Tire for agricultural vehicle including improved tread

Abstract

The present invention relates to a tyre for an agricultural vehicle, in particular to its tread, and aims to improve its field traction ability on soft ground. According to the present invention, the tyre comprises tread pattern elements (221) distributed circumferentially, the tyre having a nominal cross-sectional width L, a central tread portion (211), the central tread portion (211) being centred on the equatorial plane (E) of the tyre and having an axial width L1 which is not greater than 0.15*L and not less than 0.35*L, the tread pattern elements (221) being separated in pairs by transverse grooves (231) which form an angle of at least 30° with the circumferential direction (XX’), and the local groove volume ratio TEVL1 of the central tread portion (211) not exceeding 15%, the local groove volume ratio TEVL1 being defined as the ratio of the volume VC1 of the transverse grooves (231) to the total volume V1 of the central tread portion (21) between the load-bearing surface (24) and the tread surface (25).

Description

Technical Field

[0001] The present invention relates to a tire for an agricultural vehicle (such as an agricultural tractor or an agricultural industrial vehicle), and more particularly to the tread of said tire. Background Art

[0002] In some standards (such as the ETRTO or "European Tyre and Rim Technical Organization" standards, in its "Standard Manual - 2018" in the chapter called "Agricultural Equipment Tyres"), the dimensional specifications (section width, overall diameter, diameter and width of the mounting rim) and operating conditions (load, speed, pressure) of tires for agricultural vehicles are defined. As an example, a radial tire for a trailing wheel of an agricultural tractor is designed to be mounted on a rim with a diameter generally between 16 inches and 46 inches (or even 54 inches). It is designed to operate on agricultural tractors with a power between 50 CV and greater than 250 CV (up to 550 CV) and is capable of running at a maximum speed of 65 km / h. For this type of tire, the minimum recommended inflation pressure corresponding to the indicated load capacity is generally at most equal to 400 kPa, but for "IF" or "Improved Flexion" tires, it may be reduced to 240 kPa, and for "VF" or "Very High Flexion" tires, it may even be reduced to 160 kPa.

[0003] Like any tire, a tire for an agricultural vehicle includes a tread, which is intended to come into contact with the ground through the tread surface (the surface in contact with the solid ground), and whose two axial ends are connected to two beads by two sidewalls, said beads providing a mechanical connection between the tire and the rim intended to mount said tire.

[0004] Hereinafter, the circumferential (or longitudinal) direction, the axial (or transverse) direction, and the radial direction respectively denote the direction tangent to the tread surface and oriented in the rotational direction of the tire, the direction parallel to the rotational axis of the tire, and the direction perpendicular to the rotational axis of the tire. The radial (or meridian) plane is defined by the radial direction and the axial direction and contains the rotational axis of the tire. The circumferential plane is defined by the radial direction and the circumferential direction and is thus perpendicular to the rotational axis of the tire. The circumferential plane passing through the middle of the tread is called the equatorial plane.

[0005] The tread of a tire for an agricultural vehicle generally includes a plurality of raised elements (called tread pattern elements), said raised elements extending radially outward from the support surface up to the tread surface and being separated from each other by voids.

[0006] The proportion of voids is typically quantified by the total volume void ratio TEV, which is defined as the ratio between the volume of voids VC and the total volume V of the tread assuming no voids, which corresponds to the geometric volume delimited by the bearing surface and the tread surface. Since the tread surface varies according to the degree of wear of the tread, the total volume void ratio TEV typically (but not necessarily) varies with the degree of wear. Thus, the total volume void ratio TEV can be defined both when the tire is in a brand-new state or in a given state of wear. For example, the total volume void ratio TEV of a tire for a trailing wheel of an agricultural tractor in a brand-new state is typically at least equal to 50% and more typically at least equal to 60%. In the following, the expression "total volume void ratio TEV" implicitly means "total volume void ratio TEV when the tire is in a brand-new state".

[0007] It is also possible to define a local volume void ratio TEVL for any tread portion extending circumferentially over the entire circumference of the tire and axially from a first circumferential plane to a second circumferential plane, the distance between these two circumferential planes representing the axial width (more simply called the width) of the tread portion. The local volume void ratio TEVL is defined as the ratio between the volume of voids VCL and the total volume VL of the tread portion assuming no voids, which corresponds to the geometric volume delimited by the bearing surface, the tread surface and the two circumferential planes. As with the total volume void ratio TEV, the local volume void ratio TEVL can be defined both when the tire is in a brand-new state or in a given state of wear. In the following, the expression "local volume void ratio TEVL" implicitly means "local volume void ratio TEVL when the tire is in a brand-new state".

