of the intersectional groove may be positioned in the block in the tread planar view. 
     The block may include one tire circumferential end (circumferential end  160 ) and the other tire circumferential end (circumferential end  170 ), the block may include the step portion formed only at the one tire circumferential end, and an opening (opening) may be formed by the intersectional groove at the other tire circumferential end. 
     The lug groove may include an inner lug groove portion (inner lug groove portion  55 ) positioned at an inner side in the tread width direction from the intersectional groove, and the inner lug groove portion may be adjacent to the block while interposing the intersectional groove, in a direction away from the tire equator line among the extension directions of the inner lug groove portion. 
     According to the present invention, it is possible to improve snow traction performance and suppress the deterioration of the snow traction performance due to worn blocks, in a tire configured to include a block partitioned by a groove portion that includes a circumferential groove extending in a tire circumferential direction and a lug groove extending in a tread width direction, and configured so that a tread width direction outer end of the block is adjacent to the circumferential groove and the tread width direction outer end is distanced from a tire equator line more than a tread width direction inner end of the block. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing a tread pattern of a tire according to the present embodiment. 
         FIG. 2  is an enlarged schematic view showing a part of the tread pattern of the tire according to the present embodiment. 
         FIG. 3  is a schematic view of a block  100  according to the present embodiment. 
         FIG. 4( a )  is a schematic diagram in which the block  100  according to the present embodiment is seen from an X direction in  FIG. 3 . 
         FIG. 4( b )  is a schematic diagram in which the block  100  according to the present embodiment is seen from a Y direction in  FIG. 3 . 
         FIG. 5( a )  is an enlarged schematic view showing a part of the tread pattern of the tire according to the present embodiment. 
         FIG. 5( b )  is an enlarged schematic view showing a part of the tread pattern of the tire according to the present embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An example of a tire according to the present invention will be described with reference to the drawings. Specifically, (1) Schematic Configuration of Tread Pattern, (2) Schematic Configuration of Block  100 , (3) Operation and Effect, (4) Comparative Evaluations, and (5) Other Embodiments will be described. 
     In the following description of the drawings, the same or similar reference numerals are used to designate the same or similar parts. It will be appreciated that the drawings are schematically shown and the ratio and the like of each dimension are different from the real ones. Therefore, the specific dimensions and the like must be determined in view of the below explanation. It is needless to say that relations and ratios among the respective dimensions may differ among the diagrams. 
     (1) Schematic Configuration of Tread Pattern 
     A schematic configuration of a tread pattern in a tire according to the present embodiment will be described with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is a schematic view showing the tread pattern of the tire according to the present embodiment.  FIG. 2  is an enlarged schematic view showing a part of the tread pattern of the tire according to the present embodiment. 
     As shown in  FIG. 1 , the tire according to the present embodiment includes: a groove portion including a circumferential groove  20 , a lug groove  50 , and an intersectional groove  80 ; a block  100 ; and an end block  200 . Furthermore, the tire according to the present embodiment includes a central region CR positioned in the center in a tread width direction, and end regions TR positioned at the both outer sides in the tread width direction of the central region CR. 
     The circumferential groove  20  extends in a tire circumferential direction in a tread planar view. The circumferential groove  20  is positioned at the outer side in the tread width direction from a tire equator line CL. In the present embodiment, the circumferential groove  20  is positioned at the inner side in the tread width direction of each of the end regions TR. That is, the tread width direction inner end of the circumferential groove  20  becomes a boundary between the central region CR and end region TR. The circumferential grooves  20  are formed at the both end regions TR. Therefore, the tire according to the present embodiment includes two circumferential grooves  20 . 
     The block  100  is positioned at the inner side in the tread width direction of the circumferential groove  20 . That is, the circumferential groove  20  is adjacent to the block  100  in the tread width direction. The end block  200  is positioned at the outer side in the tread width direction of the circumferential groove  20 . 
     The lug groove  50  extends in the tread width direction in the tread planar view. In the present embodiment, the lug groove  50  extends to be inclined relative to the tread width direction. The lug groove  50  extends from one end towards the other end of a tread portion in the tread width direction. That is, the lug groove  50  extends from one end region TR towards the other end region TR through the central region CR. The lug groove  50  extends in a zigzag line in the central region CR. The lug groove  50  extends in a straight line in the end regions TR. The lug groove  50  is positioned at tire circumferential direction sides of the block  100  and end block  200 . 
