Tyre for motor vehicles

Abstract

A tyre for motor vehicles comprises a carcass ply (3), a belt structure (11) and a reinforcement structure (16) with zero-degree reinforcement cords (17). The belt structure (11) is formed by a first belt strip (12) and a second belt strip (13) mutually and axially adjacent or partially superimposed at or in proximity to a middle line plane (M) of the tyre (1). The first belt strip (12) and the second belt strip (13) have belt cords (14, 15) which delimit with the circumferential direction (C) of the tyre (1) angles (α, β) of between 25° and 40° and an angle (y) of between 50° and 80° between them. Each reinforcement cord (17) has a "tensile load - elongation" curve comprising a first section (T1), placed upstream of a predefined percentage elongation (Sp), a second section (T2), placed downstream of the predefined percentage elongation (Sp). A second slope of the second section (T2) is greater than a first slope of the first section (T1).

Description

[0001] “Tyre for motor vehicles”

[0002] DESCRIPTION

[0003] Technical field of the invention

[0004] The present invention relates to a tyre for motor vehicles. In particular, the present invention relates to low rolling resistance tyres intended to equip, for example but not exclusively, new hybrid and fully electric motor vehicles, in order to reduce the environmental impact.

[0005] Definitions

[0006] By “curvature ratio” in relation to a tyre it is meant the ratio between the distance of the radially outermost point of the tread band from the line passing through the laterally opposite ends of the tread itself, measured on a radial plane of the tyre and the distance measured along the tyre cord between said ends.

[0007] By “motor vehicle tyre” it is meant a tyre whose curvature ratio is indicatively less than 0.15, preferably between 0.03 and 0.1 , more preferably close to 0.05.

[0008] The terms “radial”, “axial” and “circumferential” refer to an axis of rotation of a tyre. By “radial plane” it is meant a plane in which the axis of rotation of the tyre lies.

[0009] “Tyre middle line plane” means a plane equidistant from the tyre beads and orthogonal to the axis of rotation of the tyre.

[0010] The above mentioned “axial widths” are measured along respective directions parallel to the axis of rotation of the tyre.

[0011] The “axial width of the belt structure” means the width of the semi-finished product constituting the belt structure in the finished tyre.

[0012] By “reinforcement cords oriented in the circumferential direction” it is meant that the reinforcement cords present are all oriented at 0°+ / -10° with respect to the circumferential direction of the tyre.

[0013] By “carcass cords oriented in the axial direction” it is meant that each of the carcass cords present lies in a respective plane which forms an angle of 90° + / - 10° with the middle line plane of the tyre.

[0014] “Overlapped part” means the overlap area between the first belt strip and the second belt strip. “Axial width of the overlapped part” means the width of the overlapped part present in the finished tyre.

[0015] The “tensile load - elongation" curve of a reinforcement cord means the curve drawn on a graph which has the elongation (for example the percentage elongation) of the reinforcement cord on the abscissa axis and the tensile force exerted on the reinforcement cord on the ordinate (for example expressed in N).

[0016] The “tensile load - elongation” curve for metal cords is obtained according to the BISFA standard - Internationally agreed methods for testing steel tyre cords 1995, Chapter E6.

[0017] The “tensile load - elongation” curve for textile cords is obtained according to the BISFA standard - Testing method for polyamide filament yams 2004, chapter 7.

[0018] By “density” it is meant the number of cords present per unit of width, for example 1 dm.

[0019] By “textile cord” it is meant a cord made of a thread or several threads of textile material.

[0020] By “hybrid cord” it is meant a cord made of threads of at least two different textile materials.

[0021] By “metal cord” it is meant a cord made of one or more metal wires.

[0022] By “diameter of a cord” it is meant the diameter of the circumference that circumscribes a cross-section of the cord.

[0023] Background art

[0024] A tyre for motor vehicles generally comprises a carcass structure associated with a belt structure.

[0025] The carcass structure comprises one or more carcass plies having respectively opposite terminal flaps engaged with respective annular anchoring structures, called bead cores, associated with a filling insert. The tyre area comprising the bead core and the filling insert forms a bead structure intended for anchoring tyre onto a corresponding mounting rim. The belt structure comprises several belt strips placed in radial overlap with respect to each other and with respect to the carcass structure, having metal reinforcement cords with a crossed orientation with respect to a circumferential development direction of the tyre. A tread band is arranged in a radially outer position with respect to the belt structure. The adoption of the strip belt structure provided with reinforcement cords crossed with respect to each other in the tyres was necessary to satisfy the request to adapt the tyres to the ever-increasing performance of motor vehicles and to improve the driving features in terms, for example, of stiffness, driving stability and driving readiness.

[0026] Tyres with a belt structure having belt strips having cross-oriented cords are in fact normally capable of providing a greater cornering force, for the same cornering angle, compared to beltless tyres. Tyres with such a belt structure are also normally capable of providing a greater self-aligning torque at low cornering angles than beltless tyres.

[0027] For several years the Applicant has paid attention to the environmental impact exerted directly and indirectly by tyres during their use. In particular, in order to contain the consumption of motor vehicles which have an impact on energy consumption and also on the emission of carbon dioxide into the atmosphere, the Applicant produces tyres with low rolling resistance, i.e. tyres with a fuel efficiency class equal to “A” (according to the European Regulation Ell 2020 / 740).

