Abstract:
A tire comprising a pair of beads; a carcass ply having ends, each ends anchored to a respective bead; at least one belt ply extending circumferentially around the tire and disposed radially outward of the carcass ply; and a tread portion disposed radially outward of the belt ply. The tread portion has a plurality of tread ribs or blocks and at least one groove disposed between adjacent tread ribs or blocks. In one aspect of the invention, the tread portion is formed from first and second rubber portions. The second rubber portion has a hysteresis value greater than the first rubber portion and a modulus value less than first rubber portion. In another aspect of the present invention, the tread portion is formed from a first and second tread compound. The hysteresis value of the second tread compound is greater than the hysteresis value of the first tread compound and the modulus value of the first tread compound is less than the modulus value of the second tread compound. In another aspect of the present invention, the tread portion is formed from a rubber with a modulus of between approximately 2 and 3 N/mm 2 , and a tan δ value of between approximately 0.2 and 0.4 under normal operating conditions. The present invention reduces the maximum longitudinal contact stress, maximum lateral contact stress, and the maximum normal contact stress on the tread layer relative to conventional tires, hence increasing the robustness of the tire with respect to tread wear.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/706,951 filed Sep. 3, 1996 now abandoned, herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a new and improved pneumatic vehicle tire and, more particularly, to a new and improved pneumatic tire having a radial ply carcass or having a bias ply carcass. 
     The present invention relates to a pneumatic vehicle tire, especially for commercial vehicles, having a radial carcass, the plies of which are made of steel or of a material of comparable high strength, and the ends of which terminate at or around the tire beads. The tire has a centrally disposed tread strip and a multi-ply belt. Typically, in such a tire, the shoulders or lateral areas of the tire tread tend to wear at a greater rate than the central portion of the tread. This necessitates the premature scrapping of such tires due to their total wear in the shoulder area although the central portion of the tread is still satisfactory for substantial additional service. The severe conditions during the service life of a commercial tire as well as the differing impact of certain forces or stresses on the tire tread across its lateral or widthwise extent significantly contributes to the non-uniform distribution of tread wear. 
     In an attempt to overcome the drawbacks in presently known tires, it has been suggested to provide a tire specifically having a rubber component of a different property to beneficially affect the tire tread performance. For example, it has been suggested that additional rubber be incorporated in the laterally outward areas of the tread so that both the central and lateral portions of the tread will wear out at approximately the same time, notwithstanding the fact that the laterally outward areas of the tread wear more quickly than the central area. This solution may, however, cause an unwanted weight increase in the shoulder region which increases the heat build-up in the tire, thereby adversely affecting tire life. Moreover, increasing the quantity of material in the shoulder regions adds to the cost of manufacture of the tire. 
     U.S. Pat. No. 3,853,164 to Mirtain proposes another solution to the problem of disproportionate tread wear. Mirtain discloses a cushion, formed of a material harder than the material of the remainder of the tread, disposed between the tread and the breaker of the tire. The cushion extends to one or both of the lateral or outside walls of the tire. This solution also is not completely satisfactory because the use of such a hard cushion results in a ride quality which is substantially rougher than that of conventional tires. Furthermore, it has been found that the use of such a hard cushion tends to reduce the traction of the tire. 
     U.S. Pat. No. 4,671,333 to Rohde et al. discloses a tire with a low-damping rubber layer disposed between plies of a multi-ply breaker belt. Such tires achieve their best results when used with commercial vehicles. By providing a step in the shoulder region, by having the belt plies extend laterally into the stepped portions, and by possibly introducing a low-damping rubber layer between plies of the belt, it was hoped to achieve a tire having a reduced resistance to rolling. German Auslegeschrift 10 07 644 to Fletcher discloses a vehicle tire having a belt of steel cord fabric with a resilient rubber underlayer of a carcass rubber mixture disposed radially outwardly from an eight ply diagonal carcass of textile fabric. The belt plies extend at an angle of 45 degrees relative to the circumferential direction of the tire. The rubber underlayer serves to prevent detachment of the belt from the carcass due to the overall rigidity of the tire and the relative movement between the tread strip and the carcass resulting therefrom. The rubber underlayer is at least 2 to 5 mm thick. 
     U.S. Pat. No. 3,931,844 to Mirtain discloses a pneumatic tire having a cushion-like support under the tread member. The support is more supple, or softer, than the rubber mixture of the tread member. The cushion extends over the tire width and has a greater thickness in the midcircumferential plane region of the tire. The cushion has only a relatively small thickness in the shoulder regions of the tire. The disclosed tire is intended to provide uniform tire wear and improved traction. 
     Nonetheless, in spite of the attempts to ameliorate the problems of the tire tread performance through the strategic placement of special property rubbers, there still remains room for improvement in this approach to an improved tire. 
     SUMMARY OF THE INVENTION 
     Thus, it is an object of the present invention to provide a tire with improved tread performance. 
     It is a further object of the present invention to provide a tire having improved tread wear resistance. 
