TIRE COMPRISING A TREAD OPTIMIZED FOR GRIP ON SNOW-COVERED GROUND

Abstract

A tire has a tread comprising at least two tread pattern elements (MA, MB) distributed periodically in the circumferential direction at pitches (PA, PB). Each tread pattern element is formed of three portions (Z1, Z2, Z3), each defining a volumetric element of which the leading edge corner is the one common to the tread surface and is the first to enter the contact patch in which the tire is in contact with the ground. With each leading edge corner being chamfered, in the portions Z1 and/or Z2, and/or Z3, the widths of the chamfers of the leading edge corners (LC.sub.i.sup.A, LC.sub.i.sup.B, i ranging from 1 to 3) satisfy the following inequalities: a) for the portion Z1:

[00001]$\left[\left[\right]\right]0.8*\frac{PA}{PB}\le \frac{L{C}_1^A}{L{C}_1^B}\le \frac{PA}{PB}*1.2;$

b) for the portion Z2:

[00002]$\left[\left[\right]\right]0.8*\frac{PA}{PB}\le \frac{L{C}_2^A}{L{C}_2^B}\le \frac{PA}{PB}*1.2;$

and c) for the portion Z3:

[00003]$\left[\left[\right]\right]0.8*\frac{PA}{PB}\le \frac{L{C}_3^A}{L{C}_3^B}\le \frac{PA}{PB}*1.2.$

Moreover, the sipes density of each tread pattern element (SDA, SDB) is at least equal to 10 mm.sup.−1 and at most equal to 70 mm.sup.−1.

Claims

1.-14. (canceled)

