WO2024223571A1 - Tire with a tread having improved endurance properties - Google Patents

Abstract

The invention relates to a tire, the tread of which has at least one kerf (8) comprising a radially inner part that is wider than the radially outer part extending to the tread surface (6). According to the invention, the tread consists of a first elastomer blend, and at least a portion of the wall of the at least one kerf surrounding the wider zone consists of a second elastomer blend over a thickness (E) of at least 1 mm, the second elastomer blend having a tear coefficient that is at least 20% greater than the tear coefficient of the first elastomer blend.

Description

TYRE WITH A TREAD WITH IMPROVED ENDURANCE PROPERTIES

[0001] The present invention relates to a tire, with radial carcass reinforcement and more particularly a tire intended to equip vehicles carrying heavy loads and traveling at high speed, such as, for example, trucks, tractors, trailers or road buses.

[0002] Generally speaking, in heavy-duty tires, the carcass reinforcement is anchored on either side in the bead area and is radially surmounted by a crown reinforcement consisting of at least two layers, superimposed and formed of parallel wires or cables in each layer and crossed from one layer to the next, making angles of between 10° and 45° with the circumferential direction. Said working layers, forming the working reinforcement, may also be covered with at least one so-called protective layer and formed of advantageously metallic and extensible reinforcement elements, called elastic. It may also comprise a layer of low-extensibility metal wires or cables forming an angle of between 45° and 90° with the circumferential direction, this ply, called the triangulation ply, being radially located between the carcass reinforcement and the first crown ply called the working ply, formed of parallel wires or cables having angles of at most 45° in absolute value. The triangulation ply forms with at least said working ply a triangulated reinforcement, which exhibits little deformation under the various stresses to which it is subjected, the triangulation ply having the essential role of absorbing the transverse compression forces to which all of the reinforcing elements are subjected in the crown area of the tire.

[0003] Cables are said to be inextensible when said cables exhibit, under a tensile force equal to 10% of the breaking force, a relative elongation of at most 0.2%.

[0004] Cables are said to be elastic when said cables exhibit, under a tensile force equal to the breaking load, a relative elongation at least equal to 3% with a maximum tangent modulus less than 150 GPa.

[0005] Circumferential reinforcing elements are reinforcing elements which make angles with the circumferential direction in the range + 2.5°, - 2.5° around 0°. [0006] The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the rolling direction of the tire.

[0007] The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.

[0008] The radial direction is a direction intersecting the axis of rotation of the tire and perpendicular to it.

[0009] The axis of rotation of the tire is the axis around which it rotates in normal use.

[0010] A radial or meridian plane is a plane that contains the axis of rotation of the tire.

[0011] The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.

[0012] Some current tires, called "road tires", are intended to run at high speed and over increasingly long distances, due to the improvement of the road network and the growth of the motorway network in the world. All the conditions under which such a tire is called upon to run undoubtedly allow an increase in the number of kilometers traveled, the wear of the tire being less; on the other hand, the endurance of the latter and in particular of the crown reinforcement is penalized.

[0013] There are indeed constraints at the level of the crown reinforcement and more particularly shear constraints between the crown layers, combined with a non-negligible increase in the operating temperature at the ends of the axially shortest crown layer, which result in the appearance and propagation of cracks in the rubber at said ends.

[0014] In order to improve the endurance of the crown reinforcement of the type of tire studied, solutions relating to the structure and quality of the layers and/or profiles of rubber compounds which are arranged between and/or around the ends of the plies and more particularly the ends of the axially shortest ply have already been provided.

[0015] It is known in particular to introduce a layer of rubber mixture between the ends of the working layers to create a decoupling between said ends to limit the shear stresses. Such decoupling layers must however exhibit very good cohesion. Such layers of rubber mixtures are for example described in patent application WO 2004/076204.

[0016] The tires thus produced effectively make it possible to improve performance, particularly in terms of endurance.

[0017] Furthermore, it is known to introduce a layer of circumferential reinforcing elements to produce tires with a very wide tread or to give tires of a given size greater load capacities. Patent application WO 99/24269 describes, for example, the presence of such a layer of circumferential reinforcing elements.

