Tire for a heavy construction vehicle with improved grip

Abstract

The present invention relates to a tire (1) for a heavy construction vehicle, and aims to improve the performance compromise between wear life, resistance to aggressions and the grip of its tread (2).According to the invention, the arrangement of the cutouts (3) evolves during tire wear, such that, for a maximum cutout depth (D) of at least 70% and at most 90% of the initial thickness (D0), the central portion (21) of the tread (2) comprises cutouts (3) of which none are effective, and every lateral portion (22, 23) of the tread (2) comprises at least one effective cutout (3), and such that, for a maximum cutout depth (D) of at least 20% and at most 40% of the initial thickness (D0), the central portion (21) of the tread (2) comprises exclusively transverse effective cutouts (32), and every lateral portion (22, 23) of the tread (2) comprises at least one effective cutout (3). Figure for the abstract: Fig. 1.

Description

Title of the invention: Pneumatic tire for a heavy civil engineering vehicle with improved grip

[0001] The present invention relates to a tire for a heavy civil engineering vehicle, intended to carry heavy loads and to roll on uneven and stony ground such as that of mines. This invention relates in particular to the tread of such a tire, the grip of which is satisfactory at any level of wear of the tire.

[0002] The invention relates more particularly to a tire intended to equip a heavy civil engineering vehicle, such as a dumper intended for the transport of materials extracted from quarries or surface mines. A dumper is subject to particularly severe driving conditions: high loads, sustained speeds, sloping and curved travel, uneven and stony ground. For example, on sites where materials, such as ores or coal, are extracted, the use of a dumper-type vehicle consists, in a simplified manner, of alternating loaded outward cycles and empty return cycles. During a loaded outward cycle, the loaded vehicle transports, mainly uphill, the extracted materials from loading areas at the bottom of the mine, or bottom of the "pit", to unloading areas: this requires good traction grip of the tires.During an empty return cycle, the empty vehicle returns, mainly downhill, to the loading areas at the bottom of the mine: this requires good braking grip from the tires. The tracks, which are usually on a slope, also often include bends, which requires good transverse grip from the tires. In addition, the tracks on which the vehicles travel are made of materials generally from the mine, for example, crushed and compacted rock, to ensure the wear layer of the track holds when the vehicles pass, and are regularly watered, which means that they are often covered with mud and water.Therefore it is necessary to allow, on the one hand, an effective evacuation of this mixture of mud and water by the tread, to guarantee satisfactory grip on this muddy ground, and, on the other hand, good resistance to wear and to attacks by stones present on the ground.

[0003] The specific use of a dumper, as previously described, requires special management of the tires fitted to it.

[0004] When new, a tire is usually mounted on the front axle, or steering axle, of the vehicle. At this front position, the load applied to the tire is generally estimated to be between 60% and 100% of its nominal load capacity, depending on whether the vehicle is running empty or loaded, this nominal load capacity being defined by, for example, the ISO 4250 standard and the Tire and Rim Association (TRA) standard. In this load range, contact between the tire and the ground is made over the entire width of the tire tread, and the tire is subjected to a limited or even low longitudinal force, but to a high transverse force, due to the tire drifting.

[0005] When the tire reaches approximately one third of its wear, that is to say when the thickness of its tread is reduced by one third compared to its initial thickness in the new condition, the tire is removed from the front axle and mounted on a rear axle, or drive axle, of the vehicle, to wear down the remaining two thirds of the tread. At this rear position, the load applied to the tire is generally estimated to be between 30% and 100% of its nominal load capacity, depending on whether the vehicle is running empty or loaded. In the lower part of this load range, corresponding to running empty, the contact between the tire and the ground is made over only part of the width of the tread.At this rear position, the tire is subjected to a significant longitudinal force (engine and brake), including when empty when only a central or median portion of the rolling surface comes into contact with the ground, and a low transverse force.

[0006] Finally, the tire is permanently removed from the drive axle when its tread reaches a residual thickness corresponding to a completely worn state in accordance with current practices.

[0007] A tire tread, intended to constitute the peripheral part of the tire, usually comprises at least one rubber-based material and is intended to be worn when it comes into contact with a ground via a rolling surface.

[0008] Usually, we designate by: - radial direction: a direction perpendicular to the axis of rotation of the tire, - axial or transverse direction: a direction parallel to the axis of rotation of the tire, - circumferential or longitudinal direction: a direction tangent to the periphery of the tire and perpendicular to the radial and axial directions respectively, - median or equatorial circumferential plane: a plane containing the radial direction and the circumferential direction, perpendicular to the axis of rotation of the tire and dividing the tire into two equal portions.

