Tire for a heavy civil engineering vehicle with improved thermal and stone-retention performance
The tire tread with stepped ventilation cavities addresses the balance of thermal performance, stone retention, and mechanical resistance by enhancing cooling efficiency and preventing stone entrapment, ensuring robustness under demanding conditions.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- MICHELIN & CO (CIE GEN DES ESTAB MICHELIN)
- Filing Date
- 2025-12-10
- Publication Date
- 2026-06-25
AI Technical Summary
Existing tire designs for heavy construction vehicles face challenges in balancing thermal performance, stone retention, and mechanical resistance, particularly at the edges of the tread, due to the design of ventilation cavities that either compromise cooling efficiency or increase the risk of foreign object entrapment and mechanical weakening.
A tire tread design featuring stepped ventilation cavities with a primary and secondary portion, each with specific angled walls, to enhance thermal dissipation while minimizing stone retention and maintaining mechanical integrity.
The design effectively cools the tread, prevents stone entrapment, and maintains structural robustness, as confirmed by customer tests, improving overall tire performance under harsh conditions.
Smart Images
Figure EP2025086286_25062026_PF_FP_ABST
Abstract
Description
Tire for a heavy construction vehicle with improved thermal performance and stone retention
[0001] The present invention relates to a tire for a heavy construction vehicle, designed to carry heavy loads and to travel on uneven and stony ground such as, for example, mine terrain. More specifically, this invention relates to the tread of such a tire, for which the compromise between thermal performance and stone retention, particularly at the edges of the tread, is improved.
[0002] The invention relates more particularly to a tire for use on heavy construction equipment, such as a dumper truck for transporting materials extracted from quarries or surface mines. A dumper truck is subjected to particularly harsh driving conditions: heavy loads, sustained speeds, inclines and curves, and travel on uneven and stony ground. For example, at material extraction sites, such as ores or coal mines, the use of a dumper truck, in simplified terms, consists of alternating loaded outbound and empty return trips. During a loaded outbound trip, the loaded vehicle transports the extracted materials, primarily uphill, from loading areas at the bottom of the mine, or pit, to unloading areas. During an empty return trip, the empty vehicle returns, primarily downhill, to the loading areas at the bottom of the mine.The tracks, which are often sloped, also frequently include curves. The driving conditions described above generate particularly high thermal stresses in the tires, especially at the edges of the tread, as well as severe mechanical stresses on the tread. Furthermore, the tracks on which the vehicles travel are generally made of materials sourced from the mine, such as crushed and compacted rock, to ensure the durability of the track's wear layer under vehicle traffic. They are also regularly watered, which means they are often covered in mud and water.Therefore, it is necessary to have a tread that allows, on the one hand, efficient evacuation of this mixture of mud and water, to guarantee satisfactory grip on this muddy ground, and on the other hand, good resistance to wear and tear and to damage from the stones present on the ground.
[0003] Generally speaking, a tire comprises a tread radially outside a crown reinforcement, itself radially outside a carcass reinforcement.
[0004] The tread, forming the peripheral part of the tire, usually comprises at least one rubber-based material and is intended to be worn down when it comes into contact with a ground via a tread surface.
[0005] In the usual way and in what follows, we will use the following: - 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.
[0006] 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.
[0007] A cutout is a space delimited by opposing walls of material separated by a distance defining the cutout's width, extending radially from the tread surface to a given depth. Depending on its width, measured perpendicular to its center line between the walls of the raised features it separates at the tread surface, a cutout is either a groove or an incision. In the case of an incision, the cutout's width is suitable to allow at least partial contact between the opposing walls delimiting the incision, at least during the tread's contact with the ground, when the tire is subjected to the recommended nominal load and pressure conditions, for example, those defined 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 nominal recommended rolling conditions.
[0008] The cutouts define raised elements of the block or rib type. A block is a polyhedron comprising a contact face, contained within the tread surface, and at least three, and most often four, lateral faces cutting across the tread surface. A rib comprises a contact face and two lateral faces extending, in the circumferential direction, along the entire length of the tread. A rib is thus delimited, in the axial direction, by one or two longitudinal cutouts.