[0008] The geometric characteristics of each tread pattern element can consist of a radial height H in the radial direction, an axial width A in the axial direction and a circumferential length B in the circumferential direction. These three dimensions H, A and B are average values and it is known that these dimensions can vary depending on the measurement points chosen on the tread pattern element. For the axial width A and the circumferential length B, due to the presence of a taper, they may increase from the tread surface towards the bearing surface at the bottom of the void. For the radial height H, for a radial tire for a trailing wheel of an agricultural tractor, the radial height H of the tread pattern element is typically at least equal to 50 mm and more typically at least equal to 60 mm. Based on these three dimensions H, A and B, for a given tread pattern element, a circumferential aspect ratio H / B, an axial aspect ratio H / A and a surface area aspect ratio B / A can be defined.

[0009] The tread for an agricultural vehicle generally includes tread pattern elements in the form of lugs. The lugs are generally parallelepipeds, continuous or discontinuous, and have an elongated shape composed of at least one straight portion or curved portion. The lugs are separated from adjacent lugs by voids or grooves. The lugs extend axially from the intermediate region of the tread to the axial ends or shoulders of the tread. The lugs include a contact surface, a leading edge surface, a trailing edge surface, and two side surfaces. The contact surface is located in the tread surface and is intended to be in full contact with the ground. The leading edge surface intersects the tread surface, and the edge formed by the intersection is intended to be the first part to contact the ground. The trailing edge surface intersects the tread surface, and the edge formed by the intersection is intended to be the last part to contact the ground.

[0010] The lugs are distributed circumferentially at a constant or variable pitch and are generally arranged on both sides of the tire equatorial plane to form a V-shaped pattern. The apex of the V-shaped pattern (or chevron pattern) is intended to be the first part to enter the ground contact patch. By rotating one half of the tread relative to the other half around the tire axis, the lugs generally exhibit symmetry with respect to the tire equatorial plane and generally have a circumferential offset between two rows of lugs.

[0011] The radial tire for an agricultural vehicle further includes reinforcement members, which are composed of a crown reinforcement member located radially inside the tread and a carcass reinforcement member located radially inside the crown reinforcement member.

[0012] The carcass reinforcement member of the radial tire for an agricultural vehicle includes at least one carcass ply connecting the two beads to each other. The reinforcement bodies of the carcass ply are substantially parallel to each other and form an angle between 75° and 105°, preferably between 85° and 95°, with the circumferential direction. The carcass ply includes reinforcement bodies (usually fabric reinforcement bodies), which are coated with an elastomer or an elastomeric polymer material (referred to as a coated compound).

[0013] The crown reinforcement member of the radial tire for an agricultural vehicle includes circumferentially extending crown plies (located radially outside the carcass reinforcement member) that are superimposed. Each crown ply is composed of reinforcement bodies, which are coated with an elastomer compound and are parallel to each other. When the crown ply reinforcement bodies form an angle less than 10° with the circumferential direction, they are referred to as circumferential or substantially circumferential and have a hoop effect that restricts the radial deformation of the tire. When the crown ply reinforcement bodies form an angle of at least 10° and generally at most 30° with the circumferential direction, they are referred to as angled reinforcement bodies and have the effect of reacting to lateral loads applied to the tire parallel to the axial direction. The crown ply reinforcement bodies can be made of a polymer material of the fabric type (such as polyester (such as polyethylene terephthalate (PET)), aliphatic polyamide (such as nylon), aromatic polyamide (such as aramid), or rayon), or can be made of a metallic material (such as steel).

[0014] Tires for agricultural vehicles are designed to operate on various types of ground, such as the more or less firm soil of fields, untreated tracks leading to the fields, and the asphalt surfaces of roads. Given the diversity of use in fields and on roads, tires for agricultural vehicles need to provide a performance compromise among field traction on soft ground, chip resistance, road wear resistance, forward running resistance, and vibration comfort on the road (this list is not exhaustive).

[0015] One fundamental problem with using tires in the field is to limit as much as possible the degree to which the tires compact the soil, which can impede crop growth.

[0016] This is the reason for the development of low-pressure (and thus high-flexion) tires in the agricultural field. Thus, the ETRTO standard can distinguish between IF (Improved Flexion) tires (whose minimum recommended inflation pressure is usually equal to 240 kPa) and VF (Very High Flexion) tires (whose minimum recommended inflation pressure is usually equal to 160 kPa). According to this standard, for an inflation pressure equal to 160 kPa, IF tires have a load-carrying capacity 20% higher than that of standard tires, and VF tires have a load-carrying capacity 40% higher.

[0017] However, using low-pressure tires has a negative impact on the handling in the field. Thus, the reduction of the inflation pressure results in a decrease in the lateral stiffness and cornering stiffness of the tire, thus reducing the lateral thrust of the tire and thus resulting in poor handling under lateral loads.