     The lug groove  50  intersects with the circumferential groove  20  in the tread planar view. The lug groove  50  extends to be inclined relative to the tread width direction, and thus, a position at which the lug groove  50  intersects with one circumferential groove  20  differs in the tire circumferential direction from a position at which the lug groove  50  intersects with the other circumferential groove  20 . In addition, an intersectional angle at this time is a smaller angle formed between the circumferential groove  20  and the lug groove  50  in the tread planar view. 
     As shown in  FIG. 2 , the lug groove  50  includes an inner lug groove portion  55  positioned at the inner side in the tread width direction from the intersectional groove  80 . In the present embodiment, the inner lug groove portion  55  is positioned on the tire equator line CL.  FIG. 2  shows extension directions in which the inner lug groove portion  55  extends. In one of the extension directions, of the inner lug groove portion  55  that is distanced from the tire equator line CL the inner lug groove portion  55  is adjacent to the block  100  while interposing the intersectional groove  80 . That is, a tread width direction outer end of the inner lug groove portion  55 , the intersectional grooves  80 , and the block  100  are positioned in this order toward the extension direction. In addition, in the present embodiment, the inner lug groove portion  55  is adjacent to the block  100  while interposing the intersectional groove  80  in either of one extension direction or the other extension direction. 
     The intersectional groove  80  extends to intersect with the lug groove  50 . In addition, an intersectional angle at this time is a smaller angle formed between the lug groove  50  and intersectional groove  80  in the tread planar view. 
     The intersectional groove  80  includes an extension direction end  88  that is an end in an extension direction in which the intersectional groove  80  extends. The intersectional groove  80  communicates to the block  100 . Thus, the extension direction end  88  is positioned in the block  100  in the tread planar view. In the present embodiment, either one or the other extension direction end  88  is positioned in the block  100 . The intersectional groove  80  extends to be bent in the block  100 . In the present embodiment, the intersectional groove  80  is positioned only in the central region CR. 
     In the present embodiment, the inner lug groove portion  55  and intersectional groove  80  configure a zigzag groove portion extending in the tire circumferential direction in a zigzag line. 
     The block  100  is partitioned by the groove portion that includes the circumferential groove  20  and the lug groove  50  in the tread planar view. Specifically, the block  100  is partitioned by the circumferential groove  20 , the lug groove  50 , and the intersectional groove  80 . The blocks  100  include a plurality of blocks  100 A positioned at one outer side in the tread width direction (the right side in  FIG. 1 ) and a plurality of blocks  100 B positioned at the other outer side in the tread width direction (the left side in  FIG. 1 ). The plurality of blocks  100 A are mutually adjacent to one another in the tire circumferential direction while interposing the lug grooves  50 . The plurality of blocks  100 B are mutually adjacent to one another in the tire circumferential direction while interposing the lug grooves  50 . That is, the tire according to the present embodiment includes a block array including the blocks  100 A and extending in the tire circumferential direction, and a block array including the blocks  100 B and extending in the tire circumferential direction. A tread pattern according to the present embodiment has a shape of point symmetry about a point positioned on the tire equator line CL, and thus, each block  100 A has the same shape as that of each block  100 B. 
     The block  100  includes an outer end  140  that is an end at the outer side in the tread width direction and an inner end  150  that is an end at the inner side in the tread width direction (see  FIG. 3 ). Furthermore, the block  100  includes a circumferential end  160  that is one end in the tire circumferential direction and a circumferential end  170  that is the other end in the tire circumferential direction (see  FIG. 3 ). Also, the block  100  includes a step portion  110  formed at the circumferential end  160 . The step portion  110  will be described later. At the circumferential end  170 , an opening  85  is formed by the intersectional groove  80 . In the present embodiment, the opening  85  is formed only at the circumferential end  170 . 
     The outer end  140  is adjacent to the circumferential groove  20 . The inner end  150  is adjacent to the intersectional groove  80 . The circumferential end  160  and circumferential end  170  are adjacent to the lug groove  50 . The outer end  140  is distanced from the tire equator line CL more than the inner end  150 . 