[0028] A tyre configured to reduce rolling resistance is illustrated in document JPH05286309A. This tyre comprises a radial carcass with polyester cords, a single belt layer formed by a right portion and a left portion adjacent to each other or partially superimposed and provided with steel cords. The steel cords of the two aforementioned portions are arranged symmetrically and form an angle of twenty- two degrees with respect to the circumferential direction of the tyre. A reinforcement layer consisting of a nylon cord tape with a rubber coating is wrapped in coils around the belt layer. Furthermore, JPH05286309A includes an additional reinforcement layer placed astride the two adjacent belt portions and configured to provide a circling effect.

[0029] In this context, the Applicant has set itself the objective of further reducing the rolling resistance of tyres, for example but not exclusively intended to equip new hybrid and fully electric motor vehicles, in particular for autonomous driving, in order to reduce the environmental impact. In particular, the Applicant has in particular set itself the goal to achieve the above objective without compromising the performance and driving features of modem tyres, in particular tyres provided with a crossed belt structure.

[0030] The Applicant has found that the above goals may be achieved by adopting a particular tyre structure which combines at least one carcass ply with two belt strips placed side by side or partially superimposed, in turn surmounted by a reinforcement structure provided with a reinforcement cord having an “elongation- tensile load” curve with two sections with different slopes.

[0031] According to a first aspect, the present invention relates to a tyre for motor vehicles having a carcass structure, a tread band, a belt structure between the carcass structure and the tread band and a reinforcement structure between the belt structure and the tread band.

[0032] Preferably, the carcass structure has a carcass ply having axially opposite end flaps engaged with respective anchoring annular structures.

[0033] Preferably, the belt structure comprises a first belt strip and a second belt strip.

[0034] Preferably, the first belt strip and the second belt strip are mutually and axially adjacent or partially superimposed at or in proximity to a middle line plane of the tyre.

[0035] Preferably, the first belt strip has first belt cords delimiting a first angle of between 25° and 40° with a circumferential direction of the tyre.

[0036] Preferably, the second belt strip has second belt cords delimiting a second angle of between -25° and -40° with the circumferential direction of the tyre.

[0037] Preferably, the first belt cords and the second belt cords delimit a third angle of between 50° and 80° between them.

[0038] Preferably, the reinforcement structure comprises reinforcement cords oriented in the circumferential direction of the tyre.

[0039] Preferably, each reinforcement cord has a predefined percentage elongation.

[0040] Preferably, each reinforcement cord has a “tensile load - elongation” curve comprising a first section, placed upstream of the predefined percentage elongation, a second section, placed downstream of the predefined percentage elongation, and a third connecting section placed between the first section and the second section. Preferably, a second slope of the second section is greater than a first slope of the first section.

[0041] According to a further aspect, the present invention relates to a tyre having a structure combining a carcass ply with a first belt strip and with a second belt strip; the carcass ply defining a carcass structure; the first belt strip and the second belt strip defining a belt structure.

[0042] Preferably, the belt structure is interposed between the carcass structure and the tread band.

[0043] Preferably, a reinforcement structure is placed between the belt structure and the tread band.

[0044] Preferably, the carcass ply has axially opposite end flaps engaged with respective anchoring annular structures.

[0045] Preferably, the first belt strip and the second belt strip are mutually and axially adjacent or partially superimposed at or in proximity to a middle line plane of the tyre.

[0046] Preferably, the first belt strip has first belt cords delimiting a first angle of between 25° and 40° with a circumferential direction of the tyre.

[0047] Preferably, the second belt strip has second belt cords delimiting a second angle of between -25° and -40° with the circumferential direction of the tyre.

[0048] Preferably, the first belt cords and the second belt cords delimit a third angle of between 50° and 80° between them.

[0049] Preferably, the reinforcement structure comprises reinforcement cords oriented in the circumferential direction of the tyre.

[0050] Preferably, each reinforcement cord has a predefined percentage elongation.

[0051] Preferably, each reinforcement cord has a “tensile load - elongation” curve comprising a first section, placed upstream of the predefined percentage elongation, a second section, placed downstream of the predefined percentage elongation, and a third connecting section placed between the first section and the second section.

[0052] Preferably, a second slope of the second section is greater than a first slope of the first section.

[0053] The Applicant believes that the present invention allows for a drastic reduction in the rolling resistance of tyres. The Applicant has also verified that the present invention allows performance and driving features suitable for the equipment of modem motor vehicles.

[0054] In particular, the Applicant has verified that the tyres made in accordance with the present invention are capable of providing a cornering force suitable for daily road use.

[0055] The present invention, in at least one of the above aspects thereof, may exhibit one or more of the following preferred features.

[0056] Preferably, the tyre is of the low rolling resistance type.

[0057] Preferably, there is only one carcass ply.

[0058] Preferably, the belt structure consists of only the first belt strip and the second belt strip.

[0059] Preferably, the first angle is between 26° and 30°.

[0060] Preferably, the first angle is equal to 26°, 27°, 28°, 29° or 30°.

[0061] Preferably, the second angle is between -26° and -30°.

[0062] Preferably, the second angle is equal to -26°, -27°, -28°, -29° or -30°.

[0063] Preferably, the first angle is equal in absolute value to the second angle. Alternatively, the first angle is different from the second angle.

[0064] Preferably, the third angle is between 52° and 60°.