[0009] The tread, for any state of wear of the tire, is geometrically characterized by an axial width, measured in the axial direction and more simply called width, and a radial thickness, measured in a radial direction and more simply called thickness. Width is defined, by convention, as the width of the portion of the rolling surface, in contact with a smooth ground, the tire being mounted on a recommended rim and subjected to nominal pressure and load conditions recommended by the usual standards. Thickness is defined, by convention, as the maximum depth measured in the cutouts, also called maximum cutout depth. In other words, the thickness of the tread corresponds to the material thickness of the raised element delimited by the deepest cutout and is equal to the maximum cutout depth.In the case of a tyre for a civil engineering vehicle, in new condition, i.e. before running, and by way of example, the width in new condition, or initial width LO, is at least equal to 600 mm and the thickness in new condition, or initial thickness DO, defined as the maximum initial cutting depth, is at least equal to 60 mm, or even 70 mm. But the characteristics of width and maximum cutting depth vary according to the state of wear of the tyre. In particular, the maximum cutting depth varies between a maximum initial cutting depth DO, in the new condition of the tyre, and a maximum residual cutting depth DR, in the worn condition of the tyre, value at which the tyre is removed from the vehicle in accordance with current practices.

[0010] To ensure satisfactory performance in longitudinal grip, under engine torque and under braking torque, as well as in transverse grip, it is necessary to form, in the tread, a sculpture which is a system of cutouts separating raised elements.

[0011] A cutout is a space delimited by walls of material facing each other and spaced from each other by a distance defining the width of the cutout, and extending from the rolling surface, in the radial direction, over a given depth. Depending on the value of its width, measured perpendicular to its mean line, between the walls of the raised elements which it separates, at the level of the rolling surface, a cutout is either an incision or a groove. In the case of an incision, this width is appropriate to allow at least partial contact of the opposite walls delimiting said incision, at least during the passage of the tread in contact with the ground, when the tire is subjected to nominal load and pressure conditions recommended, for example, by the TRA standard.In the case of a groove, the walls of this groove do not generally come into contact with each other under these recommended nominal rolling conditions.

[0012] The cutouts delimit raised elements of the block type or of the rib type. A block comprises a contact face, contained in the rolling surface, and at least three, and most often four, lateral faces intersecting the tread surface. A rib comprises a contact face and two lateral faces extending, in the circumferential direction, over the entire length of the tread. A rib is thus delimited, in the axial direction, by one or two longitudinal cutouts.

[0013] The proportion of cuts contained in the tread or in a portion of the tread can be defined, for any state of wear, by a volumetric notch rate TEV or by a surface notch rate TES.

[0014] By definition, the volumetric notch rate TEV of the tread, for a given state of wear, is equal to the ratio between the total volume VD of the cutouts, measured on the free tire, i.e. unmounted and uninflated, and the sum of the total volume VD of the cutouts and the total volume VR of the raised elements delimited by these cutouts. The sum VD+VR corresponds to the volume comprised radially between the rolling surface, at the given level of wear, and a bottom surface, translated from the rolling surface radially inwards by a radial distance equal to the maximum depth of cutout. This volumetric notch rate TEV, expressed in %, conditions the wear performance, by the volume of material to be worn available, and the longitudinal and transverse adhesion performance, by the presence of respectively transverse and longitudinal edges and cutouts having the capacity to store or evacuate water and / or mud.

[0015] By definition, the surface notch rate TES of the tread, for a given state of wear, is defined in the contact surface of the tire with a rigid ground, when the tire, mounted on its nominal rim, is inflated to its nominal pressure and crushed under its nominal load, these nominal characteristics being recommended, for example, by the TRA standard. This surface notch rate TES is equal to the ratio between the total surface SD of the cutouts, and the sum of the total surface SD of the cutouts and the total surface SR of the raised elements delimited by these cutouts, the surfaces SD and SR being determined in the contact surface. The sum SD+SR corresponds to the contact surface.This surface notch rate TES, expressed in %, conditions the wear performance, by the surface of material in contact with the ground impacting the distribution of pressures exerted by the ground on the rolling surface, and the performance in longitudinal and transverse grip, by the length of the transverse and longitudinal edges respectively conditioning the effectiveness of the indentation of the sculpture.

[0016] These notch rates, respectively volumetric TEV and surface TES, can be determined either in the new condition of the tread, before use of the tire in rolling, or for a given state of wear of the tread, characterized by a maximum remaining depth of cut.