15. A tire having a tread (10) intended to come into contact with the ground via a tread surface (20), the tread (10) comprising raised elements that are organized in tread pattern elements (MA, MB), are separated from one another at least in part by grooves (30) and extend radially toward the outside from a bottom surface (40) as far as the tread surface (20) over a radial height H at least equal to 6 mm and at most equal to a radial height H.sub.max of the tread (10), each tread pattern element (MA, MB) comprising two half-elements (MA1, MA2) and (MB1, MB2), which are symmetric with respect to an equatorial plane passing through the center of the tread (C) and are offset from one another in the circumferential direction by a distance D, each half-element (MA1, MB1) and its respective symmetric counterpart (MA2, MB2) being curved, in an axial direction (YY′), from an axial end of one edge (24G, 24D) of the tread to the center (C) of the tread (10) so as to define a preferred direction of running of the tire, and having an axial width (L), each half-element (MA1, MB1; MA2, MB2) comprising a first, lateral portion (Z3) extending from an axial end of the edge (24G, 24D) of the tread over an axial width equal to at most one third of the axial width (L) of the half-element, a second, central portion (Z1) having the same axial width as the first, lateral portion (Z3), and a third, intermediate portion (Z2) contiguous with the two other portions, each portion (Z1, Z2, Z3) of each half-element (MA1, MB1; MA2, MB2) being a volumetric element having a leading face, which is the face of which the radially outer edge corner is the first to enter the contact patch in which the tire is in contact with the ground, the edge corner of the radially outer leading face being the leading edge corner, each portion (Z1, Z2, Z3) of each half-element (MA1, MB1; MA2, MB2) having a trailing face, which is the face of which the radially outer edge corner is the last to leave the contact patch in which the tire is in contact with the ground, the edge corner of the radially outer trailing face being the trailing edge corner, the leading edge corners of each portion (Z1, Z2, Z3) respectively having a chamfered profile (51, 52, 53), with respective widths of the chamfers (LC.sub.1.sup.A, LC.sub.2.sup.A, LC.sub.3.sup.A) for the half-elements (MA1, MA2) of a first element and, respectively, (LC.sub.1.sup.B, LC.sub.2.sup.B, LC.sub.3.sup.B) for the half-elements (MB1, MB2) of a second element, the width of a chamfer in a portion (Z1, Z2, Z3) being a normal distance between the leading face of the portion and the edge corner of the chamfer belonging to the tread surface, the tread being obtained through a periodic distribution in the circumferential direction of a first tread pattern element MA formed of the first half element MA1 and of its symmetric counterpart MA2 at a pitch PA, and of a second tread pattern element MB formed of the second half element MB1 and of its symmetric counterpart MB2 at a pitch PB, where PA<PB, a sipe being a cut or void in which a distance between walls of material that delimit the sipe is less than or equal to 2 mm and a depth of which is greater than or equal to 1 mm, the sipes density of the tread pattern elements SD corresponding to a ratio between a sum of projected lengths (lpyi) of the sipes of a tread pattern element (MA, MB) along an axial direction (Y) to a product of the pitch (PA, PB) of the tread pattern element and the width (W) of the tread, the whole being multiplied by 1000, such that $\mathrm{SDA}=\frac{{.Math.}_{i=1}^{\mathrm{nay}}\mathrm{lpyi}}{\mathrm{PA}*W}*1000,\mathrm{and}\mathrm{SDB}=\frac{{.Math.}_{i=1}^{\mathrm{nby}}\mathrm{lpyi}}{\mathrm{PB}*W}*1000$ where nay and nby are a number of sipes of each tread pattern element (MA, MB) and lpyi is a projected length of the i-th sipe of a given element, wherein, in the portions Z1 and/or Z2, and/or Z3, the widths of the chamfers of the leading edge corners (LC.sub.i.sup.A, LC.sub.i.sup.B, i ranging from 1 to 3) of the half-elements (MA1, MA2) and (MB1, MB2) with respective pitches (PA, PB) satisfy the following inequalities: a) for the portion Z1: $0.8*\frac{\mathrm{PA}}{\mathrm{PB}}\le \frac{{\mathrm{LC}}_1^A}{{\mathrm{LC}}_1^B}\le \frac{\mathrm{PA}}{\mathrm{PB}}*1.2$ b) for the portion Z2: $0.8*\frac{\mathrm{PA}}{\mathrm{PB}}\le \frac{{\mathrm{LC}}_2^A}{{\mathrm{LC}}_2^B}\le \frac{\mathrm{PA}}{\mathrm{PB}}*1.2,$ and c) for the portion Z3: $0,8*\frac{\mathrm{PA}}{\mathrm{PB}}\le \frac{{\mathrm{LC}}_3^A}{{\mathrm{LC}}_3^B}\le \frac{\mathrm{PA}}{\mathrm{PB}}*1,2,$ and wherein the sipes density of each tread pattern element (SDA, SDB) is at least equal to 10 mm.sup.−1 and at most equal to 70 mm.sup.−1.

16. The tire according to claim 15, wherein the tread comprises a number (NA, NB) of tread pattern elements (MA, MB), an average sipes density SDmoy being at least equal to 10 mm.sup.−1 and at most equal to 70 mm.sup.−1, with the average sipes density being defined by: $\mathrm{SDmoy}=\frac{\mathrm{SDA}*\mathrm{NA}*\mathrm{PA}+\mathrm{SDB}*\mathrm{NB}*\mathrm{PB}}{\mathrm{NA}*\mathrm{PA}+\mathrm{NB}*\mathrm{PB}},$ where (SDA, SDB) are sipe densities of the tread pattern elements (MA, MB).

17. The tire according to claim 15, wherein a ratio between the pitch PA of the first tread pattern element MA formed of the half-elements (MA1, MA2) divided by the pitch PB of the second tread pattern element MB formed of the half-elements (MB1, MB2), PA/PB is at least equal to 0.60 and at most equal to 0.90.

18. The tire according to claim 15, wherein a ratio between the pitch PA of the first tread pattern element MA formed of the half-elements (MA1, MA2) divided by the pitch PB of the second tread pattern element MB formed of the half-elements (MB1, MB2), PA/PB is at least equal to 0.85.