[0065] Preferably, the first belt cords and the second belt cords form V shapes and the vertices of the Vs face in one rolling direction of the tyre or in a direction opposite to the rolling direction when the tyre is mounted on the motor vehicle.

[0066] Preferably, the vertices of the Vs lie on the middle line plane of the tyre.

[0067] Preferably, if the first belt strip and the second belt strip are mutually and axially abutted, an axially inner circumferential edge of the first belt strip is abutted against an axially inner circumferential edge of the second belt strip.

[0068] Preferably, if the first belt strip and the second belt strip are mutually and axially abutted, the axially inner circumferential edge of the first belt strip and the axially inner circumferential edge of the second belt strip lie on the middle line plane of the tyre or are spaced from said middle line plane.

[0069] Preferably, an overlapped part between the partially superimposed first belt layer and second belt layer is symmetric or asymmetric with respect to the middle line plane of the tyre. Preferably, a ratio between an axial width of the overlapped part and an axial width of the belt structure is smaller than 0.3.

[0070] Preferably, the ratio between the axial width of the overlapped part and the axial width of the belt structure is smaller than 0.2.

[0071] Preferably, an axial width of the overlapped part is less than 50 mm.

[0072] Preferably, an axial width of the overlapped part is less than 30 mm.

[0073] Preferably, the first section is delimited between a zero percentage elongation and a first point of the “tensile load - elongation” curve, the second section develops downstream of a second point of the “tensile load - elongation” curve, the third connecting section is delimited between the first point and the second point.

[0074] Preferably, the first point corresponds to the first point of the “tensile load - elongation” curve, starting from said zero percentage elongation, in which the concavity of the “tensile load - elongation” curve is null or facing upwards.

[0075] Preferably, if downstream of the first point the concavity of the “tensile load - elongation” curve remains upwards until failure, the second point of the “tensile load - elongation” curve is the point with maximum concavity.

[0076] Alternatively, if downstream of the first point the concavity of the upward-facing “tensile load - elongation” curve becomes downward-facing, the second point of the “tensile load - elongation” curve is an inflection point.

[0077] Preferably, the predefined percentage elongation corresponds to the intersection between a first tangent to the “tensile load - elongation” curve in the first point and a second tangent to the “tensile load - elongation” curve in the second point.

[0078] Preferably, a ratio of the second slope of the second section to the first slope of the first section is greater than 2.5.

[0079] Preferably, a ratio of the second slope of the second section to the first slope of the first section is greater than 10.

[0080] Preferably, a ratio of the second slope of the second section to the first slope of the first section is less than 50.

[0081] Preferably, a ratio of the second slope of the second section to the first slope of the first section is less than 35.

[0082] Preferably, the first slope of the first section is between 1 and 25 N / % elongation.

[0083] Preferably, the second slope of the second section is between 35 and 500 N / % elongation. Preferably, the default percentage elongation is between 1 % and 5%.

[0084] Preferably, the default percentage elongation is between 1.5% and 4.5%.

[0085] Preferably, in said “tensile load - elongation” curve a force corresponding to the predefined percentage elongation is between 5 N and 200 N.

[0086] Preferably, the force corresponding to the predefined percentage elongation is between 15 N and 90 N.

[0087] Preferably, a density of the reinforcement cords in the reinforcement structure is between 30 cords / dm and 120 cords / dm, more preferably between 40 cords / dm and 110 cords / dm.

[0088] Preferably, the reinforcement cords are metal. Alternatively, the reinforcement cords are hybrid.

[0089] Preferably, each of the metal reinforcement cords comprises at least two strands, each comprising a respective plurality of steel wires, wound together in each strand according to a predetermined winding pitch.

[0090] Preferably, said at least two strands are wound together in the same direction as said wires in said strands and with a winding pitch equal to or different from said predetermined winding pitch.

[0091] Preferably, each of the metal reinforcement cords comprises a single metal wire or at least two metal wires twisted together.

[0092] Preferably, each of the metal reinforcement cords comprises at least one helically shaped metal wire.

[0093] Preferably, each of the metal reinforcement cords has a diameter of between 0.15 mm and 0.30 mm.

[0094] Preferably, a density of the metal reinforcement cords in the reinforcement structure is between 40 cords / dm and 50 cords / dm.

[0095] Preferably, each of the hybrid reinforcement cords comprises one or more high modulus textile filaments twisted in one direction and one or more low modulus textile filaments twisted in the same direction.

[0096] Preferably, the above-mentioned high modulus textile filaments and the above- mentioned low modulus textile filaments are twisted together.

[0097] Preferably, high modulus textile filaments are made of aramid.

[0098] Preferably, low modulus textile filaments are made of nylon. Preferably, the high modulus textile filaments have a linear density of between 1100 dTex and 1200 dTex.

[0099] Preferably, the low modulus textile filaments have a linear density of between 1300 dT ex and 1600 dT ex.

[0100] Preferably, a density of the hybrid reinforcement cords in the reinforcement structure is between 70 cords / dm and 90 cords / dm.

[0101] Preferably, the first and / or second belt cords are metal.

[0102] Preferably, each of the first and / or second metal belt cords comprises at least two strands, each comprising a respective plurality of steel wires, wound together in each strand according to a predetermined winding pitch.

[0103] Preferably, a density of the first and / or second belt cords is between 60 cords / dm and 180 cords / dm, more preferably between 80 cords / dm and 135 cords / dm.

