Abstract:
An off-road tire tread for a tire includes a series of cavities extending into the tread surface, each cavity defined by sidewalls extending from a tread surface to an inclined bottom cavity surface, the inclined bottom cavity surface extending from a relatively deep leading cavity end to a relatively shallow trailing cavity end. The cavities are directionally oriented to operationally initiate contact with a ground surface at the leading cavity end and form a repeating circumferential pattern along a circumferential extent of the tread surface. The tire tread further may include one or more lug protrusions between each of the cavities extending outward from the tread surface.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to a tire tread for off-road vehicles and, in particular, to a tire tread providing suitable traction on an off-road particulate surface such as sand. 
       BACKGROUND OF THE INVENTION 
       [0002]    Traction is important in off-road vehicle usage and the tire tread performance of such vehicles is instrumental in achieving a satisfactory level of traction. Achieving satisfactory traction from a vehicle&#39;s tire tread on an off-road particulate surface such as sand is particularly problematic. Because sand has almost no cohesion and consists of large rounded particles, most lugged tires dig into the sand and the increase in motion resistance due to the sinking of the tire into the sand tends to overwhelm forward thrust. 
       SUMMARY OF THE INVENTION 
       [0003]    In one aspect of the invention, an off-road tire tread for a tire includes a series of cavities extending into the tread surface, each cavity defined by sidewalls extending from a tread surface to an inclined bottom cavity surface, the inclined bottom cavity surface extending from a relatively deep leading cavity end to a relatively shallow trailing cavity end. The cavities are directionally oriented to operationally initiate contact with a ground surface at the leading cavity end. 
         [0004]    In another aspect, each cavity has a quadrilateral sectional configuration and the cavities form a repeating circumferential pattern along a circumferential extent of the tread surface. 
         [0005]    Adjacent cavities may be spaced apart to have a centerline to centerline spacing between 3 and 5 inches; each cavity having a depth at the leading cavity end between 0.4 and 0.6 inches according to further aspects of the invention. 
         [0006]    In yet a further aspect, the tire tread further includes at least one lug protrusion between each of the cavities, the lug protrusion having a rectangular configuration in section having a width between 0.25 and 0.625 inches; a height of between 0.25 and 0.375 inches; and a length between 1 and 3 inches. 
       DEFINITIONS 
       [0007]    The following definitions are applicable to the present disclosure and are used consistently as defined below: 
         [0008]    “Apex” means an elastomeric filler located radially above the bead core and between the plies and the turnup ply. 
         [0009]    “Aspect Ratio” means the ratio of its section height to its section width. 
         [0010]    “Bead” means that part of the tire comprising an annular tensile member wrapped by ply cords and shaped, with or without other reinforcement elements such as flippers, chippers, apexes, toe guards and chafers, to fit the design rim. 
         [0011]    “Belt Structure” or “Reinforcing Belts” means at least two annular layers or plies of parallel cords, woven or unwoven, underlying the tread, unanchored to the bead, and having both left and right cord angles in the range from 17 degrees to 27 degrees with respect to the equatorial plane of the tire. 
         [0012]    “Casing” means the carcass, belt structure, beads, sidewalls, and all other components of the tire excepting the tread and undertread. The casing may be unvulcanized rubber or previously vulcanized rubber to be fitted with a new tread. 
         [0013]    “Chafers” refers to narrow strips of material placed around the outside of the bead to protect cord plies from the rim, distribute flexing above the rim, and to seal the tire. 
         [0014]    “Chipper” refers to a narrow band of fabric or steel cord located in the bead area whose function is to reinforce the bead area and stabilize the lower sidewall 
         [0015]    “Circumferential” means lines or directions extending along the perimeter of the surface of the annular tread perpendicular to the axial direction. 
         [0016]    “Cord” means one of the reinforcement strands of which the plies in the tire are comprised. 
         [0017]    “Lateral” means an axial direction. 
         [0018]    “Ply” means a continuous layer of rubber-coated parallel cords. 
         [0019]    “Radial” and “radially” mean directions radially toward or away from the axis of rotation of the tire. 
         [0020]    “Radial Ply Tire” means a belted or circumferentially-restricted pneumatic tire in which the ply cords which extend from bead to bead are laid at cord angles between 65 degrees and 90 degrees with respect to the equatorial plane of the tire. 
         [0021]    “Section Height” means the radial distance from the nominal rim diameter to the outer diameter of the tire at its equatorial plane. 