[0018] The layer of circumferential reinforcing elements is usually constituted by at least one metal cable wound to form a turn whose laying angle relative to the circumferential direction is less than 2.5°.

[0019] In combination with this internal tire structure, it is known to provide the tread, that is to say the part of the tire intended to come into contact with the ground during rolling and to wear during rolling, with a sculpture formed of relief elements delimited by grooves whether they are circumferential, transverse or oblique in orientation. The objective of such a sculpture is to give the tread good performance when rolling on dry roads and on roads covered with water, particularly in rainy weather.

[0020] In order to improve the performance of the treads without, however, excessively lowering the shear rigidities of said treads, it is known to form on the tread surface a plurality of transversely or obliquely oriented edges in order to cut the water film on a roadway to ensure good contact between the tread and the roadway. One means of obtaining such edges consists in providing the tread with a plurality of cutouts, these cutouts having the shape of grooves or the shape of incisions. In the present application, incisions are distinguished from grooves in that the incisions have an appropriate width to allow at least partial contact during rolling between the facing walls delimiting these incisions and in particular during the passage into contact with the ground, which could not be the case for grooves under normal conditions of use of the tire.

[0021] Combined with this need to improve adhesion performance by the presence of edges formed by the transverse cutouts, it is also required that the performance of a tread is sustainable, i.e. satisfactory performance is achieved even after more or less advanced partial wear. Partial wear of a tread means a state of wear corresponding to a tread thickness at most equal to the total thickness of the tread that can be worn before the tire has to be changed, particularly for regulatory reasons.

[0022] Patent application WO 02/38399-A2 describes a tread for a heavy goods vehicle tire, this tread comprising a plurality of circumferential and transverse grooves. The transverse grooves are formed by alternating hollow zones and incisions so as to have a volume of hollows opening onto the tread surface when new and a volume of hidden hollows, these hidden hollows being intended to open after partial wear of the same tread. The presence of hidden hollows - appearing with wear, makes it possible to have greater rigidity in the initial state while ensuring grip performance regardless of the level of wear of the tread.

[0023] For the purposes of the invention, a circumferentially or longitudinally oriented cutout is a cutout whose mean plane of at least part of the walls of said cutout forms an angle with a longitudinal plane of less than 10°. This angle formed with a longitudinal plane may be oriented in one direction or the other relative to said longitudinal plane. A longitudinally oriented cutout may also be a cutout whose walls undulate or zigzag around a mean plane as just described.

[0024] For the purposes of the invention, a transversely oriented cutout is a cutout whose mean plane of at least part of the walls of said cutout forms an angle with a radial plane of less than 35°. This angle formed with a radial plane may be oriented in one direction or the other relative to said radial plane. A transversely oriented cutout may also be a cutout that runs continuously on either side of a mean plane as just described; it may also be a cutout whose walls undulate or zigzag around a mean plane as just described.

[0025] For the purposes of the invention, an obliquely oriented cutout is a cutout whose mean plane of at least part of the walls of said cutout forms an angle with a radial plane of between 35° and 80°. This angle formed with a radial plane may be oriented in one direction or the other relative to said radial plane. An obliquely oriented cutout may also be a cutout that runs continuously on either side of a mean plane as just described; it may also be a cutout whose walls undulate or zigzag around a mean plane as just described. [0026] When rolling tires thus produced subjected to constraints that are particularly demanding for the tread, in particular in terms of violent impacts for example on sidewalks, damage to the tread appeared, resulting in particular in the tearing of blocks of rubber.

[0027] During tests, the inventors have demonstrated that the presence of certain types of cutouts comprising hidden hollows could lead to a deterioration in performance in terms of endurance, with the more rapid appearance of rubber block tears, in comparison with similar tires not comprising such hidden hollows for identical uses.

[0028] An aim of the invention is to provide tires whose tread endurance properties are improved regardless of the use, the properties in particular of wear and overall endurance of the tire being preserved for usual uses.