19. The tire according to claim 15, wherein the widths of the chamfers of the leading edge corners (LC.sub.1.sup.A, LC.sub.2.sup.A, LC.sub.3.sup.A) for the first tread pattern element MA formed of the half-elements (MA1, MA2) and (LC.sub.1.sup.B, LC.sub.2.sup.B, LC.sub.3.sup.B) for the second tread pattern element MB formed of the second half-elements (MB1, MB2) of the respective portions (Z1, Z2, Z3) satisfy at least one of the following relationships: LC.sub.1.sup.x belongs to the range [0.5, 2] mm, where X=A, or B, LC.sub.2.sup.x belongs to the range [1, 2.5] mm, where X=A or B, and LC.sub.3.sup.x belongs to the range [1.5, 3] mm, where X=A, or B.

20. The tire according to claim 15, the tread further comprising a third tread pattern element MC formed of two tread pattern half-elements (MC1, MC2) that are symmetric with respect to the equatorial plane (C), with a pitch PC, where PB is smaller than PC, wherein a ratio of the pitches PB/PC is greater than or equal to a ratio of the pitches PA/PB.

21. The tire according to claim 15, wherein the tread pattern elements have a radial height H.sub.max at most equal to 9 mm.

22. The tire according to claim 15, the overall volumetric void ratio TEV corresponding to a ratio of a void volume VE to a total volume VT of the tread, such that TEV=VE/VT, wherein the volumetric void ratio TEV of the tread is between 20% and 40%.

23. The tire according to claim 20, the tread comprising a third tread pattern element MC with a pitch PC, wherein the volumetric void ratio TEM of each tread pattern element (MA, MB, MC) is more or less identical.

24. The tire according to claim 20, the tread comprising a third tread pattern element MC with a pitch PC, wherein a maximum pitch of the tread pattern elements out of the pitch (PA) of the first element (MA), the pitch (PB) of the second element (MB) and the pitch (PC) of the third element (MC) is between 22 mm and 40 mm.

25. The tire according to claim 15, wherein a composition of a rubbery material of the tread has a glass transition temperature Tg of between −40° C. and −10° C. and a complex dynamic shear modulus G* measured at 60° C. of between 0.5 MPa and 2 MPa.

26. The tire according to claim 15, wherein at least 30% of the leading and/or trailing edge corners have a chamfer.

27. The tire according to claim 20, the tread comprising a number (NA, NB, NC) of tread pattern elements (MA, MB, MC), the average sipes density SDmoy being at least equal to 10 mm.sup.−1 and at most equal to 70 mm.sup.−1, with the average sipes density being defined by: $\mathrm{SDmoy}=\frac{\mathrm{SDA}*\mathrm{NA}*\mathrm{PA}+\mathrm{SDB}*\mathrm{NB}*\mathrm{PB}+\mathrm{SDC}*\mathrm{NC}*\mathrm{PC}}{\mathrm{NA}*\mathrm{PA}+\mathrm{NB}*\mathrm{PB}+\mathrm{NC}*\mathrm{PC}},$ where SDC is the sipes density of the element MC of the tread pattern.

28. The tire according to claim 15, wherein the tire has a 3PMSF (3 Peaks Mountain Snow Flake) winter certification indicated on at least one of its sidewalls.

Description

[0093] The present invention will be understood better from reading the detailed description of embodiments that are given by way of entirely non-limiting examples and are illustrated by the appended drawings, in which:

[0094] FIG. 1 is a detail view of a half-element MA1 of the tread according to the invention and denoted by the overall reference 1-A, and then three cross-sectional views (EE, FF, GG) for illustrating the chamfers made on the trailing edge corners.

[0095] FIG. 2 shows a complete element MA of the tread pattern of the tread, formed of two half-elements MA1, MA2, which are mutually symmetric with respect to the equatorial plane (“C”) passing through the centre of the tread.

[0096] FIG. 3 shows a developed view of the tread in the circumferential direction (X) according to a first embodiment of the invention, with two tread pattern elements MA and MB. The tread pattern elements differ in terms of their geometry (widths, cuts, pitches, etc.). The element MA is depicted with vertical hatching, and the element MB with small waves.

[0097] FIG. 4 shows a developed view of the tread in the circumferential direction (X) according to a second embodiment of the invention, with three tread pattern elements MA, MB and MC. The tread pattern elements differ in terms of their geometry (widths, cuts, pitches, etc.). The element MC is depicted by black dots on a white background.

