阅读说明：本技术 具有优化的结构和胎面的轮胎 (Tire with optimized structure and tread ) 是由 A·博内 F·布儒瓦 于 2018-04-25 设计创作，主要内容包括：本发明的目的是增加轮胎的耐久性,所述轮胎包括两个交叉的工作层(41、42),所述工作层(41、42)包括由单根金属线或单丝组成的增强元件,所述单根金属线或单丝具有至少等于0.20mm且至多等于0.5mm的截面。胎面的两个轴向外部部分中的至少一个轴向外部部分包含橡胶材料M1,所述橡胶材料M1具有至少等于48且至多等于60的肖氏硬度以及在23℃下测得的至少等于0.12且至多等于0.30的动力学性能tan(d)max。包含橡胶材料M1的所述至少一个轴向外部部分(22、23)的轴向外部切口(24)具有至少等于5mm的深度D和至多等于2mm的宽度W。(The aim of the invention is to increase the endurance of a tyre comprising two crossed working layers (41, 42), said working layers (41, 42) comprising reinforcing elements consisting of a single metal wire or monofilament having a section at least equal to 0.20mm and at most equal to 0.5 mm. At least one of the two axially external portions of the tread comprises a rubber material M1, said rubber material M1 having a shore hardness at least equal to 48 and at most equal to 60 and a dynamic property tan (d) max measured at 23 ℃ at least equal to 0.12 and at most equal to 0.30. The axially external cut (24) of said at least one axially external portion (22, 23) comprising rubber material M1 has a depth D at least equal to 5mm and a width W at most equal to 2 mm.)

1. Tyre (1) for passenger vehicles, said tyre (1) comprising:

a tread (2) intended to be in contact with the ground through a tread surface (21) and having an axial width LT,

the tread (2) comprising two axially external portions (22, 23), the two axially external portions (22, 23) each having an axial width (LS1, LS2) at most equal to 0.3 times the axial width LT and comprising at least one rubber material intended to be in contact with the ground during operation,

-at least one axially external portion (22, 23) comprises an axially external cut (24, 25), the axially external cut (24, 25) forming a space open to the tread surface (21) and being delimited by at least two lateral surfaces, called main lateral surfaces (241, 242), connected by a bottom surface (243),

-the axially external cut (24, 25) has a depth D defined by the maximum radial distance between the tread surface (21) and the bottom surface (243),

the tyre further comprising a crown reinforcement (3) radially inside the tread (2),

the crown reinforcement (3) comprising a working reinforcement (4) and a hooping reinforcement (5),

-the working reinforcement (4) is constituted by two working layers (41, 42), the two working layers (41, 42) each comprising reinforcing elements coated in elastomeric material, the reinforcing elements being parallel to each other and forming, respectively with the circumferential direction (XX') of the tyre, an orientation angle (A1, A2) having an absolute value at least equal to 20 ° and at most equal to 50 ° and having opposite layer-to-layer signs,

the reinforcing elements in each ply are constituted by individual wires or monofilaments having a cross section with a minimum dimension at least equal to 0.20mm and at most equal to 0.5mm and a breaking strength Rm,

the density of the reinforcing elements in each working layer is at least equal to 100 threads/dm and at most equal to 200 threads/dm,

the hooping reinforcement (5) comprising at least one hooping layer comprising reinforcing elements parallel to one another and forming, with the circumferential direction (XX') of the tyre, an angle B at most equal to 10 DEG in absolute value,

characterized in that one of the two axially external portions (22, 23) comprises at least one rubber material M1 intended to be in contact with the ground during operation, the rubber material M1 having a Shore hardness at least equal to 48 and at most equal to 60 and a dynamic property tan (d) max measured at 23 ℃ at least equal to 0.12 and at most equal to 0.30,

the axially external cuts (24, 25) in at least one axially external portion (22, 23) comprising rubber material M1 have a depth D at least equal to 5mm, have a width W at most equal to 2mm and are spaced apart in the circumferential direction (XX') of the tyre by a circumferential pitch P at least equal to 4mm,

the fracture strength Rc of each working layer (41, 42) is at least equal to 30000N/dm, Rc being defined as: rc Rm S d, where Rm is the tensile breaking strength in MPa of the monofilament and S is the mm of the monofilament²Cross-sectional area and d is the density in number of filaments/dm of filaments in the working layer under consideration.

2. Tyre according to claim 1, wherein the axially external incisions (24, 25) containing at least one axially external portion (22, 23) of rubber material M1 comprise at least one protuberance (244), said protuberance (244) locally reducing the width W of the incision to a width (W') at least equal to 0.2mm and at most equal to 0.5 mm.