[0018] One solution to re-establish the correct lateral thrust is to laterally strengthen the crown reinforcement of the tire by replacing the crown ply with a fabric reinforcement with a crown ply with a metal reinforcement. Thus, for example, a crown reinforcement including 6 crown plies with a fabric reinforcement of the rayon type has been replaced with a crown reinforcement including 2 crown plies with a steel reinforcement. Thus, document EP 2934917 describes an IF tire that includes a crown reinforcement that includes at least two crown plies with a metal reinforcement, the crown reinforcement being combined with a carcass reinforcement including at least two carcass plies with a fabric reinforcement. Summary of the Invention

[0019] Then, the objective that the inventor set for himself was to improve the field traction capacity on soft ground of tires commonly used for agricultural vehicles, in particular of tires for agricultural vehicles that include a crown reinforcement with a metal reinforcement and / or operate at low pressure (such as IF (Improved Flexion) tires or VF (Very High Flexion) tires) on soft ground.

[0020] According to the present invention, this object has been achieved by a tire for an agricultural vehicle, the tire having a nominal cross-sectional width L and comprising, radially from the outside to the inside, a tread and a crown reinforcement:

[0021] - The tread comprises tread pattern elements which are separated from one another by voids and extend radially outwards from the bearing surface to the tread surface,

[0022] - The tread has a total volume void fraction TEV which is defined as the ratio of the volume VC of the voids to the total volume V of the tread which would be assumed to be void-free between the bearing surface and the tread surface,

[0023] - The circumferential aspect ratio H / B of each tread pattern element is at most equal to 1.5, where H is the average radial height between the bearing surface and the tread surface and is at least equal to 20 mm, and B is the average circumferential length,

[0024] - The tread comprises a central portion and two intermediate portions, the central portion being centered on the equatorial plane of the tire and having an axial width L1 which is at least equal to 0.15*L and at most equal to 0.35*L, each intermediate portion extending axially outwards from the central portion for an axial distance D2 equal to 0.3*L, the axial distance D2 being measured from the equatorial plane,

[0025] - The central portion and each intermediate portion comprise tread pattern elements which are separated in pairs by transverse voids which form an angle of at least 30° with the circumferential direction of the tire,

[0026] - The crown reinforcement comprises at least two crown plies, each crown ply comprising reinforcing bodies which are parallel to one another, the reinforcing bodies being coated with an elastic material, cross over from one ply to the other and form an angle of at least 10° with the circumferential direction,

[0027] - The local volume void fraction TEVL1 of the central portion is at most equal to 15%, the local volume void fraction TEVL1 being defined as the ratio of the volume VC1 of the transverse voids to the total volume V1 of the central portion between the bearing surface and the tread surface.

[0028] According to a first feature of the invention, the tread comprises tread pattern elements, the circumferential aspect ratio H / B of which is at most equal to 1.5, H being the average radial height between the bearing surface and the tread surface and being at least equal to 20 mm, and B being the average circumferential length. For a given tread pattern element, the radial height between the bearing surface and the tread surface is substantially constant, so that the average radial height H is equal to this substantially constant radial height. In contrast, since the front (or leading) face and the rear (or trailing) face of the tread pattern element are inclined in the running direction, the circumferential length of the tread pattern element can vary substantially as a function of the depth at which it is measured; it is therefore necessary to define an average circumferential length B. The circumferential aspect ratio H / B, which geometrically characterizes the circumferential stiffness of the tread pattern element, is not necessarily constant and can vary between two tread pattern elements.

[0029] According to a second feature of the invention, the tread also comprises a central part and two intermediate parts, the central part being centered on the equatorial plane of the tyre and having an axial width L1 which is at least equal to 0.15*L and at most equal to 0.35*L, and each intermediate part extending axially outwards from the central part by an axial distance D2 equal to 0.3*L, the axial distance D2 being measured from the equatorial plane. The nominal section L of the tyre is the "design section width" defined in the ETRTO standard.

[0030] Furthermore, according to a third feature of the invention, the central part and each intermediate part comprise tread pattern elements distributed circumferentially, the tread pattern elements being separated in pairs by transverse voids which form an angle of at least 30° with the circumferential direction of the tyre. These voids are described as transverse because their direction forms a large enough angle with the circumferential direction so that they cannot be described as circumferential or longitudinal. In other words, their direction varies between an inclined position and a transverse position parallel to the axis of rotation of the tyre. By definition, the angle formed by the voids is the angle formed by their middle surface, which is generally formed by a plane perpendicular to the tread surface. If this middle surface is formed by a series of planes, each of these planes forms an angle of at least 30°.