[0029] This object is achieved according to the invention by a tire with a radial carcass reinforcement comprising a crown reinforcement, itself radially capped with a tread, said tread being joined to two beads by means of two sidewalls, said tread comprising a tread surface intended to come into contact with a roadway and form a contact surface, said tread having a plurality of cutouts consisting of at least one cutout comprising a radially inner part wider than the radially outer part opening onto the tread surface, the radially inner part and the radially outer part of said at least one cutout being connected by a widening zone, the ratio of the width measured at the bottom of said at least one cutout to the width measured on the surface of the tread on a new tire of said at least one cutout being greater than 1.2, the tread being constituted by at least one first elastomeric mixture forming at least part of the tread surface when the tyre is new, at least part of the wall of said at least one cutout, surrounding said widening zone, being made of a second elastomeric compound over a thickness of at least 1 mm and the second elastomeric compound having a tear coefficient (expressed in N/m) at least 25% greater than the tear coefficient of the first elastomeric compound, the tear coefficient being the product of the breaking force per unit thickness (expressed in N/mm thickness) by the elongation at break (expressed in %), measured at 100°C. [0030] The thickness of the second elastomeric mixture is measured locally in a direction normal to the surface of the wall of said at least one circumferential groove.

[0031] The width between the walls of a cut is measured on a new tire, in a cutting plane perpendicular to the mean plane of the walls and to the plane tangent to the surface of the tread. It is measured in a direction parallel to the plane tangent to the surface of the tread.

[0032] For the purposes of the invention, the width between the walls of a cutout in the surface of the tread corresponds to the smallest measurement in the radially outer part of the cutout and more precisely to the smallest measurement made in a zone comprised radially between a radially outermost point of said cutout and a point situated at a distance from the surface of the tread equal to 30% of the depth of said cutout.

[0033] For the purposes of the invention, the width between the walls of a cutout at the bottom of said cutout corresponds to the largest measurement in the radially inner part of the cutout and more precisely to the largest measurement made in a zone comprised radially between the radially innermost point of said cutout and a point located at a distance from the surface of the tread equal to 25% of the depth of said cutout.

[0034] For the purposes of the invention, the depth of a cut is the radial distance measured on a new tire between the surface of the tread and the radially innermost point of said cut.

[0035] The breaking force per unit thickness and the elongation at break in tearing are measured on a specimen stretched at 500 mm/min to cause the specimen to break on a dynamometric machine for tensile tests, equipped with a system for measuring and acquiring the force and the displacement of the movable crosshead. The tensile specimen consists of a rubber plate of parallelepiped shape, with a thickness equal to 2.5 mm, a length equal to 145 mm and a width equal to 10 mm. Before starting the test, three very fine notches perpendicular to the length of the specimen are made using a razor blade to a depth of 3 mm, on one edge of the specimen, one in the middle and the other two on either side of the first and 6 mm apart from it. The force (expressed in N per mm of thickness of the specimen) to be applied to obtain rupture is determined and the elongation is measured at rupture (expressed in %). The test was conducted in air, at a temperature of 100°C. High values indicate good cohesion of the rubber composition although showing crack incipients.

[0036] Preferably according to the invention, the depth, measured on a new tire, of said at least one cutout comprising a radially inner part wider than the radially outer part opening onto the tread surface is greater than or equal to 40% of the thickness of the tread.

[0037] For the purposes of the invention, the thickness of the tread, measured in a radial section of the tire, is the distance measured on a new tire between a point on the surface of the tread and the orthogonal projection of said point on the radially outer surface of the crown reinforcement.

[0038] According to a first embodiment of the invention, said at least one cutout is a cutout of longitudinal orientation.

[0039] According to a second embodiment of the invention, said at least one cutout is a cutout of transverse orientation.

[0040] According to a third embodiment of the invention, said at least one cutout is a cutout of oblique orientation.

[0041] The tires thus produced according to the invention effectively allow driving in particularly demanding conditions without seeing any tearing appear on the tread compared to tires of more conventional design.