     It is a further object of the present invention to provide a tire that exhibits reduced stresses in the contact patch. 
     It is a further object of the present invention to provide a tread having a soft rubber compound which reduces the stresses in the contact patch. 
     It is a further object of the present invention to provide a tire that exhibits a phase lag between the stresses and strains occurring in the contact patch. 
     It is a further object of the present invention to provide a tread having a highly hysteretic material to produce a phase lag between the stresses and strains occurring in the contact patch. 
     These and other objects of the present invention are achieved by a tire comprising a pair of beads; a carcass ply having ends, each ends anchored to a respective bead; at least one belt ply extending circumferentially around the tire and disposed radially outward of the carcass ply; and a tread portion disposed radially outward of the belt ply. The tread portion has a plurality of tread ribs or blocks and at least one groove disposed between adjacent tread ribs or blocks. 
     One aspect of the invention is achieved by forming the tread portion from first and second rubber portions. The second rubber portion has a hysteresis value greater than the first rubber portion and a modulus value less than first rubber portion. 
     Another aspect of the invention is achieved by forming the tread portion from a first and second tread compound. The hysteresis value of the second tread compound is greater than the hysteresis value of the first tread compound and the modulus value of the first tread compound is less than the modulus value of the second tread compound. 
     Another aspect of the present invention is achieved by forming the tread portion from a rubber with a modulus of between approximately 2 and 3 N/mm 2 , and a tan δ value of between approximately 0.2 and 0.4 when the tread is at ten percent (10%) deformation and 40° C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention will become apparent to those skilled in the art to which the present invention relates from reading the following specification with reference to the accompanying drawings in which: 
     FIG. 1 is a perspective view of a tire mounted on a rim, the tire being shown in an inflated and loaded condition as exists when the tire supports a vehicle on a ground surface and the arrows schematically illustrating the distribution of the forces exerted by the inflation gas on the tire during rolling contact of the tire along the ground surface; 
     FIG. 2A is an enlarged perspective cross-sectional view of an angular portion of a first embodiment of the tire shown in FIG. 1; 
     FIG. 2B is an enlarged perspective view of a differential element of the tire shown in FIG. 2A; 
     FIG. 3 is an enlarged cross sectional view of a portion of a first alternative embodiment of the tread portion of the tire shown in FIG. 1; 
     FIG. 4 is an enlarged cross sectional view of a portion of a second alternative embodiment of the tread portion of the tire shown in FIG. 1; 
     FIG. 5 is an enlarged cross sectional view of a portion of a third alternative embodiment of the tread portion of the tire shown in FIG. 1; 
     FIG. 6A is an enlarged cross sectional view of a possible configuration of the rubbers used in a fourth embodiment of the tread portion of the tire shown in FIG. 1; 
     FIG. 6B is an enlarged cross sectional view of another possible configuration of the rubbers used in a fourth embodiment of the tread of the tire shown in FIG. 1; 
     FIG. 6C is an enlarged perspective view in partial section of another possible configuration of the rubbers used in a fourth embodiment of the tread of the tire shown in FIG. 1; 
     FIG. 7A is an enlarged cross sectional view of a possible configuration of the rubbers used in a fifth embodiment of the tread of the tire shown in FIG. 1; 
     FIG. 7B is an enlarged cross sectional view of another possible configuration of the rubbers used in a fifth embodiment of the tread of the tire shown in FIG. 1; 
     FIG. 8 is an enlarged cross sectional view of a portion of a sixth alternative embodiment of the tread portion of the tire shown in FIG. 1; 
     FIG. 9 is a graphical representation of the measured longitudinal contact stresses (σ x ) imposed on the contact length of a tire of the present invention compared to a conventional tire; 
     FIG. 10A is a graphical representation of the measured lateral contact stresses (σ y ) imposed on the contact length of a tire of the present invention compared to a conventional tire at a given slip angle; 
     FIG. 10B is a graphical representation of the measured lateral contact stresses (σ y ) imposed on the contact length of a tire of the present invention compared to a conventional tire at another given slip angle; and 
     FIG. 11 is a graphical representation of the measured normal contact stresses (σ z ) imposed on the contact length of a tire of the present invention compared to a conventional tire. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This application uses numerous phrases and terms of art. The phrase “mid-circumferential plane” refers to the plane passing through the center of the tread and being perpendicular to the axis of rotation of the tire. 
     The term “radial” refers to the direction perpendicular to the axis of rotation of the tire. 
     The term “axial” refers to the direction parallel to the axis of rotation of the tire. 
     The term “lateral” refers to the direction along the tread of the tire going from one sidewall of the tire to the other sidewall. 
     The term “groove” refers to an elongated void area in the tread that may extend circumferentially or laterally in a straight, curved of zig-zag manner. 
     The phrase “tread width” refers to the greatest axial distance across the portion of the tread in contact with a road surface, as measured from a footprint of the tire, when the tire is mounted on a rim, subjected to a load, and inflated to a pressure corresponding to the load. All of the other tire dimensions are measured when the tire is mounted on a rim and inflated to a given pressure, but not subjected to a load. 