[0031] According to a fourth feature of the invention, the crown reinforcement comprises at least two crown plies, each crown ply comprising reinforcing bodies which are parallel to one another, the reinforcing bodies being coated with an elastic material, cross from one ply to the other, and form an angle of at least 10° with the circumferential direction.

[0032] According to a fifth and final feature of the invention, the local volume void ratio TEVL1 of the central part is at most equal to 15%, which means a small volume void ratio indicating the presence of a small void volume.

[0033] For the central portion of the tread, due to its relatively small volume void ratio and the limited circumferential aspect ratio of the tread pattern elements, the combination of these features particularly ensures a high degree of circumferential stiffness, and due to the presence of transverse voids acting as hinges, circumferential flattening is promoted upon entry into the ground contact surface; thus, the traction capacity is improved. This is because a segmented central portion composed of blocks separated by transverse voids has better circumferential flattening compared to a continuous central portion composed of at least one continuous rib without transverse voids.

[0034] Advantageously, the transverse voids of the central tread portion form an angle of at least 60° with the circumferential direction of the tire. An angle of at least 60° is characteristic of substantially transverse voids, which act as substantially transverse hinges and even further promote circumferential flattening of the tread in its central portion.

[0035] Preferably, the transverse voids of the central tread portion are transverse grooves that can close upon entry into the ground contact surface when the tire is in motion. Transverse grooves are transverse voids with a very small width as they can close upon entry into the ground contact surface. More specifically, these grooves are open upon entry into and exit from the ground contact surface and promote circumferential flattening of the tread in these regions. In the ground contact surface, these grooves close and form a continuous rib, which has a higher degree of circumferential stiffness compared to the circumferential stiffness obtained with still-open transverse voids. The ability of the transverse grooves to close upon entry into the ground contact surface is determined on a tire subjected to the pressure and load conditions defined by the ETRTO standard.

[0036] Advantageously, the transverse voids of each intermediate portion form an angle of at least 60° with the circumferential direction. An angle of at least 60° is characteristic of substantially transverse voids, which act as substantially transverse hinges and even further promote circumferential flattening of the tread in its intermediate portion.

[0037] According to a preferred embodiment, each tread pattern element of the central portion has an average radial height H1, an average circumferential length B1, and a circumferential aspect ratio H1 / B1, and each tread pattern element of each intermediate portion has an average radial height H2, an average circumferential length B2, and a circumferential aspect ratio H2 / B2, and the circumferential aspect ratio H1 / B1 is strictly greater than the circumferential aspect ratio H2 / B2. Thus, in this embodiment, there is a difference in circumferential aspect ratio between the tread pattern elements of the central portion and the tread pattern elements of each intermediate portion. The fact that the circumferential aspect ratio H1 / B1 of the tread pattern elements of the central portion is strictly greater than the circumferential aspect ratio H2 / B2 of the tread pattern elements means that the degree of circumferential stiffness of the tread pattern elements of the central portion is less than the degree of circumferential stiffness of the tread pattern elements of the intermediate portion. The relatively high degree of circumferential stiffness of the tread pattern elements of the intermediate portion advantageously ensures the traction ability of the tire when it travels on a relatively sticky ground (such as straw stubble) in the field. It should be noted that the circumferential aspect ratio defining this circumferential stiffness results from a compromise between the field traction ability on dry ground, the field traction ability on waterlogged ground (obtained through the volume void ratio adapted to each intermediate portion), and the service life of the tread (obtained by having a sufficient average radial height H2 of the tread pattern elements of the intermediate portion).

[0038] According to a preferred variant of the above preferred embodiment, the circumferential aspect ratio H2 / B2 of each tread pattern element (222) of each intermediate portion (212) is at most equal to 0.6. This condition means that, for tread pattern elements that are very elongated in the circumferential direction, when the average radial height H2 is at most equal to 60% of the average circumferential length B2, the circumferential stiffness of the tread pattern elements of the intermediate portion necessary to ensure sufficient traction ability of the tire when it travels on a relatively sticky ground in the field is obtained.

[0039] Also preferably, the local volume void ratio TEVL2 of the central tread portion is at most equal to 45%, the central tread portion being centered on the equatorial plane of the tire and having an axial width L2 equal to 2*D2 = 0.40*L, the local volume void ratio TEVL2 being defined as the ratio of the volume VC2 of the voids to the total volume V2 between the support surface and the tread surface of the central tread portion. In other words, the volume void ratio TEVL2 is greater than the volume void ratio TEVL1 of the central portion, but does not exceed 45%, the volume void ratio TEVL2 being determined on the central portion having a width L2 equal to 40% of the nominal section width L of the tire and consisting of the central portion and two intermediate portions as described above. Thus, the void volume increases with the increase in the distance from the equatorial plane of the tire, which can particularly ensure satisfactory traction on a wet ground with low viscosity.

