Patent Publication Number: US-2015083293-A1

Title: Passenger tire having low rolling resistance with improved wet traction and treadwear

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Divisional of U.S. patent application Ser. No. 12/956,870, entitled “Passenger Tire Having Low Rolling Resistance with Improved Wet Traction and Treadwear,” filed Nov. 30, 2010 (pending), which claims the benefit of and is a continuation application of U.S. patent application Ser. No. 12/943,584 entitled “Passenger Tire Having Low Rolling Resistance with Improved Wet Traction and Treadwear” filed Nov. 10, 2010 (abandoned), the disclosures of which are hereby incorporated by reference in their entirety herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally concerns pneumatic radial tires, and, more particularly, a radial tire for a passenger vehicle that maintains a low rolling resistance while having good wet traction and treadwear throughout the usable life of the tire. 
     BACKGROUND OF THE INVENTION 
     The primary purpose of the tread is to provide traction with the road surface. Sufficient traction is particularly important in inclement weather when the road surface is covered with precipitation. To maintain traction in less-than-ideal road conditions, the tread is most often designed with a tread pattern. The tread pattern consists of one or more grooves or channels in the surface of the tire. The grooves may divide the tread into regions, often referred to as “lugs.” The grooves help the tread maintain traction with the road by allowing the tread to expel precipitation from between the lugs and the road surface. Traction is also dependent upon the composition of the tread. Compositions with good traction are known, but simply selecting a composition for improved traction can have negative repercussions on the other characteristics of the tire, as set forth below, such as the wear resistance and rolling resistance. 
     The rolling resistance of the tire influences the fuel efficiency of the vehicle. It is desirable to reduce the rolling resistance to increase fuel efficiency. Generally as a tire wears, its rolling resistance decreases because the mass of the tire decreases. Like traction, rolling resistance is also dependent upon the composition of the tire. However, while compounds having low rolling resistance are desirable for improved fuel efficiency, those same compounds often do not provide the necessary minimum traction. The reverse is also true. That is, a compound with good traction properties typically exhibits an unacceptably high level of rolling resistance. 
     In addition, another characteristic of the tread is its wear resistance. The useable life of a tire is generally determined by the number of miles that can be driven on the tire before the tire tread becomes unacceptably thin, which is dictated by safety concerns. For this reason and others, it is desirable for the tread to resist wear. The compound from which the tread is made also determines the resistance of the tire to wear. However, similar to the competing characteristics between rolling resistance and traction characteristics of the tread, set forth above, compounds that are ideal for reducing treadwear typically exhibit poor traction. 
     To complicate matters, the rolling resistance and traction of a tire tread change as the tread inevitably wears away—there is a dynamic relationship between these characteristics. For instance, the rolling resistance of the tire generally decreases as the tread wears simply because of the simultaneous reduction in the weight of the tread. Furthermore, when the tread wears away or thins, the volume of the tread pattern is reduced. Since the tread pattern is primarily responsible for allowing the tread to maintain traction on wet or snow covered roads, reduction in the thickness of the tread, and the consequential loss in the void volume and sipping depth of the tread pattern, reduces the traction of the tread in these circumstances. Therefore, where rolling resistance generally decreases, which is a desirable trend, the traction of the tread in inclement weather also decreases, which is an undesirable trend. 
     As a result, it is common practice to construct a tread of a compound that is essentially a tradeoff between traction, rolling resistance, and treadwear. The application for the tire may dictate whether any one of these characteristics is to be emphasized. However, where one of the characteristics is emphasized, the remaining characteristics of the tire can be less than optimum. 
     Therefore, there remains a need for tire treads that exhibit desirable combinations of traction, rolling resistance, and treadwear characteristics that can be maintained throughout the useable life of the tire. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a pneumatic tire that maintains a low rolling resistance while having good wet traction and treadwear throughout the usable life of the tire. 
     In one embodiment, the pneumatic tire includes an outer circumferential tread having an outer tread layer in direct and continuous contact with an inner tread layer. The outer and inner tread layers together define the thickness of the tread. The inner tread layer includes one of a first elastomeric compound or a second elastomeric compound, and the outer tread layer includes the other of the first elastomeric compound or the second elastomeric compound. The second elastomeric compound having lower rolling resistance and greater resistance to treadwear than the first elastomeric compound. The inner tread layer increases in thickness across at least a widthwise portion of the outer tread layer. A maximum thickness of one of the inner tread layer and the outer tread layer is near at least one of a first shoulder and a second shoulder of the tread. As the tread wears, the inner tread layer defines a greater proportion of a running surface of the tread. 
     In another embodiment, an outer circumferential tread has an outer tread layer in direct and continuous contact with an inner tread layer. The outer and inner tread layers together define the tread thickness. A tread pattern that has tire grooves extends through the outer tread layer and into the inner tread layer. The valleys of the tire grooves are devoid of the outer tread layer. The inner tread layer has a maximum thickness near a central region of the tread and includes a first elastomeric compound. The outer tread layer includes a second elastomeric compound having lower rolling resistance and greater resistance to treadwear than the first elastomeric compound. The outer tread layer increases in thickness across at least a widthwise portion of the outer tread layer from near the central region of the tread to a maximum thickness near at least one of a first shoulder and a second shoulder of the tread to provide longer lasting wear resistance for the tire at the at least one shoulder. As the tread wears, the inner tread layer defines a greater proportion of a running surface of the tread. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described by way of example and with reference to the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a tire with a tire tread constructed according to an embodiment of the invention; 
         FIG. 2  is a cross-sectional view of the tire of  FIG. 1  showing a partially worn tread; 
         FIG. 3  is a cross-sectional view of the tire of  FIG. 2  following additional wear of the tread; 
         FIG. 4  is a cross-sectional view of a tire with a tire tread constructed according to another embodiment of the invention; 
         FIG. 5  is a cross-sectional view of a tire with a tire tread constructed according to another embodiment of the invention; and 
         FIG. 6  is a graphical representation of a relationship between temperature and tan delta (δ) for two different elastomeric compounds according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference generally to  FIGS. 1-5 , there is shown a pneumatic tire  10  that includes sidewalls  12 , inextensible beads  14 , a supporting carcass  16 , and an outer circumferential tread  18 . The individual sidewalls  12  extend radially inward from the axial outer edges or shoulders  20 ,  22  of the tread  18  to join the respective inextensible beads  14 . The opposing shoulders  20 ,  22  define the width of the tread  18  and join the respective sidewall  12  with the tread  18 . The supporting carcass  16  may include a reinforcing structure  24  that further includes a belt structure  26  disposed outwardly of the supporting carcass  16 . An innerliner (not shown) may line the interior of the supporting carcass  16  to inhibit the passage of air or other gases from within the tire  10  over prolonged periods. The outer circumferential tread  18  is adapted to contact the road during use of the tire  10 . And, in that regard, the outer circumferential tread  18  provides a combination of traction, rolling resistance, and treadwear characteristics that are maintained throughout the useable life of the tire. 