     In the block  100 , a siping  5  is formed. In the present embodiment, the siping  5  is formed in a zigzag line. It is preferable that the siping  5  formed in a region CCR positioned on the tire equator line CL, among three regions into which the central region CR is divided equally in the tread width direction, extends in the tire width direction. In a tire (for example, a tire for a light truck) in which an increase or decrease of an applied load bounces depending on the use condition, grounding contact area of the end region TR with low applied load is reduced as compared to that of the end region TR with high applied load. On the other hand, a grounding contact area of the central region CR, particularly the region CCR, is not susceptible to the increase or decrease of the applied load. Thus, by forming the siping  5  in the region CCR, it is possible to obtain an edge effect that is not susceptible to the increase or decrease of the applied load. It is preferable that an angle formed between the siping  5  formed in the region CCR and the tread width direction is smaller. Accordingly, it is possible to obtain snow traction performance that is not susceptible to the increase or decrease of the applied load. 
     It is noted that the siping has a groove width capable of closing when the block comes into contact with the ground. Specifically, the siping has a groove width of 1.5 mm or less. However, in the tire such as a TBR tire used for a large bus or a truck, a groove width of the siping may be 1.5 mm or more. 
     The block  200  is positioned at the end regions TR. The plurality of blocks  200  are mutually adjacent to one another in the tire circumferential direction while interposing the lug grooves  50 . 
     (2) Schematic Configuration of Block  100   
     A schematic configuration of the block  100  according to the present embodiment will be described with reference to  FIG. 1  to  FIG. 4 .  FIG. 3  is a schematic view of the block  100  according to the present embodiment.  FIG. 4( a )  is a schematic diagram in which the block  100  according to the present embodiment is seen from an X direction in  FIG. 3 .  FIG. 4( b )  is a schematic diagram in which the block  100  according to the present embodiment is seen from a Y direction in  FIG. 3 . In addition,  FIG. 4( a )  and  FIG. 4( b )  are schematic diagrams explaining the configuration of the block  100 , and the shape of the block  100  may be different in some parts from those of the other diagrams. In  FIG. 3 ,  FIG. 4( a ) , and  FIG. 4( b ) , the siping is not shown. 
     Also, the block  100  includes the step portion  110  formed at the circumferential end  160 . In the present embodiment, the block  100  includes the step portion  110  formed only at the circumferential end  160 . The height of the block  100  gradually reduces in the step portion  110 , as it goes towards the outer side in the tire circumferential direction from the center of the block  100 . 
     The step portion  110  is configured by a first step portion  111  and a second step portion  121 . The first step portion  111  is configured by a first step surface  111   a , a side surface  111   b , and a side surface  111   c . The second step portion  121  is configured by a second step surface  121   a , a side surface  121   b , and a side surface  121   c.    
     The step portion  110  includes a step surface  110   a  that is a surface facing the outer side in the tire radial direction. The step surface  110   a  includes at least the first step surface  111   a  that is a surface facing the outer side in the tire radial direction and lower by one step from a tread surface  101   a , and the second step surface  121   a  that is a surface facing the outer side in the tire radial direction and lower by two steps from the tread surface  101   a . Therefore, the second step surface  121   a  is lower by one step from the first step surface  111   a . In the present embodiment, the step surface  110   a  is configured by the first step surface  111   a  and second step surface  121   a.    
     The step surface  110   a  extends from the outer end  140  towards the inner end  150 . In the step surface  110   a , an extension direction of the step surface  110   a  that is a direction extending from the outer end  140  towards the inner end  150  is regarded as a first direction. In addition, the first direction coincides with an extension direction of the lug groove  50  adjacent to the circumferential end  160  of the block  100 . The step surface  110   a  extends in a straight line. The step surface  110   a  extends from the outer end  140 , and thus, the tread width direction outer end of the step surface  110   a  is positioned at the outer end  140 . Therefore, the tread width direction outer ends of the first step surface  111   a  and second step surface  121   a  are at the same position in the first direction. That is, in the outer end  140  of the block  100 , an edge portion at a tire circumferential end where the step portion  110  is formed has a stair-like shape. 
