Abstract:
A tire includes a tread band having a tread pattern defined by a plurality of grooves of substantially circumferential orientation and by a plurality of grooves of substantially transverse orientation, which grooves delimit a plurality of rubber blocks arranged in a plurality of rows of substantially circumferential orientation, each of the blocks being delimited by a contact face and at least three side faces, wherein at least some of the blocks include at least one side face of substantially transverse orientation provided with at least one irregularly shaped step having an upper face and a front face.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a national phase application based on PCT/EP2006/011489, filed Nov. 30, 2006, the content of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention concerns tire treads, especially, but not exclusively, tire treads for pneumatic tires used for heavy load automotive transport e.g. vehicles such as lorries, buses, coaches and the like. Such tires, often known generally as “truck tires”, have tread patterns which are required for all-weather type tires for travelling very long distances. 
     BACKGROUND OF THE ART 
     It is known to provide the treads of the tires with tread patterns formed of a plurality of blocks delimited by grooves of generally transverse and circumferential (or longitudinal) orientation. Each block of the tread pattern has a so-called leading face and a trailing face, each of which faces intersects the contact face of the block to form an edge (leading edge and trailing edge, respectively). The leading edge corresponds to a line of points of the contact face coming first as the tire contacts the road. 
     Trailing edges of blocks in the patterns tend to wear to a greater extent than leading edges which are brought into contact with a road surface earlier than the trailing edges, to thus produce uneven wear of the blocks in the patterns. Such a peculiar wear is referred to as “heel-and-tow wear”. 
     Another problem commonly encountered by all types of tire treads is that of stone trapping. If a stone or a piece of gravel or a like object is of similar or slightly larger size to a groove between blocks in the tire tread, it is very likely that stones or like objects lodge in the groove. As the tire rotates the stone or the like object rubs against the groove boundary and can cause damage. The likelihood of damage is even greater when the stone or like object contacts the groove base since a tear can start from this location. If a stone does lodge in a groove and can be prevented from contacting the groove base the possibility of a tear being caused is substantially reduced. 
     EP 393,873 discloses a vehicle tyre having a tread surface of ribs, blocks or pads and substantially U- or V-shaped tread grooves arranged therebetween wherein at least one of the two sidewalls of at least a part of the tread grooves is provided with grip enhancing steps which extend in the longitudinal groove direction and follow the sidewall contour. A pronounced improvement is achieved in traction, in particular on snow. 
     U.S. Pat. No. 4,423,760 discloses a tire tread for a truck tire comprising a small number of circumferential zig-zag grooves, wherein the sidewalls of each groove are each provided with a supporting buttress of varying height or width such that the maximum support is provided at or adjacent the points where the groove projects into the surrounding tread rubber. The provision of such a buttress provides a varying degree of support for each groove sidewall along the length of the groove and assists in preventing uneven wear of the tread rubber and reduces damage caused by stones trapped in the groove. 
     SUMMARY OF THE INVENTION 
     The tire according to the invention comprises a tread having a tread pattern defined by a plurality of grooves of generally circumferential orientation and by a plurality of grooves of generally transverse orientation, which grooves delimit a plurality of rubber blocks arranged in a plurality of rows, preferably at least three, having generally circumferential orientation. The outermost rows define the edges of the tread and preferably enclose at least one intermediate row of blocks. Each block is delimited by a ground contact face and at least three side faces, at least one side face having a generally transverse orientation. The number of side faces can preferably range from four to eight. 
     According to a preferred embodiment, each block comprises a ground contact face and at least four side faces comprising at least two transverse faces, represented by at least one leading face and at least one trailing face, and at least two longitudinal faces, represented by at least one right face and at least one left face. 
     According to another preferred embodiment, each block comprises a contact face and at least six side faces comprising at least four transverse faces, represented by at least two leading faces and at least two trailing faces, and at least two longitudinal faces, represented by at least one right face and at least one left face. 
     The contact face corresponds to the part of the block in contact with the ground under normal pressure and load use conditions. This contact face is delimited by leading and trailing contact edges, the leading contact edge being the location of the points of the contact face which are the first to come into contact with the ground and the trailing contact edge being the location of the points of the contact face which are the last to be in contact with the ground. The leading face and the trailing face of each block intersect the contact face along the leading contact edge and the trailing contact edge, respectively. 