[0098] FIG. 5 is a third embodiment of the invention. It differs from FIG. 4 in that both the trailing and leading edge corners are chamfered. The chamfers of the trailing edge corners are referenced (61, 62, 63).

[0099] FIG. 6 is an extract from FIG. 5, which shows an enlargement of a tread pattern element MA (MA1, MA2) with the leading and trailing edge corners chamfered.

[0100] FIG. 7 shows the footprint on the ground of a tyre of the invention inflated to its nominal pressure and subjected to a vertical load.

[0101] FIG. 8 is an extract from FIG. 7 centred on a tread pattern element in the contact patch with the leading edge corners chamfered (51, 52, 53).

[0102] In the various figures, identical or similar elements bear the same references. Given the symmetry of the tread, in order for the figures to be readable, the elements are referenced once sometimes on the side 20G and sometimes on the side 20D.

[0103] FIG. 1 shows a tread pattern half-element referenced 1-A and three cross-sectional views: the cross-sectional view EE lying on a section plane in the portion Z3, FF lying on a section plane in the portion Z2, and lastly a cross-sectional view GG lying on a section plane in the portion Z1.

[0104] The tread pattern half-element MA1 is curved from an axial end of one edge 24G of the tread to the centre (C) of the tread. The concavity of the half-element, which is oriented towards the centre of the tread (C), determines the running direction of the tyre, indicated by the reference 25. The tread pattern half-element MA1 comprises a sipe 80, which extends axially from one edge (24G, 24D) to the end of MA1 at the centre of the tread.

[0105] The half-element MA1 comprises three portions, a portion (Z3) contiguous with the edge 24G of the tread with an axial width of around one third of the total axial width of the half-element, a central portion (Z1) with the same axial width as the portion (Z3), and an intermediate portion (Z2) contiguous with the two other portions.

[0106] Each portion (Z1, Z2, Z3) is volumetric element having a leading face which is the face of which the radially outer edge corner is the first to enter the contact patch in which the tyre is in contact with the ground. The edge corner of the radially outer leading face is referred to as leading edge corner below.

[0107] Each portion (Z1, Z2, Z3) is finally provided with a chamfered edge corner, these being respectively referenced (51, 52, 53). The chamfers of the three portions (Z1, Z2, Z3) are shown in the cross sections EE, FF and GG. These chamfers (51, 52, 53) are delimited by the leading face 22 and the tread surface 20. The width of a chamfer (51, 52, 53) of a portion is the normal distance between the trailing face and the edge corner of the chamfer belonging to the tread surface. By way of illustration, the width LC.sub.3.sup.A represents the width of the chamfer of the element MA in the central portion Z3.

[0108] FIG. 2 shows a complete tread pattern element MA comprising two half-elements (MA1, MA2) that are symmetric with respect to the equatorial plane (“C”) and are offset in the circumferential direction by a length D. The repetition per wheel revolution of the tread pattern element MA formed of the half-elements (MA1, MA2) results in a tread referred to as a directional tread. The presence of a single tread pattern element MA means that the tread is a mono-pitch tread.

[0109] FIG. 3 shows a first embodiment of the invention with a circumferential developed view of the tread 10 of the tyre, which comprises two rows 20G and 20D of half-elements that are symmetric with respect to the equatorial plane C and offset in the circumferential direction by a distance D. The tread pattern elements are separated at least in part by grooves 30 and extend radially towards the outside from a bottom surface 40 to the tread surface 20 over a radial height H.

[0110] Each element (MA, MB) is made up of two half-elements (MA1, MA2) and (MB1, MB2) that are symmetric with respect to the equatorial plane C such that the element extends axially from a first edge 24G to a second edge 24D over an axial width W. A distribution pitch PA (and, respectively, PB) is associated with the element MA (and, respectively, MB). The pitch of an element is the distance measured around a circumference of the tread surface between a point of this element and the translated image of this point onto the immediately following element. The leading edge corners of each portion (Z1, Z2, Z3) of each element (MA, MB) of the tread pattern are chamfered. The chamfers are respectively referenced (51, 52, 53) with associated widths (LC.sub.1.sup.A, LC.sub.2.sup.A, LC.sub.3.sup.A) for the element MA and (LC.sub.1.sup.B, LC.sub.2.sup.B, LC.sub.3.sup.B) for the element MB.