[0042] The inventors believe they have demonstrated that when driving in conditions that are particularly stressful for the tread with tires of conventional design, the tears that occur on the tread seem to start at the widening zone between the visible part on the tread surface and the hidden part of the cutout. The inventors believe they interpret this phenomenon as a result of this widening of the cutout, which is conducive to the appearance of longitudinal breaks during violent impacts. When driving, the tire having suffered attacks on the tread such as impacts, these breaks lead to the tearing of a part of the tread. [0043] The presence of a mixture in accordance with the invention at least locally in the area sensitive to the appearance of breaks makes it possible to limit or at least delay the appearance of breaks and therefore reduces the risks of tearing off part of the tread whatever the driving conditions.

[0044] According to a preferred embodiment of the invention, the entire wall of said at least one cutout is made of the second elastomeric mixture over a thickness of at least 1 mm so that the second mixture forms the complete surface of said at least one cutout.

[0045] This preferred embodiment of the invention simplifies the production of the tire according to the invention, the second mixture being positioned before molding according to the knowledge of a person skilled in the art to arrive at the tire according to the invention after curing and molding of the tread forming the sculpture.

[0046] According to a preferred embodiment of the invention, the thickness of the second elastomeric mixture is less than 5 mm. The volume of the second mixture thus remains limited in comparison with the volume of the first mixture which thus contributes mainly to the desired properties of the tread such as grip and wear.

[0047] According to a preferred embodiment of the invention, the second elastomeric mixture has a tear coefficient at least 50% higher and preferably at least 100% higher than the tear coefficient of the first elastomeric mixture.

[0048] According to an advantageous variant of the invention, the first elastomeric mixture has a maximum value of tan(ô), noted tan(ô)max, less than 0.25.

[0049] The loss factor tan(ô) is a dynamic property of the rubber compound layer. It is measured on a viscoanalyzer (Metravib VA4000), according to the ASTM D 5992-96 standard. The response of a sample of vulcanized composition (cylindrical specimen 2 mm thick and 78 mm ² in section) is recorded, subjected to a sinusoidal stress in alternating simple shear, at a frequency of 10 Hz, at a temperature of 60°C. A strain amplitude scan is carried out from 0.1 to 50% (forward cycle), then from 50% to 1% (return cycle). For the return cycle, the maximum value of tan(ô) observed, denoted tan(ô) _ma x, is indicated. [0050] In the case where the thickness of the material is between 1 and 2 mm, the loss factor tan(δ) is measured according to the same method and under the same conditions, as described previously, on a sample of vulcanized composition which is in the form of a cylindrical test piece 1 mm thick and 78 mm2 in section.

[0051] Rolling resistance is the resistance that appears when the tire rolls. It is represented by the hysteretic losses linked to the deformation of the tire during a revolution. The frequency values linked to the revolution of the tire correspond to values of tan(ô) measured between 30 and 100°C. The value of tan(ô) at 60°C thus corresponds to an indicator of the rolling resistance of the tire while rolling.

[0052] Furthermore, the tests carried out have shown that the rolling resistance properties are preserved or even improved by the choice of the second elastomeric mixture, the latter being able to exhibit better rolling resistance performances than the first elastomeric mixture.

[0053] Advantageously according to the invention, the first elastomeric mixture is a rubber composition based on at least one elastomeric matrix comprising a copolymer based on styrene and butadiene and at least one reinforcing filler.

[0054] By “elastomer matrix” or “elastomeric matrix” is meant all of the elastomer(s) present in the rubber composition.

[0055] By “diene” elastomer (or indistinctly rubber), whether natural or synthetic, must be understood an elastomer consisting at least in part (i.e. a homopolymer or a copolymer) of diene monomer(s) (i.e., bearing(s) two carbon-carbon double bonds, conjugated or not).

[0056] By copolymer based on styrene and butadiene, is meant here a copolymer of at least one styrene monomer and at least one butadiene monomer (and of course also any mixture of such copolymers); in other words, said copolymer based on styrene and butadiene comprises by definition at least styrene units (derived from the styrene monomer) and butadiene units (derived from the butadiene monomer).

[0057] Advantageously according to the invention, the second elastomeric mixture is a rubber composition based on at least one elastomeric matrix comprising predominantly natural rubber for at least 80% and at least one reinforcing filler. [0058] According to a preferred embodiment of the invention, the second elastomeric mixture is a rubber composition based on at least one elastomeric matrix comprising 100% natural rubber.

