Tyre for a heavy duty vehicle with improved grip

Abstract

The present invention relates to a tyre (1) for a trailer of a heavy duty vehicle and aims to increase its grip performance on snow, without degrading its mechanical strength with respect to scrubbing stresses. According to the invention, the tread (2), having at least one complex circumferential cut (7) in a median portion (4), comprises a first family of transverse cuts, called transverse through-sipes (81), having a thickness (E1) at most equal to 0.8 mm, a depth (P1) at least equal to 20% of the maximum cut depth (H), and being at a distance (D1), with respect to the nearest external cavity (71), at least equal to 7 mm, and / or a second family of transverse cuts, called blind transverse cuts (82), stopping in the vicinity of an external cavity (71), having a thickness (E2) at least equal to 1 mm, a depth (P2) at most equal to 20% of the maximum cut depth (H), and being at a distance (D2), with respect to the external cavity (71), at least equal to 3 mm.

Description

Tires for heavy-duty vehicles with improved grip

[0001] The present invention relates to a tire for a heavy goods vehicle, and more particularly for a heavy goods vehicle trailer, and concerns its tread, comprising more particularly complex cutouts whose shape evolves during the wear of the tread.

[0002] A tread, made of at least one rubber material, is the peripheral part of the tire, designed to wear down upon contact with the ground via a tread surface and to grip the ground. It generally includes a tread pattern, consisting of grooves separating raised elements and thus delimited by two walls of rubber material.

[0003] By definition, the circumferential or longitudinal direction is the direction of rotation of the tire, the axial or transverse direction is the direction parallel to the axis of rotation of the tire, and the radial direction is a direction perpendicular to the axis of rotation of the tire.

[0004] Any cut in the tread, having a center line that is not necessarily straight—that is, it can be wavy or zigzag—is either circumferential or transverse. By definition, the center line of a cut is the intersection of its center surface, equidistant from the sides delimiting the cut, with the tread surface. By convention, a cut is said to be circumferential when its center line has a mean direction that is approximately circumferential, that is, forming a mean angle of less than 45° with the circumferential direction. A cut is said to be transverse when its center line has a mean direction that is approximately transverse, that is, forming a mean angle of at least 45° with the circumferential direction.

[0005] As is well known, driving conditions in wet weather for a vehicle, and particularly for a heavy goods vehicle, require rapid evacuation of water present in the contact patch between the tread and the road surface. This evacuation ensures direct contact between the tread material and the road surface via the tread. Water that is not repelled Water flows towards the front or sides of the tire, or is partially captured by grooves in the tread. Water evacuation is ensured by these grooves, which form a fluid drainage network that must be effective throughout the tire's service life, from when it is new until it reaches its maximum wear. The maximum wear level, as defined by current regulations, is the point at which the tire must be removed from the vehicle for safety reasons.

[0006] Generally, the cutouts that allow water evacuation are essentially wide cutouts called grooves. A groove is wide enough that the opposing material walls defining it do not come into contact with each other when the tread passes over the contact patch, under recommended inflation and load conditions as defined, in particular, by the European Tyre and Rim Technical Organization (ETRTO) standards in its "Standards Manual 2021 - Commercial Vehicle Tyres." The compression and shear deformations of the raised elements defining the groove determine the pressures in contact with the ground and therefore the wear.Moreover, these deformations, by generating hysteretic losses in the tread material, impact rolling resistance, and therefore the vehicle's fuel consumption.

[0007] A tread pattern may also include narrow cuts or incisions. An incision has a width such that the opposing material walls delimiting it come into at least partial contact with each other as the tread passes over the contact patch, under the tire load and pressure conditions specified by TE.TRTO as defined previously. An incision does not allow water to be evacuated, but, with regard to adhesion, has an edge effect in the contact patch, which in particular helps to break up any film of water that may be present on the ground.

