Abstract:
A pneumatic tire has a tread; the tread has a plurality of grooves, creating an initial tread configuration. The grooves and the tread elements formed thereby result in a tread have a net-to-gross ratio in the range of greater than 60%. Due to variations in the depths and variations in the heights and formations of the tread elements, after the tread has been worn, the tread has a net-to-gross ratio in the range of less than 60%.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a pneumatic tire, and more particularly to a tread of a pneumatic tire wherein the tread is configured to provide a different tread pattern as the tire is worn. 
       BACKGROUND OF THE INVENTION 
       [0002]    Tire tread patterns are provided with numerous elements such as ribs and blocks, the elements being separated by circumferential and/or transverse grooves. The grooves provide means for water evacuation and form the biting edges of the tread elements. 
         [0003]    When a tire is new, the tread has a maximum tread height. This initial height may vary depending upon the intended use of the tire; a winter tire has an increased tread depth in comparison to an all season tire. Regardless of the initial tread depth, when the tire is new, the tread elements have an initial stiffness. The actual stiffness of the tread elements is dictated by the block size, shape, and the presence of any siping. As the tread is worn, the block height decreases while the tread element stiffness increases. 
         [0004]    When a tire travels through water, two mechanisms occur: water force-back and water flow through the grooves. The water force back mechanism is always present. However, for a smooth tire, above a particular vehicle speed and water depth, a water bank is formed in front of the footprint leading edge. Slippage between the tire and the road surface will occur unless the water bank is reduced by water flow through the grooves, carrying the water through the footprint. In a new tire, there may be a compromise in the tread pattern between stiffness and wet driving performance. In such a new tire, the water force back mechanism and the non-skid tread depth is sufficient to provide for acceptable aquaplaning resistance. As the tire wears and the non-skid depth reduces, the water flow through the grooves is reduced. The present invention is directed to a tire having a tread that compensates for this reduced flow. 
       SUMMARY OF THE INVENTION 
       [0005]    Disclosed herein is a pneumatic tire. The tire is designed to have a variable tread pattern, the tread pattern changing with wear, to achieve similar tread performance for the tire when both new and worn. The changing pattern optimizes the worn tire performance in an attempt to maintain the tire&#39;s wet performance characteristics. 
         [0006]    Disclosed herein is a pneumatic tire having an equatorial plane and a tread. The tread has a plurality of grooves, creating an initial tread configuration. The grooves and the tread elements formed thereby result in a tread have a net-to-gross ratio in the range of greater than 60%; and in the range of 60% to 80% for an alternative embodiment. Due to variations in the depths and variations in the heights and formations of the tread elements, after the tread has been worn, the tread has a net-to-gross ratio in the range of less than 60%. In one embodiment, the worn tire tread net-to-gross ratio is in the range of less than 60% to 45%. 
         [0007]    According to another aspect of the invention, the unworn tire tread net-to-gross ratio is in the range of 60 to 70%. 
         [0008]    According to another aspect of the invention, the unworn tire tread net-to-gross ratio is in the range of 60 to 70% and the worn tire tread net-to-gross ratio is in the range of less than 60% to 50%. 
         [0009]    Also disclosed is a pneumatic tire having a tread defined by a plurality of grooves, creating an initial tread configuration. The grooves and the tread elements formed thereby result in a tread have a net-to-gross ratio. As the tire tread is worn, the tread configuration changes, and results in a change in the net-to-gross ratio of the tread. The change in the net-to-gross ratio, from that of the initial tire tread to the worn tread, is at least 10% less than the initial net-to-gross ratio. This change in net-to-gross ratio begins to occur when the tread depth is at most 70% of the initial tread depth. 
         [0010]    In another aspect of the invention, the worn tread has a net-to-gross ratio of at least 10% to 30% less than the initial net-to-gross ratio. Alternatively, the worn tread has a net-to-gross ratio of at least 10 to 15% less than the initial net-to-gross ratio. 
       DEFINITIONS 
       [0011]    The following definitions are controlling for the disclosed invention. 
         [0012]    “Axial” and “axially” are used herein to refer to lines or directions that are parallel to the axis of rotation of the tire. 
         [0013]    “Circumferential” means lines or directions extending along the perimeter of the surface of the annular tire parallel to the Equatorial Plane (EP) and perpendicular to the axial direction. 
         [0014]    “Groove” means an elongated void area in a tread that may extend circumferentially or laterally about the tread in a straight, curved, or zigzag manner. Circumferentially and laterally extending grooves sometimes have common portions and may be sub classified as “wide” or “narrow.” A “narrow groove” has a width in the range from about 0.8% to 3% of the compensated tread width and a “wide groove” has a width greater than 3% thereof. The “groove width” is equal to tread surface area occupied by a groove or groove portion, the width of which is in question, divided by the length of such groove or groove portion; thus, the groove width is its average width over its length. Grooves reduce the stiffness of tread regions in which they are located. Grooves may be of varying depths in a tire. The depth of a groove may vary around the circumference of the tread, or the depth of one groove may be constant but vary from the depth of another groove in the tire. 