     According to the invention, at least some blocks of the tread comprise at least one transverse face, preferably at least one leading transverse face, provided with at least one irregularly shaped step having an upper face and a front face. Preferably, the number of blocks provided with at least one irregularly shaped step according to the invention is at least 20%, more preferably at least 40%, and still more preferably at least 60% relative to the total number of the tread blocks. Most preferably, the number of blocks provided with at least one irregularly shaped step according to the invention ranges from 80% to 100% relative to the total number of the tread blocks. 
     The wording “irregularly shaped step” means that at least one of said upper and front face of such step lies on a plane forming an angle higher than 5° with respect to the plane of the block contact face or the plane of the block side face of substantially transverse orientation provided with such step, respectively. 
     In other words, an irregularly shaped step according to the invention shall have at least an upper face lying on a plane forming an angle higher than 5° with the plane of the contact face of the block, and/or at least a front face lying on a plane forming an angle higher than 5° with the plane of the transverse face provided with such step of the block. 
     More in particular, each irregularly shaped step provided to the block can have an upper face which can lie on a plane substantially parallel to, or forming an angle higher than 5°, with respect to the transverse direction, and/or to the longitudinal (or circumferential) direction, with the plane of the contact face of the block under consideration. 
     Preferably, the upper face of each step lies on a plane which can form with the plane of the contact face of the block under consideration an angle of from 5° to 60°, preferably from 5° to 45°, and most preferably from 5° to 30° with respect to the transverse direction, or an angle of from 5° to 45°, preferably from 5° to 30° with respect to the longitudinal direction, or both. 
     More in particular, each irregularly shaped step provided to the block can have a front face which can lie on a plane substantially parallel to, or forming an angle higher than 5° with respect to the transverse direction and/or to the radial direction with the plane of the leading face of the block under consideration. 
     Preferably, the front face of each step lies on a plane which can form with the plane of the contact face of the block under consideration an angle of from 5° to 30°, more preferably from 5° to 20°, either with respect to the transverse direction or to the radial direction or both. 
     For sake of clarity, when, in the whole present description and the claims thereof, an angle or an angle range is expressed as an absolute value, i.e., without the “plus” or “minus” sign, this means that the expressed angle or angle range includes both the positive and negative values. For example, the above expression “higher than 5°” means both “higher than +5°” and “lower than −5°”. On the other hand, the above expression “from 5° to 30°” means both “from −5° to −30°” and “from +5° to +30°”. 
     On the other hand, when, in the whole present description and the claims thereof, an angle or an angle range is expressed with the “plus” or “minus” sign, this means that such an angle or angle range is obtained by rotating one plane (or face) in the clockwise direction with respect to the other plane (or face) in case of “plus” sign, and in the counter-clockwise direction in case of “minus” sign. 
     Further, when, in the whole present description and the claims thereof, the term “substantially parallel” is used with respect to two planes (or two faces), this means that such planes (or such faces) form an angle ranging from −5° to +5° each other. 
     The upper face of each irregularly shaped step can have a variable width that can change in the transverse direction of the block from the intersection with one longitudinal face to the intersection with the other longitudinal face of the block. The step width (Ws) depends on the width of the groove between two faced blocks (Wg). The maximum Ws is lower than Wg. Preferably, the step width can range from 0% to 60%, preferably from 10% to 40%, relative to the width of the groove between two faced blocks. 
     The front face of each irregularly shaped step can have a variable height that can change in the transverse direction of the block from the intersection with one longitudinal face to the intersection with the other longitudinal face of the block. The step height (Hs) depends on the height of the block (Hb). The maximum Hs is lower than Hb. Preferably, the step height can range from 5% to 80%, preferably from 10% to 60% relative to the height of the block. 
     The intersection of each upper face with the corresponding each front face of each irregularly shaped step can be represented by a sharp edge or a round edge. The round edge can be formed by any radius able to join the upper face with the corresponding front face. The radius can also in turn be constant or variable along the transverse direction of the edge. 
     The Applicant has found that the tire according to the invention shows a reduced “heel-and-tow wear” compared to a tire of the prior art and allows continuous travelling for a long period of time. 
     The Applicant has also found that the tire according to the invention is less prone to entrap stones or gravels. Additionally, in case a stone or gravel was entrapped, it is prevented from contacting the groove base, it is easily throw out during the rotation of the tire, and the possibility of a tear being caused is substantially reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the plan (b), side (a,c), and perspective (d) views of a block according to a first embodiment of the present invention. 
         FIG. 2  shows the plan (b), side (a,c), and perspective (d) views of a block according to a second embodiment of the present invention. 