         [0022]    “Section Width” (“SW”) means the maximum linear distance parallel to the axis of the tire and between the exterior of its sidewalls when and after it has been inflated at normal pressure for 24 hours, but unloaded, excluding elevations of the sidewalls due to labeling, decoration or protective bands. 
         [0023]    “Shoulder” means the upper portion of sidewall just below the tread edge, tread shoulder or shoulder rib means that portion of the tread near the shoulder. 
         [0024]    “Sidewall” means that portion of a tire between the tread and the bead. 
         [0025]    “Tread” means that portion of the tire that comes into contact with the road under normal inflation and load. 
         [0026]    “Tread width (TW)” means the arc length of the tread surface in the axial direction, that is, in a plane passing through the axis of rotation of the tire. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The invention will be described by way of example and with reference to the accompanying drawings in which: 
           [0028]      FIG. 1  is a perspective view of a tire representing the prior art. 
           [0029]      FIG. 2  is a front isometric view of a tire having a tread pattern of the invention. 
           [0030]      FIG. 3  is a rear isometric view of the tire. 
           [0031]      FIG. 4  is a front plan view of the tire. 
           [0032]      FIG. 5  is an enlarged fragmentary view of the tire tread. 
           [0033]      FIG. 6  is a partial section view showing a deeper part of a tread cavity engaging sand and pushing with the tapered surface of the cavity. 
           [0034]      FIG. 7  is a plan view of the cavity footprint pattern. 
           [0035]      FIG. 8  as a section view along  8 - 8  of  FIG. 7 . 
           [0036]      FIG. 9  is a section view along  9 - 9  of  FIG. 7 . 
           [0037]      FIG. 10  is a plan view of an alternative tread pattern incorporating an array of smaller cavities oriented in an opposite direction. 
           [0038]      FIG. 11  is a section view along  11 - 11  of  FIG. 10 . 
           [0039]      FIG. 12  is a perspective view of an alternative lugged embodiment of a tire incorporating cavities pursuant to the invention. 
           [0040]      FIG. 13  is a plan view of a tread portion of the alternative embodiment of  FIG. 12 . 
           [0041]      FIG. 14  is a section view along  14 - 14  of  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    Referring to  FIG. 1 , a vehicular tire  10  is shown of a type in common use by the industry for sand road applications. The tire  10  has a circumferential tread surface  12  and a direction ribbed lug circumferential pattern extending from surface  12  and consisting of spaced apart curvilinear rib lugs  14 . Each of the rib lugs  14  has a leading edge  18  oriented to initiate contact with the ground in a direction of travel and a central lug indentation  16  oriented to scoop the granular ground surface and generate push in the direction of travel. Because sand has almost no cohesion and consists of large rounded particles, typical lugged tires with standard on/off road tread patterns, dig quickly into the sand and although they develop some forward thrust, the increase in motion resistance due to sinkage overwhelms the forward thrust. Consequently, the vehicle sinks until it bogs down. A suitable sand tire must stay on top of the sand and either just roll on top of it or paddle through it. In order to achieve acceptable sand performance, tire tread may be configured to be either totally smooth (lugless), smooth with a single centerline rib or smooth with widely spaced, very thin lugs such as shown at  10  of  FIG. 1 . However, if so designed, while working acceptably in the sand, the tires cannot be used on any other surface for any length of time due to serious deficiencies in wear (lug degradation), traction (resulting from lug degradation); ride/stability and handling. 
         [0043]    The tread pattern of the invention provides satisfactory performance in sand while maintaining functional performance on other types of surfaces. As shown in  FIGS. 2 ,  3 ,  4 ,  5 ,  6 ,  7  and  8 , the tire  20  is configured having an outer circumferential tread surface  22  in which a circumferential array of cavities  24  extend. The cavities are oriented generally according to a forward direction of travel indicated by rotation arrow  26  as will be explained. Each cavity  24  is of a general rectangular sectional configuration, defined by a pair of cavity sidewalls  28 ,  30 , a forward end wall  32 , and a rear end wall  33 . At the forward corners of each cavity  24  between each sidewall  28 ,  30  and the forward end wall  32  is a beveled surface  34 ,  36 . The forward end wall  32  represents the leading end  38  of the cavity  24  and the rearward end wall  33  the trailing end  40 . Each cavity  24  extends to a bottom inclined cavity floor surface  42 . The inclination of the bottom surface  42  is in a direction opposite to the forward direction of rotation  26  by which each cavity  24  progressively deepens from the rearward trailing end  40  to the leading end  38 . 