     The term “modulus” refers to the modulus of elasticity of the rubber at ten percent (10%) elongation and at 40° C. 
     The term “tan δ” refers to the phase lag between the stresses and strains on the rubber at ten percent (10%) elongation and at 40° C. 
     A tire  10  having improved resistance to wear that meets and achieves all the objects of the invention set forth above will now be described with reference to FIGS. 1,  2 A and  2 B. As seen in more detail in FIGS. 1 and 2A, the pneumatic tire  10  is adapted to be mounted on a rim  12 . Tire  10  comprises at least one carcass ply  16  having ends  18  each secured to one of a pair of inextensible annular bead members  20 . The bead members  20  securely mount the pneumatic tire  10  on the rim  12 . 
     The pneumatic tire  10  additionally includes a pair of sidewall portions  22 , each on opposite sides of the midcircumferential plane. Sidewall portions  22  extend from a location adjacent a bead member  20  to a shoulder region  24  at which the sidewall portion  22  is joined with a tread portion  26 . The tread portion  26  forms the portion of the pneumatic tire  10  which is in contact with the ground surface  14  during rolling movement of the tire. Tread portion  26  may include conventional tire tread sculpture features such as, for example, circumferential grooves, lateral grooves  36 , sides, or lamelles. 
     The pneumatic tire  10  also includes a plurality of circumferentially extending crown reinforcement belt plies  28  which are disposed radially intermediate the carcass ply  16  and the tread portion  26 . The belt plies  28 , as seen in FIG. 2A, may include a plurality of steel reinforcing cords  30  embedded in rubber or may alternatively include reinforcing cords of a material other than steel. 
     As seen in FIGS. 2A and 2B, tread portion  26  includes both the rubber disposed beneath the tread sculpture (i.e. the undertread) and the tread ribs or blocks. Stated differently, tread portion  26  includes all of the rubber beginning at belt ply  28  and extending radially outward therefrom. 
     A ground contacting surface forms the radially outermost surface of a body region of tread portion  26 . The numerous embodiments of the present invention, along with each alternative arrangement within an embodiment, are designed to achieve a specific balance between one or more tire performance characteristics (e.g. rolling resistance, endurance, wear resistance and traction) or tire manufacturing goals (e.g. quality, time, simplicity and the commonality of materials/processes). Other arrangements are possible which may achieve other suitable characteristics or goals. The various configurations of tread portion  26  of the present invention will now be individually described. 
     FIG. 3 demonstrates a first possible embodiment for the tread portion. Tire  110  has a tread portion  126  with a single tread rubber material  142 . Tire  110  can best achieve the benefits of the present invention by adjusting several properties of single tread rubber portion  142 . One property that could be varied in accordance with the present invention is the hardness of the rubber. A second property that could be varied in accordance with the present invention is the hysteretic properties of the rubber. 
     The modulus of elasticity can measure the hardness of a rubber. The modulus of elasticity of a composition is measured using any known technique that can provide stress measurements for a range of applied deformations under quasi-static conditions. As an example, conventional tread rubber compositions have a modulus of elasticity of between approximately 4 and 6 N/mm 2 . 
     The value tan δ can indicate the hysteresis of a rubber. The tan δ value is measured using any known technique that provides a time history of the stresses generated by an applied deformation. As an example, conventional tread rubbers have a tan δ value between approximately 0.15 to 0.2. 
     In order to increase the robustness of tire  10  with respect to uneven tread wear, it is preferable to utilize a tread rubber material that is softer and more hysteretic than conventional materials. Tread rubber material  142  should be approximately twenty to fifty percent (20-50%) softer than conventional tread materials. When compared to conventional tread rubbers, tread rubber material  142  could have a modulus of elasticity of between approximately 2 and 4 N/mm 2  under the same conditions. Preferably, tread rubber material  142  is approximately fifty percent (50%) softer than conventional tread rubber materials. In other words, tread rubber material  142  should have a modulus of elasticity of between approximately 2 and 3 N/mm 2  under the same conditions. 
     FIGS. 9,  10 A,  10 B and  11  demonstrate the benefit of using a softer tread rubber material in a tire. The figures graphically represent the measured longitudinal contact stress (σ x ), lateral contact stress (σ y ) and normal contact stress (σ z ), respectively, imposed across the contact length of the tread portion of the tire having a softer tread material as described in the present invention compared to a tire having a conventional rubber tread portion. FIG. 10A displays the lateral contact stress (σ y ) at a negative slip angle. FIG. 10B displays the lateral contact stress (σ y ) at a positive slip angle. 
     The figures clearly demonstrate a reduction in the longitudinal contact stress (σ x ), lateral contact stress (σ y ) and normal contact stress (σ z ) when the tread portion of tire  10  utilizes a softer tread rubber as compared to the use of conventional tread rubbers. The reduced maximum lateral contact stress on the tread portion of the tire leads to a reduction in the slippage of the tread on the ground. Reducing the slippage thus reduces the rate of wear of the tire. 
     Table I summarizes the maximum longitudinal contact stress (σ x ), the maximum lateral contact stress (σ y ) and the maximum normal contact stress (σ z ) exhibited in FIGS. 9,  10 A,  10 B and  11 . The table demonstrates a reduction in the maximum contact stresses when the tread portion of tire  10  utilizes a softer tread rubber as compared to the use of conventional tread rubbers. 
     