[0008] The trade-off for having cutouts in a tread is a reduction in the volume of material to be worn. To limit this reduction in tread material volume resulting from grooves and incisions, so-called complex cutouts have been proposed. Compared to conventional grooves, which are fully open across the tread surface, these cutouts allow for an increase in the volume of material in the tread. tread while respecting the cutout volume for water storage beyond a determined threshold, regardless of the level of tire wear.

[0009] Treads incorporating such complex cutouts have been described in particular in documents W02011039194A1, WO2011101495A1, WO2012130735A1, WO2016188956A1, WO2017174925A1, W02019008276A1, and WO2019122677A1. A complex cut typically opens discontinuously, at regular or irregular intervals, onto the new running surface. A complex cut thus presents external cavities, open to the running surface and separated from one another in the principal direction of the complex cut. The principal direction of the complex cut often, but not necessarily, corresponds to the direction of water flow within the complex cut when driving on a wet surface. This complex cutout includes, in addition to external cavities, internal cavities formed inside the tread, not directly open to the tread surface when new, and generally connected to the tread surface by incisions.These internal cavities are positioned radially and entirely within the tread surface when new, and are sandwiched between the external cavities. The internal cavities can be positioned at varying depths within the tread thickness. Furthermore, the external and internal cavities of a single complex cut are interconnected in such a way that continuous water flow within each complex cut is ensured at all tread wear levels, as with a conventional continuous groove. Conversely, the juxtaposition of unconnected internal and external cavities, which therefore do not allow fluid flow from one to the other, does not constitute a continuous groove and is thus not considered a complex cut.

[0010] The combined volume of all internal and external cavities in a complex tread pattern is generally less than that of all the grooves in a standard tread pattern, which are fully open across the new tread surface and have a depth corresponding to the maximum depth of the internal or external cavities. This results in a greater volume of material to be worn with a complex tread pattern. Such a tread pattern includes cutouts complex treads are also more rigid than an equivalent tread with conventional open grooves.

[0011] A tread can have both complex cutouts, opening onto the tread surface intermittently, and classic grooves, opening onto the tread surface along their entire length.

[0012] Considering the specific case of a heavy-duty trailer tire, its tread typically, but not always, comprises between four and seven circumferential rows of raised elements. For this type of tread, complex circumferential cutouts, such as those previously described, have been considered, particularly between two consecutive circumferential rows of raised elements not located at the edge of the tread. In other words, these complex circumferential cutouts are preferentially positioned in a central portion of the tread.

[0013] It is well known that a tire for a heavy-duty trailer is subjected to severe lateral stresses when the vehicle is required to take sharp turns, particularly on roundabouts. Under these cornering conditions, the tire is subjected to slippage forces that severely stress the tread, especially at its edges. This is why such a tread incorporates robust circumferential edge rows, often consisting of a continuous rib without any cuts.

[0014] It is also known that a tire for a heavy-duty trailer must have high grip performance, especially in severe driving conditions, such as driving on snow. One known solution is to equip the tread with transverse grooves whose orientation is optimized with respect to the direction of the desired grip. Typically, a transverse groove, conventionally forming an average angle of at least 45° with the circumferential direction of the tire, has an effective transverse component with respect to longitudinal grip and an effective longitudinal component with respect to transverse grip. The effectiveness of such a transverse groove with respect to grip is based on an indentation effect exerted by the outer edges of the groove on the ground. These transverse grooves are located in a central portion of the tread and are generally excluded from the edge rows which are subject to high mechanical stress, as described previously.

[0015] The inventors therefore set themselves the objective, for a tire for a heavy goods vehicle trailer, having a tread comprising complex evolving cutouts, of increasing its performance in adhesion, particularly on snowy ground, without degrading its mechanical resistance to slippage stresses.

