Patent Publication Number: US-2020276868-A1

Title: Heavy truck tire tread and heavy truck tire

Description:
FIELD OF THE INVENTION 
     This invention relates generally to tire treads and tires. More specifically, this invention relates to tire treads and tires best suitable for the drive axle(s) of heavy trucks such as the drive axle(s) of tractors used in tractor-semi-trailer combinations or of single unit straight trucks. 
     BACKGROUND OF THE INVENTION 
     Tire treads generally extend about the outer circumference of a tire to operate as the intermediary between the tire and a surface upon which it travels (the operating surface). Contact between the tire tread and the operating surface occurs along a footprint of the tire. Tire treads provide grip to resist tire slip that may result during tire acceleration, braking, and/or cornering. Tire treads may also include tread elements, such as ribs or lugs, and tread features, such as grooves and sipes, each of which may assist in providing target tire performance when a tire is operating under particular conditions. 
     One problem with treads for drive tires is the compromise between traction, rolling resistance and wear/abnormal wear. 
     It is known that adding sipes in a tire rib can improve wear rate and traction. But it has never been used successfully in the shoulder ribs of drive tires for the long-haul trucking application because it often triggers abnormal wear. The shoulders of long-haul drive tires are therefore typically designed with solid ribs, with no full-width transverse sipes or full-depth transverse grooves. As a result, the design of long-haul drive tire treads is sacrificing shoulder rib wear rate and traction in order to avoid abnormal wear. 
     SUMMARY OF THE INVENTION 
     The invention provides for a heavy truck tire tread having a longitudinal direction, a lateral direction and a thickness direction, said tread comprising:
         longitudinal grooves separating longitudinal ribs;   a pair of opposing tread edges spaced apart along the lateral direction;   a pair of shoulder ribs, each shoulder rib being adjacent to a respective tread edge of said pair of tread edges;
 
wherein the shoulder ribs are solid ribs comprising lateral full depth sipes running at a sipe angle relative to the lateral direction and;
 
wherein an average sipe angle over a center portion of said shoulder ribs is greater than 30° in absolute value.
       

     In another embodiment, the average sipe angle over the center portion of said shoulder ribs is less than 70° in absolute value. 
     In another embodiment, the average sipe angle over the center portion of said shoulder ribs is greater than 35° and less than 55° in absolute value. 
     In another embodiment, said sipe angle is less than 30° in absolute value at a point where the sipe exits the shoulder rib towards the tread edge. 
     In another embodiment, a ratio of the average sipe depth with the average distance between consecutive sipes is at least 0.3. 
     In another embodiment, the ratio of the average sipe depth with the average distance between consecutive sipes is between 0.5 and 1.5. 
     In another embodiment, said sipe exits into a shoulder notch of the shoulder rib towards the tread edge. 
     In another embodiment, the sipes are oriented relative to the rolling direction (RD) such that the interior end of the sipe at the shoulder groove makes contact with the ground before the exterior end of the sipe at the tread edge. 
     The invention also provides for a heavy truck tire comprising such a tread. 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more detailed descriptions of a particular embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  is a perspective view of a heavy truck tire comprising an embodiment of the disclosed tire tread. 
         FIG. 2  is a front view of part of the tread of  FIG. 1  showing details of its design at a much bigger scale. 
         FIGS. 3 to 6  are front views similar to that of  FIG. 2  showing other embodiments of the tread. 
     
    
    