       
         
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                 Normalized Value 
                 Normalized Value 
                   
               
               
                 Type of 
                 for Conventional 
                 for Tire of 
                 Percent 
               
               
                 Contact Stress 
                 Tire 
                 Present Invention 
                 Reduced 
               
               
                   
               
             
             
               
                 Longitudinal (σ x ) 
                 1 (reference) 
                 0.8 
                 20 
               
               
                 Lateral (σ y ) 
                 1 (reference) 
                 0.8 
                 20 
               
               
                 with negative slip 
               
               
                 angle 
               
               
                 Lateral (σ y ) 
                 1 (reference) 
                 0.7 
                 30 
               
               
                 with positive slip 
               
               
                 angle 
               
               
                 Normal (σ z ) 
                 1 (reference) 
                 0.7 
                 30 
               
               
                   
               
             
          
         
       
     
     It is also preferable, in terms of uneven tread wear characteristics, to have tread rubber material  142  more hysteretic than conventional tread rubbers. Specifically, tread rubber material  142  should be approximately thirty to one-hundred percent (30-100%) more hysteretic than conventional tread rubbers. Compared to conventional tread rubbers, tread rubber material  142  should have a tan δ value of between approximately 0.2 to 0.4 under the same conditions. Preferably, tread rubber material  142  should have a tan δ value of approximately 0.3 under the same conditions. 
     A benefit of using a more hysteretic tread rubber material in a tire can be seen in the measured longitudinal contact stresses (σ x ) imposed across the contact length of the tread portion of a tire of the present invention compared to a tire having a conventional rubber tread portion. A phase lag can be shown to exist between the longitudinal contact stresses (σ x ) of a more hysteretic tread rubber and the longitudinal contact stresses (σ x ) of a conventional tread rubber. The introduction of a phase lag leads to more stable wear on a tire. A tire using a more hysteretic tread rubber is more robust with respect to uneven tread wear as indicated by the following experimental results. 
     A conventional, or reference, tire known in the art was tested along with a tire essentially the same as the conventional tire, but having the features of the present invention. In other words, the tire of the present invention included the new tread rubber in the tread portion. The tires were heavy duty truck tires with the same size, load range, and usage as defined by the Standards of the 1997 Yearbook of The Tire and Rim Association, Inc. of Copley, Ohio. 
     The test involved two conventional tires and two tires of the present invention. The tires were tested on the steer axles of two identical long haul vehicles. The tires were rotated between the vehicles to compensate for any differences in the vehicles&#39; suspension systems. The right side tires were maintained on the right side of each vehicle when the tires were rotated. Numerous tire rotations occurred during the test period. 
     The tire testing continued until one of the tires exhibited anomalies or exhibited enough tread wear to warrant the removal of the tire from service. At that point, all of the tires were removed from service. Inspection of the tires revealed the following. 
     The conventional tires used in the experiment exhibited uneven tread wear. The tire of the present invention used in the experiment, however, did not exhibit any uneven tread wear. These results clearly establish the superior uneven tread wear characteristics of tires of the present invention. 
     Tread portion  126  with single tread rubber portion  142  can clearly provide superior uneven tread wear characteristics over conventional tread rubbers. The resulting superior uneven tread wear characteristics may, however, arise through the sacrifice of other tire characteristics. For example, tire  110  may not exhibit the rolling resistance and endurance characteristics of other tires. As a compromise between uneven tread wear characteristics, rolling resistance and endurance, it may be desirable to utilize only limited amounts of the softer, more hysteretic rubber material within the tread portion of the tire to improve the rolling resistance and endurance characteristics. 
     When limiting the amount, or volume, of the softer, more hysteretic material used in the tread portion, the remaining amount, or volume, of the tread portion of the tire should be occupied by a second tread rubber that has different properties than the first tread rubber compound. The use of the softer, more hysteretic rubber material at specific locations within the tread portion of tire  10  maintains the other tire performance characteristics while also providing superior uneven tread wear characteristics. Each alternative embodiment of the dual compound tread portion will now be described. 
     FIG. 4 demonstrates a second alternative embodiment of the present invention. Tire  210  includes a first rubber portion  232  positioned radially beneath a second rubber portion  234 . First rubber portion  232  is formed on belts  228  and extends into the tread ribs or blocks (i.e. first rubber portion  232  begins at the interface and extends into the tread ribs or blocks). First rubber portion  232  extends laterally across the entire tread width of tread portion  226 . First rubber portion  232  could use, for example, a conventional rubber composition. 