[0016] This objective was achieved by a tire for a heavy goods vehicle comprising a tread, intended to come into contact with a ground via a tread surface, comprising a central portion and two lateral portions, -the central portion being separated from each lateral portion by a circumferential cut forming, with a circumferential direction of the tire, an average angle of less than 45°, -the middle portion comprising at least one complex circumferential cut, forming, with the circumferential direction of the tire, an average angle of less than 45°, -at least one complex circumferential cut comprising, in the new condition of the tire, an alternation of external cavities, open to the tread surface, and internal cavities, hidden inside the tread and connected to the tread surface by a circumferential incision, two consecutive external and internal cavities respectively being connected to each other, -the median portion comprising a plurality of transverse cutouts, having at least one transverse portion forming, with the circumferential direction of the tire, an average angle of at least 45°, -the tread having a maximum cutting depth, -the plurality of transverse cuts comprising a first family of transverse cuts, called transverse through incisions, intersecting at least one complex circumferential cut between two consecutive external cavities, and each having a thickness of at most equal to 0.8 mm and a depth of at least equal to 20% of the maximum depth of cut, each transverse through incision being at a distance, with respect to the nearest external cavity of said complex circumferential cut, of at least 7 mm, -and / or the plurality of transverse cutouts comprising a second family of transverse cutouts, called blind transverse cutouts, stopping in the vicinity of an external cavity of a complex circumferential cutout, and each having a thickness of at least 1 mm and a depth of at most 20% of the maximum depth of cutout, each blind transverse cutout being at a distance, with respect to said external cavity, of at least 3 mm.

[0017] The tread of a tire according to the invention therefore essentially comprises a central portion with at least one complex circumferential cut having a surface opening onto the tread surface of evolving shape during tire wear, said central portion comprising, in the new state of the tire, one or the other, or both families of transverse cuts described below.

[0018] The first category of transverse cuts involves transverse cuts that pass through the complex circumferential cut, meaning they cross the complex circumferential cut outside of the external cavities of the complex circumferential cut. In a new tire, an external cavity has a cross-section opening onto the tread surface, with a significant size, typically with a smallest dimension of at least 7 mm. The intermediate portion connecting two consecutive external cavities most often comprises a narrow cut, called a circumferential cut, generally no more than 1 mm thick, extending radially outward from an internal cavity. Therefore, a transverse cut intersects a circumferential cut, extending radially outward from an internal cavity and circumferentially connecting two consecutive external cavities.

[0019] A transverse cut through the tread has a thickness of at most 0.8 mm and a depth of at least 20% of the maximum cut depth. Therefore, this transverse cut is present on the tread surface until at least 20% of the tread wear is achieved. According to the invention, this transverse cut through the tread intersects the complex circumferential cut, and more specifically the circumferential cut connecting two consecutive external cavities, at a distance of at least 7 mm from the nearest external cavity.

[0020] This minimum distance of 7 mm limits the risk of initiation and propagation of cracks between the transverse incision and the nearest external cavity, during transverse stresses known as tire tread slippage, and consequently the risk of tread damage by tearing.

[0021] The second category of transverse cuts involves blind transverse cuts, stopping close to an external cavity. Compared to a through-transverse incision of the first category, a blind transverse cut has a greater thickness, at least 1 mm, and a shallower depth, at most 20% of the maximum cut depth. Furthermore, the minimum distance from the nearest external cavity, 3 mm, for a blind transverse cut is less than that of a through-transverse incision.

[0022] As with a through-cut transverse groove, this minimum distance of 3 mm limits the risk of crack initiation and propagation between the through-cut transverse groove and the nearest external cavity during transverse stresses, known as tread slippage, and therefore the risk of tread damage from tearing. It should be noted that a blind transverse cut, which is thicker and shallower than a through-cut transverse groove, can be placed closer to an external cavity because the risk of cracking it can generate is lower.

[0023] Advantageously, the plurality of transverse cuts comprising a first family of transverse through-cuts, each transverse through-cut having a thickness of at least 0.4 mm. This minimum incision thickness results essentially from a technological constraint related to the manufacture of the metal strips intended to mold said incisions.

[0024] Even more advantageously, the plurality of transverse cuts, including a first family of through-cuts, each through-cut has a depth at most equal to the maximum cut depth. The deeper the cut, the more its edge effect, with respect to grip, is retained during tire wear, thus ensuring consistent grip throughout the tire's life. However, a deeper cut contributes to a local softening of the tread pattern, which can reduce wear life, as well as potentially weakening said pattern to tear-out resistance.