[0009] A tire tread for construction equipment typically includes cuts that can be longitudinal or transverse. By convention, a cut is considered longitudinal when its center line forms an angle of less than 45° with the longitudinal or circumferential direction of the tire. A longitudinal cut extends around the entire circumference of the tire. Either the center line forms a zero angle and is strictly longitudinal, or it includes at least one oblique portion forming a non-zero angle, for example, in the case of a cut that oscillates around the longitudinal direction. By convention, a cut is considered transverse when its center line forms an angle greater than 45° with the longitudinal or circumferential direction of the tire. A transverse cut crosses at least part of the tread.
[0010] The tread, regardless of the tire's wear condition, is geometrically characterized by an axial width, measured along the axial direction and simply called the width, and a radial thickness, measured along a radial direction and also simply called the thickness. The width is conventionally defined as the width of the portion of the tread surface in contact with a smooth surface, with the tire mounted on a recommended rim and subjected to the nominal pressure and load conditions recommended by standard practices. The thickness is conventionally defined as the maximum depth measured in the grooves, also called the maximum groove depth. In other words, the tread thickness corresponds to the thickness of the raised element delimited by the deepest groove and is equal to the maximum groove depth. In the case of a tire For a construction vehicle, in its new condition (i.e., before use), and as an example, the new width (or initial width LO) is at least 600 mm and the new tread depth (or initial tread depth DO), defined as the maximum initial cut depth, is at least 60 mm, or even 70 mm. However, the maximum width and maximum cut depth characteristics vary depending on the tire's wear. In particular, the maximum cut depth varies between a maximum initial cut depth DO, when the tire is new, and a maximum residual cut depth DR, when the tire is worn, which is the value at which the tire is removed from the vehicle in accordance with current practices.
[0011] The tread, having a high thickness and being subjected to mechanical stresses generated in the contact area with the ground, undergoes heating which can cause damage leading to premature removal of the tire, this heating within the material itself being linked to the hysteresis characteristics of the rubber materials constituting the tread.
[0012] It is known, notably from patent documents W02015140122A1, EP2655093A1, and JPH10278510A, to create cavities in the raised elements of the tread, which can be either ribs or blocks. One of the functions of these cavities is to provide thermal ventilation to dissipate the heat generated during driving. The depth and volume of these cavities are chosen according to the required temperature reduction. These cavities are generally open, either laterally on an edge of the tread or on the contact patch. The desired effect is ventilation, leading to temperature limitation within the tread, particularly near the radially inner reinforcement layers.To this end, the depth of the cavities is determined based on the tread thickness and the positioning of the ends of the top reinforcement layers. Other patent documents, such as JP2010-247707 and JP2012-171505, describe tires with treads whose edges have cavities.
[0013] To be effective, these ventilation cavities must be relatively large, both in the thickness of the strip and in the surface area of bearing, in order to create a sufficient heat exchange surface, and thus promote good ventilation by ambient air.
[0014] This cavity depth is, as indicated in patent application W02015140122A1, at least equal to 30% and even more preferably at least equal to 50% of the total height of material to be worn, i.e. the thickness of the tread.
[0015] The choice of these characteristics can lead to deep cavities opening onto the tread surface, promoting cooling but potentially causing the capture of foreign bodies or objects, such as stones, from the ground during rolling. These bodies, captured by a cavity, can be ejected if they are small compared to the dimensions of the cavity's opening, or held captive within the cavity due to the elastic and deformable nature of the rubber material forming the tread, the cavity exerting retention forces on the foreign bodies.
[0016] In the latter case, the foreign body can remain stuck in the cavity and gradually sink, during rolling, into the tread material until, if necessary, it attacks the bottom of the cavity and then, potentially, the radially inner apex reinforcement layers of the tread, directly above the cavity.