     To that end and with reference to  FIG. 1 , the outer circumferential tread  18  includes a matrix or tread pattern  28  comprising one or more grooves  30  that define lugs or tread elements  32  therebetween. The tread elements  32  collectively define the contact or running surface  34  of the tread  18  during use of the tire  10 . The grooves  30  form voids between the adjacent tread elements  32  when the elements  32  are in contact with the road surface. The voids provide a collection point for escape of material, for example, water, from between the elements  32  and the road surface thereby improving contact, and thus traction, between the tire  10  and the road. Absent the grooves  30 , the tire  10  would be more likely to hydroplane when driven through water. However, it will be appreciated that the benefit of grooves  30  may lessen as the tread  18  wears. As will be described in more detail below, the composition of the running surface  34  of the tread  18  changes as the tread  18  wears such that the tread  18  exhibits less variation in the performance over the life of the tire  10 . Furthermore, the compositional changes are location specific. Thus, the composition of the tread  18  varies by region of the running surface  34 . 
     In this regard, the outer circumferential tread  18  includes two or more layers arranged to cause variation in their exposure as the tread  18  wears. In one embodiment and with reference to  FIG. 1 , the tread  18  includes an outer tread layer  36  and an inner tread layer  38 . As shown, the outer tread layer  36  resides at a generally greater radial distance from the axis of rotation (not shown) of the tire  10  than the inner tread layer  38 . Generally, of the outer and inner tread layers  36 ,  38 , a larger proportion of the outer tread layer  36  wears away earlier in the life of the tread  18  than the inner tread layer  38  due to its relative radial position. The outer and inner tread layers  36 ,  38  may reside in direct and continuous contact with one another such that together they define the tread thickness, T, when the tire  10  is new. The outer and inner tread layers  36 ,  38  may be well-defined layers having an identifiable change in composition across an interface between the two, as set out below. Alternatively, the outer and inner tread layers  36 ,  38  may be formed by extruding multiple compositions simultaneously thereby forming a gradient in composition between the outer and inner layers  36 ,  38 . As such, the interface between the outer and inner layers  36 ,  38  may not be well defined. 
     With continuing reference to  FIG. 1 , when the tire  10  is new, the thickness, T, of the tread  18  may be defined by the combined thickness of the outer tread layer  36  and the inner tread layer  38 . The depth of the grooves  30  may be less than the thickness T of the tread  18 . As such, the grooves  30  may extend through the outer tread layer  36 , but only partially into the inner tread layer  38 . The valleys of the grooves, which is the deepest extent of the grooves  30 , may, therefore, be devoid of the outer tread layer  38 . The outer tread layer  36  may, therefore, be discontinuous or broken between elements  32  by the grooves  30 . And, the inner tread layer  38  may be exposed in the grooves  30  but the inner tread layer  38  may be continuous across the width of the tread  18 . It will be appreciated, however, that the embodiments of the invention are not so limited. For example, when the thickness of the outer tread layer  36  is approximately equal to the thickness T of the tread  18 , the grooves  30  may not extend through the outer tread layer  36 . A portion of the outer tread layer  36  would thus be left intact between adjacent tread elements  32 . However, because of variation in thickness, described below, the groove  30  may extend through the outer tread layer  36  at another widthwise location such that the inner tread layer  38  is exposed at or near the valley portion of at least one groove  30  in the tread  18 . In addition, although not shown, an undertread layer may reside between the reinforcing structure  24  and the tread  18 , for example, directly between the inner tread layer  38  and structure  24  and may be formed of a similar compound as the outer or inner tread layers  36 ,  38 . 
     In addition to the difference in radial placement, the inner tread layer  38  is made of a different elastomeric compound than the outer tread layer  36 . Unless otherwise referred to herein, the inner tread layer  38  may be made of a first elastomeric compound, and the outer tread layer  36  may be made of a second elastomeric compound. While each of the first and second elastomeric compounds are described in more detail below, in one embodiment, the second elastomeric compound may have a lower rolling resistance and a greater resistance to wear than the first elastomeric compound. Alternatively, or in addition thereto, the first elastomeric compound may have a greater wet traction than the second elastomeric compound. 
     According to embodiments of the invention, the combination of compound properties and arrangement of the tread layers  36 ,  38  provide the tread  18  with a level of rolling resistance, wet traction, and wear resistance that may be maintained during the life of the tire  10 . Overall variation of one or more of these properties over the life of the tire  10  may be less as compared to that which is observed by a tread made of a monolith of homogenous material or a tread made of uniform layers. For example, when a tread is homogenous, the wear resistance, wet traction, and rolling resistance may vary throughout the life of a tire, and in some instances the variation is significant. Specifically, if the tread on a tire is homogenous and is between 0.35 and 0.60 inches thick, the variation in the rolling resistance over the life of the tread may be from 20% to 40%. That is, the rolling resistance may decrease by as much as 40% from the new condition to near the time that the tread is worn out. Similarly, the wet traction characteristic of the tread may vary significantly. For example, hydroplaning may be more probable as the tread wears, possibly increasing in frequency by 40 to 60%. Wet traction, as measured by breaking distance on a wet surface, may be reduced by 10 to 30% over the life of a homogenous tread. The vehicle thus requires an increase in distance to come to a stop as the tread thins. The life expectancy of a homogenous tread may be in the range of 20,000 to 80,000 miles. It will be appreciated that the variation in the above characteristics of a homogenous tread may depend on the tire size, its construction, the tread pattern, the tread compound, and variation in testing procedures. 
     In one embodiment and with reference to  FIG. 1 , when the tread  18  is new or near new, the outer and inner tread layers  36 ,  38  are not of uniform thickness either individually or with respect to the other layer in a widthwise region of the tread  18 . In this respect, the thickness of each of the outer and inner tread layers  36 ,  38  may be described as one of a continuously increasing, or continuously decreasing, nature in the widthwise region. As set forth herein, the widthwise region extends over more than one tread element  32 . For example, the widthwise region may extend across two tread elements  32 , across a majority of the tread  18 , or nearly the full width of the tread  18 . 
     In particular, in one embodiment and with reference to  FIG. 1 , due to the continuous thickness variation, the thickness of the inner tread layer  38  may not be the same at any two locations along the widthwise region. By way of example, the variation in the thickness of the inner tread layer  38  from its maximum thickness to its minimum thickness in the widthwise region may be of a continuously increasing relationship. As shown in  FIG. 1 , the continuous increase in the thickness of the inner tread layer  38  may be approximately linear. Similarly, the thickness of the outer tread layer  36  also continuously varies in the widthwise region. In embodiments of the invention in which the outer tread layer  36  complements the variation in the thickness of the inner tread layer  38  to form the tread  18  having a uniform thickness, the outer tread layer  36  may also vary linearly in thickness across the widthwise region, though in a decreasing manner. 