         [0044]      FIG. 6  illustrates the tire  20  rotating against a granular ground surface  54  such as sand. In the forward direction of rotation  26 , the leading, deep end  38  of a cavity  24  is filled with sand. Due to the inclination of the bottom surface  42 , as each cavity fills, pressure is exerted against the tire at an angle normal to the bottom surface  42  of a cavity as seen by directional arrows  44 . The directional angled pressure of sand within the cavity translates in forward thrust/traction to the tire. 
         [0045]    From  FIGS. 5 ,  6 ,  7 ,  8 , and  9 , it is shown that the tread pattern of  FIG. 2  thus consists of a series of tapered cavities  24  molded into the smooth tread outer surface  22 . As the tire  20  rolls into the sand  54 , the deep end  38  of the cavity fills first and the tapered end of the cavity then pushes against the sand to push the tire forward. The cavity  24  confines/traps the sand and applies pressure (p) to it so that shear is maximized as tire slip occurs according to shear=p tan (phi) where p=normal pressure on the soil and phi=the internal angle of friction of the soil. Since there are no lugs extending from surface  22 , the tire does not sink into the sand even during slip and therefore motion resistance is minimized. The tire may also be driven on any hard surface with little compromise to wear, ride, or treadwear. Traction on hard and wet surfaces is accordingly adequate. The cavities  24  are nominally 0.5 inches (within a range of 0.4 to 0.6 inches) at the deep end and 0.1 inches (within a range of 0.05 to 0.15 inches) at the shallow end. Total net to gross is approximately 70 percent. It will further be noted from  FIG. 8  that each cavity  24  has a taper angle α of between 1 and 5 degrees or approximately 3 degrees on forward and rearward sides  32 ,  33  and on beveled surfaces  28 ,  42  as shown in  FIG. 9 . 
         [0046]    It will be appreciated that each cavity  24  may be configured other than rectangular in section if desired so long as each cavity  24  retains a concave tapered configuration. So configured, the cavity enters a sand surface deep cavity end first and allows the front part of the cavity to push on the trapped sand within the cavity to provide locomotion. 
         [0047]      FIGS. 10 and 11  show an alternative tread configuration in which a series of smaller, reversely oriented cavities  46  are placed along outer edges of the tread in order to accommodate traction in a reverse tire rotational direction  48 . The smaller cavities  46  are configured as previously described in reference to cavities  34  but oriented in an opposite direction. As such, the smaller cavities  46  do not detract significantly to the performance of the larger cavities  24  in a forward direction but provide enhanced traction by the tire in a reverse rotational direction.  FIG. 11  show the dimensional depth D 3  as within a range of 0.15 to 0.05 inches, or approximately 0.1 inches of the smaller cavities  46  at the shallow end and the deeper depth D 4  within a range of 0.15 to 0.35 inches, or approximately 0.25 inches. As with the larger cavities  24 , cavities  46  are tapered at each end by a taper angle α within a range of 1 to 5 degrees, or approximately 3 degrees. 
         [0048]      FIGS. 12 ,  13 ,  14  show an alternatively configured tire in which protruding lugs  56  are circumferentially interspersed between the cavities  24  about the tread region. The lugs  56  are provided to enhance traction when the tire traverses packed ground surfaces that would otherwise cause the tire to slip. A packed surface will generally not fill the cavities  24  as the tire rotates, whereby reducing traction and making slippage more likely. By interspersing lugs  56  between the cavities  24 , traction on packed surfaces is enhanced. As shown, at least one lug  56  is placed between adjacent cavities  24 , however, it will be appreciated that more or fewer lugs may be employed if desired. 
         [0049]    As shown in  FIGS. 13 ,  14 , each lug  56  is generally rectangular in section, having a width W 1  between 0.25 and 0.625 inches; a height D 5  of between 0.25 and 0.375 inches; and a length between 1 and 3 inches. The dimensions may be varied if desired to alter the gripping traction performances of the lugs  56 . In yet another embodiment (not shown), the combination of cavities  24 , lugs  56  ( FIG. 12 ), and smaller cavities  46  ( FIG. 10 ) along the tread region may be employed if desired. 
         [0050]    Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described which will be within the full intended scope of the invention as defined by the following appended claims.