3. Tyre according to any one of claims 1 or 2, wherein said axially external cuts (24, 25) are spaced apart in the circumferential direction (XX') of the tyre by a circumferential pitch P at most equal to 25 mm.

4. Tyre according to any one of claims 1 to 3, wherein the depth D of said axially external cuts (24, 25) is at most equal to 8 mm.

5. Tyre according to any one of claims 1 to 4, wherein the radial distance D1 between the bottom surface (243) of the axially external cut (24, 25) and the crown reinforcement (3) is at least equal to 1 mm.

6. Tyre according to any one of claims 1 to 5, wherein the radial distance D1 between the bottom surface (243) of the axially external cut (24, 25) and the crown reinforcement (3) is at most equal to 3.5 mm.

7. Tyre according to any one of claims 1 to 6, wherein each of the two axially external portions (22, 23) of the tread (2) has an axial width (LS1, LS2) at most equal to 0.2 times the axial width LT.

8. Tyre according to any one of claims 1 to 7, wherein each working layer (41, 42) comprises reinforcing elements constituted by individual wires or monofilaments having a diameter at least equal to 0.3mm and at most equal to 0.37 mm.

9. Tyre according to any one of claims 1 to 8, wherein each working layer (41, 42) comprises reinforcing elements forming an angle (A1, A2) at least equal to 22 ° and at most equal to 35 ° with the circumferential direction (XX') of the tyre.

10. Tyre according to any one of claims 1 to 9, wherein the density of reinforcing elements in each working layer (41, 42) is at least equal to 120 threads/dm and at most equal to 180 threads/dm.

11. Tyre according to any one of claims 1 to 10, wherein the reinforcing elements of the working layers (41, 42) are made of steel, preferably of carbon steel.

12. Tyre according to any one of claims 1 to 11, wherein the reinforcing elements of at least one hoop layer are made of fabric, preferably of aliphatic polyamide, aromatic polyamide or a combination of aliphatic polyamide and aromatic polyamide, polyethylene terephthalate or rayon type fabric.

13. Tyre according to any one of claims 1 to 12, wherein the hooping reinforcement (5) is radially outside the working reinforcement (4).

Technical Field

The present invention relates to tires for passenger vehicles, and more particularly to the crown of such tires.

Since a tire has a geometry that rotates about an axis of rotation, the geometry of the tire is generally described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial, axial and circumferential directions respectively denote a direction perpendicular to the axis of rotation of the tyre, a direction parallel to the axis of rotation of the tyre and a direction perpendicular to the meridian plane.

In the following, the expressions "radially on the inside" and "radially on the outside" mean "closer to the axis of rotation of the tyre in the radial direction" and "further away from the axis of rotation of the tyre in the radial direction", respectively. The expressions "axially on the inside" and "axially on the outside" mean "closer to the equatorial plane in the axial direction" and "further from the equatorial plane in the axial direction", respectively. "radial distance" is the distance relative to the axis of rotation of the tire and "axial distance" is the distance relative to the equatorial plane of the tire. The "radial thickness" is measured in the radial direction and the "axial width" is measured in the axial direction.

The tire comprises a crown comprising a tread intended to be in contact with the ground through a tread surface, two beads intended to be in contact with a rim, and two sidewalls connecting the crown to the beads. Furthermore, the tire comprises a carcass reinforcement comprising at least one carcass layer, radially inside the crown and connecting the two beads.

The tread of the tire is delimited in the radial direction by two circumferential surfaces, of which the radially outermost circumferential surface is referred to as the tread surface and the radially innermost circumferential surface is referred to as the tread bottom surface. Furthermore, the tread of the tire is delimited in the axial direction by two side surfaces. The tread is also composed of one or more rubber compounds. The expression "rubber compound" means a rubber composition comprising at least an elastomer and a filler.

The crown comprises at least one crown reinforcement located radially inside the tread. The crown reinforcement comprises at least one working reinforcement comprising at least one working layer consisting of mutually parallel reinforcing elements forming an angle of between 15 ° and 50 ° with the circumferential direction. The crown reinforcement may also comprise at least one hooping layer consisting of reinforcing elements forming an angle of between 0 ° and 10 ° with the circumferential direction, this hooping reinforcement being generally (but not necessarily) radially outside the working layer.