     The side surface  111   b  and side surface  121   b  are surfaces facing second direction sides of the first step portion  111  and second step portion  121 , respectively. Here, the second direction is a direction perpendicular to the first direction and a tire radial direction. The side surface  111   b  communicates to the tread surface  101   a  at the outer side in the tire radial direction, and to the first step surface  111   a  at the inner side in the tire radial direction. The side surface  121   b  at the outer side in the tread width direction communicates to the first step surface  111   a  at the outer side in the tire radial direction, and to the second step surface  121   a  at the inner side in the tire radial direction. The side surface  121   b  at the inner side in the tread width direction communicates to the tread surface  101   a  at the outer side in the tire radial direction, and to the second step surface  121   a  at the inner side in the tire radial direction. That is, the side surface  121   b  has an L shape as shown in  FIG. 4( a ) . A side surface  161   b  of the block  100  at the circumferential end  160  communicates to the tread surface  101   a  and second step surface  121   a  at the outer side in the tire radial direction, and to a groove bottom of the lug grooves  50  at the inner side in the tire radial direction. The side surface  161   b  at the outer side in the tire radial direction communicates to the tread surface  101   a , and the side surface  161   b  at the inner side in the tire radial direction communicates to the second step surface  121   a . That is, the side surface  161   b  has an L shape as shown in  FIG. 4( a ) . 
     The side surface  111   c  and side surface  121   c  are surfaces facing first direction sides of the first step surface  111   a  and second step surface  121   a , respectively. The side surface  111   c  communicates to the tread surface  101   a  at the outer side in the tire radial direction, and to the first step surface  111   a  at the inner side in the tire radial direction. The side surface  121   c  communicates to the tread surface  101   a  at the outer side in the tire radial direction, and to the second step surface  121   a  at the inner side in the tire radial direction. 
     The height of the block  100  gradually reduces in the step portion  110 , as it goes towards the outer side in the tire circumferential direction from the center of the block  100 . Thus, the step portion  110  has a stair-like shape including a plurality of step differences. In the present embodiment, the block  100  has a stair-like shape including a plurality of step differences in the first direction. Specifically, the step portion  110  includes the side surface  111   c  lower by one step from a side surface of the outer end  140 , and the side surface  121   c  lower by two steps from the side surface of the outer end  140 . Accordingly, in the tread surface  101   a , a block edge portion  165  that is an edge portion at the circumferential end  160  of the block  100  has a zigzag shape. 
     The step portion  110  is formed in the block  100 , and thus, the circumferential end  160  of the block  100  is cut out at the outer side in the tread width direction. Therefore, the block  100  includes, at the circumferential end  160 , a plurality of recesses recessed in the tire radial direction. The depth recessed in the tire radial direction becomes deeper in the plurality of recesses, as it goes towards the outer side in the second direction from the center of the block  100 . 
     As shown in  FIG. 4( a ) , a first direction length of the side surface  161   b  of the block  100  is regarded as a length L at the circumferential end  160 . The length L is a length of the block  100 , along the first direction, from the outer end  140  to the inner end  150 , in the side surface  161   b . The first direction length of the first step surface  111   a  is regarded as a length L 1 . The first direction length of the second step surface  121   a  is regarded as a length L 2 . It is preferable that the length L 2  is longer than the length L 1 . Furthermore, it is preferable that the length L 1  and length L 2  satisfy 0.1L≦L 1 ≦0.3L and 2L 1 ≦L 2 . 
     A height, along the tire radial direction, from the tread surface  101   a  to the groove bottom of the groove portion (namely, a tire radial direction length), that is the height of the block, is regarded as a height H. A height, along the tire radial direction, from the first step surface  111   a  to the groove bottom of the groove portion is regarded as a height h 1 . A height, along the tire radial direction, from the second step surface  121   a  to the groove bottom of the groove portion is regarded as a height h 2 . The height of the block  100  gradually reduces in the step portion  110  as it goes towards the outer side in the tire circumferential direction from the center of the block  100 , and thus, the height H, height h 1 , and height h 2  satisfy h 2 &lt;h 1 &lt;H. 
     A tire radial direction length from the tread surface  101   a  to the first step surface  111   a  is regarded as a length D 1 . A tire radial direction length from the first step surface  111   a  to the second step surface  121   a  is regarded as a length D 2 . It is preferable that the length D 1  and length D 2  satisfy D 2 &lt;D 1 . Furthermore, it is preferable that the D 1  and D 2  satisfy 0.1H≦D 2 &lt;D 1 ≦0.3H. Here, the 0.1H denotes 0.1 times the height H and the 0.3H denotes 0.3 times the height H. 