[0059] The rubber compositions according to the invention of the first mixture or of the second mixture may comprise one or more reinforcing fillers.

[0060] Any type of so-called reinforcing filler, known for its ability to reinforce a rubber composition which can be used in particular for the manufacture of tires, can be used, for example an organic filler such as carbon black, an inorganic filler such as silica or even a mixture of these two types of fillers.

[0061] Suitable carbon blacks include all carbon blacks, in particular the blacks conventionally used in tires or their treads. Among the latter, mention will be made more particularly of the reinforcing carbon blacks of the 100, 200, 300 series, or the blacks of the 500, 600 or 700 series (ASTM D-1765-2017 grades), such as for example blacks NI 15, N134, N234, N326, N330, N339, N347, N375, N550, N683, N772).

[0062] By “reinforcing inorganic filler” is meant here any inorganic or mineral filler, whatever its color and origin (natural or synthetic), also called “white” filler, “light” filler or even “non-black” filler (“non-black filler”) as opposed to carbon black; this inorganic filler being capable of reinforcing on its own, without any other means than an intermediate coupling agent, a rubber composition intended for the manufacture of pneumatic tires, in other words capable of replacing, in its reinforcing function, a conventional tire-grade carbon black. Such a filler is generally characterized, in a known manner, by the presence of hydroxyl groups (-OH) on its surface, requiring, in order to be used as a reinforcing filler, the use of a coupling agent or system intended to ensure a stable chemical bond between the filler and the elastomeric matrix.

[0063] Suitable inorganic reinforcing fillers include inorganic fillers of the siliceous type, preferably silica (SiCL). The silica used may be any reinforcing silica known to those skilled in the art, in particular any precipitated or pyrogenic silica having a BET surface area and a CTAB specific surface area both of less than 450 m ² /g, preferably from 30 to 400 m ² /g, in particular between 60 and 300 m ² /g. [0064] Of course, the term inorganic reinforcing filler also means mixtures of different reinforcing inorganic fillers, in particular highly dispersible silicas as described above or a mixture of inorganic fillers of the siliceous type and non-siliceous inorganic fillers. As non-siliceous inorganic fillers, mention may be made of mineral fillers of the aluminous type, in particular alumina (Al2O3) or aluminum (oxide)hydroxides, or reinforcing titanium oxides, for example described in US 6,610,261 and US 6,747,087. The non-siliceous inorganic fillers, when present, are in the minority in the reinforcing filler.

[0065] The physical state in which the inorganic reinforcing filler is presented is indifferent, whether in the form of powder, microbeads, granules, or even balls.

[0066] A person skilled in the art will understand that, as a filler equivalent to the reinforcing inorganic filler described in this paragraph, a reinforcing filler of another nature could be used, in particular organic such as carbon black, provided that this reinforcing filler would be covered with an inorganic layer such as silica, or would have functional sites on its surface, in particular hydroxyl sites, requiring the use of a coupling agent to establish the bond between the filler and the elastomer. As an example, mention may be made, for example, of carbon blacks for tires as described, for example, in patent documents WO 96/37547, WO 99/28380.

[0067] According to a variant of the invention, the reinforcing filler of the first mixture and/or of the second mixture is predominantly carbon black, that is to say that it comprises more than 50% (>50%) by weight of carbon black relative to the total weight of reinforcing filler. Optionally, according to this variant, the reinforcing filler may also comprise silica or another reinforcing inorganic filler.

[0068] According to another variant of the invention, the reinforcing filler of the first mixture and/or of the second mixture consists of carbon black.

[0069] According to another variant of the invention, the reinforcing filler of the first mixture and/or of the second mixture is predominantly an inorganic reinforcing filler (preferably silica), that is to say that it comprises more than 50% (>50%) by weight of an inorganic reinforcing filler such as silica relative to the total weight of the reinforcing filler. Optionally according to this variant, the reinforcing filler also comprises carbon black. According to this option, the carbon black is used at a rate less than or equal to 20 pce, more preferably less than or equal to 10 pce (for example, the carbon black content may be in a range from 0.5 to 20 pce, in particular from 1 to 10 pce). In the indicated ranges, the coloring (black pigmenting agent) and anti-UV properties of carbon blacks are benefited from, without otherwise penalizing the typical performances provided by the reinforcing inorganic filler.