     The use of the same or similar reference numerals in the figures denotes the same or similar features. 
     DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS 
     Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the drawings. These examples are provided by way of explanation of the invention. 
     As shown in  FIG. 1 , a heavy truck tire  1  generally comprises a crown portion C connected by respective sidewalls SW, SW′ to beads portions B, B′. The crown portion comprises a tread  2  according to an embodiment of the invention. The design of the tread is substantially symmetric, that is to say that the tread features are arranged substantially symmetrically about the center plane of the tread. This tread is said to be of a directional design because it has a different appearance according to which side it is oriented. A directional tire or tread does not only look differently but it also performs differently if used in one rolling direction or the other. This is why directional treads or tires conventionally bear markings that indicate the designed rolling direction. Such markings may take the form of arrows RD pointing in the designed rolling direction. Using the tire for rolling in the opposite direction would be detrimental to its best performance. 
       FIG. 2  is a magnified and flattened projection view of a portion of the tread  2  of  FIG. 1 . As shown in  FIG. 2 , the tread has a longitudinal direction X (also referred to as the circumferential direction of the tire), a lateral direction Y (also referred to as the axial or transverse direction) and a thickness direction Z (also referred to as the tread depth direction). 
     The tread depth is generally defined as the distance between the tread contact surface and a translation of this contact surface to be tangent to the deepest features in the tread. 
     The tread has respective tread edges  21 ,  21 ′ on each side and longitudinal ribs defined by longitudinal grooves  20  separating the ribs. Longitudinal grooves may be straight or undulate along their main direction as represented in the FIGS. The ribs defined between the respective shoulder grooves and tread edges are referred to as shoulder ribs  22 ,  22 ′. Shoulder ribs are solid ribs comprising lateral sipes  23  running across them and connecting the shoulder grooves to the tread edges. A sipe is the narrow space formed in a tread between walls of material over a depth at most equal to the tread depth, said walls being able, at least in part, to come into contact with one another in the usual running conditions of the tire. Sipes are generally made as thin as manufacturing would reasonably allow, most of the time under 1 mm and preferably at around 0.5 mm. Sipes  23  are full depth sipes. Sipes are said to be full depth sipes when their average depth is at least 50% of the tread depth. 
     As shown on the left side of  FIG. 2 , a center portion CP of the shoulder rib is defined between an outer boundary line OBL and an inner boundary line IBL. The outer boundary line OBL is a longitudinal straight line running at an average distance of 8 mm from the tread edge  21 . The inner boundary line IBL is a longitudinal straight line running at an average distance of 5 mm from the interior edge of the shoulder rib, adjacent to the shoulder groove  20 . 
     The orientation of a lateral sipe  23  is defined by its angle α relative to the lateral direction Y. A certain angle α can be measured in any location along the sipe. This local angle α can be a constant value in the case of a straight sipe but α can also vary significantly along the length of the sipe. To characterize the main orientation of the sipe, an average sipe angle αa is defined in the center portion CP of the rib. The average sipe angle αa is defined as the angle relative to the lateral direction Y of a straight line connecting the points where the sipe intersects the inner and outer boundary lines IBL, OBL. According to the invention, this average angle is greater than 30° and preferably less than 70° in absolute value. Using absolute value to characterize an angle is a way to focus on its magnitude and ignore its direction. 
     A distance d can be measured between consecutive sipes. A sipe density SD can be established as the ratio of the average sipe depth ASD with the average distance d (SD=ASD/d). 
       FIG. 3  shows another embodiment where the sipes are undulating (zigzagging) along their main direction. Undulated sipes promote tread stiffness due to the sipe walls interlocking when loaded on the ground. Undulations may have many different shapes and can typically be one-directional (so called zigzag sipes) or bi-directional (so called egg-crate sipes). This FIG also illustrates the fact that the local sipe angle α may vary to a large extent while the average sipe angle αa is maintained between 30° and 70°. 
       FIG. 4  shows yet another embodiment where the sipes exit to the sides of the shoulder rib at a lower angle, typically less than 30°. 
       FIG. 5  shows yet another embodiment where the sipes exit to the outside of the shoulder rib into notches  24  that are recessed from the tread edges. Tread edge notches do not affect the definition of the location of the outer boundary line OBL. 
     In  FIGS. 1 to 5 , each side of the tread is represented as being symmetric to the other side of the tread relative to a center (or equatorial) plane of the tread. But a tread according to the invention may also comprise tread halves that are notably different as long as each tread half remains within the scope of the invention as limited by the claims. 
       FIG. 6  shows a tread similar to that of  FIG. 2  except for the fact that its pattern is made non-directional by having the sipe angles on one side of the tread reversed. This illustrates the fact that the tread may be directional as shown in  FIGS. 1 to 5  as well as non-directional. 
     In the drawings, the grooves are shown in the generic and conventional shape of fully open grooves but they could be of many other forms. They may for instance be partially hidden grooves, that is to say, grooves that may not be always fully open to the tread surface. Such grooves may for example undulate along their length between a lower position where they are only connected to the surface by a sipe and a higher position where they are fully open at the surface. Partially hidden grooves may also consist in an under-surface duct connected to the surface by a series of radially extending passages. 
     The tread may also have shallow depressions, markings or engravings in otherwise solid shoulder ribs. Such shallow features and are intended to wear out during the early wear life of the tread and do not affect the stiffness of the ribs. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. As already discussed above, a tread or tire according to the invention may also comprise tread halves that are notably different from one another as long as each tread half remains within the scope of the invention as limited by the claims. Thus, it is intended that the present invention covers such modifications and variations as they fall within the scope of the appended claims and their equivalents.