[0017] A tire tread for a civil engineering vehicle usually comprises cutouts that may be longitudinal or transverse. A longitudinal cutout has a mean line forming, with the longitudinal or circumferential direction of the tire, an angle of less than 45°, and has the particularity of extending over the entire circumference of the tire. Either the mean line forms a zero angle and is strictly longitudinal, or it comprises at least one oblique portion forming a non-zero angle, for example, in the case of a cutout oscillating around the circumferential direction. A transverse cutout has a mean line forming, with the longitudinal or circumferential direction of the tire, an angle of greater than 45°, and has the particularity of crossing at least part of the tread.Generally, the width of a cutout decreases progressively from the rolling surface to the bottom of the groove, due to the inclination of the walls of the raised elements delimiting said cutouts. However, a high volumetric notch rate in the new condition has a certain number of disadvantages. First of all, it promotes the capture and retention of stones in the cutouts, these being likely to damage the crown of the tire by the cracks that they potentially induce. Then, a high volumetric notch rate in the new condition implies an equally high surface notch rate, therefore a rather reduced contact surface with the ground of the raised elements, and, consequently, high pressures on the ground which accentuate the phenomenon of abrasion of the tread and therefore its wear.Finally, a high volumetric notch rate in new condition allows lateral deformations, called "barrel", of the raised elements, by Poisson effect: which reduces the effective volume of the cutouts, characterizing their capacity for storing and evacuating water or muddy mixture, hence a loss of grip of the tire on muddy ground. However, these deformations by Poisson effect tend to decrease when the wear of the tread increases, due to the reduction in the height of the raised elements.

[0018] In document WO 2022064134 A1, for a tire of size 24.00R35 intended to equip a civil engineering vehicle, a system of cutouts with widths that evolve according to the level of wear of the tread has been proposed. The principle of this invention is to propose a tire for a heavy civil engineering vehicle whose grip, more particularly on wet and / or muddy ground, is guaranteed at any level of wear. During the first third of wear, the tire is mounted at the front of the vehicle and is subjected to a load at least equal to 60%, for a vehicle traveling unladen, and at most equal to 100%, for a vehicle traveling fully laden, of the recommended load Zn. During the remaining two-thirds of wear, the tire is mounted at the rear of the vehicle and is subjected to a load at least equal to 30%, for a vehicle traveling unladen, and at most equal to 100%, for a vehicle running at full load, of the recommended load Zn. This invention aims more specifically to improve the performance compromise between wear life, resistance to aggression and grip of the tire. According to the invention, the tread having an axial width LO and comprising, on each side of an equatorial plane, at least one external longitudinal cutout at an axial distance LE at least equal to 0.5*L0 / 2, and at least one internal longitudinal cutout at an axial distance LI at most equal to 0.4*L0 / 2, the at least one external longitudinal cutout comprises an external radial portion opening onto the rolling surface and having an average width at least equal to 0.6 times its height, and the at least one internal longitudinal cutout comprises an internal radial portion not opening onto the rolling surface and having an average width at least equal to 0.6 times its height.

[0019] The inventors set themselves the objective of designing a tread for a tire for a heavy civil engineering vehicle, making it possible to further improve, compared to the aforementioned state of the art, the performance compromise between wear life, resistance to aggression and grip, when used on tracks which may be covered with water and mud, while guaranteeing the durability of grip throughout the life of the tire.

[0020] This objective is achieved, according to the invention, by a tire for a heavy civil engineering vehicle comprising a tread comprising an arrangement of cutouts separating raised elements, - the tread having, in the new condition of the tire, an initial width, measured in an axial direction parallel to the axis of rotation of the tire and between two edges of the tread, and having an initial thickness, measured in a radial direction perpendicular to a rolling surface and defined as the maximum initial cutting depth, -the tread comprising a median portion extending symmetrically on either side of a median plane of the tire and having an initial median width equal to 50% of the initial width, and two lateral portions each extending from the median portion to a tread edge, - the cutouts being either longitudinal cutouts having a mean line forming, with a circumferential direction, an angle less than 45°, or transverse cutouts having a mean line forming, with the circumferential direction, an angle greater than 45°, - a cut-out being said to be effective, at a given level of wear, when its width, measured perpendicular to its mean line, at the level of wear considered, between the walls of the raised elements which it separates, at the level of the rolling surface, is at least equal to 25% of the initial thickness, - the arrangement of the cutouts being evolutive during the wear of the tire, such that, for a maximum cutout depth at least equal to 70% and at most equal to 90% of the initial thickness, the median portion of the tread comprises cutouts among which none is effective, and any lateral portion of the tread comprises at least one effective cutout, and such that, for a maximum cutout depth at least equal to 20% and at most equal to 40% of the initial thickness, the median portion of the tread comprises exclusively transverse effective cutouts, and any lateral portion of the tread comprises at least one effective cutout.

[0021] The principle of the invention is to have a tread having, at any level of wear, good performance in wear, resistance to aggression and grip; that is to say to guarantee the durability of these three essential performances throughout the life of the tire, thanks to a sculpture which evolves during wear. Thus the contributions of the respectively median and lateral portions of the tread to the grip performance will vary according to the degree of wear of the tire.