     As introduced above, there may be no relationship between the thickness of the outer tread layer  36  and thickness of the inner tread layer  38 , except that at any particular location in the widthwise region, the two thicknesses may define the thickness T of the tread  18  at that location. As such, the outer tread layer  36  may be substantially thicker than the inner tread layer  38  at all points across the widthwise region. Alternatively, the inner tread layer  38  may be substantially thicker than the outer tread layer  36  at all locations along the widthwise region. It will be appreciated that the relative thickness of the layers  36 ,  38  may depend on the depth of the grooves  30  and the compositions of the outer and inner tread layers  36 ,  38 , among other factors. In one embodiment, however, the maximum thickness of the outer tread layer  36  may nearly coincide with the minimum thickness of the inner tread layer  38 . While the maximum thickness of each of the outer and inner tread layers  36 ,  38  are shown as being approximately equal, the embodiments of the invention are not so limited, as set out above. 
     With continuing reference to  FIG. 1 , the widthwise region extends from near one shoulder  20  to near the other shoulder  22 . The maximum thickness of the outer tread layer  36  occurs near the shoulder  20  and the maximum thickness of the inner tread layer  38  occurs near the opposing shoulder  22 . To that end, the respective minimum thicknesses of the outer and inner tread layers  36 ,  38  occur at shoulder  22  and shoulder  20 , respectively. The inner tread layer  38  continuously increases, and the outer tread layer  36  continuously decreases, in thickness from the minimum thickness to the maximum thickness between the shoulders  20 ,  22 . As shown, the variation in thickness is approximately linear for the outer and inner tread layers  36 ,  38  between the shoulders  20 ,  22 . Although not shown, it will be appreciated that the thickness of the inner tread layer  38  may decrease, rather than increase, from shoulder  20  to shoulder  22  with the outer tread layer  36  increasing rather than decreasing. It will also be appreciated that while a linear relationship or an approximation thereof is shown and described, the relative thickness variation between the outer and inner tread layers  36 ,  38  may be represented by another mathematical expression, such as, a quadratic relationship, and still maintain a continuously increasing, or decreasing, thickness relationship. 
     As a result of the tread configuration shown in  FIG. 1 , as the tread  18  wears, portions of both of the tread layers  36 ,  38  are removed. By this configuration, a continuously increasing proportion of the inner tread layer  38 , and a continuously decreasing proportion of the outer tread layer  36 , forms the running surface  34  along the width of the tread  18 . With reference now to  FIGS. 1 and 2 , since the outer tread layer  36  is made of the second elastomeric compound, i.e., a compound that is more resistant to wear and has a lower rolling resistance than the compound of the inner tread layer  38 , the outer tread layer  18  lasts longer than a tread made of a homogenous material having greater wet traction than the second elastomeric compound. Thus, to wear the tread  18  from the new or near new condition of  FIG. 1  to the partially worn condition of  FIG. 2  may take longer than it would for such a homogenous material. Further, in one embodiment, the thickness of the outer tread layer  36  can be increased at locations subject to high wear or loading that would likely accelerate treadwear. 
     At some time during the life of the tire  10 , the tread  18  is worn to the condition shown in  FIG. 2 . As shown, the inner tread layer  38  may form the running surface  34  from the shoulder  22  to about the midpoint of the tread width. The inner tread layer  38  therefore forms a continuous portion of the running surface  34  from the shoulder  22  to the location on the running surface  34  where the outer tread layer  36  remains. Because the first elastomeric compound exhibits improved wet traction versus the second elastomeric compound, the tread  18  exhibits improved wet traction in the region including the shoulder  22  versus the second elastomeric compound. The tread  18  also exhibits resistance to treadwear and lower rolling resistance because the outer tread layer  36  is exposed in the region of the tread that includes the shoulder  20 . Thus, as the tread  18  wears from the condition of  FIG. 1  to the condition of  FIG. 2 , the influence of the properties of the first elastomeric compound on the tread  18  performance increases and may be generally proportional to the gradual increase in the exposed surface area of the first elastomeric compound along the running surface  34 . Similarly, the influence of the properties of the second elastomeric compound on the tread  18  gradually decreases because the surface area of the outer tread layer  36  decreases as a proportion of the running surface  34  also decreases. Thus, by the continuously increasing or decreasing thickness relationship, the overall tread properties change as the tread  18  wears. The tire  10  may thus be characterized by decreased rolling resistance and improved wear resistance initially with a gradual shift to improved wet traction as the tread wears. Further, the gradual shift is predetermined according to the thickness relationship between the outer and inner tread layers  36 ,  38 . It will be appreciated that the continuous thickness variation and composition of each of the outer and inner tread layers  36 ,  38  may be adjusted to obtain a predetermined balance between rolling resistance, wet traction, and wear resistance not observed in homogenous treads or in uniformly layered treads at similar periods during the life of the tire  10 , as set forth above. 
     With reference to  FIG. 3 , after the tread  18  is further worn, in which case the running surface  34  of the tread  10  may approach at least one of the deepest or valley portions of the grooves  30 , the inner tread layer  38  forms a majority of the running surface  34 , possibly leaving only a small portion of the outer tread layer  36  near the shoulder  20 . However, it will be appreciated that over the life of the tire  10 , the outer tread layer  36  may be completely removed with only the inner tread layer  38  remaining, such as, up to a point that is slightly less than the depth of the grooves  30 . The reinforcing structure  24  may, thus, never contact the road surface during use of the tire  10 . 
     Advantageously, the tire  10  with the new configuration shown in  FIG. 1  and wear pattern shown in  FIGS. 2 and 3  may be constructed with the maximum thickness of the outer tread layer  36  positioned at a location of greatest wear or deformation to improve the endurance or life of the tire  10 . In addition, the continuous change in thickness relation between the outer and inner tread layers  36 ,  38  may provide a predetermined, graduated offset of the loss in volume of the grooves  30  with improvement in the wet traction provided by the inner tread layer  38 . The relationship of the outer and inner tread layers  36 ,  38  may be such that the rate of exposure of the inner tread layer  38  substantially compensates for a reduction in the volume of the tire grooves  30  continuously over the life of the tire  10 . The thickness relationship of the outer and inner tread layers  36 ,  38  may also be configured to maintain a minimal rolling resistance of the tread  18  while maintaining the wet traction of the tread  18  near the level observed in the “new” condition. The minimal rolling resistance is that which provides acceptable fuel efficiency. It will be appreciated that the continuous thickness variation over the particular widthwise region between the outer and inner tread layers  36 ,  38  may be adjusted such that the outer tread layer  36  may wear away sooner or later during the life of the tire  10 . The timing of any complete removal of the outer tread layer  36  may be predetermined and may depend upon the compositions of the first and second elastomeric compounds, the thickness of the tread  18 , and the depth of the grooves  30 , among other factors. 