[0017] To overcome this drawback of foreign body trapping, the invention, described in document WO2018096259A1, aims to optimize the geometry of ventilation cavities designed to promote heat exchange between the solid parts of the tread and the surrounding air, while reducing the risk of damage from foreign bodies that could enter these cavities and become trapped there. According to this invention, for a tread with cutouts delimiting sections of material forming raised elements, each comprising a contact face, at least one ventilation cavity is formed on a plurality of raised elements. This cavity has a depth at least equal to 70% of the tread material thickness, and each ventilation cavity is delimited by a wall surface ending in a bottom surface.Furthermore, each ventilation cavity comprises a first cavity section extended in depth by a second cavity section. In the first cavity section located between the bearing surface and a level of... In the intermediate depth HI, located between 30% and 70% of the maximum depth H of the ventilation cavity, the average draft angle A of the wall is at least 20 degrees, while in the second cavity section extending in depth from the first section to the bottom of the cavity, the average draft angle B of the wall is at most 15 degrees. In other words, the inclination of the cavity walls decreases as one descends into the cavity.
[0018] This type of ventilation cavity is likely to be less effective when the block-like relief elements have significantly different axial and circumferential dimensions, that is, when the blocks are elongated either axially or circumferentially. In such cases, to maintain its cooling effectiveness, the ventilation cavity with at least one double slope, as previously described, must be widened along the direction of the block's largest dimension. However, such a widened cavity risks, on the one hand, mechanically weakening the block and, on the other hand, increasing the risk of foreign bodies, particularly stones, becoming trapped inside the block.
[0019] The inventors set themselves the objective of designing a tire for a heavy construction vehicle with a tread comprising blocks, particularly at the edges of the tread, with ventilation cavities opening onto the contact faces of said blocks, allowing to improve the performance compromise of the tread between its thermal endurance, its ability not to retain foreign bodies and its mechanical resistance.
[0020] This objective is achieved, according to the invention, by a tire for a heavy construction vehicle having a tread intended to come into contact with the ground via a tread surface, -the tread comprising an arrangement of cutouts separating blocks and having a radial thickness E, measured, along a radial direction perpendicular to an axis of rotation of the tire, between the tread surface and the bottom of the deepest cutout, -any block having a contact face, contained within the rolling surface, and being delimited by at least three lateral faces intersecting the contact face, said contact face having a smaller dimension Lmin and a larger dimension Lmax, -at least one block comprising a ventilation cavity, extending radially inwards from the contact face to a depth H and opening onto the contact face via an opening surface having a smaller dimension Dmin and a larger dimension Dmax, -the ventilation cavity comprising a first radially external portion, consisting of a single primary cavity, extended radially inwards by a second radially internal portion, comprising at least two independent secondary cavities separated from each other by a protrusion, -and any secondary cavity of the second radially interior portion having a double-sloped inclined wall, the radially exterior portion of the inclined wall forming with the radial direction an angle A2 of at least 20° and the radially interior portion of the inclined wall forming with the radial direction an angle A3 of at most 15°.
[0021] The principle of the invention is to provide a tread block, which may be subjected to a high temperature rise during tire rolling, with a stepped ventilation cavity designed to evacuate at least part of the heat generated in said block, while limiting the retention of stones and ensuring sufficient mechanical resistance of the block.
[0022] A stepped ventilation cavity essentially comprises a first radially external portion, consisting of a single primary cavity with an effective ventilation volume over a first radially external portion of the block. Furthermore, this single primary cavity is designed to prevent the retention of stones that might otherwise become trapped within it.
[0023] A stepped ventilation cavity also essentially comprises a second, radially interior portion extending from the first, radially exterior portion with a single primary cavity. This second, radially interior portion includes a juxtaposition of at least two secondary cavities constituting additional ventilation volumes that allow for the removal, at least in part, of the heat generated within the depth of the block. Each secondary cavity is designed to prevent the retention of stones that may have entered the single primary cavity. Furthermore, two adjacent secondary cavities are separated from each other. by a protrusion. The alternation of secondary cavities and protrusions limits the mechanical weakening of the block.
[0024] Essentially, every secondary cavity of the second radially interior portion has a double-sloped inclined wall, the radially exterior portion of the inclined wall forming an angle A2 of at least 20° with the radial direction and the radially interior portion of the inclined wall forming an angle A3 of at most 15° with the radial direction.