[0022] According to a first essential characteristic of the invention, for a maximum cutout depth at least equal to 70% and at most equal to 90% of the initial thickness, the middle portion of the tread comprises cutouts among which none is effective, and any lateral portion of the tread comprises at least one effective cutout. In other words, at the start of wear, the middle portion of the tread only comprises cutouts said to be ineffective, from the point of view of grip, with a width, at the level of wear considered, less than 25% of the initial thickness: the middle portion of the tread is not effective in grip. Conversely, each lateral portion of the tread contributes to grip thanks to the presence of at least one effective cutout.

[0023] At the start of the tire's life, therefore at the start of tread wear, the tire is generally mounted on the front axle of the vehicle. When the vehicle is laden, which corresponds to a load applied to the tire substantially equal to the nominal load Zn, in the normative sense, the contact of the tread with the ground is made over the entire initial width of the tread. When the vehicle is unladen, which corresponds to a load applied to the tire approximately equal to 60% of the nominal load Zn, the contact of the tread with the ground is also made substantially over the entire initial width of the tread. Consequently, when fully laden or unladen, the respective median and lateral portions of the tread come into contact with the ground during rolling. Under these conditions, the median portion of the tread, devoid of effective cutouts, therefore weakly notched, on the one hand ensures good resistance to attacks in this high pressure zone, and on the other hand guarantees the presence of a high volume of rubber material favorable to wear life. Grip is then essentially guaranteed by the lateral portions of the tread comprising effective cutouts, the depth of which is sufficient, at the start of life, to evacuate the necessary volume of water and mud.

[0024] According to a second essential characteristic of the invention, for a maximum cutout depth at least equal to 20% and at most equal to 40% of the initial thickness, the middle portion of the tread comprises exclusively transverse effective cutouts, and any lateral portion of the tread comprises at least one effective cutout. In other words, at the end of wear, the middle portion of the tread comprises effective cutouts, from the point of view of grip, of exclusively transverse orientation, therefore capable of evacuating water and mud towards the lateral portions of the tread: the middle portion then contributes to grip. In parallel, each lateral portion of the tread also contributes to grip thanks to the presence of at least one effective cutout.The contribution of the middle portion makes it possible to compensate for the reduction in the contribution of the lateral portions, due to the reduction in the depth of the cuts at the end of wear, which makes it possible to evacuate a volume of water and mud substantially identical to that evacuated at the start of wear.

[0025] At the end of the tire's life, therefore at the end of tread wear, the tire is generally mounted on a rear axle of the vehicle. When the vehicle is laden, which corresponds to a load applied to the tire substantially equal to the nominal load Zn, in the normative sense, the contact of the tread with the ground is made over the entire width of the tread. On the other hand, when the vehicle is unladen, which corresponds to a load applied to the tire approximately equal to 30% of the nominal load Zn, the contact of the tread with the ground is limited substantially to the middle portion of the tread. Whether the vehicle is laden or unladen, sufficient traction is consequently ensured by the presence of effective transverse cutouts in the middle portion of the tread.

[0026] In a first preferred embodiment, for a maximum cutout depth at least equal to 70% and at most equal to 90% of the initial thickness, any lateral tread portion comprises at least one effective transverse cutout. The presence of effective transverse cutouts on the lateral tread portions, at the start of wear, when the tire is mounted on the steering axle, contributes to the transmission of braking forces.

[0027] In a second preferred embodiment, for a maximum depth of cutout at least equal to 70% and at most equal to 90% of the initial thickness, any lateral portion of the tread comprises at least one effective longitudinal cutout. In the particular case where the only effective cutouts present on the lateral portions of the tread, at the start of wear, when the tire is mounted on the steering axle, are longitudinal, the shoulder ribs, delimited axially inwards by an effective longitudinal cutout, are continuous, since they are not cut transverse effective cutouts. However, a continuous edge rib is rigid and therefore less sensitive to the appearance of wear patterns, the risk of which is high for a tire mounted on a steering axle and therefore not subject to torque forces.

[0028] According to an advantageous embodiment, the median portion of the tread having a median surface notch rate TESm equal to the ratio between the total surface area SDm of its cutouts and the sum of the total surface area SDm of its cutouts and the total surface area SRm of the raised elements delimited by these cutouts, and any lateral portion of the tread having a lateral surface notch rate TES1 equal to the ratio between the total surface area SD1 of its cutouts and the sum of the total surface area SD1 of its cutouts and the total surface area SR1 of the raised elements delimited by these cutouts, for a maximum cutting depth at least equal to 70% and at most equal to 90% of the initial thickness, the median surface notch rate TESm is at most equal to the lateral surface notch rate TES1, preferably at most equal to 70% of the surface notch rate lateral TES1.In other words, at the beginning of wear, the middle portion of the tread is less open than the lateral portions of the tread. This implies that the middle portion of the tread contributes mainly to the performance of resistance to aggression and life with respect to wear, while the lateral portions of the tread guarantee grip.