[0040] According to the first embodiment variant, the tread only includes lateral voids. Thus, the tread does not include any circumferential voids.

[0041] According to the second embodiment variant, the central tread portion is axially delimited on both sides by circumferential voids. Circumferential voids are understood to mean strictly circumferential voids, or voids inclined at an inclination angle of at most equal to 45° with respect to the circumferential direction.

[0042] Advantageously, the average radial height H of each tread pattern element is at most equal to 55 mm. When above this value, the circumferential slenderness ratio H / B of the tread pattern element has a risk of being higher than 0.8. As a result, the buckling side deflection stiffness and shear stiffness of each tread pattern element in the circumferential direction become too low to ensure sufficient overall circumferential stiffness for the desired traction level, especially in the central portion of the tread. In addition, an excessive radial height will adversely affect the heat level at the tire crown and thus adversely affect its durability.

[0043] Also advantageously, the total volume void ratio TEV of the tread is at most equal to 56%. When above this value of the total volume void ratio TEV, the void volume becomes too high. Accordingly, the volume of the material becomes too low to ensure a sufficient service life in terms of wear.

[0044] In a particular embodiment of the lateral voids, at least a part of the lateral voids includes at least one chamfer that opens on the tread surface, forms an angle D with the radial direction of at least equal to 30° and at most equal to 70°, and has a radial height C of at least equal to 3 mm and at most equal to 10 mm. Generally, all lateral voids include at least one chamfer. According to the first variant, all lateral voids include a single chamfer. According to the second variant, all lateral voids include two facing chamfers. The presence of the chamfer helps to significantly improve the traction of the tread.

[0045] According to a preferred embodiment of the tire crown reinforcement, the crown reinforcement includes a crown ply containing a metallic reinforcement, preferably including at most two crown plies containing a metallic reinforcement. The presence of the metallic reinforcement enables obtaining the desired crown stiffness with a limited number of crown plies (which means a limited crown thickness). Compared with the conventional crowns of the prior art, this results in a lower degree of buckling stiffness of the crown, thus promoting the flattening of the tire. As a result, the area of the contact patch in contact with the ground increases, thereby on the one hand reducing the ground pressure and thus the compaction of the ground, and on the other hand increasing the traction capacity.

[0046] In a first preferred tire portion, the tire for an agricultural vehicle is an "IF" or "Improved Flexion" tire within the meaning of the "ETRTO" or "European Tyre and Rim Technical Organization" standard (in its "Standard Manual - 2018" in the chapter called "Agricultural Equipment Tires"), and the load - carrying capacity of said tire is 20% higher than that of a standard tire at the same pressure.

[0047] In a second preferred tire portion, the tire for an agricultural vehicle is a "VF" or "Very High Flexion" tire within the meaning of the "ETRTO" or "European Tyre and Rim Technical Organization" standard (in its "Standard Manual - 2018" in the chapter called "Agricultural Equipment Tires"), and the load - carrying capacity of said tire is 40% higher than that of a standard tire at the same pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The features of the present invention are illustrated by means of schematic Figures 1 to 12 drawings which are not to scale:

[0049] - Figure 1 : Perspective view of a tire for an agricultural vehicle according to a first embodiment variant of the present invention.

[0050] - Figure 2 : Detail perspective view of the tread of a tire for an agricultural vehicle according to a first embodiment variant of the present invention ( Figure 1 detail C1).

[0051] - Figure 3 : Top view of a tire for an agricultural vehicle according to a first embodiment variant of the present invention.

[0052] - Figure 4 : Detail top view of the tread of a tire for an agricultural vehicle according to a first embodiment variant of the present invention ( Figure 3 detail D1).

[0053] - Figure 5 : Circumferential cross - section of the tread of a tire for an agricultural vehicle according to a first embodiment variant of the present invention ( Figure 3 cross - section A - A).

[0054] - Figure 6 : Meridional half - cross - section of a tire for an agricultural vehicle according to the present invention ( Figure 3 cross - section B - B).

[0055] - Figure 7 : Perspective view of a tire for an agricultural vehicle according to a second embodiment variant of the present invention.

[0056] - Figure 8 : Detail perspective view of the tread of a tire for an agricultural vehicle according to a second embodiment variant of the present invention (Figure 7 Detail C2).

[0057] - Figure 9 : Top view of a tyre for an agricultural vehicle according to a variant of the third embodiment of the invention.

[0058] - Figure 10 : Detail perspective view of the tread of a tyre for an agricultural vehicle according to a variant of the third embodiment of the invention ( Figure 9 Detail D3).