[0025] Preferably, for the plurality of transverse cuts comprising a first family of through-transverse incisions, the distance between each through-transverse incision and the nearest external cavity is at least 10 mm. The greater the minimum distance between the through-transverse incision and the nearest external cavity, the lower the risk of cracking, due to a greater crack propagation distance.

[0026] Advantageously, the plurality of transverse cuts includes a second family of blind transverse cuts, each blind transverse cut having a thickness of at least 2 mm. A higher minimum thickness reduces the risk of crack initiation at the end of the blind transverse cut, in the vicinity of the external cavity.

[0027] Even more advantageously, the plurality of transverse cutouts includes a second family of blind transverse cutouts, each blind transverse cutout having a thickness of at most 5 mm, preferably at most 4 mm. The greater the thickness of the blind transverse cutout, the higher the rolling noise generated, hence the benefit of limiting this thickness.

[0028] Also advantageously, the plurality of transverse cuts includes a second family of blind transverse cuts, each blind transverse cut having a depth at least equal to 10% of the maximum cut depth. A blind transverse cut must have sufficient depth, when the tire is new, to have a significant edge effect on grip.

[0029] According to a particular embodiment, the plurality of transverse cuts comprising a second family of blind transverse cuts, at least one blind transverse cut comprises a first transverse portion, forming, with the circumferential direction of the tire, an average angle of at least 45°, and a blind end portion forming, with the circumferential direction of the tire, an average angle of less than 45°, preferably less than 30° and even more preferably less than 25°. The blind end portion is therefore circumferential, in the sense of the convention as previously defined. In this particular embodiment, the blind cross-section comprises a first transverse portion, forming a mean angle of at least 45° with the transverse direction relative to the circumferential direction. This first transverse portion is extended by a second circumferential blind end portion, forming a mean angle of less than 45° with the circumferential direction. The blind cross-section therefore has a broken mean line with two inclinations, the mean angle of the blind end portion being a design parameter for adjusting the minimum distance between the blind end and the nearest external cavity.

[0030] Advantageously, the circumferential pitch between two consecutive crosscuts of the plurality of crosscuts is at least 10 mm, preferably at least 20 mm. The circumferential pitch is the maximum circumferential distance measured between two consecutive crosscuts, not necessarily of the same type. Below 10 mm, the circumferential pitch becomes too small, and a high number of crosscuts increases, in particular, rolling noise and the risk of tread detachment.

[0031] Advantageously, the circumferential spacing between two consecutive transverse cuts in the plurality of transverse cuts is at most 50 mm, preferably at most 35 mm. Above 50 mm, the circumferential spacing becomes too large, and the small number of transverse cuts reduces, in particular, the grip on snow-covered ground.

[0032] The features of the invention are illustrated by schematic figures 1 and 2, which are not shown to scale: -Figure 1: Top view of a portion of the tread of a tire according to the invention, -Figure 2: Median circumferential section of a portion of tread of a tire according to the invention.