[0029] According to an advantageous variant of the preceding embodiment, the median surface notch rate TESm is at most equal to 12%. At the start of wear, this upper limit of the median surface notch rate TESm limits the risk of attacks in the middle part of the tread, an area of ​​high pressure in contact with the ground, and therefore the most exposed to attacks.

[0030] Advantageously, for a maximum cutout depth at least equal to 20% and at most equal to 40% of the initial thickness, any lateral portion of the tread comprises at least one effective transverse cutout. The presence of effective transverse cutouts, at the end of wear, on the lateral portions of the tread of a tire mounted on a drive axle of a vehicle, particularly under load, contributes to the grip and the transmission of longitudinal forces under engine or braking torque.

[0031] According to an advantageous embodiment, the median portion of the tread having a median surface notch rate TESm equal to the ratio between the total surface area SDm of its cutouts and the sum of the total surface area SDm of its cutouts and the total surface area SRm of the raised elements delimited by these cutouts, and any lateral portion of the tread having a lateral surface notch rate TES1 equal to the ratio between the total surface area SD1 of its cutouts and the sum of the total surface area SD1 of its cutouts and the total surface area SRI of the raised elements delimited by these cutouts, for a maximum depth of cut at least equal to 20% and at most equal to 40% of the initial thickness, the median surface notch rate TESm is at least equal to 70% and at most equal to 130% of the lateral surface notch rate TES1.At the end of wear, for a tire mounted on a vehicle's drive axle, the median and lateral surface notch rates remain close enough to guarantee an equivalent level of traction between an unladen vehicle and a laden vehicle. Thus, the median and lateral portions of the tread all contribute to grip.

[0032] According to an advantageous variant of the preceding embodiment, the median surface notch rate TESm is at least equal to 10%. At the end of wear, this lower limit of the median surface notch rate TESm guarantees a traction capacity by the middle part of the tread, in particular when the vehicle is running empty.

[0033] Advantageously, the median surface notch rate TESm, for a maximum cutting depth at least equal to 70% and at most equal to 90% of the initial thickness, is at most equal to the median surface notch rate TESm, for a maximum cutting depth at least equal to 20% and at most equal to 40% of the initial thickness. This condition aims to maximize, at the start of wear, the volume of rubber material in the middle part of the tread, the area most stressed with regard to wear.

[0034] The characteristics of the invention are illustrated by schematic figures 1 to 8, not shown to scale: -[Fig.l]: Partial front view of a tire tread according to a first embodiment of the invention, at the start of wear, -[Fig.2]: Circumferential sectional view of a tire tread according to the first embodiment of the invention, at the start of wear, -[Fig.3]: Partial front view of a tire tread according to the first embodiment of the invention, at the end of wear, -[Fig.4]: Partial front view of a tire tread according to a second embodiment of the invention, at the start of wear, -[Fig.5]: Partial front view of a tire tread according to a preferred variant of the first embodiment of the invention, at the start of wear, -[Fig.6]: Partial front view of a tire tread according to the preferred variant of the first embodiment of the invention, at the end of wear, -[Fig.7]: Partial front view of a tire tread according to a preferred variant of the second embodiment of the invention, at the start of wear, -[Fig.8]: Partial front view of a tire tread according to the preferred variant of the second embodiment of the invention, at the end of wear.

[0035] [Fig. 1] is a partial front view of a tread 2 of a tire 1, for a heavy civil engineering vehicle, according to a first embodiment of the invention, at the start of wear. The tread 2 comprises an arrangement of cutouts 3 separating raised elements 4. The tread 2 has, in the new state of the tire, an initial width LO, measured in an axial direction YY' parallel to the axis of rotation of the tire and between two tread edges (24, 25), and having an initial thickness DO (not shown in [Fig.l]), measured in a radial direction ZZ' perpendicular to a tread surface 20 and defined as the maximum initial cutout depth.The tread 2 comprises a median portion 21 extending symmetrically on either side of a median plane XZ of the tire and having an initial median width LCO equal to 50% of the initial width LO, and two lateral portions (22, 23) each extending from the median portion 21 to a tread edge (24, 25). The cutouts are either longitudinal cutouts 31 having a mean line forming, with a circumferential direction XX', an angle less than 45°, or transverse cutouts 32 having a mean line forming, with the circumferential direction XX', an angle greater than 45°. A cutout 3 is said to be effective, at a given level of wear, when its width W, measured, perpendicular to its mean line, between the walls of the raised elements 4 which it separates, at the level of the rolling surface 20, is at least equal to 25% of the initial thickness D0 (not shown in [Fig.l]), at the level of wear considered.According to the invention, the arrangement of the cutouts 3 changes during wear of the tire. According to a first characteristic of the invention, as shown in [Fig.l], for a maximum cutout depth D at least equal to 70% and at most equal to 90% of the initial thickness D0, the middle portion 21 of tread 2 comprises cutouts 3 among which none is effective, and any lateral portion (22, 23) of tread 2 comprises at least one effective cutout 3. A maximum cutout depth D at least equal to 70% and at most equal to 90% of the initial thickness D0 corresponds to a state at the start of wear. A middle portion 21 of tread 2 comprising cutouts 3 among which none is effective is a closed portion which essentially contributes to resistance to aggression and to the service life. in wear, but not in grip on wet or muddy ground. On the other hand, lateral portions (22, 23) of tread 2 comprising effective cutouts 3, transverse in this case, contribute to grip on wet or muddy ground.