     With reference now to  FIG. 4 , in another embodiment of the present invention, the continuous thickness variation between layers  36 ,  38  and compositional differences thereof may extend over two widthwise regions ( 40 A and  40 B) of the tread  18 . As with the embodiments described above, the widthwise regions  40 A,  40 B may include more than one tread element  32 . The widthwise regions  40 A,  40 B may be approximately the same overall width, or they may be of different overall widths. One region  40 A may be between the shoulder  20  and central region or the centerline (C) of the tread  18 , and the other region  40 B may be between the centerline (C) of the tread  18  and the shoulder  22 . By way of example only, the two regions  40 A,  40 B may be arranged symmetrically on either side of the centerline (C), as shown. Thus, the outer and inner tread layers  36 ,  38  may be arranged nearly symmetrically with respect to the centerline (C) of the tread  18 . It will be appreciated that while two widthwise regions  40 A,  40 B of the tread  18  are shown and described herein, multiple widthwise regions may be formed in accordance with the embodiments of the invention shown and described herein. 
     With continuing reference to  FIG. 4 , the widthwise regions  40 A,  40 B extend from the shoulder  20  to the centerline (C) of the tread width and from the centerline (C) to the shoulder  22 . Thus, the two regions  40 A,  40 B essentially extend the full width of the tread  18 . A maximum thickness of the inner tread layer  38  may be located near the centerline (C) of the tread  18 . As such, the inner tread layer  38  may continuously increase in thickness from near the shoulder  20  to the centerline (C) of the tread  18  and then continuously decreases in thickness from centerline (C) of the tread  18  to the shoulder  22 . The individual thicknesses of the outer and inner tread layers  36 ,  38  may be unrelated. The thickness variation of the inner tread layer  38  shown is approximately linear across each region  40 A,  40 B. Thus, the thickness of the inner tread layer  38  varies linearly with respect to the widthwise location within the respective widthwise region  40 A,  40 B. Where the tread  18  is of a uniform thickness, the outer tread layer  36  varies in thickness such that it complements the thickness variation of the inner tread layer  38 . The outer tread layer  36  gradually decreases from the shoulder  20  to the centerline (C) and then increases in thickness from the centerline (C) to the shoulder  22 . By way of example, the outer tread layer  36  may vary linearly across both widthwise regions  40 A,  40 B. It will be appreciated that while a linear relationship or an approximation thereof is shown and described, the relative thickness variation between the outer and inner tread layers  36 ,  38  may be represented by other mathematical expressions and still maintain a continuously increasing, or decreasing, thickness relationship. The combined thicknesses of the outer and inner tread layers  36 ,  38  may thus represent the thickness of the tread  18  at any widthwise location on the tread  18 . It will be appreciated that the invention is not limited to having a uniformly thick tread. Furthermore, while the linear relationships shown are similar in each respective widthwise region  40 A,  40 B, embodiments of the present invention are not limited to similar linear relationships between the two regions  40 A,  40 B. It will be appreciated that one region may be a linear relationship while the second or other widthwise regions may be a nonlinear, though of a continuously increasing or decreasing thickness nature. 
     Similar to the embodiments of the tire  10  shown in  FIGS. 1-3 , the inner tread layer  38  of  FIG. 4  may be made of the first elastomeric compound and the outer tread layer  36  may be made of the second elastomeric compound. The second elastomeric compound may have a greater wear resistance and a lower rolling resistance than the first elastomeric compound. Alternatively or in addition thereto, the first elastomeric compound may have a greater wet traction than the second elastomeric compound. However, where an alternative wear pattern or a change in wet traction and/or rolling resistance is desirable to address a particular tire configuration, the inner tread layer  38  in  FIGS. 1 and 4  may be made of the second elastomeric compound and the outer tread layer  36  may be made of the first elastomeric compound. 
     During wear of the tread  18  having the configuration shown in  FIG. 4 , the outer tread layer  36  in the central region of the tread  18  may initially wear away, though it forms a portion of the tread  18  at each shoulder  20 ,  22 , exposing the inner tread layer  38  in the central region. Thus, in embodiments in which the second elastomeric compound has a greater wear resistance and lower rolling resistance than the first elastomeric compound, the tire  10  exhibits longer life versus a tread made of a homogenous compound having a greater wet traction. In addition, in one embodiment, when the first elastomeric compound has a greater wet traction than the second elastomeric compound, the inner tread layer  38  in the central region may initially compensate for the loss of the depth of the grooves  30  in the central region. It will be appreciated that in one embodiment in which the two widthwise regions are not arranged symmetrically, exposure of the inner tread layer  38  may also not be symmetrical with respect to the centerline or midpoint of the tread  18 . An asymmetrical exposure of the inner tread layer  38  with respect to the centerline of the tread  18  may compensate for known asymmetrical loading on the tread  18  or another characteristic of the tire  10 . 
     With reference now to  FIG. 5 , in another embodiment of the present invention, the continuous thickness variation between layers  36 ,  38  and compositional differences thereof may extend over two widthwise regions  42 A and  42 B of the tread  18 . Similar to the embodiment shown in  FIG. 4 , the widthwise regions  42 A,  42 B of  FIG. 5  define the total width of the tread  18  and may include more than one tread element  32 . The widthwise regions  42 A,  42 B may be approximately the same overall width, or they may be different overall widths. One region  42 A may be between the shoulder  20  and central region or the centerline (C) of the tread  18 , and the other region  42 B may be between the centerline (C) of the tread  18  and the shoulder  22 . By way of example only, the two regions  42 A,  42 B may be arranged symmetrically on either side of the centerline (C), as shown. Thus, the outer and inner tread layers  36 ,  38  may be arranged nearly symmetrically with respect to the centerline (C) of the tread  18 . It will be appreciated that while two widthwise regions  42 A,  42 B of the tread  18  are shown and described herein, multiple widthwise regions may be formed in accordance with the embodiments of the invention shown and described herein. 
     With continuing reference to  FIG. 5 , the widthwise regions  42 A,  42 B extend from the shoulder  20  to the centerline (C) of the tread width and from the centerline (C) to the shoulder  22 , respectively. Thus, the two regions  42 A,  42 B arranged end-to-end essentially extend the full width of the tread  18 . In contrast to the embodiment of the tread  18  shown in  FIG. 4 , a maximum thickness of the inner tread layer  38  of  FIG. 5  may be located near one or both of the shoulders  20 ,  22  of the tread  18 . As such, the inner tread layer  38  may continuously decrease in thickness from near the shoulder  20  to the centerline (C) of the tread  18  and then continuously increases in thickness from centerline (C) of the tread  18  to the shoulder  22 . The individual thicknesses of the outer and inner tread layers  36 ,  38  may be unrelated. The thickness variation of the inner tread layer  38  shown is approximately linear across each region  42 A,  42 B. Thus, the thickness of the inner tread layer  38  varies linearly with respect to the widthwise location within the respective widthwise region  42 A,  42 B. Where the tread  18  is of a uniform thickness, the outer tread layer  36  varies in thickness such that it complements the thickness variation of the inner tread layer  38 . The outer tread layer  36  gradually increases in thickness from the shoulder  20  to the centerline (C) and then decreases in thickness from the centerline (C) to the shoulder  22 . By way of example, the outer tread layer  36  may vary linearly across both widthwise regions  42 A,  42 B. It will be appreciated that while a linear relationship or an approximation thereof is shown and described, the relative thickness variation between the outer and inner tread layers  36 ,  38  may be represented by other mathematical expressions and still maintain a continuously increasing, or decreasing, thickness relationship. The combined thicknesses of the outer and inner tread layers  36 ,  38  may thus represent the thickness of the tread  18  at any widthwise location on the tread  18 . It will be appreciated that the invention is not limited to having a uniformly thick tread. Furthermore, while the linear relationships shown are similar in each respective widthwise region  42 A,  42 B, embodiments of the present invention are not limited to similar linear relationships between the two regions  42 A,  42 B. It will be appreciated that one region may have a linear relationship while the second or other widthwise regions may be nonlinear in nature, though of a continuously increasing or decreasing thickness. 