     Second rubber portion  234  is positioned radially outward from first rubber portion  232 . Second rubber portion  234  does not substantially extend into the shoulder area of tire  210 . Rather, second tread portion  234  extends between approximately the laterally outermost grooves  236  in tread portion  226 . As seen in the figure, second rubber portion  234  forms the ground contacting surface of tread portion  226  or tire  210 . Tread portion  226 , having first rubber portion  232  and second rubber portion  234 , is manufactured using known techniques. 
     Second rubber portion  234  should occupy between approximately twenty to fifty percent (20-50%) of the volume of tread portion  226 . Since second rubber portion  234  maintains a relatively constant thickness across the tread width in this embodiment, the volume of tread portion  226  occupied by second rubber portion  234  can be approximated by its thickness. Thus, second rubber portion  234  should encompass between approximately twenty to fifty percent (20-50%) of the depth of tread portion  226 . Recalling the discussion earlier, the depth of tread portion  226  includes the intertread. Preferably, second rubber portion  234  occupies approximately one-third of the total volume of tread portion  226 . As stated above, this can be approximated by second rubber portion  234  encompassing approximately one-third of the total depth of tread portion  226 . 
     Second rubber portion  234  can have the same properties as tread rubber material  142  in the first embodiment. Thus, second rubber portion  234  should be softer than first rubber portion  132 . Specifically, second rubber portion  234  should be between approximately twenty and fifty percent (20-50%) softer than first rubber portion  232 . Preferably, second rubber portion  234  is approximately fifty percent (50%) softer than first rubber portion  232 . 
     In addition, second rubber portion  234  should be more hysteretic than first rubber portion  232 . Specifically, second rubber portion  234  should be approximately thirty to one-hundred percent (30-100%) more hysteretic than first rubber portion  232 . Preferably, second rubber portion  234  is fifty percent (50%) more hysteretic than first rubber portion  232 . 
     The present invention does not require the second tread portion to be positioned as shown in FIG.  4 . Applicant recognizes that the second rubber portion can be positioned at any suitable location within the tread portion of the tire. The volume occupied by the second rubber portion in the tread portion is more important to the present invention than the specific radial position of the second rubber portion in the tread portion. The following alternative embodiments establish that the second rubber portion can be positioned at numerous locations within the tread portion of the tire. 
     FIG. 5 demonstrates a third alternative embodiment of the present invention. Tire  310  includes a second rubber portion  334  positioned within each main tread rib. A first rubber portion  332  is formed on belt ply  328  and is positioned radially outward of second rubber portion  334 . In other words, first rubber portion  332  envelops second rubber portion  334  in tread portion  326 . Tread portion  326 , having first rubber portion  332  and second rubber portion  334 , is manufactured using known techniques. 
     Second rubber portion  334  should occupy between approximately twenty to fifty percent (20-50%) of the volume of tread portion  326 . Since second rubber portion  334  maintains a relatively constant thickness across the tread width in this embodiment, the volume of tread portion  326  occupied by second rubber portion  334  can be approximated by its thickness. Thus, second rubber portion  334  should encompass between approximately twenty to fifty percent (20-50%) of the depth of tread portion  326 . Recalling the discussion earlier, the depth of tread portion  326  includes the undertread. Preferably, second rubber portion  334  occupies approximately one-third of the total volume of tread portion  326 . As stated above, this can be approximated by second rubber portion  334  encompassing approximately one-third of the total depth of tread portion  326 . Groove  336  is formed, or cut, into both first rubber portion  332  and second rubber portion  334 . 
     Second rubber portion  334  is not required to be positioned as shown in FIG.  5 . The volume occupied by second rubber portion  334  in tread portion  326  is more important to the present invention than the specific radial position of second rubber portion  334  in tread portion  326 . In this embodiment, second rubber portion  334  can be positioned at any location, or altitude, within tread portion  326  of tire  310 . The placement of second rubber portion  334  could be based on, for example, manufacturing considerations. 
     Second rubber portion  334  can also have the same properties as second rubber portion  234  in the second embodiment. Thus, second rubber portion  334  should be softer and more hysteretic than first rubber portion  332 . Specifically, second rubber portion  334  should be between approximately twenty and fifty percent (20-50%) softer than first rubber portion  332 . Preferably, second rubber portion  334  is approximately fifty percent (50%) softer than first rubber portion  332 . Second rubber portion  334  should be approximately thirty to one-hundred percent (30-100%) more hysteretic than first rubber portion  332 . Preferably, second rubber portion  334  is fifty percent (50%) more hysteretic than first rubber portion  332 . 
     FIG. 6A-C demonstrate several possible configurations of a fourth alternative embodiment of the present invention. Tire  410  includes a second rubber portion  434  formed on belts  228 ; and a first rubber portion  432  positioned radially outward from second rubber portion  434 . First rubber portion  432  extends laterally across the entire tread width of tread portion  426 . First rubber portion  432  could be, for example, a conventional rubber. 