[0036] [Fig. 2] is a circumferential sectional view of a tire tread 2 according to the first embodiment of the invention, at the start of wear. The circumferential sectional view is taken along the section plane AA shown in [Fig. 1]. [Fig. 2] more particularly shows the effective transverse cutouts 32, separating raised elements 4, present in a lateral portion 23 and extending radially inwards from the tread surface 20. Such a cutout 32 is defined by its width W, measured at the tread surface 20 between the walls of the two raised elements 4 that it delimits, and by its depth D, measured along the radial direction ZZ' between the tread surface 20 and the bottom of the cutout.The initial thickness D0, defined in the new condition of the tire before rolling as the initial maximum cutting depth, measured over all the cuts in the tread, whether effective or not, is represented by a dotted line. At the start of wear, by convention, the maximum cutting depth D is at least equal to 70% and at most equal to 90% of the initial thickness D0.

[0037] [Fig. 3] is a partial front view of a tread 2 of a tire 1, for a heavy civil engineering vehicle, according to the first embodiment of the invention, at the end of wear. The elements of [Fig. 3] are referenced identically to those of [Fig. 1]. [Fig. 3] illustrates the second essential characteristic of the invention: for a maximum cutout depth D at least equal to 20% and at most equal to 40% of the initial thickness D0, the median portion 21 of the tread 2 comprises exclusively transverse effective cutouts 32, and any lateral portion (22, 23) of the tread 2 comprises at least one effective cutout (3). A maximum cutout depth D at least equal to 20% and at most equal to 40% of the initial thickness D0 corresponds to a state at the end of wear. [Fig.3] shows, in particular, the presence of effective transverse grooves 32, present both in each lateral portion (22, 23) and in the middle portion 21. In the case shown, the effective transverse cutouts 32 of the lateral portions (22, 23) extend into the middle portion 21. This makes it possible to constitute continuous channels allowing water and mud to be evacuated transversely, from the middle portion 21 towards each lateral portion (22, 23). Thus the middle portion 21 and the lateral portions (22, 23) contribute together to ensuring grip on wet or muddy ground.

[0038] [Fig.4] is a partial front view of a tread 2 of a tire 1 according to a second embodiment of the invention, at the start of wear. This second embodiment of the invention, shown in [Fig.4], differs from the first embodiment of the invention, shown in [Fig.l], by the presence, in the lateral portions (22, 23), of a network of effective longitudinal 31 and transverse 32 cutouts, interconnected. This two-dimensional network of effective cutouts, at the edge of the tread, is particularly efficient with regard to grip on wet or muddy ground, thanks to a high notch rate. The middle portion of this second embodiment of the invention remains identical to that of [Fig.l].

[0039] [Fig. 5] is a partial front view of a tread 2 of a tire 1 according to a preferred variant of the first embodiment of the invention, at the start of wear. In this preferred variant, the raised elements of the lateral portions of the tread comprise cavities or wells, extending radially inwards from the rolling surface. These wells make it possible in particular to limit the temperature rise in the lateral portions of the crown of the tire, and therefore contribute to increasing the thermal endurance of the tire. The raised elements of the middle portion further comprise incisions which contribute to longitudinal grip under braking or engine torque.

[0040] [Fig. 6] is a partial front view of a tire tread 2 according to the preferred variant of the first embodiment of the invention, shown in [Fig. 5], at the end of wear. At the end of wear, the lateral tread portions comprise exclusively transverse effective cutouts, which extend into the median tread portion, which comprises in its central part a circumferential distribution of the adjacent raised elements. It should be noted that the wells and incisions, mentioned in the description of [Fig. 5], have partially disappeared during the wear of the tire.

[0041] [Fig. 7] is a partial front view of a tread 2 of a tire 1 according to a preferred variant of the second embodiment of the invention, at the start of wear. In this preferred variant, the raised elements of the lateral portions and of the median portion of the tread comprise incisions which contribute to longitudinal grip under braking or engine torque.

[0042] [Fig. 8] is a partial front view of a tire tread 2 according to the preferred variant of the second embodiment of the invention, at the end of wear. At the end of wear, the lateral tread portions comprise exclusively transverse effective cutouts, which extend into the median tread portion, which comprises in its central part a circumferential distribution of the adjacent raised elements. It should be noted that the incisions, mentioned in the description of [Fig. 7], have completely disappeared during the wear of the tire.