In order to obtain good grip on wet ground, a cut is made in the tread. The incisions represent indentations or grooves or sipes or circumferential grooves and form spaces to the tread surface. At the tread surface, the indentations have no characteristic major dimension. On the tread surface, a sipe or groove has two characteristic main dimensions: width W and length Lo such that length Lo is at least equal to twice width W. The sipe or groove is thus delimited by at least two main side surfaces, which determine its length Lo and are connected by the bottom surface, the two main side surfaces being separated from each other by a non-zero distance, referred to as the width W of the sipe or groove.

The definition of a sipe or groove is by definition as follows:

sipes or grooves delimited only by two major side surfaces are called open,

the sipes or grooves delimited by three side surfaces (two of which are the main surfaces determining the length of the incisions) are called blind grooves,

the siping or groove delimited by four side surfaces, two of which are the main surfaces determining the length of the cut, is called a double blind groove.

The difference between the sipe and the groove is the average value of the distance separating the two main side surfaces of the incision, i.e. its width W. In the case of sipes, this distance is adapted to allow the contact of two major side surfaces facing each other when the sipe enters a contact patch where the tire is in contact with the road surface. In the case of a trench, the major side surfaces of the trench cannot contact each other under normal operating conditions. For passenger vehicle tires, this distance of sipes is typically at most equal to 1 millimeter (mm). The circumferential groove is a cut substantially in the circumferential direction, which is substantially continuous over the entire circumference of the tire.

More specifically, the width W is the average distance determined along the length of the cut and along the radial portion of the cut between a first circumferential surface located radially inside the tread surface at a radial distance of 1mm and a second circumferential surface located radially outside the bottom surface at a radial distance of 1mm, thereby avoiding any measurement problems associated with the junction where the two major side surfaces intersect the tread surface and the bottom surface.

The depth of the cut is the maximum radial distance between the tread surface and the bottom of the cut. The maximum value of the depth of the cut is called the tread depth D. The tread bottom surface, or bottom surface, is defined as the surface of the tread surface that translates radially inward by a radial distance equal to the tread depth. The incisions delimit relief elements, called ribs, on the tread.

Background

In the context of current sustainable development, saving energy and thus raw materials is one of the key objectives of the industry. One of the research routes to achieve this aim, for passenger vehicle tyres, is to replace the metal cords usually used as reinforcing elements in the various layers of the crown reinforcement with independent threads or monofilaments as described in document EP 0043563, wherein the dual purpose of using such reinforcing elements is to reduce the weight and to reduce the rolling resistance.

However, the use of such reinforcing elements has the drawback of causing these monofilaments to buckle under compression, so that the tire exhibits insufficient durability, as described in document EP 2537686. As described in this same document, the person skilled in the art proposes a specific layout of the various layers of the crown reinforcement and a specific quality of the material constituting the reinforcing elements of the crown reinforcement, in order to solve this problem.

Detailed analysis of the physical phenomena shows that buckling of the monofilaments occurs in the axially outermost part of the tread below the cut, as described in documents JP 2012071791, EP 2016075729, EP 2016075721, EP 2016075725, EP 2016075741. The tire area is particularly characterized by high compressive loads experienced when the vehicle is operating in a curved path. The flex resistance of the monofilaments is dependent on the geometry of the cut, thus demonstrating that the tread pattern has an unexpected effect on the durability of the monofilaments.

Disclosure of Invention

The main object of the present invention is to optimize optimally the tread of such a tire, with the dual purpose of improving the rolling resistance of the tire and the durability of the monofilaments that make up the reinforcing elements of the working layers of the tire, by adjusting the mechanical properties of the rubber compound of the tread and the geometry and layout of the incisions.

This object is achieved by a passenger vehicle tyre comprising:

a tread intended to be in contact with the ground through a tread surface and having an axial width LT,

the tread comprises two axially external portions each having an axial width at most equal to 0.3 times the axial width LT and comprising at least one rubber material intended to come into contact with the ground during operation,

at least one axially external portion comprises an axially external cut forming a space open to the tread surface and delimited by at least two lateral surfaces, called main lateral surfaces, connected by a bottom surface,

said axially external cut has a depth D defined by the maximum radial distance between the tread surface and the bottom surface,

the tire further comprises a crown reinforcement located radially inside the tread,

the crown reinforcement comprising a working reinforcement and a hooping reinforcement,

the working reinforcement is constituted by two working layers, each comprising reinforcing elements encased in an elastomeric material, the reinforcing elements being parallel to each other and forming, respectively with the circumferential direction (XX') of the tyre, an orientation angle (A1, A2) having an absolute value at least equal to 20 ° and at most equal to 50 ° and having opposite signs from layer to layer,

the reinforcing elements in each ply are constituted by individual metal wires or monofilaments having a cross section with a minimum dimension at least equal to 0.20mm and at most equal to 0.5mm and a breaking strength Rm,