[0059] - Figure 11 : Cross-sectional view of a lateral void including a single chamfer.

[0060] - Figure 12 : Cross-sectional view of a lateral void including two facing chamfers. Detailed description

[0061] Figures 1 to 5 Shows a tyre 1 for an agricultural vehicle according to a variant of the first embodiment of the invention. The tread 2 includes tread pattern elements 22 which are separated from each other by voids 23 and extend radially outwards from a bearing surface 24 to a tread surface 25 (the bearing surface and the tread surface as Figure 6 shown). In this first embodiment variant, the tread 2 only includes lateral voids 231 ( Figure 2 、 Figure 4 and Figure 5 ). In the central tread portion 211, the lateral voids 231 are lateral grooves having a width I which are capable of closing when the tyre enters the ground contact surface during travel when the tyre is subjected to the pressure and load conditions defined by the ETRTO standard ( Figure 4 and Figure 5 ). The tread pattern elements 221 of the central portion 211 of the tread separated by said lateral grooves have an average radial height H and an average circumferential length B ( Figure 5 ), and thus have a circumferential aspect ratio H / B.

[0062] Figure 6 Is a meridian half-section of a tyre for an agricultural vehicle according to the invention. This radial section is along Figure 3is formed by plane B-B. This figure shows a tire 1 for an agricultural vehicle, the tire 1 having a nominal sectional semi-width L / 2 and including, radially from the outside to the inside, a tread 2 and a crown reinforcement 3. The tread 2 includes tread pattern elements 22 which are separated from each other by voids 23 and extend radially outwards from a support surface 24 to a tread surface 25. The overall volume void ratio TEV of the tread 2 is at least equal to 35%, the overall volume void ratio TEV being defined as the ratio of the volume VC of the voids 23 to the overall volume V of the tread 2 which is assumed to have no voids between the support surface 24 and the tread surface 25. The circumferential aspect ratio H / B of each tread pattern element 22 is at most equal to 0.8, H being the average radial height between the support surface 24 and the tread surface 25 and at least equal to 20 mm, and B being the average circumferential length (not shown) of the tread pattern element 22. The crown reinforcement 3 includes two crown layers 31, 32, each crown layer 31, 32 including reinforcement bodies which are parallel to each other, preferably metallic, and which are coated with an elastic material, cross from one layer to the other, and form an angle of at least 10° with the circumferential direction XX' of the tire. The carcass reinforcement 4 is located radially inside the crown reinforcement 3. According to the invention, the central tread portion 211 includes tread pattern elements 221 (identified by the general reference numeral 22 of the tread pattern element in Figure 6 ), which are circumferentially distributed, the central tread portion 211 being centered on the equatorial plane E of the tire and having an axial width L1 of at least equal to 0.15*L and at most equal to 0.35*L, the tread pattern elements 221 being spaced apart in pairs by transverse voids 231 ( Figure 6 not shown in), the transverse voids 231 forming an angle of at least equal to 30° with the circumferential direction XX', and the local volume void ratio TEVL1 of the central tread portion 211 being at most equal to 15%, the local volume void ratio TEVL1 being defined as the ratio of the volume VC1 of the transverse voids 231 to the overall volume V1 of the central tread portion 211 between the support surface 24 and the tread surface 25. Figure 6 shows the central tread half with width L1 / 2. The tread 2 also includes two intermediate portions 212, each intermediate portion 212 extending axially outwards from the central portion 211 for an axial distance D2 equal to 0.3*L, the axial distance D2 being measured from the equatorial plane E. Figure 6Shows a single intermediate tread portion 212 having a width D2 - L1 / 2. Preferably, the local volume void fraction TEVL2 of the central tread portion 20 is at most equal to 45%, the central tread portion 20 being centered on the equatorial plane of the tire and having an axial width L2 equal to 2*D2 = 0.60*L, and being composed of the entire central portion 211 and two intermediate portions 212, the local volume void fraction TEVL2 being defined as the ratio of the volume of voids VC2 to the total volume V2 of the central tread portion 20 between the support surface 24 and the tread surface 25. Figure 6 Shows a central tread half - portion having a width D2 = L2 / 2.

[0063] Figure 7 and Figure 8 Show an overall view of a tire 1 for an agricultural vehicle and a view of detail C2 according to a variant of a second embodiment of the invention, respectively. The tread 2 includes tread pattern elements 22, which are separated from each other by voids 23. In this variant of the second embodiment, the tread 2 includes a central portion 211, the central portion 211 including circumferentially - distributed tread pattern elements 221, which are separated from each other by lateral - groove - type voids 231 that are capable of closing when they enter the ground - contacting ground surface during the tire's travel. In addition, the central tread portion 211 is axially bounded on both sides by circumferential voids 233.