[0070] The elastomeric compositions of the tread of the tire according to the invention may optionally also comprise all or part of the usual additives, known to those skilled in the art and usually used in treads, such as for example processing aids, fillers (reinforcing or non-reinforcing, other than those mentioned above), pigments, protective agents such as anti-ozone waxes, chemical anti-ozonants (6PPD, TMQ), antioxidants, anti-fatigue agents and reinforcing resins (such as those described for example in application WO 02/10269).

[0071] According to an alternative embodiment of the invention, the crown reinforcement of the tire is formed of at least two working crown layers of reinforcing elements.

[0072] According to a preferred embodiment of the invention, the reinforcing elements of the working crown layers are inextensible metal cables.

[0073] The metal elements are preferably steel cables.

[0074] Advantageously according to the invention, the reinforcing elements of the working crown layers of the crown reinforcement are crossed from one layer to the other, making angles of between 10° and 45° with the circumferential direction.

[0075] Advantageously still according to the invention, the working crown reinforcement comprises a layer of circumferential reinforcing elements, preferably arranged radially between two working crown layers.

[0076] According to an advantageous embodiment of the invention, the reinforcing elements of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a secant modulus at 0.7% elongation of between 10 and 120 GPa and a maximum tangent modulus of less than 150 GPa.

[0077] A preferred embodiment of the invention further provides that the crown reinforcement is completed radially on the outside by at least one additional layer, said to be protective, of so-called elastic reinforcing elements, oriented relative to the circumferential direction with an angle between 10° and 45° and in the same direction as the angle formed by the inextensible elements of the working layer which is radially adjacent to it.

[0078] According to any of the embodiments of the invention mentioned above, the crown reinforcement can also be completed, radially on the inside between the carcass reinforcement and the radially inner working layer closest to said carcass reinforcement, by a triangulation layer of inextensible steel metal reinforcing elements making, with the circumferential direction, an angle greater than 45° and in the same direction as that of the angle formed by the reinforcing elements of the layer radially closest to the carcass reinforcement.

[0079] Other advantageous details and characteristics of the invention will emerge below from the description of the exemplary embodiments of the invention with reference to FIGS. 1 to 4 which represent: FIG. 1, a meridian view of a diagram of a tire according to the invention, FIG. 2, a projection view of a diagram of a portion of the surface of a tread of a tire according to the invention, FIG. 3, a sectional view along the section plane PI of a longitudinal cutout of a tire according to the invention, FIG. 4, a sectional view along the section plane P2 of a transverse cutout of a tire according to the invention.

[0080] The figures are not drawn to scale to simplify understanding.

[0081] In Figure 1, the tire 1, of size 315/70 R 22.5, comprises a radial carcass reinforcement 2 anchored in two beads 3 by turning around bead wires 4. The carcass reinforcement is formed of a single layer of metal cables. This carcass reinforcement 2 is hooped by a crown reinforcement 5, formed radially from the inside to the outside: a first working layer formed of non-hooked inextensible metal cables 9.35, continuous over the entire width of the ply, oriented at an angle equal to 22°, a layer of circumferential reinforcing elements formed of 21x23 steel metal cables, of a second working layer formed of non-fretted inextensible metal cables 9.35, continuous over the entire width of the sheet, oriented at an angle equal to 18° and crossed with the metal cables of the first working layer, of a protective layer formed of elastic metal cables 6.35.

[0082] All of these layers forming the crown reinforcement 5 are not shown in the figures.

[0083] The crown reinforcement is itself capped with a tread having a surface 6 intended to come into contact with the ground. The surface 6 of the tread is formed of four circumferential cutouts 7, 8 forming circumferential ribs constituting the tread.

[0084] Figure 2 illustrates a projection view of a diagram of a portion of the surface 6 of a tread of a tire 1. The surface 6 of the tread is formed of longitudinal cutouts 7 and 8, and transverse cutouts 9 and 10. The longitudinal cutouts 7 and transverse cutouts 9 are grooves and the longitudinal cutouts 8 and transverse cutouts 10 are incisions. All of these cutouts 7, 8, 9 and 10 form the tread elements 11 constituting the tread.