the density of the reinforcing elements in each working layer is at least equal to 100 threads/dm and at most equal to 200 threads/dm,

the hooping reinforcement comprising at least one hooping layer, the hooping layer comprising reinforcing elements parallel to one another and forming, with the circumferential direction (XX') of the tire, an angle B at most equal to 10 DEG in absolute value,

one of the two axially external portions comprises at least one rubber material M1 intended to be in contact with the running surface, said rubber material M1 having a Shore hardness at least equal to 48 and at most equal to 60 and a dynamic property tan (d) max measured at 23 ℃ at least equal to 0.12 and at most equal to 0.30,

axially external cuts in at least one axially external portion comprising rubber material M1, having a depth D at least equal to 5mm, having a width W at most equal to 2mm, and being spaced at a circumferential pitch P at least equal to 4mm in the circumferential direction of the tyre,

the breaking strength Rc of each working layer is at least equal to 30000N/dm, Rc being defined as: rc Rm S d, where Rm is the tensile breaking strength in MPa of the monofilament and S is the mm of the monofilament²Cross-sectional area of the meter, and d is taken into accountThe density of the filaments in the working layer of (2) in terms of number of filaments per dm.

From the point of view of mechanical operation, buckling of the reinforcing elements occurs under compression. The buckling occurs only in those regions of the working layer which are radially inside the axially outermost part of the tread, since the compressive load is highest in this region in the case of transverse loads. The maximum axial width of each of these axially outermost portions is 0.3 times the total width of the tire tread.

The buckling resistance of a monofilament also depends on the resistance of axially adjacent filaments, where the onset of buckling of one can cause buckling of the other through the influence of the load distribution around the buckling monofilament. In order to obtain improved durability, it is appropriate not only to take care of the filament density and diameter conditions, but also to satisfy the conditions relating to the strength of the working layers, i.e. the breaking strength R of each working layer_CIt is necessary to be at least equal to 30000N/dm, Rc being defined as: rc Rm S d, where Rm is the tensile breaking strength in MPa of the monofilament and S is the mm of the monofilament²Cross-sectional area and d is the density in number of filaments/dm of filaments in the working layer under consideration.

Buckling is a complex and unstable phenomenon that can cause fatigue fracture of at least one object (e.g., a beam or shell) that is an order of magnitude smaller than the major dimension. A monofilament is such an object with a cross-section much smaller than its length. The phenomenon starts when the major dimension is compressed. This phenomenon persists due to the asymmetry of the monofilament geometry or due to the presence of transverse forces caused by bending of the monofilament, which is a highly destructive stress load for metallic materials. This complex phenomenon is highly dependent in particular on the boundary conditions, the movability of the element, the direction in which the load is applied and the deformations resulting from this load. Buckling does not occur if the deformation does not occur substantially in the direction of the main dimension of the monofilament, the load being absorbed by shearing of the rubber compound between the monofilaments in the case of monofilaments surrounded by a matrix of rubber compound, such as those of a working layer of a tire.

However, research work has proved advantageous to use, as such material, in one of the two axially outer portions (LS1, LS2), preferably the one that is the most heavily loaded in the tread, a rubber material with low stiffness, intended to be in contact with the ground when the tire is new or when the tire is not new but is normally used on a vehicle under conditions of legal safety wear (hereinafter referred to as tread material); the rubber material with low rigidity is a rubber material with the following characteristics:

a Shore hardness measured according to the standard ASTM 2240 or DIN 53505 at least equal to 48 and at most equal to 60,

the kinetic properties tan (d) max, measured at 23 ℃, being at least equal to 0.12 and at most equal to 0.30. The kinetic properties tan (D) max were measured on a viscosity analyzer (Metravib VA4000) according to the standard ASTM D5992-96. The response of a sample of the vulcanized composition (cylindrical test specimen having a thickness of 4mm and a cross-section of 400mm 2) subjected to a simple alternating sinusoidal shear stress at a frequency of 10Hz at 23 ℃ was recorded. The strain amplitude sweep was performed from 0% to 50% (outward cycle) and then from 50% to 0% (return cycle). For the return cycle, the maximum value tan (d) max of tan (d) observed was measured. The lower the value of tan (d) at 23 ℃, the better the rolling resistance of the tire.

The use of such a material in the tread makes it possible to reduce the magnitude of the stress load in the monofilaments by more than 5%. In addition to this property, these materials also offer the advantage of generally having high grip properties.