[0025] An angle A2 of at least 20° on the radially outer portion of the inclined wall of a secondary cavity promotes the ejection of stones that could be captured by the secondary cavity, thus preventing the risk of stones being retained by a secondary cavity. An angle A3 of less than 15° on the radially inner portion of the inclined wall of a secondary cavity, and at most 15°, limits the size of stones that could enter the secondary cavity and damage the top reinforcement.
[0026] Preferably the depth H of the ventilation cavity, measured, along a radial direction perpendicular to an axis of rotation of the tire, between the contact face of the block and the bottom of the deepest secondary cavity, is at least equal to 70% of the radial thickness E of the tread.
[0027] This minimum value of the depth H of the ventilation cavity, measured, along a radial direction perpendicular to an axis of rotation of the tire, between the contact face of the block and the bottom of the deepest secondary cavity, is necessary for effective ventilation, and therefore allows cooling of the block throughout its thickness.
[0028] Preferably, the first radially outer portion has a depth Hl, measured between the contact face and the top of the protrusion of the second highest radially inner portion, at least equal to 30% and at most equal to 60% of the depth H of the ventilation hollow.
[0029] A depth Hl of the first radially outer portion is, by convention, measured between the contact face and the apex of the protuberance of the second, highest radially inner portion. Indeed, the apexes of the various protuberances are not necessarily positioned radially at the same level.
[0030] The HI depth of the first radially outer portion must be sufficient to guarantee a minimum ventilation volume allowing at least partial evacuation of the heat generated in a radially outer portion of the block. However, this value must not be too high, firstly, to limit the loss of block rigidity due to material removal, and therefore the risk of mechanical weakening of the block, and secondly, to avoid increasing the risk of stone retention that could migrate through the rubber material of the tread and, consequently, approach the crown reinforcement of the tire and, if necessary, damage it.
[0031] Advantageously the unique primary cavity of the first radially external portion has an inclined wall, forming with the radial direction, an angle Al of at least 20°.
[0032] An inclined wall of the single primary cavity of the first radially external portion, with an angle Al, usually called the draft angle, of at least 20°, promotes the ejection of stones that could be captured by the single primary cavity, i.e. prevents the risk of stone retention by the latter.
[0033] Preferably the second radially inner portion has a depth H2, measured between the top of the highest protuberance and the bottom of the deepest secondary cavity, at least equal to 40% and at most equal to 70% of the depth H of the ventilation hollow.
[0034] A depth H2 of the second radially inner portion is, by convention, measured between the apex of the highest protuberance and the bottom of the deepest secondary cavity. As seen previously, the apexes of the various protuberances are not necessarily positioned radially at the same level. Similarly, the bottoms of the secondary cavities are not necessarily positioned radially at the same level.
[0035] The depth H2 of the second radially inner portion must be sufficient to guarantee a minimum ventilation volume allowing at least partial heat removal in a radially inner portion of the block. However, this value must not be too high to limit the protrusion heights, and therefore to guarantee sufficient rigidity of the protrusions and, consequently, their mechanical resistance.
[0036] According to a particular embodiment of the invention, the apexes of a portion of the protuberances of the second radially inner portion are positioned, relative to the contact face, at a radial distance strictly greater than the depth H 1 of the first radially outer portion.
[0037] According to another particular embodiment of the invention, the bottoms of a part of the secondary cavities of the second radially inner portion are positioned, relative to the contact face, at a radial distance strictly less than the depth H of the ventilation hollow.
[0038] According to a preferred embodiment of the invention, the smallest dimension Dmin of the opening surface of the ventilation cavity has a direction parallel to the direction of the smallest dimension Lmin of the contact face of the block comprising the ventilation cavity, and the largest dimension Dmax of the opening surface of the ventilation cavity has a direction parallel to the direction of the largest dimension Lmax of the contact face of the block comprising the ventilation cavity.
[0039] This orientation of the ventilation cavity relative to the orientation of the block is optimal from the point of view of ventilation efficiency, because the respective maximum dimensions are oriented in the same first direction and the respective minimum dimensions are oriented in the same second direction.