[0033] Figure 1 is a top view of a portion of the tread 2 of a tire 1 according to the invention, more specifically a tire of size 245 / 70R17.5. The tire 1, intended to equip a heavy goods vehicle, and more particularly a heavy goods vehicle trailer, comprises a tread 2, intended to come into contact with a ground via a tread 3, comprising a central portion 4 and two lateral portions 5. The central portion 4 is separated from each lateral portion 5 by a circumferential cut 6 forming, with a circumferential direction XX' of the tire, a mean angle of 0°, in the embodiment shown. The central portion 4 comprises a single complex circumferential cut 7, forming, with the circumferential direction XX' of the tire, a mean angle of 0°. The complex circumferential cut 7 comprises, in the new condition of the tire, an alternation of external cavities 71, open onto the tread. 3, and internal cavities 72, concealed within the tread 2 (not visible in Figure 1) and connected to the tread surface 3 by a circumferential incision 721, two consecutive external cavities 71 and internal 72 being connected to each other. The central portion 4 comprises a plurality of transverse cutouts (81, 82), having at least one portion forming, with the circumferential direction XX' of the tire, a mean angle of 60°. The tread 2 has a maximum cutout depth H (not visible in Figure 1) of 13 mm. According to the invention, the plurality of transverse cutouts (81, 82) comprises a first family of transverse cutouts, called through transverse incisions 81, intersecting the complex circumferential cutout 7 between two consecutive external cavities 71, and each having a thickness El equal to 0.6 mm and a depth PI of 4 mm, representing 31% of the maximum depth of cut H. Each transverse incision 81 is at a distance D1, relative to the nearest external cavity 71 of said complex circumferential cut 7, equal to 10 mm. The plurality of transverse cuts (81, 82) also includes a second family of transverse cuts, called blind transverse cuts 82, stopping in the vicinity of an external cavity 71 of the complex circumferential cut 7, and each having a thickness E2 equal to 2 mm and a depth P2 equal to 2 mm, representing 16% of the maximum depth of cut H. Each blind transverse cut 82 is at a distance D2, relative to said external cavity 71, equal to 3 mm.Each blind transverse cut 82 comprises a first transverse portion 821, forming, with the circumferential direction XX' of the tire, a mean angle B1 equal to 60°, and a blind end portion 822 forming, with the circumferential direction XX' of the tire, a mean angle B2 equal to 25°. The circumferential pitch Pc between two consecutive transverse cuts of the plurality of. transverse cuts (81, 82), respectively between a transverse through cut 81 and a transverse blind cut 82, is equal to 35 mm.

[0034] Figure 2 is a median circumferential section of a portion of the tread 2 of a tire 1 according to the invention. This median circumferential section is made along the cutting plane AA of Figure 1, corresponding to the median circumferential plane XZ, passing through the middle of the tread, and intersects the complex circumferential cut 7 along its midline. The complex circumferential cut 7, located in the median portion 4 of the tread 2, comprises, in the new condition of the tire, an alternation of external cavities 71, open onto the tread surface 3, and internal cavities 72, concealed within the tread 2 and connected to the tread surface 3 by a circumferential incision 721, with two consecutive external cavities 71 and internal 72 being connected to each other.In the embodiment shown, the complex circumferential cut 7 extends radially from the rolling surface 3 to a depth H, corresponding to the maximum cutting depth. In Figure 2, two blind transverse cuts 82 with a depth P2 are shown in dashed lines (because they do not open in the XZ plane). Also shown in cross-section are through transverse incisions 81, having a thickness El and a depth PL.

[0035] The inventors have more particularly studied this invention for a tire I of size 245 / 70R17.5, intended to equip the axles of a heavy goods vehicle trailer, and having a maximum load capacity of 2725kg for an inflation pressure of 9 bars.

[0036] Table 1 below summarizes the characteristics of tire I according to the invention, studied by the inventors: [Table 1]

[0037] The results of the performance tests, carried out on the previously described 245 / 70 R 17.5 test size, showed compliance with the expected results. In particular, grip on snow, tested according to the method described in Annex 7 of UN 117 regulation of the United Nations Economic Commission for Europe, is significantly improved compared to the Michelin reference tire 245 / 70 R 17.5 X MULTI T2, with a 35% increase in traction. Furthermore, wet grip performance, measured according to the method described in Annex 5 of UN 117 regulation, and rolling noise performance, measured according to the method described in Annex 3 of UN 117 regulation, are at the same level as those of the reference tire. Finally, tear resistance performance is currently being confirmed through customer road tests.