Their low hysteresis and low stiffness properties mean that they are not intended to be incorporated into the tread material that is in contact with the ground during operation, but rather as a material inside the tire to achieve rolling resistance benefits. In internal use, the applied deformation is mostly an imposed deformation. Thus, the lower hysteresis of this material results in improved rolling resistance for the same deformation. However, since the tread material operates under imposed stresses: the softer the material, the more it deforms. Upon leaving the contact patch, the rib regains its geometry and slides on the ground on which the tire is running for a distance known as the sliding distance, due to the reduction in contact pressure. The sliding distance of the ribs containing the softer material is greater than the sliding distance of the ribs containing the harder material. Rib wear is proportional to the sliding distance. Therefore, these materials are disadvantageous in terms of wear.

The invention is characterized in that: the use of a soft tread material maintains the durability of the monofilaments, and adjusts the tread pattern in the axially outer portion of the tread to make the tread perform sufficiently well in terms of wear. This effect is obtained by: for incisions having a depth greater than 5mm, the width of the incision in the axially outer portion of the tread is defined as 2 mm. In this case, since the width of the incision is small, in the contact patch, the ribs made of a soft material abut against each other, act like large ribs having a large rigidity, deform less and slide over a short sliding distance. The wear performance obtained by such a tread pattern associated with such a tread material is of the same order of magnitude as that obtained by a prior art tread pattern having a harder tread material.

For incisions of shallow depth, i.e. less than 5mm, it is for this characteristic reason that the rigidity of the ribs of the tread pattern is also higher.

Furthermore, by limiting the movement of the ribs of the tread pattern in the axially outer portion of the tread, the tread pattern according to the invention enables a better exploitation of the low hysteresis potential of the tread material. This makes it possible to reduce the rolling resistance of the tire, which is one of the main purposes of using monofilaments.

Furthermore, such a tread pattern limits the compression movements in the axially outer portions of the tread, making it possible to reduce the stresses in the monofilaments of the working layer, thereby improving the endurance performance related to buckling.

Thus, according to the invention, the structure, material and tread pattern thus "cooperate" to achieve the primary objective of reducing buckling in the monofilaments of the working layer, but also to achieve the final design objective of providing better rolling resistance.

In order to maintain tread pattern stiffness to allow sufficient wear performance and behaviour, narrow incisions having a width less than 2mm and a depth D at least equal to 5mm are spaced apart in the circumferential direction (XX') of the tyre by a circumferential pitch P at least equal to 4 mm. The circumferential pitch is the average circumferential distance between the average linear profiles of two circumferentially consecutive axially outer incisions on the axially outermost portion considered in the tread. In general, tire treads may have a significantly variable circumferential spacing to limit road noise.

In the case of incisions that are not axially external and therefore in the case of portions of the tread that are not axially external, the compressive load under lateral loading of the tire is too low to cause buckling. In the case of such a part, the design used on the axially outer part can be used universally or designs known in the art can be used.

The axially external cut may be of any shape: curved, sinusoidal, zigzag, and thus by appropriate design may or may not allow relative movement of its major side surfaces during operation. In order to prevent relative movements of the main side surfaces of the incisions, it is sufficient to create tread pattern elements that are such that one surface abuts the other when the movement in question is about to occur. A sipe that zigzags or sinusoidally curves along the length of the incision is an example of such an incision if the two major side surfaces are designed to abut each other. If the corrugation of the sipe is in the depth direction, relative radial movement of the major side surfaces will be prevented. If the sipe has a double wave shape in its length and depth directions, both movements will be prevented. Since large relative movements of the crown element cause the monofilaments of the working layer to buckle, particularly movements allowed by the tread pattern, any resistance to movement within the tread pattern may result in improved durability of the monofilaments.

It is therefore particularly advantageous for the axially external cut comprising at least one axially external portion of rubber material M1 to comprise at least one protrusion which locally reduces the width W of the cut to a width at least equal to 0.2mm and at most equal to 0.5 mm. Such a protrusion allows the tread ribs provided with the protrusion to contact each other at lower stress load levels, so that the stiffness increase produced by such contact is also obtained for these stress load levels. They thus make it possible to limit the amount of wear under light loads, so that the wear performance as a whole is improved, and more particularly local wear patterns can be avoided. A protuberance refers to a local variation in relief of at least one lateral surface of the incision.

The two axially outer portions of the tread may comprise one or more circumferential grooves in order to reduce the risk of hydroplaning on wet ground. For passenger vehicle tires, these circumferential grooves typically represent small width contact patches and have no known effect on monofilament buckling.