     In addition, it will be appreciated that the two widthwise regions  42 A,  42 B may not be arranged symmetrically with respect to the centerline (C). Thus, exposure of the inner tread layer  38  may also not be symmetrical with respect to the centerline or midpoint of the tread  18 . An asymmetrical exposure of the inner tread layer  38  with respect to the centerline of the tread  18  may compensate for known asymmetrical loading on the tread  18  or another characteristic of the tire  10 . 
     As with the exemplary embodiments of the invention shown in  FIGS. 1 and 4 , the inner tread layer  38  of  FIG. 5  may be made of the first elastomeric compound and the outer tread layer  36  may be made of the second elastomeric compound. Alternatively, the inner tread layer  38  may be made of the second elastomeric compound and the outer tread layer  36  may be made of the first elastomeric compound. The selection of the compounds for either of the layers  36 ,  38  may depend upon various factors including the type of tire and the desired performance characteristics, among others. 
     The composition of the compounds together with the continuous variation in thickness between the outer and inner tread layers  36 ,  38  provides improved wear resistance and wet traction over the life of the tire  10 . As introduced above, the second elastomeric compound may have a greater wear resistance and/or lower rolling resistance than the first elastomeric compound. In addition, or alternatively, the first elastomeric compound may have a greater wet traction than the second elastomeric compound. A greater wear resistance generally means that a given thickness of the compound thins at a lower rate than a compound having a relatively lower wear resistance. And, a lower rolling resistance generally means that less energy is required to roll a given tread over a given distance than another compound. The wet traction is representative of a coefficient of friction in contact with a road surface when that surface is wet. A compound with a relatively high wet traction has a relatively high coefficient of friction and thus is not as likely to slide when in contact with the wet surface when all other factors remain the same. This is particularly relevant during braking. 
     Generally, the second elastomeric compound may be positioned to reduce the rate of treadwear. And, the first elastomeric compound may be positioned to compensate for the loss in traction that is the result of the reduction in the depth of the grooves  30  in the tread  18  as the tread  18  wears. By such a construction, wear of the tread first removes the outer tread layer  36  of the second elastomeric compound, thereby exposing the inner tread layer  38  of the first elastomeric compound, which compensates for the loss in wet traction due to treadwear. It is also contemplated that the outer and inner tread layer  36 ,  38  may alternatively be made of the first and second elastomeric compounds, respectively. The alternative arrangement of the outer and inner tread layers  36 ,  38  and compounds may be made to achieve a particular performance improvement not possible with a tire having a tread made of a homogenous compound. 
     The material properties of a particular elastomeric compound may be indicative of the compound&#39;s wet traction, rolling resistance, and wear resistance. Generally, in addition to the glass transition temperature or T g , material properties that provide some indication of wet traction and rolling resistance include measurements of rebound and tan delta (δ). The material property that indicates the resistance of the tread to tread wear includes the measurement of the Grosch abrasion resistance or rate. 
     Tan δ is a ratio of dynamic loss modulus, G″, to dynamic storage modulus, G′, and is dependent on the temperature of the compound during measurement. Tan δ is a ratio of the viscous response to the elastic response and is a measure of hysteresis. As such, tan δ may be proportional to the energy loss generated under conditions of constant energy input. A lower hysteresis, as indicated by a relatively small tan δ value, is typically indicative of lower energy loss and less heat generation when the compound is cyclically loaded. The lower the hysteresis energy loss is, or the smaller value of the tan δ is, the lower the rolling resistance. A relatively high value of tan δ is indicative of a better wet traction. 
     While the value of tan δ is dependent on the composition, it is also dependent on temperature as shown in  FIG. 6 . With reference to  FIG. 6 , typically, the relationship between the values of tan δ and temperature result in one or more peaks or maximums in the values of tan δ at a particular temperature (labeled “Tan delta peak” in  FIG. 6 ). The peak in tan δ generally represents the maximum absorption of energy of the compound at a constant loading frequency. The differences in properties between the compounds may be indicated by a difference between the temperatures at which the peak in the tan δ occurs. By way of example, the first elastomeric compound may be a blend of about 75 parts of high styrene SSBR to about 25 parts BR with resins, and the second elastomeric compound may be a blend of about 70 parts low styrene SSBR to about 30 parts BR without resins. In this case, the first elastomeric compound may have a peak in the temperature versus tan δ plot that occurs at a higher temperature than a peak in the temperature plot versus tan δ of the second elastomeric compound. 
     Rather than measuring tan δ across a large range of temperatures, tan δ may be measured according to industry standards, such as, ASTM D5992, at specific measurement temperatures. Usually, these temperatures are 0°, 60°, 90°, and 100° C. It is believed that a relatively low value of tan δ at about 60° C. is an indication of better rolling resistance. It is also believed that a relatively high value of tan δ at about 0° C. is an indication of a better wet traction. By way of example, the first elastomeric compound may have a tan δ value at around 0° C. that is greater than a corresponding tan δ value of the second elastomeric compound, and thus the first elastomeric compound may have a better wet traction than the second elastomeric compound. By way of additional example, the second elastomeric compound may have a tan δ value at around 60° C. that is less than the first elastomeric compound at the same temperature, and thus the second elastomeric compound may have a better rolling resistance than the first elastomeric compound. In one embodiment, the tan δ values for the first elastomeric compound are greater than the tan δ values for the second elastomeric compound at all temperatures greater than 0° C. 
     Rebound is also a measure of hysteresis of the compound when subject to loading, though rebound is measured by ASTM D1054. Generally, the lower the measured rebound at 0° C., the greater the wet traction. The higher the measured rebound at 60° C., the lower the rolling resistance. 