     FIG. 6A provides the first possible configuration. In this configuration of tire  410 , second rubber portion  434  extends the full lateral extent of tread portion  426 . First rubber portion  432  is positioned radially outward from second rubber portion  434 . As seen in the figure, first rubber portion  432  forms the tread features of tread portion  426 . Grooves  436 , the tread blocks or ribs, and the sides are formed into first rubber portion  432 . Second rubber portion  434  remains in the undertread of tread portion  426 . In other words, second tread portion  434  does not extend into the tread blocks or ribs. 
     FIG. 6B provides the second possible configuration. In this configuration of tire  410 , second rubber portion  434  extends laterally between the outermost grooves  436  in tread portion  426 . First rubber portion  432  is positioned radially outward from second rubber portion  434 . Slightly different than the configuration in FIG. 3A, second rubber portion  434  does not extend substantially into the laterally outermost tread ribs, which can be a decoupling, or sacrificial, rib. 
     Second rubber portion  434  does not maintain a flat, or planar, interface with first rubber portion  432 . Second rubber portion  434  can have an undulating upper surface. Portions of second rubber portion  434  partially extend into the tread ribs or blocks. The greatest thicknesses of second rubber portion  434  occurs underneath the edges of the tread ribs or blocks. The smallest thicknesses of second rubber portion  434  occurs both beneath the grooves and in the medial portion of the tread ribs or blocks. Second rubber portion  434  does not extend to grooves  436 . Grooves  436  are formed, or cut, into first rubber portion  432 . 
     FIG. 6C provides the third possible configuration. In this configuration of tire  410 , second rubber portion  434  extends the full lateral extent of tread portion  426 . First rubber portion  432  is positioned radially outward from second rubber portion  434 . Second rubber portion  434  does not maintain a flat, or planar, interface with first rubber portion  432 . Second rubber portion  434  partially extends into the tread ribs. Second rubber portion extends to the bottom of grooves  436 . Grooves  436  are formed, or cut, into tread portion  426  so that the groove bottom comprises the second rubber material. The remainder of groove  436  is formed, or cut, into first rubber portion  432 . 
     In each of the possible configurations of the fourth embodiment, second rubber portion  434  should occupy between approximately twenty to fifty percent (20-50%) of the total volume of tread portion  426 . Preferably, second rubber portion  434  occupies approximately one-third of the total volume of tread portion  426 . 
     Since second rubber portion  434  in both FIGS. 6A and 6C maintains a relatively constant thickness across the tread width, the volume of tread portion  426  occupied by second rubber portion  434  can be approximated by its thickness. Thus, second rubber portion  434  in the configurations shown in FIGS. 6A and 6C should encompass between approximately twenty to fifty percent (20-50%) of the depth of tread portion  426 . Recalling the discussion earlier, the depth of tread portion  426  includes the undertread. Preferably, second rubber portion  434  encompasses approximately one-third of the total depth of tread portion  426 . 
     Second rubber portion  434  can also have the same properties as second rubber portion  234  in the second embodiment. Thus, second rubber portion  434  should be softer and more hysteretic than first rubber portion  432 . Specifically, second rubber portion  434  should be between approximately twenty and fifty percent (20-50%) softer than first rubber portion  432 . Preferably, second rubber portion  434  is approximately fifty percent (50%) softer than first rubber portion  432 . Second rubber portion  434  should be approximately thirty to one-hundred percent (30-100%) more hysteretic than first rubber portion  432 . Preferably, second rubber portion  434  is fifty percent (50%) more hysteretic than first rubber portion  432 . 
     The previous alternative embodiments demonstrated the second rubber portion extending across substantially the entire lateral extent of the tread portion of the tire. The following alternative embodiment demonstrates the second rubber portion only extending across a portion of the lateral extent of the tread portion of the tire. 
     FIG. 7A and 7B demonstrate two possible configurations of a fifth alternative embodiment of the present invention. Tire  510  includes a first rubber portion  532  and a second rubber portion  534 . As discussed above, second rubber portion  534  has a limited lateral extent. First rubber portion  532  forms the remainder of tread portion  526 . In the areas of tread portion  526  without second rubber portion  534 , first rubber portion  532  encompasses the full depth of tread portion  526 . First rubber portion  532  extends across the entire lateral extent of tread portion  526 . First rubber portion  532  could be, for example, a conventional rubber. Two possible locations of second rubber portion  534  having a limited lateral extent will now be described. 
     FIG. 7A provides the first possible configuration. In this configuration of tire  510 , second rubber portion  534  is positioned in the shoulder area of tread portion  526 . First rubber portion  532  extends the entire tread width of tread portion  526 . The specific configuration shown in FIG. 7A positions second rubber portion  534  on belt ply  528  and first rubber portion  532  radially outward from second rubber portion  534 . The tread features of tread portion  526  shown in the figure are formed in first rubber portion  532 . Second rubber portion  534  does not extend into the tread ribs. In other words, grooves  536 , tread blocks or ribs, and sipes are formed into first rubber portion  532 . 