[0104] Preferably, the carcass ply has carcass cords.

[0105] Preferably, at a tyre crown comprising the belt structure and the tread band, the carcass cords are oriented in an axial direction of the tyre.

[0106] Preferably, the carcass cords lie in respective planes each forming an angle of 90° + / - 10° with the middle line plane of the tyre.

[0107] Preferably, the carcass cords are textile or hybrid.

[0108] Preferably, a density of the carcass cords is between 60 cords / dm and 140 cords / dm, more preferably between 85 cords / dm and 130 cords / dm.

[0109] Preferably, the first belt strip and the second belt strip are placed in contact with the carcass structure.

[0110] Preferably, the reinforcement structure is placed in contact with the first belt strip and the second belt strip.

[0111] Preferably, the tread band is placed in contact with the reinforcement structure.

[0112] Further features and advantages will appear more clearly from the detailed description of preferred but non-exclusive embodiments of a tyre for motor vehicles according to the present invention.

[0113] Such description is given hereinafter with reference to the accompanying drawings, provided only for illustrative and, therefore, non-limiting purposes, in which: - Figure 1 illustrates a half-section along a radial plane of an embodiment of a tyre for motor vehicles according to the present invention;

[0114] - Figure 1 A is an enlargement of a portion of Figure 1 ;

[0115] - Figure 2 illustrates a half-section along a radial plane of a different embodiment of the tyre for motor vehicles according to the present invention;

[0116] - Figure 2A is an enlargement of a portion of Figure 2;

[0117] - Figure 3 is schematic top view of elements of the tyre of Figure 1 ;

[0118] - Figure 4 is schematic top view of elements of the tyre of Figure 2;

[0119] - Figure 5 is a further enlarged portion of Figure 1 or 2;

[0120] - Figure 6 illustrates a continuous elongated element used for manufacturing the tyres shown in the preceding figures;

[0121] - Figure 7 illustrates a “tensile load - elongation” curve of a reinforcement cord of a tyre referred to in the preceding figures;

[0122] - Figure 7A is an enlargement of a portion of the curve of Figure 7;

[0123] - Figure 8 illustrates a “tensile load - elongation” curve of a different reinforcement cord;

[0124] - Figure 9 illustrates rolling resistances of tyres according to the present invention normalised with respect to a reference tyre;

[0125] - Figure 10 illustrates cornering stiffnesses as a function of the vertical load of the tyres in Figure 9;

[0126] - Figure 11 illustrates points corresponding to the standardised rolling resistances and standardised cornering stiffnesses of the tyres in Figures 9 and 10;

[0127] - Figure 12 illustrates rolling resistances of one of the tyres shown in Figures 9, 10 and 11 and of two variants thereof;

[0128] - Figure 13 illustrates the cornering force as a function of the cornering angle of the tyres in Figure 12.

[0129] Detailed

[0130] Figure 1 illustrates a half-section along a radial plane of a tyre 1 for motor vehicles according to the present invention. The tyre 1 for motor vehicles shown in Figure 1 has a curvature ratio of approximately 0.05. The tyre 1 comprises a carcass structure 2 formed by a carcass ply 3. The carcass ply 3 has a first end flap 3A thereof engaged with a first annular anchoring structure 4A, called rim, associated with a first filling insert 5A, and a second end flap 3B engaged with a second annular anchoring structure 4B associated with a second filling insert 5B. The two areas of the tyre 1 each comprising the annular anchoring structure 4A, 4B and the filling insert 5A, 5B form a first and a second bead structure 6, 7 intended for anchoring the tyre 1 on a corresponding mounting rim, not illustrated.

[0131] The carcass ply 3 comprises a plurality of carcass cords 8, illustrated schematically in Figure 3, parallel to each other and immersed in or covered by elastomeric material. The carcass cords 8 lie substantially in respective radial planes, i.e. in planes containing a rotation axis of the tyre 1 . At a crown of the tyre 1 , the carcass cords 8 are oriented according to a substantially axial direction of the tyre 1. In embodiment variants not shown, the carcass cords lie in respective planes each forming an angle of 90° + / - 10° with the middle line plane “M” of the tyre 1 . The carcass ply 3 is therefore of the radial type. The carcass cords 8 may be textile or hybrid. For example, the carcass cords 8 are made of Rayon. A density of the carcass cords 8 in the carcass ply 3 is for example between 60 cords / dm and 140 cords / dm or more preferably between 85 cords / dm and 130 cords / dm.

[0132] One or more layers of elastomeric material generally known as “liner” 9, which provides the necessary impermeability to the inflation air of the tyre 1 , and “underliner” 10, are arranged in a radially inner position with respect to the carcass ply 3. Liner 9 and underliner 10 are applied against the carcass structure 2. Associated with the carcass structure 2 is a belt structure 11 formed by a first belt strip 12 and a second belt strip 13. Each of the aforementioned first belt strip 12 and second belt strip 13 extends circumferentially around a radially outer portion of the carcass structure 2 and in contact with the latter. In the embodiment of Figures 1 , 1A and 3, the first belt strip 12 and the second belt strip 13 are mutually and axially adjacent to each other at the middle line plane “M” of the tyre 1 . Therefore, the first belt strip 12 and the second belt strip 13 form a single belt layer. The first belt strip 12 has an axially outer circumferential edge 12A and an axially inner opposing circumferential edge 12B. The second belt strip 13 has an axially outer circumferential edge 13A and an axially inner opposing circumferential edge 13B. The axially inner circumferential edge 12B of the first belt strip 12 abuts the axially inner circumferential edge 13B of the second belt strip 13 and these circumferential edges lie on the middle line plane “M” of the tyre 1. In embodiments not illustrated in the accompanying figures, the axially inner circumferential edge 12B of the first belt strip 12 and the axially inner circumferential edge 13B of the second belt strip 13 are staggered with respect to the middle line plane “M” of the tyre 1 .