     The resistance to abrasion for the compounds may be evaluated by measuring the Grosch abrasion rate. A lower Grosch abrasion rate indicates a more wear resistant compound. To measure the Grosch abrasion rate, a sample of the compound is placed at a slip angle under constant load as it traverses a given distance on a rotating abrasive disk (from HB Schleifmittel GmbH). Frictional forces, both lateral and circumferential, generated by the abrading sample can be measured together with the load using a custom tri-axial load cell. The surface temperature of the abrading wheel is monitored during testing and is reported as an average temperature. 
     The Grosch abrasion rate test may be run on a LAT-100 abrader and is measured in terms of mg/km of material abraded away. The compound may be subject to three levels of abrasion severity (low, medium, and high). In a low severity test, the sample is loaded with 20 Newtons at a slip angle of 2° on the disk, which is run at a speed of 20 or 40 kilometers per hour (kph) for a distance of 7,500 meters. According to the medium severity test, a load of 40 Newtons is used on the sample oriented at a slip angle of 6°. The disk speed is 20 kph and the sample travel distance is 1,000 meters. A high abrasion severity test may be run at a load of 70 Newtons at a slip angle of 12°, a disk speed of 20 kilometers per hour, and a sample travel distance of 250 m. In one embodiment, the measured Grosch abrasion rate of the second elastomeric compound is less than the measured Grosch abrasion rate of the first elastomeric compound. 
     Another material property that may be used as an indicator of the performance of the compounds is the glass transition temperature or T g  of the compound. The glass transition temperature may be defined as the temperature above which the compound behaves in an elastic or rubbery manner rather than in a glassy or brittle manner. Where the compound is composed of two or more immiscible materials, each of those materials may exhibit a T g  such that the compound may have multiple glass transition temperatures, one for each phase in the compound. In one embodiment, the first elastomeric compound has a T g  that is greater than the T g  of the second elastomeric compound. By way of example, the T g  of the first elastomeric compound may be in the range of about −10° C. to about −40° C., and the T g  of the second elastomeric compound may be in the range of about −30° C. to about −60° C. 
     Fillers may be added to one or both of the first and second elastomeric compositions to change the respective properties thereof. However, while addition of a filler to the compound may increase a desired characteristic, such as wet traction, the addition may also cause deterioration of another characteristic, such as the rolling resistance. According to one embodiment, silica may be added to the compound to improve the wet traction of the compound. However, addition of silica may also increase the rolling resistance. High levels of silica may be one component of the first elastomeric compound. In another embodiment, the first and second elastomeric compositions may include silica but differ in silica content. The first elastomeric composition may have a higher silica content than the silica content of the second elastomeric composition. 
     By way of example, the first elastomeric composition may include from about 70 phr to about 110 phr silica, and the second elastomeric composition may include less than 70 phr silica. By way of further example, the second elastomeric composition may include an amount of silica in the range of about 50 phr to about 70 phr silica, and the first elastomeric composition may include an amount of silica in an amount greater than 70 phr silica. It will be appreciated, however, that as little as 40 phr silica is possible in the second elastomeric composition when silica is included therein. It will also be appreciated that other fillers, such as, carbon black, may be used in the composition to cause a difference in the wet traction characteristic between the first and the second elastomeric compositions. In addition, one or both of the compositions may also include one or more filler components, which can include calcium carbonate, clay, mica, silicates, talc, titanium dioxide, starch and other organic fillers, such as wood flour. 
     In addition, to achieve a relative difference in temperature in the tan δ peak, resins may be added to one or both of the first and second elastomeric compounds to change the respective properties thereof. However, while addition of resins to the compound may increase a desired characteristic, such as wet traction, the addition may also cause deterioration of another characteristic, such as the rolling resistance and treadwear. The reverse may also be true, i.e., addition of resins may also increase the rolling resistance and decrease treadwear resistance. In one embodiment, the first elastomeric compound may contain relatively high percentages of resins. In another embodiment, the first and second elastomeric compositions may include resins, but the resin composition may differ. In one embodiment, the first elastomeric composition may include from about 5 phr to about 20 phr resins, and the second elastomeric composition may include up to about 10 phr resins. 
     It will be appreciated that there are a wide variety of polymers and resins that may be used in the manufacture of the outer and inner tread layers  36 ,  38  to obtain a relative difference in T g  or difference in temperature between locations of the peak in the tan δ curve, as described herein. It will also be appreciated that numerous additional additives are known in the art and may also be added to provide a compound having desired physical properties. Such known and commonly used additive materials are activators, retarders and accelerators, rubber processing oils, resins including tackifying resins, plasticizers, fatty acids, zinc oxide, waxes, antidegradant, antiozonants, peptizing agents, and curatives. As is known to those having ordinary skill in the art, depending on the intended use of the tire  10 , the additives are selected and used in conventional amounts. 
     According to one embodiment, the mixing of all of the components of the compositions for the tread  18  can be accomplished by methods known to those having ordinary skill in the art. For example, the ingredients can be mixed in at least two non-productive stages followed by a productive mix stage. The final curatives are typically mixed in the final stage, which is conventionally called the “productive” mix stage in which the mixing typically occurs at a temperature, or ultimate temperature, lower than the vulcanization temperature of the elastomer. The terms “non-productive” and “productive” mix stages are well known to those having skill in the runner mixing art. The layer  36  and/or layer  38  may be provided as a sheet that is formed, e.g., by extrusion casting. Alternatively, the tread  18  may be made by introducing the compositions into either a quad-extruder equipped with different zone dies or into a gear pump extruder so as to form a green tread with different compounds that, when cured, result in the different layers described above. 
     Furthermore, the pneumatic tire  10  can be built on a tire forming drum (not shown), vulcanization temperature can be from about 100° C. to about 250° C. For example, passenger tires might be cured at a temperature ranging from about 150° C. to about 180° C. and truck tires might be cured at a temperature ranging from about 130° C. to about 170° C. Cure time may vary from about five minutes to several hours. Cure time and temperature depend on many variables well known in the art, including the composition of the tire components, including the cure systems in each of the layers, the overall tire size and thickness, etc. Vulcanization of the assembled tire results in complete or substantially complete vulcanization or crosslinking of all elements or layers of the tire assembly, e.g. the tread  18 , the carcass  16 , and the sidewalls  12 . In addition to developing the desired strength characteristics of each layer and the overall structure, vulcanization enhances adhesion between these elements, resulting in a cured, unitary tire  10  from what were separate, multiple layers. 
     Non-limiting examples of the first and second elastomeric compounds for use in the outer and inner layers  36 ,  38  in accordance with the detailed description are disclosed below. The example is merely for the purpose of illustration and is not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Other examples will be appreciated by a person having ordinary skill in the art. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 1 st  elastomeric 
                 2 nd  elastomeric 
               