     The length D 1  and length D 2  may satisfy D 1 =D 2 . Furthermore, the length D 1  and length D 2  may satisfy D 1 &lt;D 2 . Moreover, the D 1  and D 2  may satisfy 0.1H≦D 1 &lt;D 2 ≦0.3H. By satisfying 0.1H≦D 1 , in the early stage of using the tire, it is possible to prevent a first edge portion  115  that is an edge portion formed between the first step surface  111   a  and side surface  121   b  from coming in contact with a road surface. If the first edge portion  115  comes into contact with the road surface, an edge pressure at the block edge portion  165  formed between the tread surface  101   a  and side surface  111   b  is reduced. Edge effects are reduced if the edge pressure is reduced, and as a result, the snow traction performance is deteriorated. By preventing the first edge portion  115  from coming in contact with the road surface, edge effects in the early stage of using the tire can be secured. When a step difference that is a difference of the step surface  110   a  in the tire radial direction is smaller, a side bulging (so-called crushing) in which a side surface bulges is less likely to occur. The edge pressure is reduced if the side bulging occurs, and as a result, edge effects are reduced. The side bulging becomes less likely to occur in the side surface  121   b  by satisfying D 2 ≦0.3H, and thus, it is possible to obtain sufficient edge effects of the first edge portion  115  when the block  100  is worn down. 
     The length D 1  is 15 to 25% of the height H and it is preferable that the sum of the length D 1  and length D 2  is 40 to 50% of the height H. 
     An average length of the block  100  in the second direction is regarded as a width W. The width W is an average length, along the second direction, from the side surface  161   b  at the circumferential end  160  to a side surface  171   b  at the circumferential end  170 . The portion formed by the intersectional groove  80  is not included in the average length. As shown in  FIG. 4( b ) , a length of the first step surface  111   a  in the second direction is regarded as a width W 1 . The width W 1  is a length of the side surface  111   c  along the second direction. A length of the second step surface  121   a  in the second direction is regarded as a width W 2 . The width W 2  is a length of the side surface  121   c  along the second direction. It is preferable that the width W 1  and width W 2  satisfy 0.02W≦W 1 ≦0.05W and 0.02W≦W 2 ≦0.05W. That is, it is preferable that the width W 1  and width W 2  are 2% or more and 5% or less the width W. 
     (3) Operation and Effect 
     In the tire according to the present embodiment, the block  100  includes the step portion  110  formed at the circumferential end  160 , in which the height of the block  100  gradually reduces as it goes towards the outer side in the tire circumferential direction from the center of the block  100 , the step portion  110  includes the first step surface  111   a  and second step surface  121   a , and the step surface  110   a  extends from the outer end  140  towards the inner end  150 . 
     The step portion  110  that includes the step surface  110   a  extending from the outer end  140  is formed in the block  100 , and thus, a length, along the tire circumferential direction, of the outer end  140  becomes shorter as compared to the case in which the step portion  110  is not formed. Therefore, a grounding pressure on the outer end  140  can be increased, and thus, an edge pressure on the outer end  140  is increased. Accordingly, it is possible to obtain sufficient edge effects of the block  100  as a whole. As a result, the snow traction performance can be improved. In addition, a difference of grounding pressures between the outer end  140  and inner end  150  is reduced, and thus, it is possible to keep balance of the grounding pressure of the block  100  and prevent the block  100  from being unevenly worn. 
     The step portion  110  includes the first step surface  111   a  and second step surface  121   a , and therefore, the block  100  includes a plurality of edge portions (the first edge portion  115  and the second edge portion  125 ) in the tire radial direction. Thus, edge effects by the plurality of edge portions can be obtained even if the block  100  is worn out by use, and therefore, it is possible to suppress the deterioration of the snow traction performance due to the worn block. 
     In the tire according to the present embodiment, the first direction length of the second step surface  121   a  is longer than the first direction length of the first step surface  111   a . Therefore, it is possible to obtain more enhanced edge effects when the block  100  is worn out by use, as compared to the case in which the plurality of edge portions has the same first direction length. As a result, it is possible to more suppress the deterioration of the snow traction performance due to wear. 