[0133] The first belt strip 12 comprises a plurality of first belt cords 14 which are parallel to each other and immersed in or covered by elastomeric material (Figure 3). The second belt strip 13 comprises a plurality of second belt cords 15 which are parallel to each other and immersed in or covered by elastomeric material. The first belt cords 14 delimit with a circumferential direction “C” of the tyre 1 a first angle “a” between 25° and 40°, the second belt cords 15 delimit with the circumferential direction “C” a second angle “|3” between -25° and -40° and furthermore the first belt cords 14 and the second belt cords 15 delimit a third angle “y” between 50° and 80° between them. The third angle “y” is the sum of the first angle “a” and the second angle “|3”.

[0134] In Figure 3, the first angle “a” and the second angle “|3” are smaller in absolute value by about 40°. In embodiments not illustrated, the first angle “a” and the second angle “|3” are included in absolute value between 26° and 30°, for example equal to 26°, 27°, 28°, 29° or 30°, and therefore the third angle “y” is included between 52° and 60°.

[0135] The first belt cords 14 and the second belt cords 15 therefore form V shapes, i.e. a herringbone pattern. The vertices of the Vs may face in one rolling direction of the tyre 1 or in a direction opposite to the rolling direction when the tyre 1 is mounted on a motor vehicle. In the embodiment of Figures 1 , 1A and 3, the vertices of the Vs lie on the middle line plane “M” of tyre 1 and furthermore the Vs are symmetrical with respect to the middle line plane “M” as the first angle “a” is equal to the second angle “|3”. In embodiment variants not illustrated in the accompanying figures, the first angle “a” is different from the second angle “|3” and therefore the Vs are asymmetrical with respect to the middle line plane “M”. The first belt cords 14 and the second belt cords 15 are metal. For example, each of such first belt cords 14 and second belt cords 15 comprises two or more strands, each comprising a respective plurality of steel wires, wound together in each strand according to a predetermined winding pitch. A density of the first belt cords 14 and / or the second belt cords 15 in the respective first belt strip 12 and second belt strip 13 is between 60 cords / dm and 180 cords / dm, more preferably between 80 cords / dm and 135 cords / dm.

[0136] Associated with the belt structure 11 is a reinforcement structure 16 (zero degree layer) comprising reinforcement cords 17 oriented according to the circumferential direction “C” of the tyre 1. The reinforcement cords 17 are wound in a plurality of circumferential turns arranged in a radially outer position to the belt structure 11 . The circumferential turns are arranged so as to form an angle of between 0° and 10° with a circumferential direction C of the tyre 1 . The reinforcement structure 16, better illustrated in Figures 1 A, 1 B and 5, comprises a layer of elastomeric material in which the reinforcement cords 17 are immersed. The reinforcement cords 17 may for example be deposited by winding a continuous elongated element 18 (partially illustrated in Figure 6), which comprises one or more reinforcement cords 17 immersed in or covered by elastomeric material 19, in turns placed side by side or partially superimposed. A density of the reinforcement cords 17 in the reinforcement structure 16 is for example between 30 cords / dm and 120 cords / dm, more preferably between 40 cords / dm and 110 cords / dm.

[0137] The reinforcement structure 16 is positioned directly against the first belt strip 12 and against the second belt strip 13 and, in the embodiment of Figure 1 , protrudes axially beyond the axially outer circumferential edge 12A of the first belt strip 12 and beyond the axially outer circumferential edge 13A of the second belt strip 13. A ratio of an axial width “Li” of the belt structure 11 to an axial width “L of the reinforcement structure 16 is for example equal to 0.9. Opposite axial edges of the reinforcement structure 16 therefore rest against the carcass ply 3 or against under-belt inserts 16A, as illustrated in Figure 5.

[0138] The reinforcement cords 17 may be metal or hybrid. Furthermore, each reinforcement cord 17 has a “tensile load - elongation” curve provided with sections with different slopes. For example, each of the metal reinforcement cords 17 comprises two or more strands, each comprising a respective plurality of steel wires, wound together in each strand according to a predetermined winding pitch. The two or more strands may be wound together in the same direction as said wires in said strands and with a winding pitch equal to or different from said predetermined winding pitch. For example, each of the metal reinforcement cords 17 comprises a single metal wire or two or more metal wires twisted together. For example, each of the metal reinforcement cords 17 comprises one or more helically shaped metal wires. Each of the metal reinforcement cords 17 preferably has a diameter of between 0.15 mm and 0.30 mm. A density of the metal reinforcement cords 17 in the reinforcement structure 16 is for example between 40 cords / dm and 50 cords / dm.

[0139] The “tensile load - elongation” curve of a metal reinforcement cord 17 is illustrated for example in Figures 7 and 7A. Such “tensile load - elongation” curve is obtained according to the BISFA standard - Internationally agreed methods for testing steel tyre cords 1995, Chapter E6. The metal reinforcement cord 17 is a steel cord of the type 3x4x0.20 HEHT.