     In this configuration, second rubber portion  534  should occupy between approximately twenty to fifty percent (20-50%) of the volume of the shoulder area of tread portion  526 . Since second rubber portion  534  maintains a relatively constant thickness within the shoulder area of tread portion  526 , the volume of the shoulder area of tread portion  526  occupied by second rubber portion  534  can be approximated by its thickness. Thus, second rubber portion  534  should encompass between approximately twenty to fifty percent (20-50%) of the depth of the shoulder area of tread portion  526 . Recalling the discussion earlier, the depth of tread portion  526  includes the undertread. Preferably, second rubber portion  534  occupies approximately one-third of the total volume of the shoulder area of tread portion  526 . As stated above, this can be approximated by second rubber portion  534  encompassing approximately one-third of the total depth of the shoulder area of tread portion  526 . 
     Applicant recognizes that second rubber portion  534  is not required to be adjacent belt ply  528 . The volume of second rubber portion  534  is more important to the present invention than the specific radial position of second rubber portion  534  in the shoulder area of tread portion  526 . In fact, Applicant believes second rubber portion  534  can be positioned at any position, or altitude, within the shoulder area of tire  510 . The placement of second rubber portion  534  could be based, for example, on manufacturing considerations. 
     FIG. 7B provides the second possible configuration. In this configuration of tire  510 , second rubber portion  434  is associated with only one tread block or rib. First rubber portion  532  extends the entire tread width of tread portion  526 . The specific configuration shown in FIG. 7B positions second rubber portion  534  on belt ply  528  and first rubber portion  532  radially outward from second rubber portion  534 . The tread features of tread portion  526  shown in the figure are formed in first rubber portion  532 . Second rubber portion  534  partially extends into the tread rib or block. However, second rubber portion  534  does not extend to grooves  536 . Grooves  536  are formed, or cut, into first rubber portion  532 . 
     In this configuration, second rubber portion  534  should occupy between approximately twenty to fifty percent (20-50%) of the volume of the area including, and beneath, the specific tread rib or block. Since second rubber portion  534  maintains a relatively constant thickness within, and beneath, the specific tread rib or block, the volume of the area including, and beneath, tread rib or block occupied by second rubber portion  534  can be approximated by its thickness. Thus, second rubber portion  534  should encompass between approximately twenty to fifty percent (20-50%) of the depth of the area including, and beneath, the specific tread rib or block in tread portion  526 . Recalling the discussion earlier, the depth of tread portion  526  includes the undertread. Preferably, second rubber portion  534  occupies approximately one-third of the total volume of the area including, and beneath, the specific tread rib or block. As stated above, this can be approximated by second rubber portion  534  encompassing approximately one-third of the total depth in the area including, and beneath, the specific tread rib or block. 
     As with the first configuration, Applicant recognizes that rubber portion  534  is not required to be positioned in the tread rib or block as specifically shown in the figure. The volume of second rubber portion  534  is more important to the present invention than the specific radial position of second rubber portion  534  in the tread rib or block. In fact, Applicant believes second rubber portion  534  can be positioned at any position, or altitude, within the tread rib or block. The placement of second rubber portion  534  could be based, for example, on manufacturing considerations. 
     Furthermore, second rubber portion  534  is not limited to placement under a single tread rib or block. If appropriate, second rubber portion  534  could be positioned under a plurality of tread ribs or blocks in tire  510 . 
     Second rubber portion  534  can also have the same properties as second rubber portion  234  in the second embodiment. Thus, second rubber portion  534  should be softer and more hysteretic than first rubber portion  532 . Specifically, second rubber portion  534  should be between approximately twenty and fifty percent (20-50%) softer than first rubber portion  532 . Preferably, second rubber portion  534  is approximately fifty percent (50%) softer than first rubber portion  532 . Second rubber portion  534  should be approximately thirty to one-hundred percent (30-100%) more hysteretic than first rubber portion  532 . Preferably, second rubber portion  534  is fifty percent (50%) more hysteretic than first rubber portion  532 . 
     FIG. 8 demonstrates a sixth alternative embodiment of the present invention. The placement of the two rubber portions in this embodiment is similar to the placement of first rubber portion  432  and second rubber portion  434  in FIG.  6 C. In this embodiment, tire  610  includes a tread portion  626  having a second rubber portion  640  formed on belts  628  (i.e. second rubber portion  640  forms at least part of the undertread). A first rubber portion  638  is positioned radially outward from second rubber portion  640 . Second rubber portion  640  does not extend substantially into the laterally outermost tread ribs, which can be a decoupling rib. 
     Second rubber portion  640  does not maintain a flat, or planar, interface with first rubber portion  638 . Second rubber portion  640  can have an undulating upper surface. The greatest thicknesses of second rubber portion  640  occurs in the medial portion of the tread ribs or blocks. The smallest thicknesses of second rubber portion  640  occurs beneath grooves  636 . Portions of second rubber portion  640  partially extend into the tread ribs or blocks. However, second rubber portion  640  does not extend to grooves  636 . Grooves  636  are formed, or cut, into first rubber portion  638 . 