[0111] The “edges” 24G, 24D of the tread 10 are understood to be the surfaces that limit the boundaries between the tread 10 and the sidewalls 60. These two edges 24G, 24D are at a distance from one another by a value W corresponding to the width of the tread 10. These two edges 24D, 24G are situated at equal distances from a central axis C. This central axis C divides the tread 10 into two half-treads.

[0112] Still in FIG. 3, the tread pattern of the tread comprises an element MA, depicted with vertical hatching and an element MB depicted with small waves. The number of tread pattern elements distributed in the circumferential direction results from the minimization of the running noise emitted by the tyre depending on the geometry of the elements (width, pitch, cuts, etc.). The tread depicted in FIG. 3 is referred to as a multi-pitch tread with two tread pattern elements MA and MB with respective pitches PA and PB. Each half-element (MA1, MA2) and (MB1, MB2) of the tread pattern elements (MA, MB) comprises a sipe 80 which extends from one edge (24G, 24G) to the end of the half-element at the centre of the tread.

[0113] FIG. 4 shows a second embodiment of the invention, in which the tread pattern of the tread comprises a third tread pattern element MC. Here again, the element MC comprises two half-elements (MC1, MC2) that are symmetric with respect to the equatorial plane C. This third element is depicted with graphical elements in the form of dots on a white background. The pitch associated with this element is PC. The half-elements of MC likewise comprise a sipe 80. FIG. 4 differs from FIG. 3 through the addition of the additional element MC with a pitch PC.

[0114] FIG. 5 is a third embodiment of the invention. It differs from FIG. 4 in that both the trailing and leading edge corners are chamfered. The chamfers of the trailing edge corners are referenced (61, 62, 63).

[0115] FIG. 6 is an enlargement of a tread pattern element of the third embodiment of the invention, in which the leading edge corners (51, 52, 53) and trailing edge corners (61, 62, 63) are chamfered in the zones (Z1, Z2, Z3). The chamfers of the trailing edge corners have widths (KC.sub.1.sup.A, KC.sub.2.sup.A, KC.sub.3.sup.A) which do not necessarily have the same values as the widths of the chamfers of the leading edge corners.

[0116] FIG. 7 shows the footprint on the ground of a tyre of the invention inflated to its nominal pressure and subjected to a vertical load. The perimeter of the contact patch is indicated by the broken-line curve. Inside this perimeter, the geometric elements with a light background represent the voids of the tread pattern of the tread, grooves 30 and sipes 80, and the geometric elements on a dark background represent the material surface in contact with the ground, made up of the tread pattern elements. The tyre compressed by the load has its deformed tread pattern extending from an edge 24G to an edge 24D over a width W.

[0117] FIG. 8 is an extract from FIG. 7 centred on a tread pattern element in the contact patch with the leading edge corners chamfered (51, 52, 53). The half-elements (MA1, MA2) comprise sipes 80 running from the edge (24G, 24D) to the ends of (MA1, MA2). This figure illustrates the calculation of the sipes density. The projection of the sipe 80 of the half-element MA1 onto the axial direction is Lpy1, and the projection of the sipe 80 of the half-element MA2 onto the axial direction is Lpy2. By definition, the sipes density SDA, according to FIG. 8, is the ratio 1000*(LPy1+LPy2)/(PA*W).

[0118] The invention was studied more particularly in the case of a passenger vehicle tyre of standardized designation, according to the ETRTO (European Tyre and Rim Technical Organisation), 205/65 R16 94V. For this size, a version according to the invention with three tread pattern elements MA, MB and MC, with respective variable pitches PA, PB and PC, was produced.

[0119] To optimize the arrangement of the elements so as to reduce the whining and beating noise, each tread pattern element is associated with an elementary, for example sinusoidal, signal. For one complete revolution of the wheel, the associated signal is periodic and results from the sum of the elementary signals.