The monofilament can have any cross-sectional shape, and an elliptical cross-section is known to have advantages (even when having smaller dimensions) over a circular cross-section because of its higher inertia when bent, and thus its higher resistance to buckling. In the case of a circular cross-section, the smallest dimension corresponds to the diameter of the cross-section. In order to guarantee the fatigue fracture strength of the filaments and the shear resistance of the rubber compound located between the filaments, the density of the reinforcing elements of each working layer is at least equal to 100 threads/dm and at most equal to 200 threads/dm. Density means the average number of filaments within a 10 cm width of the working layer, measured perpendicular to the direction of the filaments of the working layer under consideration. The distance between successive reinforcing elements may be fixed or variable. The reinforcing elements may be laid down in layers or strips during manufacture or laid down separately.

The buckling resistance of a monofilament also depends on the resistance of axially adjacent filaments, where the onset of buckling of one can cause buckling of the other through the influence of the load distribution around the buckling monofilament. In order to obtain improved durability, it is appropriate not only to take care of the filament density and diameter conditions, but also to satisfy the conditions relating to the strength of the working layers, i.e. the breaking strength R of each working layer_CIt is necessary to be at least equal to 30000N/dm, Rc being defined as: rc Rm S d, where Rm is the tensile breaking strength in MPa of the monofilament and S is the mm of the monofilament²Cross-sectional area and d is the density in number of filaments/dm of filaments in the working layer under consideration.

For tires that do not impose a particular mounting direction, one solution involves applying the invention to the two axially outermost portions of the tread.

For tires that impose a particular mounting direction, one option is to apply the invention only to the axially outermost portion of the tread that is located on the outside of the vehicle.

The tread pattern of a passenger vehicle tyre is generally substantially symmetrical or substantially antisymmetric or substantially asymmetrical.

It is advantageous for the axially external incisions to be open axially to the outside of the axially external portion of the tread, so that water stored in the incisions can be displaced outside the contact patch if running on a wet road.

Preferably, when the tread comprises at least one circumferential groove, the axially external cut opens axially to the inside of the circumferential groove of the tread, so as to enable the water stored in the cut to be removed to the main water reservoir if running on a wet road.

For grip performance, it is also preferable that the axially external cuts are spaced in the circumferential direction (XX') of the tyre by a circumferential pitch P at most equal to 25mm, so as to avoid a density of cuts that is too low, resulting in insufficient grip performance.

Preferably, the depth D of the axially external incisions is at most equal to 8mm, in order to prevent excessive flexibility of the tread pattern and loss of performance in terms of wear and rolling resistance.

It is particularly advantageous for the radial distance D1 between the bottom surface of the axially external cut and the crown reinforcement to be at least equal to 1 mm. This is because this minimum amount of rubber material protects the crown from being attacked and pierced by obstacles, stones or any debris laid on the ground.

It is preferable that the radial distance D1 between the bottom surface of the axially external incisions and the crown reinforcement be at most equal to 3.5mm, in order to obtain a tire that performs well in terms of rolling resistance.

Advantageously, the axial width (LS1, LS2) of each of the two axially external portions of the tread is at most equal to 0.2 times the axial width LT of the tread.

Preferably, each working layer comprises reinforcing elements consisting of individual wires or monofilaments having a minimum dimension of the cross section at least equal to 0.3mm and at most equal to 0.37mm, which constitutes an optimal balance in terms of target performance (weight reduction and buckling durability of the reinforcing elements of the working layer).

One preferred solution is that each working layer comprises reinforcing elements forming an angle at least equal to 22 ° and at most equal to 35 ° with the circumferential direction (XX') of the tyre, which constitutes the best compromise between tyre behaviour and tyre endurance performance.

It is advantageous that the density of the reinforcing elements in each working layer is at least equal to 120 threads/dm and at most equal to 180 threads/dm, so as to guarantee the durability of the rubber compound operating under shear between the reinforcing elements and its tensile and compressive durability.

The reinforcing elements of the working layer may be linear or may be non-linear. The reinforcing elements may be preformed in a sinusoidal, zig-zag or corrugated shape, or spiraled along a row. The reinforcing elements of the working layer are made of steel (preferably carbon steel, such as that used in cords of the "steel cord" type), although it is also possible to use other steels (for example stainless steel) or other alloys.

When carbon steel is used, the carbon content thereof (% by weight of steel) is preferably in the range of 0.8% to 1.2%. The invention is particularly applicable to very High strength, referred to as "SHT" ("Super High strength"), Ultra High strength, referred to as "UHT" ("Ultra High strength"), or curtain line type steel of "MT" ("Mega strength") steel. The carbon steel reinforcement then has a tensile breaking strength (Rm) preferably higher than 3000MPa, more preferably higher than 3500 MPa. Their total elongation At break (At), which is the sum of the elastic and plastic elongations, is preferably greater than 2.0%.