[0140] Figures 7 and 7A show the percentage elongation (%) on the abscissa and the tensile force (expressed in Newton) on the ordinate.

[0141] As may be seen, the “tensile load - elongation” curve in Figures 7 and 7A has a first section T1 with the concavity facing downwards (visible in the enlargement of Figure 7A), followed by a third connecting section K with the concavity facing upwards and then by a second section T2 with the concavity facing first upwards and then downwards. The curve in Figure 7 therefore has an inflection point between the first section T1 and the third connecting section K and an inflection point at the beginning of the second section T2.

[0142] Having defined “first point P1” as the first point of the “tensile load - elongation” curve encountered, starting from zero elongation, in which the concavity of the “tensile load - elongation” curve is facing upwards (in the graph in Figure 7A, the first point P1 is placed just after the inflection point of the first section T1) and having defined “second point P2” as the inflection point at the beginning of the second section T2, “predefined percentage elongation Sp” is defined as the elongation placed at the intersection between a first tangent to the “tensile load - elongation” curve in the first point P1 and a second tangent to the “tensile load - elongation” curve in the second point P2.

[0143] The first section T1 is delimited between the zero elongation and the first point P1 of the “tensile load - elongation” curve, the second section T2 develops downstream of the second point P2 of the “tensile load - elongation” curve, the third connecting section K is delimited between the first point P1 and the second point P2. The first section T1 and the second section T2 of the “tensile load - elongation” curve are connected by the third connecting section K with an increasing slope. The third connecting section K forms a sort of elbow or knee with a concavity facing upwards.

[0144] In the graph of Figure 7, the default percentage elongation Sp is equal to approximately 1.7% and a force F corresponding to such default percentage elongation Sp is equal to approximately 35 N.

[0145] The first section T 1 , located upstream of the predefined percentage elongation Sp, is provided with a first slope measured at the first point P1 , and the second section T2, located downstream of the predefined percentage elongation Sp, is provided with a second slope measured at the second point P2. The second slope is greater than the first slope.

[0146] For example, each of the hybrid reinforcement cords 17 comprises one or more high modulus textile filaments twisted in one direction and one or more low modulus textile filaments twisted in the same direction. The above-mentioned high modulus textile filaments and the above-mentioned low modulus textile filaments are twisted together. For example, the high modulus textile filaments are made of aramid and have a linear density of between 1100 dTex and 1200 dTex. For example, the low modulus textile filaments are made of nylon and have a linear density of between 1300 dTex and 1600 dTex.

[0147] A density of the hybrid reinforcement cords 17 in the reinforcement structure 16 is for example between 70 cords / dm and 90 cords / dm.

[0148] The “tensile load - elongation” curve of a hybrid reinforcement cord 17 is illustrated for example in Figure 8. The “tensile load - elongation” curve for textile or hybrid cords is obtained according to the BISFA standard - Testing method for polyamide filament yams 2004, Chapter 7. The hybrid reinforcement cord 17 is a cord of the Ar / Ny 1100 / 1400 type. As may be seen, the “tensile load - elongation” curve in Figure 8 has the first section T1 with the concavity facing downwards (as shown in Figure 7A), followed by the third connecting section K with the concavity facing upwards and then by the second section T2, which unlike the curve in Figure 7, has a concavity always facing upwards (up to the breaking load).

[0149] As for Figure 7, the first point P1 corresponds to the first point of the “tensile load - elongation” curve encountered, starting from zero elongation, in which the concavity of the “tensile load - elongation” curve is facing upwards. Unlike Figure 7, the second point P2 of the “tensile load - elongation” curve in Figure 8 is the point with maximum concavity.

[0150] In the graph of Figure 8, the default percentage elongation Sp is equal to approximately 2.9% and a force F corresponding to such default percentage elongation Sp is equal to approximately 23.6 N.

[0151] Irrespective of the specific examples illustrated, the predefined percentage elongation Sp of the reinforcement cords 17 adopted in the present invention is between 1 % and 5%, preferably between 1.5% and 4.5%. A force corresponding to the predefined percentage elongation is preferably between 5 N and 200 N, more preferably between 15 N and 90 N. Preferably, the first slope of the first section is between 1 and 25 N / % elongation and the second slope of the second section is between 35 and 500 N / % elongation. The ratio of the second slope of the second section T2 to the first slope of the first section T 1 is preferably between 2.5 and 50, preferably between 10 and 35.

[0152] In the example of the graph of Figure 7, the first slope of the first section T1 is approximately 22 N / % elongation and the second slope of the second section T2 is approximately 470 N / % elongation, so the aforementioned ratio is approximately 21 .4. In the example of the graph in Figure 8, the first slope of the first section T1 is approximately 7 N / % elongation and the second slope of the second section T2 is approximately 18 N / % elongation, so the aforementioned ratio is approximately 2.6.

[0153] In other words, the reinforcement cord 17 increases its stiffness when it is stretched beyond the predefined percentage elongation Sp. Given that in the finished tyre 1 the reinforcement cord 17 has such predefined percentage elongation Sp, then this means that when, during the rolling of the tyre 1 on the road, said tyre 1 deforms so as to put the reinforcement cord 17 under traction or portions thereof, then the reinforcement cord 17 behaves as if it had a high stiffness corresponding to the slopes of the second section T2.