     Although similarly shaped, the physical properties of the rubber portions of this embodiment are different than the previously described embodiments. There are two possible arrangements for this embodiment. 
     The first possible configuration of this embodiment requires first tread compound  638  to have a modulus similar to conventional rubber compounds, but be more hysteretic than conventional rubber compounds. In other words, first tread compound  638  should be approximately thirty to one-hundred percent (30-100%) more hysteretic than conventional rubber compounds, yet essentially maintaining a conventional modulus. Preferably, first rubber portion  638  is fifty percent (50%) more hysteretic than conventional rubber compounds. 
     Conversely, second tread compound  640  should have a hysteresis similar to conventional rubber compounds, but be softer than conventional rubber compounds. In other words, second tread compound  640  should be between approximately twenty and fifty percent (20-50%) softer than conventional rubber compounds, yet essentially maintaining a conventional hysteresis. Preferably, second rubber portion  640  is approximately fifty percent (50%) softer than conventional rubber compounds. 
     In this configuration, first tread compound  638  should occupy between approximately one-half (½) to two-thirds (⅔) of the volume of tread portion  626  of tire  610 . Second tread compound  640  preferably occupies between approximately one-third (⅓) to one-half (½) of the volume of tread portion  626  of tire  610 . Preferably, first tread compound  638  and second tread compound  640  both occupy one-half (½) of the volume of tread portion  626 . 
     Since first tread compound  638  and second tread compound  640  maintain a relatively constant thickness across the tread width, the volume of tread portion  626  occupied by the tread compounds can be approximated by their thicknesses. Thus, first tread compound  638  should occupy between approximately one-half (½) to two-thirds (⅔) of the depth of tread portion  626 . Second tread compound  640  should occupy between approximately one-third (⅓) to one-half (½) of the depth of tread portion  626 . Recalling the discussion earlier, the depth of tread portion  626  includes the undertread. 
     The second possible configuration of this embodiment rearranges the tread rubber compounds. First tread compound  638  should have a hysteresis similar to conventional rubber compounds, but be softer than conventional rubber compounds. In other words, first tread compound  638  should be between approximately twenty and fifty percent (20-50%) softer than conventional rubber compounds, yet essentially maintaining a conventional hysteresis. Preferably, first rubber portion  638  is approximately fifty percent (50%) softer than conventional rubber compounds. 
     Conversely, second tread compound  640  should have a modulus similar to conventional rubber compounds, but be more hysteretic than conventional rubber compounds. In other words, second tread compound  640  should be approximately thirty to one-hundred percent (30-100%) more hysteretic than conventional rubber compounds, yet maintaining essentially a conventional modulus. Preferably, second rubber portion  640  is fifty percent (50%) more hysteretic than conventional rubber compounds. 
     In this configuration, first tread compound  638  should occupy between approximately one-third (⅓) to one-half (½) of the volume of tread portion  626 . Second tread compound  640  should occupy between approximately one-half (½) to two-thirds (⅔) of the volume of tread portion  626 . Preferably, first tread compound  638  and second tread compound  640  both occupy one-half (½) of the volume of tread portion  626 . 
     As discussed earlier, the volume of tread portion  626  occupied by the tread compounds can be approximated by their thicknesses. Thus, first tread compound  638  should occupy between approximately one-third (⅓) to one-half (½) of the depth of tread portion  626 . Second tread compound  640  should occupy between approximately one-half (½) to two-thirds (⅔) of the depth of tread portion  626 . Recalling the discussion earlier, the depth of tread portion  626  includes the undertread. 
     The present invention exploits the heretofore unrecognized relationship between the maximum longitudinal and lateral contact stresses imposed on the tire and the wear of the tire to provide a tire with improved wear endurance. It is understood that the invention has a scope sufficient to include the full range of values of the maximum longitudinal and lateral contact stresses to the extent that the wear improvement potential of the invention is achieved. Thus, the invention covers those tire design situations in which the geometry, composition, and location of the second rubber portion is selected to only slightly reduce the maximum longitudinal contact stresses while relatively significantly reducing the maximum lateral contact stresses. 
     Conversely, the invention covers those tire design situations in which the geometry, composition, and location of the second rubber portion is selected to only slightly reduce the maximum lateral contact stresses while relatively significantly reducing the maximum longitudinal contact stresses. In any event, it is to be understood that the reductions in the maximum longitudinal and lateral contact stresses are optimally selected in coordination with the impact of the second rubber portion on the handling and rolling resistance characteristics of the tire. Additionally, it will often be prudent to take into account the impact of the reduced rigidity rubber portion on the manufacturing complexity of the tire. 
     Applicant understands that many other variations are apparent to one of ordinary skill in the art from a reading of the above specification. For instance, the present invention is not limited to new tires. The present invention can also be used with retreaded tires and tire treads in strip form which are ultimately cured before or after mounting on a tire casing. These variations and other variations are within the spirit and scope of the instant invention as defined by the following appended claims.