         FIG. 3  shows the plan (b), side (a,c), and perspective (d) views of a block according to a third embodiment of the present invention. 
         FIG. 4  shows the plan (b), side (a,c), and perspective (d) views of a block according to a fourth embodiment of the present invention. 
         FIG. 5  shows the plan (b), side (a,c), and perspective (d) views of a block according to a fifth embodiment of the present invention. 
         FIG. 6  shows the plan view of a tire comprising a plurality of blocks according to an embodiment of the present invention. 
         FIG. 7  shows the plan view of a tire comprising a plurality of blocks according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the  FIGS. 1 to 7 , the x, y, and z lines represent the longitudinal, radial and transverse direction of a tyre tread, respectively. The arrow on the x line indicates the tyre rolling direction. The length l, the width w and the height h of the blocks represented in  FIGS. 1 to 7  can have any value known in the art of tyre manufacturing. Preferably, the blocks have a length l in the range of from 10 to 100 mm, a width w in the range of from 10 to 100 mm, and a height h in the range of from 10 to 100 mm. 
       FIG. 1  shows a block  10  according to a first embodiment of the present invention, comprising a contact face  2 , two transverse faces  3  and  4 , represented by one trailing face  3  and one leading face  4 , and two longitudinal faces  5  and  6 , represented by one right face  5  and one left face  6 . 
     Typically, the block  10  can represent an embodiment of one block of a plurality of blocks forming an outermost row situated on the edges of the tread and forming the shoulders of the tire. 
     The block  10  is characterized by the presence of a step  11  formed on the leading face  4 . The step  11  is delimited by an upper face  12 , a front face  13 , and two longitudinal faces  5   a  and  6   a  prosecuting the longitudinal faces  5  and  6  of the block  10 . 
     In the embodiment of  FIG. 1 , the upper face  12  is substantially parallel to the contact face  2 , both with respect to the transverse direction z, and to the longitudinal direction x. 
     In the embodiment of  FIG. 1 , the front face  13  is substantially parallel with the leading face  4  with respect to the radial direction y, but it forms an angle of from 5° to 30° with respect to the transverse direction z. 
     The heights h 1  and h 2  of the step  11  are equal and can range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  10 . 
     The widths w 1  and w 2  of the step  11  are different and can independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. While in the embodiment of  FIG. 1  width w 1  is greater than width w 2 , in a different embodiment width w 1  can also be lower than width w 2 . 
       FIG. 2  shows a block  20  according to a second embodiment of the present invention, comprising a contact face  2 , two transverse faces  3  and  4 , represented by one trailing face  3  and one leading face  4 , and two longitudinal faces  5  and  6 , represented by one right face  5  and one left face  6 . 
     Typically, the block  20  can represent an embodiment of one block of a plurality of blocks forming an outermost row situated on the edges of the tread and forming the shoulders of the tire. 
     The block  20  is characterized by the presence of two steps  21  and  22  formed on the leading face  4 . The step  21  is delimited by an upper face  23 , a front face  24 , and a first portion of the two longitudinal faces  5   a  and  6   a  prosecuting the longitudinal faces  5  and  6  of the block  20 . The step  22  is delimited by an upper face  25 , a front face  26 , and a second portion of the two longitudinal faces  5   a  and  6   a  prosecuting the longitudinal faces  5  and  6  of the block  20 . 
     In the embodiment of  FIG. 2 , the upper faces  23  and  25  are substantially parallel to the contact face  2  both with respect to the transverse direction z, and to the longitudinal direction x. 
     In the embodiment of  FIG. 2 , the front faces  24  and  26  are substantially parallel with the leading face  4  with respect to the radial direction y, but they form, each independently, an angle of from 5° to 30° with respect to the transverse direction z. 
     In the embodiment of  FIG. 2 , the heights h 1  and h 2  of the step  21  are equal and can range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  20 . Similarly, the heights h 3  and h 4  of the step  22  are equal and can range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  20 . 
     The widths w 1  and w 2  of the step  21  are different and can independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. Similarly, the widths w 3  and w 4  of the step  22  are different and can independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. 
     Additionally, while in the embodiment of  FIG. 2  the widths w 1  and w 3  as well as w 2  and w 4  are represented with the same value, they can also be different each other and independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. 
       FIG. 3  shows a block  30  according to a third embodiment of the present invention, comprising a contact face  2 , two transverse faces  3  and  4 , represented by one trailing face  3  and one leading face  4 , and two longitudinal faces  5  and  6 , represented by one right face  5  and one left face  6 . 