               
                 Ingredient 
                 compound (parts) 
                 compound (parts) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 40% Styrene SSBR 1   
                 103 
                   
               
               
                 21% SSBR 2   
                 — 
                 70 
               
               
                 Cis 1,4-polybutadiene A 3   
                 — 
                 30 
               
               
                 Cis 1,4-polybutadiene B 4   
                 25 
                 — 
               
               
                 Precipitated Silica 5   
                 90 
                 60 
               
               
                 Carbon black A 6   
                 9 
                 — 
               
               
                 Carbon black B 7   
                 — 
                 4.8 
               
               
                 Rubber processing aid 8   
                 1.5 
                 16.5 
               
               
                 Coupling agent 9   
                 7.6 
                 4.8 
               
               
                 Resins 10   
                 15 
                 — 
               
               
                 Antidegradant 
                 4 
                 3 
               
               
                 Stearic acid 
                 4 
                 3 
               
               
                 Zinc oxide 
                 0.5 
                 1.8 
               
               
                 Sulfur 
                 0.6 
                 1.9 
               
               
                 Accelerators 
                 1.9 
                 3.2 
               
               
                   
               
               
                   1 oil extended, methoxysilano-functionalized solution polymerized copolymer of butadiene and styrene, SSBR, with 40% bound styrene, 14.4% vinyl content, available commercially as SE SLR-6430 from Dow Deutschland Anlagengesellschaft GmbH 
               
               
                   2 functionalized SBR as a solution polymerization prepared styrene/butadiene rubber functionalized with an alkoxysilane group and functional group comprised of a thiol group and having 21% bound styrene as SLR SE4602 ™ from Dow Europe GmbH 
               
               
                   3 high cis 1,4-polybutadiene rubber, available commercially as Bud1208 ™ from The Goodyear Tire &amp; Rubber Company 
               
               
                   4 about 95% to about 98% cis 1,4-isomeric units, about 1% to about 3% cis 1,4-isomeric trans units, and about 0.3% to about 9% vinyl 1,2-content; a number average molecular weight (Mn) in a range of from about 230,000 to about 250,000 with a heterogeneity index (Mw/Mn) in a range of from about 1.5 to 1 to about 2 to 1, and a T g  in a range of from about −104° to about −109° C., available commercially from Lanxess as CB25 ™  
               