[0043] The invention has been more particularly studied for a tire for a civil engineering vehicle of the dumper type in the dimension 53 / 80R63, according to two variants A and B, variant A corresponding to figures 5 and 6 and variant B to figures 7 and 8, the tire dimension taken as reference being 24.00R35.

[0044] Table 1 below presents the characteristics of the reference dimension, as well as those of variants A and B of the invention: [Tables 1] Characteristics 24.00R35 (Reference) 53 / 80R63 (Variant A) 53 / 80R63 (Variant B) Initial tread width LO 600mm 1188mm 1188mm Initial tread thickness DO in new condition, before rolling 74mm 121mm 121mm Width W of transverse cut in each lateral portion (at 20% wear) Effective 22 mm (30%D0) Effective 60 mm (50%D0) Effective 42 mm (35%D0) Width W of longitudinal cut in each lateral portion (at 20% wear) Effective 21 mm (28%D0) Not effective 12mm (10%D0) Effective 36 mm (30%D0) Width W of transverse cut in the medial portion (at 20% wear) Not effective 2 mm (3%D0) Not effective 12mm(10%D0) Not effective 12 mm (10%D0) Width W of longitudinal cut in the middle portion (at 20% wear) Not effective 6 mm (8%D0) Not effective 12 mm (10% D0) Not effective 12 mm (10% D0) Width W of transverse cut in each lateral portion (at 70% wear) Effective 21 mm (28% D0) Effective 42 mm (35% D0) Effective 36 mm(30% D0) Width W of longitudinal cut in each lateral portion (at 70% wear) Not effective 2 mm (3% D0) Not effective 12 mm (10% D0) Not effective 0 mm (0% D0) Width W of transverse cut in the medial portion (at 70% wear) Effective 21 mm (28% D0) Effective 36 mm (30% D0) Effective 36 mm (30% D0) Width W of longitudinal cut in the portion Effective 21 mm (28% D0) Not effective 24 mm (20% D0) Not effective 24 mm (20% D0) median (at 70% wear) Overall volumetric notch rate TEV new 16.4% 16.1% 16% Overall surface notch rate TES at 20% wear 14.6% 18.2% 17.8% Lateral surface notch rate TES1 at 20% wear 23.2% 28.1% 27.3% Median surface notch rate TESm at 20% wear 5.8% 8.3% 8.3% Overall surface notch rate TES at 70% wear 16.9% 15.9% 15.6% Lateral surface notch rate TES1 at 70% wear 13.9% 17.3% 16.6% Notch rate median surface area TESm at 70% wear 20% 14.5% 14.5% Effective notch of lateral portion at 20% wear Longitudinal and transverse Transverse Longitudinal and transverse Effective notch of median portion at 20% wear None None None Effective notch of lateral portion at 70% wear Transverse Transverse Transverse Effective notch of median portion at 70% wear Longitudinal and transverse Transverse Transverse Note: (1) A wear percentage quantifies a loss of material height (elements in relief), therefore a reduction in initial thickness DO (2) 20% wear is a state of wear at the beginning of life (or beginning of wear) (3) 70% wear is an end-of-life (or end-of-wear) wear condition

[0045] Table 2 below shows the estimated braking and wear performance for the reference dimension, as well as for variants A and B of the invention: [Tables 2] Characteristics 24.00R35 (Reference) 53 / 80R63 (Variant A) 53 / 80R63 (Variant B) Braking performance, at 20% wear, empty or loaded 100% 103% 102% Braking performance, at 70% wear, empty 106% 96% 96% Braking performance, at 70% wear, loaded 101% 99% 98% Wear performance at 20% wear, empty and loaded 100% 98% 99% Wear performance, at 70% wear, empty 96% 103% 103% Wear performance, at 70% wear, loaded 100% 101% 101%

[0046] Concerning the braking performance, the reference in base 100 is the reference dimension at 20% wear, for a loaded vehicle. The braking performance indicator is defined as follows: Braking performance at x% wear, for the dimension considered = braking distance at 20% wear, for the reference dimension / braking distance at x% wear, for the dimension considered. A braking performance indicator greater than 100% indicates a reduction in the braking distance of the dimension considered compared to the reference dimension at 20% wear. Table 1 shows that variants A and B according to the invention, at 20% wear, are more efficient in braking than the reference dimension, but the trend is reversed at 70% wear.

[0047] Concerning wear performance, the reference in base 100 is the reference dimension at 20% wear, for a loaded vehicle. The performance indicator in wear is defined as follows: Wear performance at x% wear, for the dimension concerned = (1 - TES at x% wear of the dimension concerned) / (1 — TES at 20% wear of the reference dimension). Under these conditions, a wear performance indicator greater than 100% indicates a gain in the rate of material in contact with the ground of the dimension considered compared to the reference dimension at 20% wear. Table 2 shows that variants A and B according to the invention, at 70% wear, are more efficient in wear than the reference dimension, but the trend is reversed at 20% wear.