     Furthermore, the first direction length of the first step surface  111   a  is shorter than the first direction length of the second step surface  121   a , and thus, it is possible to ensure a larger area of the tread surface  101   a  of the block  100 . As a result, a surface friction effect between the tread surface  101   a  and the snow surface can be secured, and thus, the snow traction performance can be improved. 
     In the tire according to the present embodiment, it is preferable that the length L 1  and length L 2  satisfy 0.1L≦L 1 ≦0.3L, and 2L 1 ≦L 2 . By satisfying 0.1L≦L 1  and 2L 1 ≦L 2 , it is possible to obtain sufficient edge effects by the first edge portion  115  and second edge portion  125 . 
     In the tire according to the present embodiment, it is preferable that the length D 1  and length D 2  satisfy D 2 &lt;D 1 . The first edge portion  115  is distanced from the tread surface  101   a , and thus, it is possible to prevent the first edge portion  115  from coming in contact with the road surface. Accordingly, edge effects in the early stage of using the tire can be secured, and thus, the snow traction performance is improved. When a step difference that is a difference of the step surface  110   a  in the tire radial direction is smaller, a side bulging (so-called crushing) in which a side surface bulges is less likely to occur. The edge pressure is reduced if the side bulging occurs, and as a result, edge effects are reduced. The side bulging becomes less likely to occur in the side surface  121   b  by satisfying D 2 &lt;D 1  as compared to the side surface  111   b , and thus, it is possible to obtain sufficient edge effects of the first edge portion  115  when the block  100  is worn down. As a result, it is possible to more suppress the deterioration of the snow traction performance due to wear. 
     Furthermore, in the tire according to the present embodiment, it is preferable that the length D 1  and length D 2  satisfy 0.1H≦D 2 &lt;D 1 ≦0.3H. When the block  100  is worn down, an edge pressure at the first edge portion  115  is reduced if the second edge portion  125  comes into contact with the road surface, when the first edge portion  115  becomes an edge portion at the outermost side in the tire radial direction. Accordingly, edge effects obtained by the first edge portion  115  are reduced, and as a result, the snow traction performance is deteriorated. It is possible to prevent the second edge portion  125  from coming in contact with the road surface by satisfying 0.1H≦D 2 , and thus, edge effects of the first edge portion  115  at the time when the block  100  is worn down can be secured. As a result, it is possible to more suppress the deterioration of the snow traction performance due to wear. Furthermore, the side bulging becomes less likely to occur in the side surface  111   b  by satisfying D 1 ≦0.3H, and thus, it is possible to prevent edge effects of the block edge portion  165  from being reduced. 
     In the tire according to the present embodiment, it is preferable that the width W 1  and width W 2  satisfy 0.02W≦W 1 ≦0.05W and 0.02W≦W 2 ≦0.05W. By satisfying that the width W 1  and width W 2  are equal to or more than 2% of the width W, it is possible to obtain sufficient edge effects by the first edge portion  115  and the second edge portion  125 . 
     In the tire according to the present embodiment, the groove portion includes the intersectional groove  80 , the intersectional groove  80  communicates to the block  100 , and the extension direction end  88  of the intersectional groove  80  is positioned in the block  100  in the tread planar view. When the intersectional groove  80  passes through the block  100 , the block  100  is divided into a plurality of small blocks  100 . Each of the small blocks  100  has weak block stiffness, and thus, the block  100  is easy to fall down when the tire rotates. If the block  100  falls down, a grounding property of the block  100  is deteriorated, and as a result, it is not possible to obtain sufficient edge effects. In the present embodiment, the extension direction end  88  is positioned in the block  100 , and therefore, the intersectional groove  80  does not pass through the block  100 . Therefore, the block stiffness of the block  100  can be secured, and thus, it is possible to obtain sufficient edge effects. Furthermore, the extension direction end  88  is positioned in the block  100 , and thus, a groove area ratio (a negative ratio) in the tread surface increases. A snow shearing force is more likely to be obtained as the groove area ratio is larger, and thus, the snow traction performance is increased. 