[0120] With the aid of a digital tool, the initial arrangement is optimized with respect to the whining and beating noise by carrying out simulations on different arrangements. Using a Fourier transform on the signal associated with the arrangement, the spectrum of the signal is analysed in the frequency domain. The criteria for stopping the optimization process are linked to the amplitude of the whining and beating features, and to their spread along the frequency axis.

[0121] At the end of this iterative approach, for the tyre size studied, 205/65R16 94V, the total number of elements of the tread is established at 73 per wheel revolution, arranged in the sequence: MB-MC-MC-MC-MA-MC-MB-MB-MA-MB-MA-MB-MC-MA-MA-MA-MA-MA-MA-MB-MC-MC-MB-MA-MA-MC-MB-MC-MA-MC-MB-MA-MA-MB-MB-MB-MC-MC-MA-MA-MA-MA-MB-MB-MB-MC-MC-MC-MB-MB-MA-MA-MA-MB-MA-MC-MA-MB-MA-MC-MA-MC-MC-MB-MB-MA-MA-MA-MA-MA-MB-MB-MC.

[0122] The circumference of the tyre is equal to 2017.5 mm and the width of the tread is 161 mm. The tread pattern of the tread of the manufactured tyre comprises 3 tread pattern elements (MA, MB, MC) which are distributed in 30 elements MA, in 18 elements MB and finally in 15 elements MC.

[0123] The following table recaps the features of the tread pattern elements (MA, MB, MC):

TABLE-US-00001 TABLE 1 Nominal width of Volumetric Number the elements (mm) void ratio Pitch of tread Cen- Inter- of the of the pattern tre mediate Edge elements elements elements Z1 Z2 Z3 (%) (mm) Elements 30 3.9 7.1 8.1 30.5 23.56 MA Elements 18 4.6 8.3 9.6 30.5 27.71 MB Elements 15 5.6 10.1 11.4 30.5 33.66 MC

[0124] Each tread pattern element (MA, MB, MC) is formed of two half-elements (MA1, MA2), (MB1, MB2) and (MC1, MC2). The data in Table 1 are consolidated for the complete tread pattern element (MA, MB, MC) cumulating the contributions of the two half-elements as regards the void ratios.

[0125] Each tread pattern half-element is cut into three portions (Z1, Z2, Z3). The mean normal width of each portion is measured by measuring the normal distance between the leading and trailing edge corners without taking the chamfers into account.

[0126] The average of the sipes densities of the tread pattern elements (MA, MB, MC) per wheel revolution is 36 mm.sup.−1. This is the weighted average of the sipes densities of each of the tread pattern elements. For example, the sipes density of the tread pattern element MB is 36 mm.sup.−1.

[0127] The surface void ratio is 48.6±0.02. Its value increases by 8% with the presence of the chamfers at the trailing edge corners (15% with the trailing and leading edge corners). Under these conditions, the performance is improved by the effect of contact pressure on the trailing edge corners.

[0128] The volumetric void ratio is 30.5%±0.004, making it possible to ensure a sufficient performance on wet ground for an “all-season” tyre. The following table summarizes the characteristics of the chamfers:

TABLE-US-00002 TABLE 2 Chamfer width Chamfer width Chamfer width in the edge in the centre in the intermediate portion (Z1) portion (Z3) portion (Z2) mm mm mm Elements MA 1.3 0.85 1.7 Elements MB 1.5 1 2 Elements MC 1.8 1.2 2.4

[0129] The width of a chamfer in a portion (Z1, Z2, Z3) is the normal distance between the trailing face of the portion and the edge corner of the chamfer belonging to the tread surface.

[0130] The presence of the chamfers on the leading edges of the tread pattern elements improves grip on snow by around 3 to 5%.

[0131] The presence of the chamfers on the leading edges of the tread pattern elements does not have a detrimental effect on the running noise, which remains in accordance with UNECE (United Nations Economic Commission for Europe) regulation R117 with a level of radiated acoustic power below the threshold provided by the regulation.

[0132] The invention is not limited to the embodiments and variants presented and other embodiments and variants will become clearly apparent to a person skilled in the art.