[0064] Figure 9 and Figure 10Shows respectively an overall view of a tire for an agricultural vehicle and a view of detail D3 according to a variant of the third embodiment of the present invention. In this variant of the third embodiment, the tread 2 of a tire having a nominal cross-sectional width L includes a central portion 211 and two intermediate portions 212. The central portion 211 is centered on the equatorial plane E of the tire and has an axial width L1 of at least equal to 0.15*L and at most equal to 0.35*L. Each intermediate portion 212 extends axially outwards from the central portion 211 for an axial distance D2 equal to 0.3*L, the axial distance D2 being measured from the equatorial plane E. The assembly consisting of the central portion 211 and the two intermediate portions 212 forms a central portion 20, which is centered on the equatorial plane E of the tire and has an axial width L2 equal to 2*D2 = 0.60*L. The central portion 211 includes circumferentially distributed tread pattern elements 221, which are separated in pairs by transverse voids 231, the transverse voids 231 forming an angle of at least equal to 30° (in the present case, at least equal to 60°) with the circumferential direction XX' of the tire. The transverse voids 231 of the central portion 211 are transverse sipes, which are capable of closing when they enter the ground contact surface during tire travel. The circumferential aspect ratio H1 / B1 of each tread pattern element 221 is at most equal to 1.5, where H1 is the average radial height between the support surface and the tread surface and is at least equal to 20 mm ( Figure 8 and Figure 9 not shown), and B1 is the average circumferential length. B1 is measured at the tread surface because the leading and trailing faces of the tread pattern element 221 are substantially radial. According to the present invention, the local volume void ratio TEVL1 of the central portion 211 is at least equal to 15%, the local volume void ratio TEVL1 being defined as the ratio of the volume VC1 of the transverse voids 231 to the total volume V1 of the central portion 211 between the support surface and the tread surface. Each central portion 212 includes circumferentially distributed tread pattern elements 222, which are separated in pairs by transverse voids 232, the transverse voids 232 forming an angle of at least equal to 30° (in the present case, at least equal to 60°) with the circumferential direction XX' of the tire. The circumferential aspect ratio H2 / B2 of each tread pattern element 222 is at most equal to 1.5, where H2 is the average radial height between the support surface and the tread surface and is at least equal to 20 mm ( Figure 8 and Figure 9 not shown), and B2 is the average circumferential length. B2 is measured between two points located substantially at the middle of the leading face and the middle of the trailing face respectively, which are known to exhibit an inclination called conicity with respect to the radial plane YZ. At Figure 9 and Figure 10In the embodiment shown, the circumferential slenderness ratio H1 / B1 is strictly greater than the circumferential slenderness ratio H2 / B2, and the circumferential slenderness ratio H2 / B2 itself is strictly less than 0.6. Finally, the local volume void ratio TEVL2 of the central part 20 is at most equal to 45%, the central part 20 being centered on the equatorial plane E of the tire and having an axial width L2 equal to 2*D2 = 0.60*L, the local volume void ratio TEVL2 being defined as the ratio of the volume VC2 of the voids to the total volume V2 of the central part 20 between the support surface and the tread surface.

[0065] Figure 11 and Figure 12 respectively show a cross-sectional view of a lateral void including a single chamfer and a cross-sectional view of a lateral void including two facing chamfers. Each chamfer 26 opens onto the tread surface 25, forming an angle D with the radial direction of at least 30° and at most 70°, and having a radial height C of at least 3 mm and at most 10 mm.

[0066] The invention has been more particularly studied for a tire for agricultural vehicles of size VF 600 / 70R30 165D, which corresponds to Figure 9 and Figure 10 the embodiment of the invention shown, in which the circumferential elements of the tread pattern elements are different between the central part and the intermediate part.

[0067] Table 1 below shows the characteristics of the examples studied by the inventors:

[0068] [Table 1]

[0069]

[0070] The inventors have found that for a low ground slip level between 4% and 10%, the traction of a tire according to the invention having the characteristics described in Table 1 is increased by about 27% compared to a reference tire of the prior art, i.e. it is capable of generating a traction about 27% higher than that generated by the tires of the prior art.