     In the tire according to the present embodiment, the block  100  includes the circumferential end  160  and circumferential end  170 , and also includes the step portion  110  formed only at the circumferential end  160  and the opening  85  formed at the circumferential end  170  by the intersectional groove  80 . The circumferential end  170  with the opening  85  formed thereat has weaker block stiffness as compared to the circumferential end  160  without the opening  85  formed thereat. The block stiffness of the circumferential end  170  is further reduced if the step portion  110  is formed at the circumferential end  170 . Therefore, it is not possible to obtain sufficient edge effects due to the fall even if the step portion  110  is formed. By preventing the block stiffness of the circumferential end  170  from being reduced by forming the step portion  110  only at the circumferential end  160 , edge effects by the block edge portion  165  are secured. Furthermore, it is possible to prevent the circumferential end  170  from being unevenly worn due to the fall. 
     In the tire according to the present embodiment, the lug groove  50  includes the inner lug groove portion  55 , and the inner lug groove portion  55  is adjacent to the block  100  while interposing the intersectional groove  80  in one of the extension directions of the inner lug groove portion  55  that is distanced from the tire equator line. As shown in  FIG. 5( a ) , the intersectional groove  80  intersects with the lug groove  50 . Therefore, the tire according to the present embodiment includes an intersectional region R 1  that is a region where the lug groove  50  and the intersectional groove  80  intersect with each other. By the rotation of the tire, snow enters the intersectional groove  80  at the extension direction end  88  side. The entered snow is pushed out by a load towards the center of the intersectional groove  80 . Furthermore, a grounding length that is a circumferential length of the tread surface  101   a  in the tire circumferential direction becomes shorter as the grounding length is distanced from the tire equator line. Thus, when the tread surface  101   a  comes into contact with the ground, the part, closer to the tire equator line, of the tread surface  101   a  comes into contact with the road surface earlier, and the part, farther away from the tire equator line, of the tread surface  101   a  comes into contact with the road surface later. Accordingly, snow enters the inner lug groove portion  55  first. Then, the entered snow is pushed out by a load in one of the extension directions that is distanced from the tire equator line. The inner lug groove portion  55  is adjacent to the block  100  while interposing the intersectional groove  80  in the extension directions, and thus, the block  100  dams up the pushed out snow. The snow pushed out in the intersectional groove  80  hits the snow pushed out from the inner lug groove portion  55  and dammed up by the block  100  in the intersectional region R 1 , thereby a strong snow column with high density is formed in the intersectional region R 1 . Accordingly, a large snow shearing force can be obtained, and thus, the snow traction performance is improved. It is possible to prevent the formed snow column from remaining in the intersectional region R 1  because the snow column is discharged along the extension direction of the intersectional groove  80 , and thus, it is possible to suppress the deterioration of the snow traction performance. 
     In addition, as shown in  FIG. 5( b ) , the circumferential groove  20  intersects with the lug groove  50 , and thus, the tire according to the present embodiment includes an intersectional region R 2  that is a region where the circumferential groove  20  and lug groove  50  intersect with each other. Snow having entered the circumferential groove  20  is pushed out in the rotation direction by the rotation of the tire. Furthermore, as described above, the snow having entered the lug groove  50  is pushed out by the load in one of the extension directions, of the lug groove  50 , that is distanced from the tire equator line. Accordingly, the snow pushed out in the circumferential groove  20  hits the snow pushed out in the lug groove  50  in the intersectional region R 2 , thereby a strong snow column with high density is formed in the intersectional region R 2 . Accordingly, a large snow shearing force can be obtained, and thus, the snow traction performance is improved. Further, the lug groove  50  extends to the end region TR, and thus, the snow column formed in the intersectional region R 2  is discharged along the extension directions of the circumferential groove  20  and the lug groove  50 . Thus, it is possible to prevent the snow column from remaining in the intersectional region R 2 , and thus, it is possible to suppress the deterioration of the snow traction performance. 
     (4) Comparative Evaluations 
     In order to investigate an effect of the present invention, the following comparative evaluations were performed. Further, the present invention is not limited to the following Examples. 
     The comparative evaluations used tires including blocks having characteristics shown in Table 1 under the following condition.
         Tire size: 235/65R16C 115R   Tire pattern: LR50CZ   Inner Pressure: 475 kPa   Rim Size: 7J×16       

     Specifically, a tire according to Comparative Example 1 includes a block without a step portion formed therein. Tires according to Example 1 to Example 3 include blocks with step portions formed therein, each step portion including a first step surface and a second step surface. In the tire according to the Example 1, the length D 1  and the length D 2  have the same length. In the tire according to the Example 2, the length L 1  and the length L 2  have the same length. That is, in the tire according to the Example 2, a first direction length of the first step surface and a first direction length of the second step surface have the same length. In the tire according to the Example 3, the first direction length of the second step surface is longer than the first direction length of the first step surface. In addition, in the tires according to the Example 1 to the Example 3, each step portion is formed at the outer side in a tread width direction of a tire circumferential end. 