For steel reinforcements, the measurements of the breaking strength (in MPa) expressed in Rm and the elongation At break (total elongation in%) expressed in At are carried out under tension according to ISO standard 6892 of 1984.

The steel used, whether it is in particular carbon steel or stainless steel, may itself be coated with a metal layer which, for example, improves the workability or reinforcement of the steel monofilament and/or the wear properties of the tire itself, such as adhesion, corrosion resistance or ageing resistance. According to a preferred embodiment, the steel used is coated with a layer of brass (Zn-Cu alloy) or a layer of zinc; it will be recalled that the brass or zinc coating makes the wire easier to draw and better adheres to the rubber during its manufacture. However, the reinforcement may be covered with a thin layer of a metal other than brass or zinc, for example having the function of improving the corrosion resistance of these wires and/or their adhesion to rubber, such as thin layers of Co, Ni, Al, and of alloys of two or more of Cu, Zn, Al, Ni, Co, Sn compounds.

Preferably, the reinforcing elements of at least one hoop layer are made of a fabric, which is of aliphatic polyamide, aromatic polyamide, a combination of aliphatic polyamide and aromatic polyamide, polyethylene terephthalate or rayon type, since fabric materials are particularly suitable for this use due to their light weight and high stiffness. The distance between successive reinforcing elements in the hoop layer may be fixed or variable. The reinforcing elements may be laid down in layers or strips during manufacture or laid down separately.

It is advantageous that the hoop reinforcement is located radially outside the working reinforcement to ensure good durability of the working reinforcement.

Drawings

The features and other advantages of the present invention will be better understood with the aid of fig. 1 to 4, which are not drawn to scale but are drawn in a simplified manner to make the invention easier to understand:

figure 1 shows a perspective view of a portion of a tyre according to the invention, in particular its structure and its tread.

Figure 2 shows a meridional section through the crown and shows the axially external portions 22 and 23 of the tread and their width.

Figures 3A and 3B show the meridian profiles of the two radially external portions of the tread of a passenger vehicle tyre.

Fig. 4A and 4B show two cross sections of an axially external cut in which there are projections locally reducing its width W.

Detailed Description

Fig. 1 shows a crown portion of a tire. The tire comprises a tread 2, said tread 2 being intended to be in contact with the ground through a tread surface 21. In the axially external portions 22 and 23 of the tread, there are main grooves 26, the axially external cuts comprising sipes 24 and grooves 25. The tyre further comprises a crown reinforcement 3, said crown reinforcement 3 comprising a working reinforcement 4 comprising two working layers 41 and 42 and a hooping reinforcement 5. Fig. 1 further shows double-blind incisions, incisions of simple and complex type (i.e. with parallel side surfaces, or with side surfaces with a zigzag or sinusoidal portion in the main direction of the incision or its depth, to prevent some relative movement of the two side surfaces) that are open axially to the outside or inside.

Fig. 1 shows only axial, axially outer cuts in the axially outer portions 22 and 23 of the tread along the axial axis (YY'). In fact, this depiction is merely for the sake of facilitating the readability of fig. 1, it being possible to make the axially external incisions in the tread of the passenger vehicle form an angle of between plus or minus 60 ° with the axial direction (YY'), depending on the performance aimed, in particular in terms of wet grip.

Figure 2 schematically shows a meridian section through the crown of a tyre according to the invention. It particularly shows the widths LS1 and LS2 of the axially outer portions 22 and 23 of the tread, as well as the overall width LT of the tire tread. The depth D of the axially external incisions 24, 25, measured along a meridian section of the tyre, and the distance D1 between the bottom surface 243 of the axially external incisions 24, 25 and the crown reinforcement 3 are also shown. A meridian section through the tyre is obtained by cutting the tyre in two meridian planes. For example, a meridian section of the tire has a thickness of about 60mm at the tread in the circumferential direction. The measurement is carried out with the distance between the two beads remaining the same as that of the tyre mounted on its rim and slightly inflated.

In fig. 3A and 3B, the axial edges 7 of the tread are determined, said axial edges 7 making it possible to measure the tread width. In fig. 3A, the tread surface 21 intersects the axially outer surface 8 of the tire, the axial edge 7 being readily determined by one skilled in the art. In fig. 3B, the tread surface 21 is continuous with the axially outer surface 8 of the tire, on a meridional section of the tire a tangent to the tread surface at any point on said tread surface in the region of the transition towards the sidewall is drawn. The first axial edge 7 is the point where the angle β between said tangent and the axial direction YY' is equal to 30 °. When there are several points where the angle J between the tangent and the axial direction is equal to 30 °, the radially outermost point is used. The second axial edge of the tread is determined using the same method.