     Typically, the block  30  can represent an embodiment of one block of a plurality of blocks forming an outermost row situated on the edges of the tread and forming the shoulders of the tire. 
     Similarly to the block  20  of  FIG. 2 , the block  30  of  FIG. 3  is characterized by the presence of two steps  31  and  32  formed on the leading face  4 . The step  31  is delimited by an upper face  33 , a front face  34 , and a first portion of the two longitudinal faces  5   a  and  6   a  prosecuting the longitudinal faces  5  and  6  of the block  30 . The step  32  is delimited by an upper face  35 , a front face  36 , and a second portion of the two longitudinal faces  5   a  and  6   a  prosecuting the longitudinal faces  5  and  6  of the block  30 . 
     In the embodiment of  FIG. 3 , the upper faces  33  and  35  are substantially parallel to the contact face  2  with respect to the longitudinal direction x, but they form, each independently, an angle of from 5° to 60°, preferably from 5° to 45°, with respect to the transverse direction z. 
     In the embodiment of  FIG. 3 , the front faces  34  and  36  are substantially parallel with the leading face  4  with respect to the radial direction y, but they form, each independently, an angle of from 5° to 30°, preferably from 5° to 20°, with respect to the transverse direction z. 
     In the embodiment of  FIG. 3 , the heights h 1  and h 2  of the step  31  are different and can independently range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  30 . Similarly, the heights h 3  and h 4  of the step  32  are different and can independently range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  30 . 
     The widths w 1  and w 2  of the step  31  are different and can independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. Similarly, the widths w 3  and w 4  of the step  32  are different and can independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. 
     Additionally, while in the embodiment of  FIG. 3  the widths w 1  and w 3  as well as w 2  and w 4  are represented with the same value, they can also be different each other and independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. 
       FIG. 4  shows a block  40  according to a fourth embodiment of the present invention, comprising a contact face  2 , two transverse faces  3  and  4 , represented by one trailing face  3  and one leading face  4 , and two longitudinal faces  5  and  6 , represented by one right face  5  and one left face  6 . 
     Typically, the block  40  can represent an embodiment of one block of a plurality of blocks forming an outermost row situated on the edges of the tread and forming the shoulders of the tire. 
     Similarly to the block  20  of  FIG. 2 , the block  40  of  FIG. 4  is characterized by the presence of two steps  41  and  42  formed on the leading face  4 . The step  41  is delimited by an upper face  43 , a front face  44 , and a first portion of the two longitudinal faces  5   a  and  6   a  prosecuting the longitudinal faces  5  and  6  of the block  40 . The step  42  is delimited by an upper face  45 , a front face  46 , and a second portion of the two longitudinal faces  5   a  and  6   a  prosecuting the longitudinal faces  5  and  6  of the block  40 . 
     In the embodiment of  FIG. 4 , the upper faces  43  and  45  lie on planes forming with the plane of the contact face  2 , each independently, an angle of from 5° to 45°, preferably from 5° to 30°, with respect to the longitudinal direction x, and an angle of from 5° to 60°, preferably from 5° to 45°, with respect to the transverse direction z. 
     In the embodiment of  FIG. 4 , the front faces  44  and  46  lie on planes forming with the plane of the leading face  4 , each independently, an angle of from 5° to 30°, preferably from 5° to 20°, both with respect to the transverse direction z and to the radial direction y. 
     In the embodiment of  FIG. 4 , the heights h 1  and h 2  of the step  41  are different and can independently range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  40 . Similarly, the heights h 3  and h 4  of the step  42  are different and can independently range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  40 . 
     The widths w 1  and w 2  of the step  41  are different and can independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. Similarly, the widths w 3  and w 4  of the step  42  are different and can independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. 
     Additionally, while in the embodiment of  FIG. 4  the widths w 1  and w 3  as well as w 2  and w 4  are represented with the same value, they can also be different each other and independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. 
       FIG. 5  shows a block  50  according to a fifth embodiment of the present invention, comprising a contact face  2 , two transverse faces  3  and  4 , represented by one trailing face  3  and one leading face  4 , and two longitudinal faces  5  and  6 , represented by one right face  5  and one left face  6 . 
     Typically, the block  50  can represent an embodiment of one block of a plurality of blocks forming an intermediate row situated between the outermost rows forming the shoulders of the tire. 