[0085] In Figure 3, a longitudinal cutout 8 is schematically shown in section along the cutting plane Pi. This cutting plane Pi is perpendicular to the mean plane of the walls and to the plane tangent to the surface 6 of the tread. The width Df of the cutout 8 at the bottom of the cutout is equal to 6.4 mm. The width _ds of the longitudinal cutout 8 on the surface of the tread is measured between the ends 12 and 13 of said longitudinal cutout 8 on the surface 6 of the tread. It is equal to 0.8 mm. The ratio Df / _ds is equal to 8.

[0086] In Figure 4, a transverse cutout 10 is schematically shown in section along the cutting plane P2. This cutting plane P2 is perpendicular to the mean plane of the walls and to the plane tangent to the surface 6 of the tread. The width Df' of the transverse cutout 10 at the bottom of the cutout is equal to 4 mm. The width _ds ' of the transverse cutout 10 of the tread is measured between the ends 14 and 15 of said transverse cutout 10 on the surface 6 of the tread 5. It is equal to 0.6 mm. The ratio Df7 _ds ' is equal to 6.7.

[0087] In the case of Figures 3 and 4, the cutouts 8 and 10 form an incision in the surface of the tread which, in accordance with the definition given above, has a width of less than 2 mm. After the tread wears, the hidden hollow under the said incision reveals a groove. As explained above, when the tire is new, the incision makes it possible to form edges while maintaining significant rigidity of the tread, the walls coming into contact with each other at the moment of contact with the ground. After wear, when the hidden hollow appears it forms a groove and therefore edges, the loss of rigidity being limited due to the lesser depth of the cut in the tread.

[0088] The tread is mainly made up of a first mixture. In Figures 3 and 4, a thickness E of a second mixture is also shown forming the wall of the circumferential 8 and transverse 10 cutouts.

[0089] The average thickness E of the second mixture is equal to 2 mm. This thickness E is measured in the direction normal to the inner surface of the cutouts 8, 10.

[0090] Tests are carried out with tires according to the invention.

[0091] The same tests are carried out with reference tires. The reference tires differ from the tires according to the invention by the absence of the second mixture, that is to say that the tread is entirely made up of the first mixture.

[0092] The different mixtures used for the treads are listed below:

(1) Natural rubber

(2) Polybutadiene neodymium 98% of 1,4-cis unit; Tg=-108°C

(3) Anionic styrene-butadiene copolymer containing 15% by weight of styrene unit and 24% of vinyl of the butadiene part (Tg -65°C)

(4) Carbon black grade N 134 according to ASTM D-1765 -2017

(5) Carbon black grade N115 according to ASTM D-1765 -2017

(6) SER 6266 from SER

(7) N-(l,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine “Santoflex 6PPD” from Flexys

(8) N-cyclohexyl-2-benzothiazol-sulfenamide “Santocure CBS” from Flexsys company

(9) N-cyclohexylthio-phthalimide (CTP/PVI) marketed by Shandong Derek New Materials Co.

(10)Stearic acid “Pristerene 4931” from Uniqema company

(11) Industrial grade Zinc Oxide from Umicore

[0093] The values of the constituents are expressed in pce (parts by weight per hundred parts of elastomers).

[0094] The measured properties are expressed in the following table for each of the mixtures:

[0095] Initial endurance tests were carried out on a test machine requiring each of the tires to run in a straight line at a speed equal to the maximum speed index prescribed for said tire (speed index) under an initial load of 4000 kg, gradually increased to reduce the duration of the test.

[0096] Further endurance tests were carried out on a test machine cyclically imposing a transverse force and a dynamic overload on the tires. tests were carried out for the tires according to the invention with conditions identical to those applied to the reference tires.

[0097] The tests thus carried out showed that the distances covered during each of these tests are at least as great or even greater for the tires according to the invention than the reference tires.

[0098] A final test aimed at reproducing sidewalk impacts that could be sustained by the tires was carried out.

