Abstract:
A tire tread provided with a plurality of grooves delimiting blocks and/or ribs and having an average depth H. This tread has a tread surface designed to come into contact with the roadway only along a plurality of raised contact lines. The raised contact lines intersect one another to delimit, on the tread surface, a plurality of contiguous first cavities each formed by a wall which is recessed radially inwardly from the tread surface. The walls of the first cavities are intersected by respective second cavities situated radially.

Description:
BACKGROUND 
       [0001]    The field of the invention is that of passenger vehicle tire treads and more specifically the design of the rolling surface of such treads. 
         [0002]    Winter driving on roads covered with snow and ice requires a tread that has a great many ridges and recesses in order to promote better grip by storing and/or clearing the fluid or solid particles present on these roads. 
         [0003]    Clearing means that the fluid or solid particles are drained away from direct contact between the tread and the roadway so that the material of which the tread is made is in direct contact with the roadway. 
         [0004]    While it is possible to use means that allow this drainage to be performed once the tire has undergone an initial degree of wear (particularly once cavities initially present within the tread have reached the tread surface), it has been found that before it reaches this initial degree of wear, a tread may exhibit inferior performance. 
         [0005]    The object of the invention is therefore to provide a technical solution to the problem of insufficient grip (“grip” being the accepted English term for adhesion) in the first few kilometers of running with a new tread. 
       SUMMARY 
       [0006]    To this end, the tire tread according to the invention is provided with a plurality of grooves delimiting blocks and/or ribs, these grooves having an average depth H. This tread has a surface radially on the outside of which part forms a tread surface designed to come into contact with the roadway during rolling. This tread surface is characterized in that it is in contact with the roadway only along a plurality of contact raised lines, these contact raised lines intersecting one another to delimit, on the tread surface a plurality of small cavities, that is to say of parts situated radially on the inside of the said tread surface. These cavities which are recessed with respect to the tread surface significantly increase the contact pressure along the contact lines. This encourages the clearing of loose particles present on the roadway towards the cavities and is therefore beneficial to ensuring contact between the tire and the roadway. 
         [0007]    Advantageously, the contact raised lines are organized into a plurality of first contact lines and a plurality of second contact lines, the first contact raised lines being directed substantially in a first direction, the second contact raised lines being directed substantially in a second direction. The first contact raised lines intersect the second contact raised lines to form, on the tread surface, at the intersection with the said tread surface, first cavities, each first cavity being delimited in the thickness of the tread by a main wall. In this alternative form of the invention, the first and the second contact lines may follow paths which are either rectilinear or non-rectilinear. What is essential is that the tread be in contact, at least at light loads, only along the contact lines, all the other points of the surface of the tread being radially offset towards the inside (that is to say towards the axis of rotation of the tire fitted with the tread) in order to increase the contact pressure on these lines and thus be able to direct any loose particles present on the roadway towards the cavities. 
         [0008]    For preference, the largest dimension of the first cavities on the tread surface is at least equal to 0.2 mm and at most equal to 1.5 mm for a tire intended to be fitted to a passenger vehicle. 
         [0009]    Advantageously, the maximum depths of the first cavities are at most equal to 10% of the depth of the grooves. 
         [0010]    According to an improvement of this invention, the tread according to the invention may further comprise a plurality of second cavities of depth P 2 , each second cavity opening onto the main wall delimiting a first cavity, this second cavity being delimited by a second wall. 
         [0011]    In the latter alternative form, the sum of the depths of the first and second cavities is at most equal to 20% of the depth H of the grooves. Thus it becomes possible to create an initial volume suited to capturing the particles present on the roadway without in any way adversely affecting the rigidity of the tread. 
         [0012]    In order not to cause too much disruption to the rigidity of the tread near the tread surface, it is sensible for the projected area of the surface delimiting each second cavity onto the tread surface to be at least equal to 10% and at most equal to 40% of the projected area of the first cavities onto the tread surface. 
         [0013]    It is also sensible for the intersection between the wall delimiting the second cavity and the main wall of the first cavity to be a closed geometric line the length of which differs from the length of the geometric line of intersection of the first cavity with the tread surface. 