Claims

Tyre Claims (1) for a heavy goods vehicle comprising a tread (2), intended to come into contact with a ground by means of a tread surface (3), comprising a central portion (4) and two lateral portions (5), -the median portion (4) being separated from each lateral portion (5) by a circumferential cut (6) forming, with a circumferential direction (XX') of the tire, a mean angle of less than 45°, -the median portion (4) comprising at least one complex circumferential cutout (7), forming, with the circumferential direction (XX') of the tire, a mean angle of less than 45°, -at least one complex circumferential cut (7) comprising, in the new condition of the tire, an alternation of external cavities (71), open to the tread surface (3), and internal cavities (72), hidden inside the tread (2) and connected to the tread surface (3) by a circumferential incision (721), two consecutive external (71) and internal (72) cavities being connected to each other, -the middle portion (4) comprising a plurality of transverse cuts (81, 82), having at least one transverse portion forming, with the circumferential direction (XX') of the tire, a mean angle (T, Bl) of at least 45°, -the tread (2) having a maximum cutting depth (H), characterized in that the plurality of transverse cuts (81, 82) comprises a first family of transverse cuts, called transverse through cuts (81), intersecting at least one complex circumferential cut (7) between two consecutive external cavities (71), and each having a thickness (El) at most equal to 0.8 mm and a depth (PI) at least equal to 20% of the maximum cutting depth (H), each transverse through incision (81) being at a distance (Dl), relative to the nearest external cavity (71) of said complex circumferential cut (7), at least equal to 7 mm, and / or in that the plurality of transverse cuts (81, 82) comprises a second family of transverse cuts, called blind transverse cuts (82), stopping in the vicinity of an external cavity (71) of a complex circumferential cut (7), and each having a. thickness (E2) at least equal to 1 mm and a depth (P2) at most equal to 20% of the maximum cutting depth (H), each blind transverse cut (82) being at a distance (D2), relative to said external cavity (71), of at least equal to 3 mm. Pneumatic (1) according to claim 1, the plurality of transverse cuts (81, 82) comprising a first family of through transverse incisions (81), in which each through transverse incision (81) has a thickness (E1) at least equal to 0.4 mm. Pneumatic (1) according to claim 1 or 2, the plurality of transverse cuts (81, 82) comprising a first family of through transverse incisions (81), in which each through transverse incision (81) has a depth (P1) at most equal to the maximum cutting depth (H).Pneumatic (1) according to any one of claims 1 to 3, the plurality of transverse cutouts (81, 82) comprising a first family of through transverse cutouts (81), wherein the distance (Dl) between each through transverse cutout (81) and the nearest external cavity (71) is at least 10 mm. Pneumatic (1) according to any one of claims 1 to 4, the plurality of transverse cutouts (81, 82) comprising a second family of blind transverse cutouts (82), wherein each blind transverse cutout (82) has a thickness (E2) at least 2 mm. Pneumatic (1) according to any one of claims 1 to 5, the plurality of transverse cutouts (81, 82) comprising a second family of blind transverse cutouts (82), in which each blind transverse cutout (82) has a thickness (E2) at most equal to 5 mm, preferably at most equal to 4 mm.Pneumatic (1) according to any one of claims 1 to 6, the plurality of transverse cutouts (81, 82) comprising a second family of blind transverse cutouts (82), wherein each blind transverse cutout (82) has a depth (P2) at least equal to 10% of the maximum cutout depth (H). Pneumatic (1) according to any one of claims 1 to 7, the plurality of transverse cutouts (81, 82) comprising a second family of blind transverse cutouts (82), wherein at least one blind transverse cutout (82) comprises a first transverse portion (821), forming, with the circumferential direction (XX') of the. pneumatic, an average angle (Bl) of at least 45°, and a blind end portion (822) forming, with the circumferential direction (XX') of the pneumatic, an average angle (B2) of less than 45°, preferably less than 30° and even more preferably less than 25°.

9. Pneumatic (1) according to any one of claims 1 to 8, wherein the circumferential pitch (Pc) between two consecutive transverse cutouts of the plurality of transverse cutouts (81, 82) is at least 10 mm, preferably at least 20 mm.

10. Pneumatic (1) according to any one of claims 1 to 9, wherein the circumferential pitch (Pc) between two consecutive transverse cutouts of the plurality of transverse cutouts (81, 82) is at most equal to 50 mm, preferably at most equal to 35 mm.

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