               
                   5 precipitated Silica available commercially as Zeosil 1165MP ™ from Rhodia 
               
               
                   6 ASTM designation N134 
               
               
                   7 ASTM designation N330 
               
               
                   8 rubber processing oil and wax 
               
               
                   9 bis-(3-triethoxysilylpropyl) poly sulfide having an average in a range of from about 3.6 to about 4 connecting sulfur atoms in its polysulfidic bridge available commercially as Si69 ™ from Evonic Degussa 
               
               
                   10 styrene/alphamethylstyrene resin and medium hard Coumarone Indene resin 
               
            
           
         
       
     
     The first and second elastomeric compositions were individually prepared according to industry standard practices. The compositions were cured at a temperature of about 160° C. for about 14 minutes and the physical properties of the compounds were evaluated. The physical property data is provided in Table 2. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 1st elastomeric 
                 2nd elastomeric 
               
               
                 Properties 
                 compound 
                 compound 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Wet traction 
                   
                   
               
               
                 0° C. Rebound 1   
                 8.2 
                 25.2 
               
               
                 tan δ (3%; 0° C.; 10 Hz) 2   
                 0.615 
                 0.337 
               
               
                 Rolling Resistance 
                   
                   
               
               
                 Room Temp. Reboud 1   
                 23.6 
                 51.2 
               
               
                 100° C. Rebound 
                 61.1 
                 75.5 
               
               
                 tan δ (3%; 30° C.; 10 Hz) 2   
                 0.287 
                 0.12 
               
               
                 tan δ (3%; 60° C.; 10 Hz) 2   
                 0.176 
                 0.076 
               
               
                 tan δ (10%; 100° C.; 11 Hz) 3   
                 0.112 
                 0.069 
               
               
                 Treadwear 
                   
                   
               
               
                 Grosch Abrasion rate 4  @ Low, mg/km 
                 5.67 
                 2.14 
               
               
                 Grosch Abrasion rate 4  @ Med, mg/km 
                 82.8 
                 31.5 
               
               
                 Grosch Abrasion rate 4  @ High, mg/km 
                 410 
                 297 
               
               
                 Other properties 5   
                   
                   
               
               
                 100% Modulus, MPa 
                 2.65 
                 1.80 
               
               
                 300% Modulus, MPa 
                 13.9 
                 13.4 
               
               
                 Tensile Strength, MPa 
                 21.0 
                 17.3 
               
               
                 Elongation at Break, % 
                 441 
                 384 
               
               
                 Room Temp. Hardness 
                 75.9 
                 60.8 
               
               
                 100° C. Hardness 
                 66.3 
                 58.6 
               
               
                   
               
               
                   1 rebound as measured by ASTM D1054 
               
               
                   2 dynamic measurement from the temperature sweep at 3% strain and 10 Hz by ASTM B5992 
               
               
                   3 rubber process analyzer (RPA) instrument 
               
               
                   4 measured on a LAT-100 Abrader 
               
               
                   5 Automated Testing System (ATS) instrument 
               
            
           
         
       
     
     As indicated by the data in Table 2, the first elastomeric compound has a greater wet traction than the second elastomeric compound. This translates to shorter vehicular stopping distances for equal exposure between the first elastomeric compound and the second elastomeric compound. As set forth above, a lower rebound value is an indication of better wet traction. Referring to Table 2, the value of 0° C. rebound of 8.2 of the first elastomeric compound indicates that it has a better wet traction than the second elastomeric composition which has a 0° C. rebound value of 25.2. The relative improvement between the wet traction of the first elastomeric compound and the second elastomeric compound is indicated by the ratio between the two, which is slightly greater than 3 to 1. 
     The tan δ value at 0° C., 3% strain, and 10 Hz for each compound is also a relative indication of wet traction. A higher value of tan δ at 0° C. is an indication of better wet traction. As shown, the first elastomeric composition has a tan δ value at 0° C. of 0.615 and the second elastomeric composition has a tan δ value of 0.337 or a ratio of about 1.8 to 1. The relative difference in values between the first and second elastomeric compounds indicates a proportional and significant difference in wet traction. 
     As is also indicated by the data in Table 2, the second elastomeric compound is characterized as having a lower rolling resistance than the first elastomeric compound. This is so because all of the room temperature and elevated temperature rebound values are greater than the respective room and elevated temperature rebound values for the first elastomeric composition. As shown, the measured room temperature rebound for the second elastomeric compound is about 51.2, whereas the respective rebound for the first elastomeric compound is about 23.6. The ratio of the two is consequently slightly greater than 2 to 1 for relative room temperature rebound. The elevated temperature rebound values are similarly different with the second elastomeric compound being greater than the first elastomeric compound by a ratio of about 1.2 to 1. 
     Further, Table 2 also provides data that indicates that the 30° C. and 60° C. tan δ values of the second elastomeric compound are lower than the 30° C. and 60° C. tan delta values for the first elastomeric compound. Accordingly, the second elastomeric compound has a lower hysteresis energy loss than the first elastomeric compound at these temperatures. As such, the second elastomeric compound has a lower rolling resistance than the first elastomeric compound. As calculated from their respective values, the second elastomeric compound has tan δ values that are greater by a ratio of about 2.3 to 1 for each of the measurements at 30° C. and 60° C. relative to the first elastomeric composition. Similarly, the tan δ values at 10% strain, 100° C., and 11 Hz for the second elastomeric compound is greater than the corresponding value for the first elastomeric compound by a ratio of about 1.6 to 1. This is considered to be significant as a tread configured with the first and second elastomeric compound as described herein may have a reduced hysteresis loss that promotes low rolling resistance and consequently improved fuel efficiency versus a homogenous tire, though the tread would be expected to maintain wet traction as the tread wears. 
     With regard to wear resistance, the data in Table 2 indicates that the second elastomeric compound has relatively high resistance to wear as compared to the first elastomeric compound. In particular, each of the abrasion rates for the second elastomeric compound measured lower than the first elastomeric compound. According to the data, the second elastomeric compound abrades at a rate that is less than half of the abrasion rate measured for the first elastomeric compound with the “severe” abrasion rate being around 38% less than the abrasion rate of the first elastomeric compound. 
     Also according to Table 2, the mechanical properties indicate that the two compounds meet or exceed industry norms for use of the materials in a tread. 
     While the present invention has been illustrated by the description of one or more embodiments thereof, and while the embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantage and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of applicant&#39;s general inventive concept.