         [0014]    Advantageously, the second cavities have dimensions suited to containing a spherical body of a diameter of 0.2 mm. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]    Other features and advantages of the invention will emerge from the description given hereinafter with reference to the attached drawings which, by way of nonlimiting examples, show some embodiments of the subject matter of the invention. 
           [0016]      FIG. 1  shows part of a tread surface of a tread according to the invention, intended for a passenger vehicle tire; 
           [0017]      FIG. 2  shows a section on II-II of  FIG. 2 ; 
           [0018]      FIG. 3  shows another alternative form of tread according to the invention; 
           [0019]      FIG. 4  shows the tread surface of a tread according to the invention intended for a passenger vehicle tire; 
           [0020]      FIG. 5  shows an enlarged local view of the tread shown in  FIG. 4 ; 
           [0021]      FIG. 6  shows a section on VI-VI of  FIG. 5 ; 
       
    
    
       [0022]    To make the figures and the associated description easier to understand, the same references are used to denote identical structural elements irrespective of the alternative form of embodiment considered. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]      FIG. 1  shows the contact face  40  of a tread element  4  of rectangular shape delimited by ridges  41 ,  42 ,  43  and  44 , this contact face being intended to come into contact with the roadway during running. 
         [0024]    This contact face comprises a plurality of raised contact lines, this plurality being made up of three series of contact lines  7 ,  8 ,  9  respectively parallel to three different directions, two of these directions being parallel to the ridges of the tread element. 
         [0025]    These contact lines  7 ,  8 ,  9  intersect one another in such a way as to delimit surface portions of substantially triangular shape. Furthermore, in each of the said triangles and with the exception of the points of the contact lines, all the other points are set back from the plane surface passing through the sides of the said triangle so as to form kinds of small cavities  50 . In this way, a kind of partition of the external surface of the block  4  is created whereby all the points on this surface, with the exception of the points of contact lines, are offset towards the inside of the tread. Thus, during running, this element comes into contact along its contact raised lines, and then along part or even all of the surfaces of cavities formed, as will be explained more specifically in connection with  FIGS. 4-6 . 
         [0026]    It is essential that, on the external face of each element of a tread intended to come into contact with the roadway, there should be no contact surface other than the contact lines (in particular it is recommended that the formation of flaps on the contact lines be avoided), in order to enjoy the full technical benefit of the invention. 
         [0027]      FIG. 2  shows a sectioned view of the tread element of  FIG. 1 , this section being taken along the section line II-II. This cross section shows that the contact face  40  of the tread element comprises a plurality of cavities  50  opening to the outside of the tread and delimited on the outside by the contact lines  7 ,  8 ,  9 . The depth of the cavities here is 0.2 mm and the max length of the sides delimiting each cavity on the tread surface here is of the order of 1.0 mm 
         [0028]    In the alternative form of the invention shown by  FIG. 3 , the exterior face of a tread element comprises a plurality of first cavities  50  delimited by raised contact lines  7  and  8 . This exterior face comprises a first plurality  7  of raised contact lines with non-rectilinear paths and a second plurality  8  of raised contact lines with non-rectilinear paths. Each of the contact lines  7  of the first plurality of lines is directed in the first direction D 7 . Each of the lines of the second plurality of contact lines  8  is directed in a second direction D 8 . Aside from the first cavities  50  that open onto the contact face, there are second cavities  60  of cylindrical shape and which open onto the wall delimiting the first cavities  50 . These second cavities are formed to create a kind of reservoir in which the loose particles on the roadway can be temporarily collected. The total depth of a first and of a second cavity in this instance is equal to 1 mm. The fact that the contact pressures generated on the contact lines  7  and  8  are increased in comparison with the other points of the first cavities allows the loose particles to be directed towards the first and second cavities. In the case of the prior art, the fact that there were contact surfaces rather than contact lines made it impossible to direct the said loose particles towards the cavities, and this resulted in a loss of contact with the roadway. 