[0040] According to a preferred variant of the preceding preferred embodiment, the following two conditions are met: - P = K*H, where P is the distance between the axes of two successive secondary cavities of the second radially internal portion, K is a coefficient between 1 / 3 and 1, and H is the depth of the ventilation cavity (8). - N = E ((Lmax - Lmin) / (K*H)), N being the number of secondary cavities of the second radially inner portion and E being the integer part (in the mathematical sense) of the ratio (Lmax - Lmin) / (K*H).
[0041] These two relationships allow us to determine the optimal pitch P and number N of secondary cavities in the radially inner portion, as a function of the depth H of the ventilation cavities and the respective minimum (Lmin) and maximum (Lmax) dimensions of the block's contact face. This arrangement of secondary cavities allows for an optimal compromise between thermal ventilation, preventing stone retention, and the block's mechanical robustness.
[0042] Preferably the largest dimension Lmax of a block is greater than or equal to 1.5 times its smallest dimension Lmin.
[0043] A stepped ventilation cavity, as defined by the invention, is particularly effective for an elongated block, whose largest dimension Lmax is significantly greater than its smallest dimension Lmin, typically greater than or equal to 1.5 times its smallest dimension Lmin.
[0044] Preferably, the tread comprises, at each of its axial ends, a circumferential row of blocks provided with a ventilation hollow.
[0045] The presence of ventilation channels in the blocks distributed circumferentially at the axial ends of the tread, commonly referred to as tread shoulders, allows for efficient cooling of the uppermost portions of the tire's crown, which are generally the thickest and therefore subject to the highest temperatures. Thermal ventilation is particularly necessary in these areas.
[0046] The features of the invention are illustrated by schematic figures 1 to 6, which are not shown to scale: -Figure 1: Top view of a portion of the tread of a tire according to the invention, -Figure 2: Cross-sectional view of a stepped ventilation groove in the tread of Figure 1, along the AA direction of the groove's largest dimension, -Figure 3: Schematic cross-sectional view of an example of a stepped ventilation groove, along its largest dimension, -Figure 4: Schematic cross-sectional view of an example of a stepped ventilation shaft, along its longest dimension, in its new condition and in the presence of stones, -Figure 5: Schematic cross-sectional view of an example of a stepped ventilation shaft, along its longest dimension, worn down to the level of its second portion radially inward and in the presence of stones, -Figure 6: Schematic top view of a stepped ventilation cavity, whose largest and smallest dimensions (Dmax, Dmin) are respectively parallel to the largest and smallest dimensions (Lmax, Lmin) of the contact face of the block.
[0047] Figure 1 is a top view, along a radial axis ZZ', of a portion of the tread 2 of a tire 1 according to the invention. The tire 1 for a heavy construction vehicle has a tread 2 intended to make contact with the ground via a tread surface 3. The tread 2 comprises an arrangement of cutouts 4 separating blocks 5. Each block 5 has a contact face 6, contained within the tread surface 3, and is delimited by four lateral faces 7 intersecting the contact face 6. In the embodiment of the invention shown, the tread 2 comprises, at each of its axial ends, a circumferential row of blocks 5, each provided with a ventilation groove 8, opening onto the contact face 6 via a through-surface 9.
[0048] Figure 2 is a cross-sectional view of a stepped ventilation groove 8 of the tread of Figure 1, along the AA direction of the largest dimension of the groove. The ventilation groove 8 comprises a first radially external portion 10, consisting of a single primary cavity 11, extended radially inwards by a second radially internal portion 12, comprising two independent secondary cavities 13 separated from each other by a protrusion 14.