         [0015]    “Inner” means toward the inside of the tire and “outer” means toward its exterior. 
         [0016]    “Lateral” means an axial direction. 
         [0017]    “Nonskid” means the depth of grooves in a tire tread. 
         [0018]    “Radial” and “radially” are used to mean directions radially toward or away from the axis of rotation of the tire. 
         [0019]    “Sipe” means a void area in a tread that may extend circumferentially or laterally in the tread in a straight, curved, or zigzag manner. A sipe typically is formed by steel blades inserted into a cast or machined mold or tread ring therefor. In the appended drawings, excluding close up drawings, sipes are illustrated by single lines because they are so narrow. A “sipe” is a groove having a width in the range from about 0.2% to 0.8% of the compensated tread width. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention will be described by way of example and with reference to the accompanying drawings in which: 
           [0021]      FIG. 1  is a perspective view of an unworn tire; 
           [0022]      FIG. 1A  is an enlarged view of section  1 A from  FIG. 1 ; 
           [0023]      FIG. 2  is a perspective view of a tire following wear of the tire tread; 
           [0024]      FIG. 2A  is an enlarged view of section  2 A from  FIG. 2 ; 
           [0025]      FIG. 3  is a flat view of the tread of  FIG. 1 ; 
           [0026]      FIG. 4  is a flat view of the tread of  FIG. 2 ; 
           [0027]      FIG. 5  is a cross-sectional view along line  5 - 5  of  FIG. 3 ; 
           [0028]      FIG. 6  is a cross-sectional view along line  6 - 6  of  FIG. 4 ; 
           [0029]      FIG. 7  is a perspective view of one design pitch of the unworn tread of  FIG. 1 ; 
           [0030]      FIG. 8  is an unworn tire tread illustrating another embodiment; and 
           [0031]      FIG. 9  is a flat view of the tread of  FIG. 8  following wear of the tread. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    The following language is of the best presently contemplated mode or modes of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0033]    A tire is illustrated in  FIG. 1 . The tire  10  has a tread  12  comprising a plurality of tread elements  14  separated by inclined lateral grooves  16 . The tread elements  14  extend continuously between the tread shoulders  18 . Circumferentially adjacent tread elements  14  are connected in the region of the tire equatorial plane EP, creating a quasi rib  20  in the tread, see also  FIG. 3 . The tread  14  is also provided with sipes  22  that extend between the circumferentially adjacent lateral grooves  16 . 
         [0034]    Radially inward of the tread surface  24  are a plurality of groove voids. In the tread of  FIG. 1 , in each design pitch, extending between a tread shoulder edge and the equatorial plane, radially inward of the tread surface  24  of each tread element are three groove voids  26 ,  28 ,  30 , see  FIGS. 1A ,  5 , and  7 . The groove voids have a primary axis of orientation along the circumferential length of the tire. This axis of orientation is inclined at an angle of 0° to 35° relative to the equatorial plane of the tire. The groove voids  26 ,  28 ,  30  also have opposing circumferential ends  26 E,  28 E,  30 E, coincident with the lateral grooves  16 . Groove voids  26  have a construct of a type that may be identified as a sipe-groove, wherein, at the tread surface, there is a sipe and at a defined depth of the tread, the sipe transforms to a groove void. As the respective groove voids  26 ,  28 ,  30  in circumferentially adjacent tread elements  14  are circumferentially aligned, the groove voids  26 ,  28 ,  30  create multiple tunnel-like features in the tread  12 . 
         [0035]    However, as there is tread rubber located above each groove void, when the tread is unworn or has been subjected to a limited amount of tread wear, an obstruction is created in what may be considered a water flow channel. Due to the absence of continuous unobstructed circumferential grooves, the main water flow mechanism that occurs in the tread of  FIG. 1  is water force back. 
         [0036]    The tread  14  has an initial non-skid tread depth D calculated between the tread surface of the tire when unworn, and the radially innermost base of a groove in the tread, see  FIG. 5 . The tread, when unworn, also has a net-to-gross ratio. The net-to-gross ratio is the ratio of the total surface area of the normally loaded and normally inflated tire tread that will contact with a hard flat surface such as the ground, divided by the total area of the tread, including non-contacting portions such as grooves, as measured around the entire circumference of the tire, i.e. the ratio of contact area to total tread area. For the illustrated embodiments, the treads have a net-to-gross ratio greater than 60%, more specifically, the net-to-gross ratios for the illustrated treads are in the range of 60 to 80%, with the net-to-gross ratio of the tire of  FIG. 1  in the range of 60 to 70%. 