Claims

Claims

1. A tire (1) for a heavy civil engineering vehicle, comprising a tread (2) comprising an arrangement of cutouts (3) separating raised elements (4), -the tread (2) having, in the new condition of the tire, an initial width (LO), measured in an axial direction (YY') parallel to the axis of rotation of the tire and between two tread edges (24, 25), and having an initial thickness (DO), measured in a radial direction (ZZ') perpendicular to a tread surface (20) and defined as the maximum initial cutout depth, -the tread (2) comprising a median portion (21) extending symmetrically on either side of a median plane (XZ) of the tire and having an initial median width (LCO) equal to 50% of the initial width (LO), and two lateral portions (22, 23) each extending from the median portion (21) up to a tread edge (24, 25), -the cutouts being either longitudinal cutouts (31) having a mean line forming, with a circumferential direction (XX'), an angle less than 45°, or transverse cutouts (32) having a mean line forming, with the circumferential direction (XX'), an angle greater than 45°, -a cutout (3) being said to be effective, at a given level of wear, when its width (W), measured, perpendicular to its mean line, between the walls of the raised elements (4) which it separates, at the level of the tread surface (20), is at least equal to 25% of the maximum tread depth in new condition (DO), characterized in that the arrangement of the cutouts (3) changes during wear of the tire, such that, for a maximum cutout depth (D) at least equal to 70% and at most equal to 90% of the initial thickness (D0), the median portion (21) of the tread (2) comprises cutouts (3) among which none is effective, and any lateral portion (22, 23) of the tread (2) comprises at least one effective cutout (3), and such that, for a maximum cutout depth (D) at least equal to 20% and at most equal to 40% of the initial pure depth (D0),the middle portion (21) of the tread (2) comprises exclusively transverse effective cutouts (32), and any portion, lateral (22, 23) of tread (2) comprises at least one effective cutout (3).

2. Tire (1) according to claim 1 wherein, for a maximum cutout depth (D) at least equal to 70% and at most equal to 90% of the initial thickness (DO), any lateral portion (22, 23) of tread (2) comprises at least one effective transverse cutout (32).

3. Tire (1) according to one of claims 1 or 2 wherein, for a maximum cutout depth (D) at least equal to 70% and at most equal to 90% of the initial thickness (D0), any lateral portion (22, 23) of tread (2) comprises at least one effective longitudinal cutout (31).

4. Tire (1) according to any one of claims 1 to 3, the median tread portion (21) having a median surface notch rate TESm equal to the ratio between the total surface area SDm of its cutouts and the sum of the total surface area SDm of its cutouts and the total surface area SRm of the raised elements delimited by these cutouts, and any lateral tread portion (22, 23) having a lateral surface notch rate TES1 equal to the ratio between the total surface area SD1 of its cutouts and the sum of the total surface area SD1 of its cutouts and the total surface area SR1 of the raised elements delimited by these cutouts, in which, for a maximum cutout depth (D) at least equal to 70% and at most equal to 90% of the initial thickness (D0), the median surface notch rate TESm is at most equal to the rate TES1 lateral surface notch,preferably at most equal to 70% of the lateral surface notch rate TES1.,

5. Tire (1) according to claim 4 in which the median surface notch rate TESm is at most equal to 12%.

6. Tire (1) according to any one of claims 1 to 5 wherein, for a maximum cutout depth (D) at least equal to 20% and at most equal to 40% of the initial thickness (D0), any lateral portion (22, 23) of tread (2) comprises at least one effective transverse cutout (32).

7. Tire (1) according to any one of claims 1 to 6, the median tread portion (21) having a median surface notch rate TESm equal to the ratio between the total surface area SDm of its cutouts and the sum of the total surface area SDm of its cutouts and the total surface area SRm of the raised elements delimited by these cutouts, and any lateral portion of tread (22, 23) having a lateral surface notch rate TES1 equal to the ratio between the total surface area SD1 of its cutouts and the sum of the total surface area SD1 of its cutouts and the total surface area SRI of the raised elements delimited by these cutouts, in which, for a maximum depth of cutout (D) at least equal to 20% and at most equal to 40% of the initial thickness (DO), the median surface notch rate TESm is at least equal to 70% and at most equal to 130% of the lateral surface notch rate TES1.

8. Tire (1) according to claim 7 in which the median surface notch rate TESm is at least equal to 10%.

9. Tire (1) according to any one of claims 1 to 8 in which the median surface notch rate TESm, for a maximum cutting depth (D) at least equal to 70% and at most equal to 90% of the initial thickness (D0), is at most equal to the median surface notch rate TESm, for a maximum cutting depth (D) at least equal to 20% and at most equal to 40% of the initial thickness (D0).

Images