         [0029]      FIGS. 4-6  show an alternative form of the invention forming the tread surface  10  of a passenger car tire tread  1 . This tread  1  comprises a plurality of longitudinal  21  and transverse  22  grooves delimiting tread pattern blocks  4  of which one of the surfaces directed towards the outside of the tread forms part of the tread surface  10  of the said tread. 
         [0030]    These grooves have an average height H equal in this instance to 5 mm. 
         [0031]    Furthermore, in the as-new state of this tread  1  and on each tread pattern block  4 , a plurality of first cavities  50  opening onto the exterior surface of the said block has been molded, each first cavity  50  being extended towards the inside of the tread by a second cavity  60 . The intersections of the first cavities with the tread surface of the tread follow lines of contact  7  and  8 , all the other points of the main surfaces of the first cavities being situated towards the inside relative to the said lines of contact. In this way, the tread surface comes into contact with the roadway along the lines of contact  7  and  8  first; thereafter, contact may be had over at least some of the main walls of the main cavities or even over the entirety thereof. What is essential is that initially, contact established along the lines of contact, generates forces that drive any loose particles present on the roadway towards the inside of the first cavities. 
         [0032]      FIG. 5  shows an enlarged view of part of a block  4  of such a tread, and reveals a plurality of the first cavities  50  and the second cavities  60 . 
         [0033]    As shown by  FIG. 6  which is a section on VI-VI of  FIG. 5 , each first cavity  50  of depth P 1  (equal to 0.1 mm) is delimited by a main wall  51  which, in this instance, has the overall shape of a truncated pyramid. The lines  52  along which the faces of the said pyramid intersect make an angle of 12° with the tread surface. The main wall  51  of this first cavity  50  intersects the tread surface  10  along a geometric line  53  of closed contour of total length C 1 . This geometric line  53  is formed of portions of first and second lines of contact  7 ,  8  and is able to apply a force that causes the tire to engage with the roadway and with the materials covering the roadway (such as snow). This geometric line  53  formed by the intersection of the main wall  51  of the first cavity  50  with the tread surface  10  has a substantially square shape with a side length A equal in this instance to 1.2 mm. 
         [0034]    The first cavities  50  are formed in such a way as to be contiguous with one another on each tread pattern block  4  (“contiguous” here means that part of a line of contact of a first cavity is also part of a line of contact of another first cavity). Thanks to this arrangement, it is possible to improve the efficiency with which particles on the roadway are collected and thus improve the efficiency of the ridges in contact with the road surface. In this instance, one side of a first square is also a side of square adjacent to the first square. 
         [0035]    The purpose of the first cavity  50  is to increase the pressure loading on the line of contact in contact with the road surface while at the same time capturing the loose particles present on the road surface and directing them towards the bottom of the said first cavity. 
         [0036]    Furthermore, each first cavity  50  is extended within the thickness of the tread by a second cavity  60 , of depth P 2 , in this instance equal to 0.4 mm, opening onto the main wall  51  of the first cavity  50 . This second cavity  60 , which acts as a reservoir for the loose particles, is delimited by a wall  61  which intersects the main wall  51  of the first cavity  50  along a circular line of closed contour the length C 2  of which is shorter than the length C 1  of the line of intersection  53  between the first cavity and the tread surface  10 . 
         [0037]    In this instance, the second cavity  60  is of cylindrical circular shape with a diameter D equal to 0.8 mm and opens onto the bottom end of a first cavity  50 . The total depth (the sum of the depths P 1  and P 2 ) of the first and second cavities is equal to 0.4 mm. 
         [0038]    In order to optimize the collection of particles and their temporary storage during transient contact with the road surface, it is recommended that the choice of total volume of first and second cavities per unit area on the tread surface be at least equal to 5 cubic millimeters (5 mm 3 ). This then gives enough first cavities that the loose particles can be drained away to the reservoirs formed by the second cavities. 
         [0039]    For preference, the second cavities have any shape of appropriate size that they can contain at least one spherical body of a diameter equal to 0.2 millimeters. 
         [0040]    The invention is not restricted to the examples described and depicted and various modifications can be made thereto without departing from its scope.