[0154] A tread band 20 is applied in a position radially outer to the reinforcement structure 16, made of an elastomeric compound like other semi-finished products making up tyre 1. The tread band 20 is positioned directly against the reinforcement structure 16. Moreover, respective sidewalls 21 of elastomeric compound are applied in an axially external position on the lateral surfaces of the carcass structure 2, each extending from one of the lateral edges of the tread band 20 at the respective bead structure 6, 7.

[0155] The tyre in Figures 2 and 4 differs from that in Figures 1 and 3 as the first belt strip 12 and the second belt strip 13 are partially overlapped at the middle line plane “M” of the tyre 1 instead of being placed side by side. In the illustrated embodiment, an overlapped part 22 between the first belt strip 12 and the second belt strip 13 is symmetrical with respect to the middle line plane “M” and a ratio of an axial width “L3” of such overlapped part 22 to the axial width “Li” of the belt structure 11 is about 0.25. Such ratio is preferably less than 0.3. In embodiment variants, such ratio is less than 0.2. For example, the axial width “L3” of the overlapped part 22 is less than 50 mm, preferably less than 30 mm.

[0156] Tests

[0157] Four tyres B, C, D, E according to the present invention were compared through road tests with a reference tyre A. All tyres represent a PZERO model and in size 235 / 50 R20 XL 104Y. The reference tyre A comprises a radial mono-ply carcass structure, a two-layer belt structure with crossed cords and a hybrid zero-degree layer, i.e. made up of hybrid cords. The four tyres B, C, D, E comprise a radial single-ply carcass structure, two adjacent or partially superimposed belt strips with belt cords inclined symmetrically with respect to the circumferential direction and a zero-degree metal or hybrid layer with double-slope cords as illustrated above.

[0158] Cord materials:

[0159] - carcass ply: RY 1840 / 2 (48x48) EPDM 120;

[0160] - belt strips (steel): 2+2x0.25; - metal reinforcement structure (steel): 3x4x0.2 HE EPDM 45 (“tensile load - elongation” curve in Figure 7);

[0161] - hybrid reinforcement structure: AR / NY 1100 / 1400 (28 / 7x28) EPDM 79 (“tensile load - elongation” curve in Figure 8).

[0162] The tests were performed according to the following regulations: UNECE 117R02 / 15 (2023), Annex 6 (Torque method) - ISO 28580:2018 (par. 4b (Torque method)) - Notification No. 2022-165 (Korea MOTIE).

[0163] Rim: 20x7.5”

[0164] Pressure: 2.5 bars

[0165] Vertical load: 720 kg

[0166] The features of tyres A, B, C, D, E are shown in the following Table 1 .

[0167] Table 1

[0168] Figure 9 shows the rolling resistance coefficient RR% of tyres A, B, C, D, E. The rolling coefficient RR has been normalised by assigning the value 100 to the reference tyre A. As may be seen, tyres B, C, D, E have a RR (RR%B = 91 .8, RR%c = 87.7, RR%D = 86.9, RR%E = 86.9) lower than the rolling resistance coefficient RR of the reference tyre A (RR%A = 100).

[0169] In the following Table 2, in addition to the normalised RR values illustrated in Figure 9, the experimental RR values in N / kN and the percentage decreases in the rolling resistance coefficient RR of tyres B, C, D, E compared to the reference A are also reported. Table 2

[0170] The advantages in terms of reduction of rolling resistance RR shown by tyres B, C, D, E are evident and significant.

[0171] Tests were also performed to detect the cornering stiffness.

[0172] Rim: 20x8”

[0173] Pressure: 2.5 bars

[0174] Figure 10 illustrates the cornering stiffness (CS) as a function of the vertical load (VL) in Newton of tyres A, B, C, D, E normalised by assigning the value 100 to the maximum cornering stiffness of the reference A corresponding to a vertical load of about 8000 N. Tyres B, C and D have cornering stiffness curves close to that of the reference tyre A. Tyre B has a reduction in the cornering stiffness of about 5% at low vertical loads, below 8000 N, and an increase in cornering stiffness of up to about 10% more at higher vertical loads, above 8000 N. Tyres C and D have a reduction in the cornering stiffness of less than 10% along the entire curve. The curve of the tyre E is lower but still such that it does not give rise to a significant penalty in non-sporty driving on the road.

[0175] This result was obtained by adopting the belt structure described and claimed herein and without the need for additional reinforcement elements, such as the reinforcement layer placed astride the two adjacent belt portions described in the prior art document JPH05286309A cited above.

[0176] Figure 11 illustrates, in a graph having on the abscissa the normalised cornering stiffness at a vertical load of 6750 N and on the ordinate the normalised rolling resistance coefficient, points corresponding to the tyres A, B, C, D, E. As may be seen, all the points of tyres B, C, D, E are grouped at the bottom and towards the right, i.e. at low rolling resistances and sufficiently high cornering stiffnesses.

[0177] The variation of the rolling resistance coefficient and of the cornering force Fs (in Newton) as a function of the asymmetry of the belt strip cords with respect to the middle line plane “M” of the tyre, i.e. of the first and second angles “a” and “|3” , was evaluated through a finite element simulation (FEA). The features of the simulated tyre are those of tyre C in Table 1 , wherein the third angle “y” between the first belt cords and the second belt cords is kept constant and equal to 52° while the first and second angles “a” and “|3” have been varied as per the following Table 3.