[0049] Figure 3 is a schematic cross-sectional view of an example of a stepped ventilation groove 8, along its longest dimension. The tread comprises an arrangement of cutouts separating blocks 5 and has a radial thickness E, measured, along a radial direction ZZ' perpendicular to an axis of rotation of the tire, between the tread surface and the bottom of the deepest cutout. Each block 5 has a contact face 6, contained within the tread surface, and is bounded by at least three lateral faces 7 intersecting the contact face 6. The block 5 shown in Figure 3 has a radial thickness E and includes a ventilation groove 8, extending radially inward from the contact face 6 to a depth H and opening onto the contact face 6 through a through-surface 9. The ventilation groove 8 comprises a first radially outer portion 10, consisting of a single primary cavity 11, extended radially inward by a second radially inward portion 12, comprising four independent secondary cavities 13 separated from each other by a protrusion 14. The depth H of the ventilation cavity 8, measured along the radial direction ZZ', between the contact face 6 of the block 5 and the bottom 16 of the deepest secondary cavity 13, is at least equal to 70% of the radial thickness E of the tread. The first radially outward portion 10 has a depth Hl, measured between the contact face 6 and the apex 15 of the protrusion 14 of the second radially inward portion 12, at least equal to 30% and at most equal to 60% of the depth H of the ventilation cavity 8. The single primary cavity 11 of the first radially outward portion 10 has an inclined wall 16, forming with the radial direction ZZ', an angle Al of at least 20°.The second radially internal portion 12 has a depth H2, measured between the top 15 of the highest protuberance 14 and the bottom 16 of the deepest secondary cavity 13, at least equal to 40% and at most equal to 70% of the depth H of the ventilation hollow 8. Every secondary cavity 13 of the second radially internal portion 11 has a double-sloped inclined wall 17, the radially external portion 18 of the inclined wall 17, forming with the radial direction ZZ', an angle A2 of at least 20° and the radially internal portion 19 of the inclined wall 17, forming with the radial direction ZZ', an angle A3 of at most 15°. The apexes 15 of the protuberances 14 of the second radially inner portion 11 are positioned, with respect to the contact face 6, at a radial distance equal to the depth H 1 of the first radially outer portion 10.The bottoms 16 of a part of the secondary cavities 13 of the second radially inner portion 11 are positioned, with respect to the contact face 6, at a radial distance strictly less than the depth H of the ventilation hollow 8.
[0050] Figure 4 is a schematic cross-sectional view of an example of a stepped ventilation cavity 8, along its largest dimension, in its new condition and in the presence of stones. The stones 20 are of such a size that they can penetrate, through the opening surface 9, into the single primary cavity 11, without however being able to penetrate into the secondary cavities 13, two by two separated by a protrusion 14. In addition, the inclined wall of the single primary cavity 11 is configured to allow the evacuation, and therefore the non-retention, of the stones 20.
[0051] Figure 5 is a schematic cross-sectional view of an example of a stepped ventilation shaft 8, along its longest dimension, worn down to its second radially inner portion and containing stones. The stones 21 are of such a size that they cannot penetrate the secondary cavities 13, which are separated in pairs by a protrusion 14. Furthermore, the double-sloped inclined wall of each secondary cavity 13 is configured to allow the stones 21 to be evacuated, and thus not retained, at its first radially outer portion, and to prevent their penetration at its second radially inner portion.
[0052] Figure 6 is a schematic top view of a stepped ventilation cavity 8, the largest and smallest dimensions of which (Dmax, Dmin) are respectively parallel to the largest and smallest dimensions (Lmax, Lmin) of the contact face of the block 5, according to a preferred embodiment of the invention. The distance between the axes of two successive secondary cavities 13, separated in pairs by a protrusion 14, is equal to the pitch P. Preferably, the pitch P satisfies the relation: P = K*H, K being a coefficient between 1 / 3 and 1 and H being the depth of the ventilation cavity 8. Furthermore, the number N of cavities, equal to 4 in Figure 6, preferentially satisfies the relation: N = E ((Lmax - Lmin) / (K*H)), E being the integer part (in the mathematical sense) of the ratio (Lmax - Lmin) / (K*H).
[0053] The invention was studied more particularly for a tire for a dumper-type civil engineering vehicle in the size 59 / 80 R 63.
[0054] Table 1 below presents the characteristics of the example studied by the inventors: [Table 1]
[0055] Customer tests have confirmed the thermal gain provided by the ventilation grooves in the blocks at the axial ends of the tread, the absence of stone retention by the ventilation grooves and the mechanical robustness of the blocks including the ventilation grooves.