         [0037]    As the tread  14  wears, the non-skid tread depth D is reduced, and the groove voids  26 ,  28 ,  30  are exposed, creating new unobstructed circumferential grooves  32 ,  34 ,  36  in the tread, see  FIGS. 2 ,  4 , and  6 ; hence the term “groove void.” A groove void is a the tread feature that is an void to the initial tread surface that generates a groove when exposed; thus the groove void has a width in the range of groove widths as defined above. Exposure of the groove voids is based upon the radial depth DR of the rubber above the groove voids. The rubber radial depth DR is 30% to 70% of the full nonskid D; thus the grooves are exposed anywhere from 30 to 70% wear of the full tread depth D. The groove voids  26 ,  28 ,  30  ideally also have a radially innermost base aligned at the same tread radial depth as the grooves  16 . 
         [0038]    The circumferential grooves  32 ,  34 ,  36  transform the tread of  FIG. 1  from one of connected tread elements  14  to a tread having a plurality of non-connected tread blocks  40 ,  42 ,  44  located on each side of a center rib  20 . 
         [0039]    As seen in  FIG. 5 , the portions of tread rubber radially outward of each groove void all have the same rubber radial depth; i.e. all of the groove voids  26 ,  28 ,  30  have the same groove void depth DV. For groove voids  26 , that have a sipe component located radially outward thereof, connecting it to the tread surface, the void depth DV, is measured for only that portion of the void which has a width within the definition of a groove width; this is best illustrated in  FIG. 5 . However, to gradually increase the amount of unobstructed circumferential grooving in the tread, the groove void depths DV may vary. The variation may be selected based upon the axial position of the groove in the tread or the total cross-sectional area of the groove void. For example, if based upon the axial position of the to-be-created groove, it may be desired to have the axially outermost grooves  32  appear before the grooves  36  located nearest the equatorial plane. For such a tread variation, the groove void depth DV of the axially outermost groove void  26  is greater than the groove void depth of the axially inner groove void  30 . If it is desired that the groove voids have a constant cross sectional area, the axially wider groove voids would have a reduced groove void depth DV in comparison to the other groove voids. Alternatively, the groove voids may be placed such that the radial depth D R  of the rubber above the groove voids  26 ,  28 ,  30  is constant, while the groove void depths D V  are varied. Such a tread results in varying depths to the exposed grooves, creating more cross-sectional area through which the water may flow. 
         [0040]    As the new grooves  32 ,  34 ,  36  are continuous, as the tire travels through water, water is free to flow unobstructed from the leading edge of the footprint to the trailing edge of the footprint. To improve water flow in the worn tire tread, it is one aspect of the invention that the newly created grooves fully extend between the leading and trailing footprint edges, contacting, and preferably extending beyond, the leading and trailing edges of the tire footprint. 
         [0041]    Following the creation of the unobstructed grooves  32 ,  34 ,  36 , the illustrated tread has a net-to-gross ratio of less than 60%; the worn tread net-to-gross ratio is in the range of less than 60% to 45%. Alternatively expressed, with the creation of the new grooves in the tread, the net-to-gross ratio decreases by at least 10%, in the range of 10 to 30%; alternatively, the decrease in the net-to-gross ratio is in the range of 10 to 20% or 10 to 15%. For the remaining effective life of the tread, the net-to-gross ratio remains less than the original net-to-gross ratio of the tire tread—the cross-sectional area of grooves in the tread does not decrease. 
         [0042]    An alternative embodiment of the tire tread is shown in  FIGS. 8 and 9 . The tread  50  of  FIG. 8  is that of an unworn tread. The tread has a combination of connected circumferentially adjacent tread elements  52  creating a rib-like feature at the tire equatorial plane EP. On each lateral side of the central rib-like tread feature is a row of alternating width tread blocks  54 ,  56 . Axially outward of this row of tread blocks is a shoulder row of tread blocks  58 . The alternating width tread blocks  54 ,  56  and shoulder tread blocks  58  are separated by inclined lateral grooves  60 . Separating the two rows of blocks is a continuous, non-obstructed circumferential groove. 
         [0043]    In the row of alternating width blocks, the greater width blocks  56  are provided with a groove void  62 . The groove void  62  is aligned with a short length circumferential groove  64 . In the central rib-like feature, groove voids  66  are provided on each side of the equatorial plane EP. As the tread wears, continuous unobstructed circumferential grooves  68 ,  70  are generated as the rubber above the groove voids  62 ,  66  is worn away. This results in two rows  72 ,  74  of individual blocks between a center rib and the shoulder blocks  58 . A tread of lower net-to-gross with an increased number of continuous unobstructed circumferential grooves is generated. 
         [0044]    While two definitive embodiments of the tread have been provided, other tread element configurations are conceivable wherein the aspects of a lower net-to-gross ratio tread within defined ranges are met.