Fig. 4 shows two cross sections of the axially external cuts (24, 25), wherein the presence of at least one protrusion (244) locally reduces the cut width W at most equal to 2mm to a width W' at least equal to 0.2mm and at most equal to 0.5 mm. Fig. 4A shows a cross-section where there are two protrusions (244) on the side surfaces (241, 242) of the cut-out. Fig. 4A shows two asymmetric protrusions, but symmetric protrusions may be used in the cut-outs according to the invention. Fig. 4B shows a cross section of the cut (24, 25) wherein a single protrusion (244) locally reducing the cut width W to a width W' at least equal to 0.2mm and at most equal to 0.5mm is provided on the side surface 241 (or possibly 242).

The inventors have calculated, on the basis of the invention, a tyre of size 205/55R16 inflated to a pressure of 2 bar comprising two working layers of steel monofilaments having a diameter of 0.3mm, distributed at a density of 158 wires/dm and forming angles with the circumferential direction XX' equal to 27 ° and-27 °, respectively. The monofilaments have a breaking strength Rm equal to 3500MPa and the working layers each have a breaking strength Rc equal to 39000N/dm. The tire comprises axially external incisions on two axially external portions of the tread of the tire, said two axially external portions having an axial width equal to 0.21 times the axial width of the tread. The radial distance D1 between the bottom surface of the axially external cut and the crown reinforcement is at least equal to 2 mm.

Calculations were performed for each tire. The tyre a not according to the invention comprises incisions having a rectangular cross section, the width of which remains constant over the width of between 2.5mm and 3.5mm, the depth of the incisions being equal to 6.5 mm. The average circumferential spacing of the incisions was 27 mm. The rubber material of the two axially external portions of the tread of this tire a, not conforming to the invention, intended to be in contact with the ground during operation, is a material characterized by a high stiffness enabling a good compromise between wear and behaviour. The properties of this rubber material are a shore hardness equal to 67 and a tan (δ) max equal to 0.35.

The tire B not according to the invention is identical in all respects to the tire a, with the difference that the rubber material of the two outer portions in the tread of this tire B (intended to be in contact with the ground during operation) is a material characterized by its low stiffness: the properties of this rubber material are a shore hardness equal to 52 and a tan (δ) max equal to 0.17.

The tyre C according to the invention comprises incisions having a rectangular cross section, the width of which, kept constant over the width, is between 0.5mm and 1.5mm, the depth of the incisions being equal to 6.5 mm. The average circumferential spacing of the cuts was 22 mm. The rubber material of the two outer portions of the tread of this tire C according to the invention (intended to be in contact with the ground during operation) is, like that of tire B, a material characterized by its low rigidity: the properties of this rubber material are a shore hardness equal to 52 and a tan (δ) max equal to 0.17.

The conditions used for the calculation reproduce the running conditions of the front tires outside the curve, i.e. the most heavily loaded tires in passenger vehicles. Two cases were simulated, representing the conditions experienced by the tires when the vehicle was cornering at lateral accelerations of 0.3g and 0.7 g. Under a lateral acceleration of 0.3g, the tyre is subjected to a lateral load (Fy) of 150daN and a vertical load (Fz) of 568daN, with a camber angle of 0.68 °. At a lateral acceleration of 0.7g, the tire is subjected to a lateral load (Fy) of 424daN and a vertical load (Fz) of 701daN, with a camber angle of 3 °. In tyres B and C, the use of rubber material intended to be in contact with the ground during operation in the two axially external portions of the tread makes it possible to reduce the stress amplitude calculated for the two stress load levels in the heaviest loaded monofilament by 6.5% compared to those same stress loads calculated for tyre a under the same stress load conditions, adding to 6.5% by the combined effect of tread pattern and material only for tyre C by 10%.

On a highly rough road surface, tires A, B and C were subjected to wear tests on a vehicle at a distance exceeding 10000 km. The wear performance of tire B was reduced by more than 10% compared to tire a, while tire C was improved in this performance by nearly 40% relative to tire a.

Further, the rolling resistance of the tire C according to the present invention was improved by 2% as compared with the tire B, which showed an improvement of 15% with respect to the rolling resistance of the tire a.

Thus, the present invention does synergistically improve the durability of the monofilaments in the working layer as well as the wear and rolling resistance properties of the tire.

Results above 100 represent performance improvements based on base 100.