     Similarly to the block  20  of  FIG. 2 , the block  50  of  FIG. 5  is characterized by the presence of two steps  51  and  52  formed on the leading face  4 . The step  51  is delimited by an upper face  53 , a front face  54 , and a first portion of the two longitudinal faces  5   a  and  6   a  prosecuting the longitudinal faces  5  and  6  of the block  50 . The step  52  is delimited by an upper face  55 , a front face  56 , and a second portion of the two longitudinal faces  5   a  and  6   a  prosecuting the longitudinal faces  5  and  6  of the block  50 . 
     The upper face  53  is formed by two half-faces  53   a  and  53   b  having different orientation each other. 
     More in particular, the half-face  53   a  lies on a plane forming with the plane of the contact face  2  an angle of from +5° to +60° with respect to the transverse direction z (while being substantially parallel with respect to the longitudinal direction x). The half-face  53   b  lies on a plane forming with the plane of the contact face  2  an angle of from −5° to −60° with respect to the transverse direction z (while being substantially parallel with respect to the longitudinal direction x). 
     Similarly, the upper face  55  is formed by two half-faces  55   a  and  55   b  having different orientation each other. 
     More in particular, the half-face  55   a  lies on a plane forming with the plane of the contact face  2  an angle of from −5° to −60° with respect to the transverse direction z (while being substantially parallel with respect to the longitudinal direction x). The half-face  55   b  lies on a plane forming with the plane of the contact face  2  an angle of from +5° to +60° with respect to the transverse direction z (while being substantially parallel with respect to the longitudinal direction x). 
     The front face  54  is formed by two half-faces  54   a  and  54   b  having different orientation each other. Similarly, the front face  56  is formed by two half-faces  56   a  and  56   b  having different orientation each other. 
     In the embodiment of  FIG. 5 , the second half-faces  54   a  and  56   a  lie on planes forming with the plane of the leading face  4 , each independently, an angle of from +5° to +30° with respect to the transverse direction z (while being substantial parallel with respect to the radial direction y). The second half-faces  54   b  and  56   b  lie on planes forming with the plane of the leading face  4 , each independently, an angle of from −5° to −30° with respect to the transverse direction z (while being substantial parallel with respect to the radial direction y). 
     In the embodiment of  FIG. 5 , two half-faces have been provided for each of the upper faces  53  and  55  and the front faces  54  and  56 . However, it must be understood that any upper and/or front face of any irregularly shaped step according to the invention can be formed by a plurality of sub-faces having different orientation each other. Preferably, the number of sub-faces ranges from 1 to 6, more preferably from 1 to 4, and most preferably from 1 to 3. 
     In the embodiment of  FIG. 5 , the heights h 1  (or h 2 ) and h a  of the step  51  are different and can independently range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  50 . Similarly, the heights h 3  (or h 4 ) and h b  of the step  52  are different and can independently range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  50 . 
     The widths w 1  (or w 2 ) and w a  of the step  51  are different and can independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. Similarly, the widths w 3  (or w 4 ) and w b  of the step  52  are different and can independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. 
     In the embodiment of  FIG. 5 , the heights h 1  and h 2  as well as h 3  and h 4  are represented with the same value. However, they can also be different each other and independently range from 5% to 80%, preferably from 10% to 60%, relative to the height h of the block  50 . 
     Additionally, while in the embodiment of  FIG. 5  the widths w 1 , w 2 , w 3  and w 4  are represented with the same value, they can also be different each other and independently range from 0% to 60%, preferably from 10% to 40%, relative to the maximum width between two faced blocks. 
     For sake of simplicity, in the above described examples the number of steps has been limited to two. However, the number of steps for each block is not particularly limited and any number can be contemplated. In practice, it is preferable to have a number of steps from 1 to 8, more preferably from a 1 to 5, and most preferably from 1 to 3. 
     Also, the steps can be formed only on the leading transverse face, as represented in the above examples, but can also be formed only on the trailing transverse face, or on both transverse faces. In case of a plurality of transverse faces, the steps can be formed on any number of transverse faces. However, the presence of at least one step according to the invention on at least one leading transverse face is preferred. 
     For sake of simplicity, the basic shapes of blocks  10  to  50  of  FIG. 1 to 5  are represented as regular cubes. It is understood that the blocks provided with at least one irregular step according to the invention are not limited to the cubic shape, but can have any useful shape known in the art of tread for tyres provided that they have at least one side face of substantially transverse orientation z, preferably at least one transverse leading face, provided with at least one irregularly shaped step having an upper face and a front face. 