[0178] Table 3

[0179] Figure 12 illustrates the rolling resistance coefficient RR of tyres C, C’, C”. The rolling coefficient has been normalised by assigning the value 100 to the reference C. As may be seen, the variation of the rolling resistance coefficient due to the asymmetry is less than 0.5% and therefore negligible.

[0180] Figure 13 illustrates the variation of the cornering force Fs as a function of the cornering angle SA for tyres C, C’, C”. As may be seen, the curves are parallel to each other and translated according to the asymmetry of the belt strips. The cornering force Fs of the symmetric tyre C passes through zero at a substantially zero cornering angle while the cornering force Fs of the asymmetric tyres C’, C” passes through zero at cornering angles other than zero and this allows, for example, the straight-line centring performance to be modified.

Claims

CLAIMS1. Tyre for motor vehicles having a carcass structure (2), a tread band (20), a belt structure (11 ) between the carcass structure (2) and the tread band (20) and a reinforcement structure (16) between the belt structure (11) and the tread band (20); wherein the carcass structure (2) has a carcass ply (3) having axially opposite end flaps (3A, 3B) engaged with respective anchoring annular structures (4A, 4B); wherein the belt structure (11) comprises a first belt layer (12) and a second belt layer (13); wherein the first belt layer (12) and the second belt layer (13) are mutually and axially side-by-side or partially superimposed at or in proximity to a middle line plane (M) of the tyre (1); wherein the first belt layer (12) has first belt cords (14) delimiting, with a circumferential direction (C) of the tyre (1 ), a first angle (a) comprised between 25° and 40°, wherein the second belt layer (13) has second belt cords (15) delimiting, with the circumferential direction (C) of the tyre (1 ), a second angle ([3) comprised between -25° and -40°, wherein the first belt cords (14) and the second belt cords (15) between them delimit a third angle (y) comprised between 50° and 80°; wherein the reinforcement structure (16) comprises reinforcement cords (17) oriented according to the circumferential direction (C) of the tyre (1 ); wherein each reinforcement cord (17) has a predefined percentage elongation (Sp); wherein each reinforcement cord (17) has an “elongation-tensile load” curve comprising a first section (T1 ), placed upstream of the predefined percentage elongation (Sp), a second section (T2), placed downstream of the predefined percentage elongation (Sp), and a third connector section (K) placed between the first section (T1 ) and the second section (T2); wherein a second slope of the second section (T2) is greater than a first slope of the first section (T1 ).

2. Tyre according to claim 1 , wherein the first angle (a) is comprised between 26° and 30° and the second angle ([3) is comprised between -26° and -30°.

3. Tyre according to claim 1 or 2, wherein the first angle (a) is equal in absolute value to the second angle ([3).

4. Tyre according to one of claims 1 to 3, wherein an overlapped part (22) between the partially superimposed first belt layer (12) and second belt layer (13) is symmetric or asymmetric with respect to the middle line plane (M) of the tyre (1 ).

5. Tyre according to claim 4, wherein a ratio between an axial width (L3) of the overlapped part (22) and an axial width (Li) of the belt structure (11) is smaller than 0.3.

6. Tyre according to claim 5, wherein the ratio between the axial width (L3) of the overlapped part (22) and the axial width (Li) of the belt structure (11 ) is smaller than 0.2.

7. Tyre according to any one of the preceding claims, wherein a ratio between the second slope of the second section (T2) and the first slope of the first section (T1 ) is greater than 2.5 and smaller than 50.

8. Tyre according to the preceding claim, wherein the ratio between the second slope of the second section (T2) and the first slope of the first section (T1 ) is comprised between 10 and 35.

9. Tyre according to any one of the preceding claims, wherein the first slope of the first section (T1 ) is comprised between 1 and 25 N / elongation % and wherein the second slope of the second section (T2) is comprised between 35 and 500 N / elongation %.

10. Tyre according to any one of the preceding claims, wherein the predefined percentage elongation (Sp) is comprised between 1 % and 5%.

11. Tyre according to any one of the preceding claims, wherein in said “tensile load - elongation” curve, a force corresponding to the predefined percentage elongation (Sp) is comprised between 5 N and 200 N.

12. Tyre according to any one of the preceding claims, wherein the reinforcement cords (17) are metal or hybrid.

13. Tyre according to claim 12, wherein the reinforcement cords (17) are metal and each comprises at least two strands, each comprising a respective plurality of steel wires, wound together in each strand according to a predetermined winding pitch, wherein said at least two strands are wound together in the same sense of said wires in said strands and with a winding pitch equal to or different from said predetermined winding pitch.

14. Tyre according to claim 12, wherein the reinforcement cords (17) are hybrid and each comprises one or more textile filaments with high elastic modulus twisted in one direction and one or more textile filaments with low elastic modulus twisted in the same direction, wherein the aforesaid textile filaments with high elastic modulus and the aforesaid textile filaments with low elastic modulus are twisted together.

15. Tyre according to any one of the preceding claims, wherein the first belt cords (14) and the second belt cords (15) are metal.

16. Tyre according to any one of the preceding claims, wherein the carcass ply (3) has carcass cords (8); wherein, at a crown of the tyre (1 ) comprising the belt structure (11) and the tread band (20), the carcass cords (8) are oriented according to an axial direction of the tyre (1 ).

Images