     By using aforementioned respective tires, snow traction performance and snow traction performance after wear were evaluated. The measurement of the snow traction performance and the snow traction performance after wear were performed according to the ASTM standard. The snow traction performance after wear was measured by using respective tires applied on a vehicle that had traveled 10000 km on a dry road. The results are shown in Table 1. In addition, the tire according to the Example 3 is used as a reference (100), and measured results of the respective tires were indexed. It is shown that any performance is more excellent as the value of the index is larger. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Com. Ex. 1 
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 whether step portion exists 
                 x 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                 length D1 (%) 
                 — 
                 20 
                 20 
                 20 
               
               
                 length D2 (%) 
                 — 
                 20 
                 45 
                 45 
               
               
                 length L1 (mm) 
                 — 
                 8 
                 8 
                 8 
               
               
                 length L2 (mm) 
                 — 
                 24 
                 8 
                 24 
               
               
                 snow traction performance 
                 92 
                 100 
                 100 
                 100 
               
               
                 snow traction performance 
                 91 
                 92 
                 94 
                 100 
               
               
                 after wear 
               
               
                   
               
             
          
         
       
     
     As shown in the Table 1, it can be seen that the snow traction performance is improved and at the same time, the snow traction performance due to the worn block is suppressed in the tire according to the Example 3 as compared to the Comparative Example 1. Specifically, it can be seen that the snow traction performance is improved by the step portion formed in the block. Furthermore, it can be seen that the snow traction performance after wear is improved by adjusting the first direction lengths and tire radial direction lengths of respective step surfaces. 
     (5) Other Embodiments 
     The contents of the present invention are disclosed through the above embodiments of the present invention. However, it should not be interpreted that the statements and drawings constituting a part of the present disclosure limit the present invention. The present invention includes various embodiments not described here. 
     For example, in the tire according to the present embodiment, the step portion  110  includes the step surface  110   a  configured only by the first step surface  111   a  and second step surface  121   a . However, such a configuration is not limiting. The step portion  110  may include a third step surface lower by three steps from the tread surface  101   a  and that is a surface facing the outer side in the tread width direction. The step portion  110  may include at least four step surfaces. 
     Furthermore, in the tire according to the present embodiment, the first direction length of the second step surface  121   a  is longer than the first direction length of the first step surface  111   a . However, such a configuration is not limiting. The first direction length of the first step surface and the first direction length of the second step surface may have the same length. 
     Furthermore, in the tire according to the present embodiment, the step portion  110  is formed only at the circumferential end  160 . However, such a configuration is not limiting. The step portion  110  may be formed at the both circumferential ends, that is, the circumferential end  160  and circumferential end  170 . 
     Furthermore, the tire according to the present embodiment includes the two block arrays of the block array including the blocks  100 A and the block array including the blocks  100 B. However, such a configuration is not limiting. The number of the block arrays may be one or at least three. When the number of the block arrays is one, the center of each block in the tread width direction slides towards one end side in the tread width direction, and a tread width direction end of the block at the one end side in the tread width direction is distanced from the tire equator line, more than a tread width direction end of the block at the other end side in the tread width direction. 
     Furthermore, in the tire according to the present embodiment, the block  100  and block  200  include a siping. However, such a configuration is not limiting. The block  100  and block  200  may not include a siping. 
     The tire according to the present invention may be a pneumatic tire with air or a so-called solid tire filled with rubber. Further, a pneumatic tire filled with rare gas such as argon rather than air may be used. 
     The technical range of the present invention is to be defined only by the inventive specific matter according to the adequate claims from all the above description. 
     In addition, the entire content of Japanese Patent Application No. 2011-140437 (filed on Jun. 24, 2011) is incorporated in the present description by reference. 
     INDUSTRIAL APPLICABILITY 
     As described above, the tire according to the present invention is able to improve snow traction performance and suppress the deterioration of the snow traction performance due to worn blocks, and thus is available for a tire manufacturing field.