     Also for sake of simplicity, any face of the blocks and irregular steps of  FIG. 1 to 5  has been represented as a planar surface. It is understood that the faces of the blocks and irregular steps provided in the tire according to the invention are not limited to a planar surface, but can have any useful shape known in the art of tread for tyres. In case of a non planar surface, the reference plane to be considered for the scope of the present invention shall be the plane passing trough the edges of the face under consideration. 
     The tread according to the present invention is realized on a tyre having any kind of conventional structure such as, for example, those comprising a carcass, a tread band located on the crown of said carcass, a pair of axially superimposed sidewalls terminating in beads reinforced with bead wires and corresponding bead fillers, for securing said tyre to a corresponding mounting rim. The tyre preferably also comprises a belt structure interposed between the carcass and the tread band. The carcass is usually formed by one or more carcass plies associated to said bead wires, while the belt structure generally comprises at least two belt layers, usually comprising metal cords, parallel to each other in each layer and crossing over those of the adjacent layers, preferably inclined symmetrically with respect to the equatorial plane, and radially superimposed on each other. Optionally, the structure may also comprise additional belt layers, formed by cords, usually metal cords, oriented circumferentially, i.e. with a disposition at substantially zero degrees with respect to said equatorial plane. 
       FIG. 6  is a partial plan view of a tread according to an embodiment of the present invention. As illustrated in  FIG. 6 , the above described tyre has a tread of elastomeric material, provided with two circumferential grooves  101  and  102  and a plurality of transverse grooves  103 . Circumferential grooves  101  and  102  divide a tread central region  105  from two shoulder regions  104  and  106 , located on the left ( 104 ) and on the right ( 106 ) of the equatorial plane, respectively. The plurality of transverse grooves  103  divides the tread in a plurality of modules  107 , each module comprising a central block  112  belonging to tread central region  105  and two shoulder blocks belonging to tread shoulder regions  104  and  106 , namely an external shoulder block  111  and an internal shoulder block  113 . 
     Accordingly, the tread central region  105  comprises a plurality of blocks  112  arranged in a central row and divided by transverse grooves  103 . The blocks  112  of  FIG. 6  are identical to the above described block  50  of  FIG. 5 . However, blocks  112  can have any other different irregular shape according to the present invention. 
     On the other hand, the tread shoulder regions  104  and  106  comprise a plurality of blocks  111  and  113  arranged in an external and internal row, respectively, and divided by transverse grooves  103 . The external blocks  111  of  FIG. 6  are identical to the above described block  30  of  FIG. 3 . However, blocks  111  can have any other different irregular shape according to the present invention, such as, for example, those illustrated in  FIGS. 1 ,  2 , and  4 . The internal blocks  113  of  FIG. 6  are specular to blocks  111  and are specularly identical to the above described block  30  of  FIG. 3 . However, blocks  113  can have any other different irregular shape according to the present invention, such as, for example, those illustrated in  FIGS. 1 ,  2 , and  4 . 
       FIG. 7  is a partial plan view of a tread according to another embodiment of the present invention. For sake of simplicity, only half a surface of the tread is illustrated in  FIG. 7 , the other half being specular with the illustrated surface respective to the equatorial line  100 . As illustrated in  FIG. 7 , the above described tyre has a tread pattern comprising two zig-zag circumferential grooves  201 , and  202  and a plurality of zig-zag transverse grooves  203 . 
     The circumferential groove  201  and  202  and the transverse grooves  203  divide the tread in a shoulder circumferential row of blocks  204 , a central circumferential row of blocks  205 , and a central circumferential continuous rib  206 . 
     The blocks  204  according to the present invention have all the same shape, and comprise two leading transverse faces  210  and  211 , two trailing transverse faces  212  and  213 , and two longitudinal faces, namely a shoulder face  214  and a central face  215 . The leading transverse faces  210  and  211  of blocks  204  comprise two irregularly shaped steps  216  and  217  having an increased width when proceeding from the shoulder face  214  to the central face  215 . 
     The blocks  205  according to the present invention have all the same shape, and comprise three leading transverse faces  220 ,  221 , and  222 , two trailing transverse faces  223  and  224 , and two longitudinal faces, namely a shoulder face  225  and a central face  226 . Two leading transverse faces  221  and  222  of blocks  205  comprise two irregularly shaped steps  227  and  228  having an increased width when proceeding from the shoulder face  225  to the central face  226 .