Claims
Demands 1. Tyre (1) for a heavy construction vehicle having a tread (2) intended to come into contact with a ground via a tread surface (3), -the tread (2) comprising an arrangement of cutouts (4) separating blocks (5) and having a radial thickness E, measured, along a radial direction (ZZ') perpendicular to an axis of rotation of the tire, between the tread surface (3) and the bottom of the deepest cutout (4), -any block (5) having a contact face (6), contained within the rolling surface (3), and being delimited by at least three lateral faces (7) intersecting the contact face (6), said contact face (6) having a smaller dimension Lmin and a larger dimension Lmax, -at least one block (5) comprising a ventilation cavity (8), extending radially inwards from the contact face (6) to a depth H and opening onto the contact face (6) by means of an opening surface (9) having a smaller dimension Dmin and a larger dimension Dmax, characterized in that the ventilation cavity (8) comprises a first radially external portion (10), consisting of a single primary cavity (11), extended radially inwards by a second radially internal portion (12), comprising at least two independent secondary cavities (13) separated from each other by a protrusion (14), and in that each secondary cavity (13) of the second radially internal portion (12) has a double-sloped inclined wall (17), the radially external portion (18) of the inclined wall (17), forming with the radial direction (ZZ'),an angle A2 of at least 20° and the radially inner portion (19) of the inclined wall (17), forming with the radial direction (ZZ'), an angle A3 of at most 15°.
2. Pneumatic (1) according to claim 1, wherein the depth H of the ventilation hollow (8), measured, along the radial direction (ZZ'), between the contact face (6) of the block (5) and the bottom (16) of the deepest secondary cavity (13), is at least equal to 70% of the radial thickness E of the tread (2).
3. Pneumatic (1) according to claim 1 or 2, wherein the first radially outer portion (10) has a depth Hl, measured between the contact face (6) and the apex (15) of the protrusion (14) of the second radially inner portion (12) the highest, at least equal to 30% and at most equal to 60% of the depth H of the ventilation hollow (8).
4. Pneumatic (1) according to any one of claims 1 to 3, wherein the single primary cavity (11) of the first radially outer portion (10) has an inclined wall (16), forming with the radial direction (ZZ'), an angle Al of at least 20°.
5. Pneumatic (1) according to any one of claims 1 to 4, wherein the second radially inner portion (12) has a depth H2, measured between the apex (15) of the highest protrusion (14) and the bottom (16) of the secondary cavity (13) the deepest, at least equal to 40% and at most equal to 70% of the depth H of the ventilation hollow (8).
6. Pneumatic (1) according to any one of claims 1 to 5, wherein the apexes (15) of a portion of the protuberances (14) of the second radially inner portion (12) are positioned, relative to the contact face (6), at a radial distance strictly greater than the depth Hl of the first radially outer portion (10).
7. Pneumatic (1) according to any one of claims 1 to 6, wherein the bottoms (16) of a portion of the secondary cavities (13) of the second radially inner portion (12) are positioned, relative to the contact face (6), at a radial distance strictly less than the depth H of the ventilation hollow (8).
8. Pneumatic (1) according to any one of claims 1 to 7, wherein the smallest dimension Dmin of the opening surface (9) of the ventilation recess (8) has a direction parallel to the direction of the smallest dimension Lmin of the contact face (6) of the block (5) comprising the ventilation recess (8), and wherein the largest dimension Dmax of the opening surface (9) of the ventilation recess (8) has a direction parallel to the direction of the largest dimension Lmax of the contact face (6) of the block (5) including the ventilation hollow (8).
9. Pneumatic (1) according to claim 8, wherein the following two conditions are met: - P = K*H, P being the distance between the axes of two successive secondary cavities (13) of the second radially internal portion (12), K being a coefficient between 1 / 3 and 1 and H being the depth of the ventilation cavity (8), - N = E ((Lmax - Lmin) / (K*H)), N being the number of secondary cavities (13) of the second radially internal portion (12) and E being the integer part (in the mathematical sense) of the ratio (Lmax - Lmin) / (K*H).
10. Pneumatic (1) according to any one of claims 1 to 9, wherein the largest dimension Lmax of a block (5) is greater than or equal to 1.5 times its smallest dimension Lmin.
11. Pneumatic (1) according to any one of claims 1 to 10, wherein the tread (2) comprises, at each of its axial ends, a circumferential row of blocks (5) provided with a ventilation hollow (8).