[0099] This test consists of a preliminary step of rolling for an hour and a half at 120 km/h, the tires being subjected to a load of 4000 kg.

[00100] The vehicle then follows a trajectory causing the tires to pass over a sidewalk with a height of between 15 cm and 20 cm.

[00101] The tires are made to follow three different trajectories in succession in order to have more or less pronounced overlaps of a few centimeters of the tire tread on the sidewalk at a speed of 10 km/h. These three successive trajectories are reproduced four times in order to thus make twelve passes on the sidewalk.

[00102] The entire test is repeated so that each tire undergoes it twice.

[00103] At the end of these tests, the tires are checked by shearography and dissected to analyze any damage. This is a visual analysis to compare any cracks and their propagation. The tires are rated and compared with each other. A rating greater than 100 corresponds to a less damaged tire. A value of 100 is assigned to the reference tire that is the most damaged.

[00104] At the end of the rolling, the tires according to the invention present less extensive damage than the reference tires.

[00105] Furthermore, rolling resistance measurements were carried out. [00106] The test results are presented in the following table. The rolling resistance measurements are expressed in kg/t, with a value of 100 being assigned to the reference tire. Values greater than 100 show better rolling resistance performance.

[00107] It emerges from these tests that the tires according to the invention make it possible to maintain, or even slightly improve, performance in terms of rolling resistance in comparison with the reference tire.

Claims

1 - A tire (1) with a radial carcass reinforcement (2) comprising a crown reinforcement (5), itself radially capped with a tread, said tread being joined to two beads (3) by means of two sidewalls, said tread comprising a tread surface (6) intended to come into contact with a roadway and form a contact surface, said tread having a plurality of cutouts consisting of at least one cutout (8, 10) comprising a radially inner part wider than the radially outer part opening onto the tread surface (6), the radially inner part and the radially outer part of said at least one cutout being connected by a widening zone, the ratio of the width (Df, D'f) measured at the bottom of said at least one cutout (8, 10) to the width (ds, d's) measured on the surface of the tread on a new tire of said at least one cutout (8, 10) being greater than 1.2, characterized in that the tread is made up of at least one first elastomeric mixture forming at least part of the rolling surface when the tire is new, in that at least part of the wall of said at least one cutout (8, 10), surrounding said widening zone, is made up of a second elastomeric mixture over a thickness of at least 1 mm and in that the second elastomeric mixture has a tear coefficient (expressed in N/m) at least 25% greater than the tear coefficient of the first elastomeric mixture, the tear coefficient being the product of the breaking force per unit of thickness (expressed in N/mm of thickness) by the elongation at break (expressed in %), measured at 100 °C. 2 - Tire (1) according to claim 1, characterized in that said at least one cutout (8) is a cutout of longitudinal orientation. 3 - Tire (1) according to claim 1, characterized in that said at least one cutout is a cutout (10) of transverse orientation. 4 - Tire (1) according to claim 1, characterized in that said at least one cutout is a cutout of oblique orientation. 5 - Tire (1) according to one of claims 1 to 4, characterized in that the entire wall of said at least one cutout (8, 10) is made of the second elastomeric mixture over a thickness of at least 1 mm. 6 - Tire (1) according to one of the preceding claims, characterized in that the thickness of the second elastomeric mixture is less than 5 mm. 7 - Tire (1) according to one of the preceding claims, characterized in that the second elastomeric mixture has a tear coefficient at least 50% greater than the tear coefficient of the first elastomeric mixture. 8 - Tire (1) according to one of the preceding claims, characterized in that the first elastomeric mixture has a maximum value of tan(ô), noted tan(ô) _ma x, less than 0.25, the measurement of tan(ô) being carried out at 60°C according to standard ASTM D 5992-96. 9 - Tire (1) according to one of the preceding claims, characterized in that the first elastomeric mixture is a rubber composition based on at least one elastomeric matrix comprising a copolymer based on styrene and butadiene and at least one reinforcing filler. 10 - Tire (1) according to one of the preceding claims, characterized in that the second elastomeric mixture is a rubber composition based on at least one elastomeric matrix comprising mainly natural rubber for at least 80% and at least one reinforcing filler.