Claims

1. Tire (1) for an agricultural vehicle, said tire (1) having a nominal cross-sectional width L and comprising, radially from the outside to the inside, a tread (2) and a crown reinforcement (3); - The tread (2) comprises tread pattern elements (22), said tread pattern elements (22) being separated from each other by voids (23) and extending radially outwards from a support surface (24) to a tread surface (25), - The tread (2) has an overall volume void ratio TEV, said overall volume void ratio TEV being defined as the ratio of the volume VC of the voids (23) to the overall volume V of the tread (2) that would be assumed to have no voids between the support surface (24) and the tread surface (25), - The circumferential aspect ratio H / B of each tread pattern element (22) is at most equal to 1.5, H being the average radial height between the support surface (24) and the tread surface (25) and at least equal to 20 mm, and B being the average circumferential length, - The tread (2) comprises a central part (211) and two intermediate parts (212), said central part (211) being centered on the equatorial plane (E) of the tire and having an axial width L1 of at least equal to 0.15*L and at most equal to 0.35*L, each intermediate part (212) extending axially outwards from the central part (211) for an axial distance D2 equal to 0.3*L, said axial distance D2 being measured from the equatorial plane (E), - The central part (211) and each intermediate part (212) respectively comprise circumferentially distributed tread pattern elements (221, 222), said tread pattern elements (221, 222) being separated in pairs by transverse voids (231, 232), said transverse voids (231, 232) forming an angle of at least equal to 30° with the circumferential direction (XX’) of the tire, - The crown reinforcement (3) comprises at least two crown plies (31, 32), each crown ply (31, 32) comprising reinforcing bodies that are parallel to each other, said reinforcing bodies being coated with an elastic material, crossing from one ply to the other, and forming an angle of at least equal to 10° with the circumferential direction (XX’), It is characterized in that The local volume void ratio TEVL1 of the central part (211) is at most equal to 15%, said local volume void ratio TEVL1 being defined as the ratio of the volume VC1 of the transverse voids (231) of the central part (211) to the overall volume V1 of the central part (211) between the support surface (24) and the tread surface (25).

2. The tire (1) according to claim 1, wherein, The transverse voids (231) of the central part (211) form an angle of at least equal to 60° with the circumferential direction (XX’).

3. The tire (1) according to any one of claims 1 and 2, wherein, The transverse voids (231) of the central part (211) are transverse grooves that can close when they enter the ground contact surface when the tire is in motion.

4. The tire (1) according to claim 1, wherein The transverse voids (232) of each intermediate part (212) form an angle of at least equal to 60° with the circumferential direction (XX’).

5. The tire (1) according to claim 1, each tread pattern element (221) of the central portion (211) has an average radial height H1, an average circumferential length B1, and a circumferential aspect ratio H1 / B1, and each tread pattern element (222) of each intermediate portion (212) has an average radial height H2, an average circumferential length B2, and a circumferential aspect ratio H2 / B2, wherein, The circumferential aspect ratio H1 / B1 is strictly greater than the circumferential aspect ratio H2 / B2.

6. The tire (1) according to claim 5, wherein, For each tread pattern element (222) of each intermediate portion (212), the circumferential aspect ratio H2 / B2 is at most equal to 0.

6.

7. The tire (1) according to claim 1, wherein, The local volume void ratio TEVL2 of the central portion (20) is at most equal to 45%, the central portion (20) being centered on the equatorial plane (E) of the tire and having an axial width L2 equal to 2*D2 = 0.60*L, the local volume void ratio TEVL2 being defined as the ratio of the volume VC2 of the voids (23) of the central portion (20) to the total volume V2 of the central portion (20) between the bearing surface (24) and the tread surface (25).

8. The tire (1) according to claim 1, wherein, The tread (2) only includes transverse voids (231, 232).

9. The tire (1) according to claim 1, wherein, The central portion (211) is axially delimited on both sides by circumferential voids (233).

10. The tire (1) according to claim 1, wherein, The average radial height H of each tread pattern element (22) is at most equal to 55 mm.

11. The tire (1) according to claim 1, wherein, The total volume void ratio TEV of the tread (2) is at most equal to 56%.

12. The tire (1) according to claim 1, wherein, At least a portion of the tread pattern elements (22) includes at least one chamfer (26), the chamfer (26) opening onto the tread surface (25) and forming an angle D with the radial direction (ZZ') of at least 30° and at most 70°, and having a radial height C of at least 3 mm and at most 10 mm.

13. The tire (1) according to claim 1, wherein, The crown reinforcement (3) includes crown plies (31, 32) containing metallic reinforcement.

14. The tire (1) according to claim 13, wherein, The crown reinforcement (3) includes at most two crown plies (31, 32) containing metallic reinforcement.

15. The tire (1) according to claim 1, wherein, The tire for an agricultural vehicle is an "improved flex" tire within the meaning of the chapter called "Agricultural Equipment Tires" in the "Standard Manual - 2018" of the standards of the "European Tyre and Rim Technical Organization".

16. The tire (1) according to claim 1, wherein, The tire for an agricultural vehicle is an "extremely high flex" tire within the meaning of the chapter called "Agricultural Equipment Tires" in the "Standard Manual - 2018" of the standards of the "European Tyre and Rim Technical Organization".

Images