Tire with optimised tread

Abstract

A tire, the speed rating of which is at least V, comprises a tread, having a volumetric void ratio at least equal to 2% and at most equal to 20%, having a central part Pc comprising at least one groove, and two lateral parts PI each having a volumetric void ratio at most equal to 2%. The central part Pc of the tread has an axial width at most equal to one third of the axial width LBDR of the tread, and any portion of the central part Pc of the tread that is circumferentially delimited by two meridian planes separated by a circumferential distance dc equal to one tenth of the outer perimeter P of the tire comprises at least one and at most three grooves that open onto the meridian planes delimiting the portion in question.

Claims

1. A tire, the speed rating of which is at least V, that is to say one that is adapted to be mounted on a vehicle of which the maximum permissible speed is at least equal to 240 km/h, the axial width of which is at least equal to 200 mm, and having an outer perimeter P, comprising: a tread, adapted to come into contact with the ground via a tread surface, having an axial width LBDR and a volumetric void ratio at least equal to 2% and at most equal to 20%; the tread having a central part Pc, centred on an equatorial plane C and delimited by two circumferential planes, and two lateral parts PI positioned axially on either side of the central part Pc, each lateral part PI being devoid of cuts and having a volumetric void ratio equal to 0%; the central part Pc of the tread comprising at least one groove, forming a space opening onto the tread surface and being delimited by at least two main lateral faces connected by a bottom face; the at least one groove in the central part Pc of the tread having a mean width W at least equal to 6 mm and at most equal to 30 mm, and a depth H at least equal to 3 mm and at most equal to 6 mm, wherein the central part Pc of the tread has an axial width at most equal to one third of the axial width LBDR of the tread, and wherein any portion of the central part Pc of the tread that is circumferentially delimited by two meridian planes separated by a circumferential distance dc equal to one tenth of the outer perimeter P of the tire comprises at least one and at most three grooves that open onto the meridian planes delimiting said portion.

2. The tire according to claim 1, wherein the tread comprises a rubber material, a loss factor P60 of which, measured at 60° C. and 10 Hz, is at least equal to 25%.

3. The tire according to claim 1, wherein the tread comprises a rubber material, a loss factor P60 of which, measured at 60° C. and 10 Hz, is at least equal to 35%.

4. The tire according to claim 1, wherein the tread has a maximum radial thickness E in a range of 3 mm to 7 mm.

5. The tire according to claim 4, wherein the tread has the maximum radial thickness E equal to 5.5 mm.

6. The tire according to claim 1, wherein the central part Pc of the tread has an axial width equal to one quarter of the axial width LBDR of the tread.

7. The tire according to claim 1, comprising the tread having two axial edges, one of which, positioned on the inner side of the vehicle, is referred to as the inner axial edge, wherein at least one groove in the tread has an inner main lateral face, axially closest to the inner axial edge, that forms a mean taper angle with a circumferential plane that is at least equal to 10°.

8. The tire according to claim 7, wherein the mean taper angle of the inner main lateral face is at least equal to 30°.

9. The tire according to claim 7, wherein the at least one groove in the tread has an outer main lateral face, axially furthest away from the inner axial edge, that forms a mean taper angle with a circumferential plane that is at most equal to the mean taper angle of the inner main lateral face.

10. The tire according to claim 1, wherein at least one groove in the central part Pc of the tread is obtained by moulding.

11. The tire according to claim 1, wherein at least one groove in the central part Pc of the tread is obtained by a mechanical cutting or machining operation.

12. The tire according to claim 1, wherein the central part Pc of the tread contains a single, substantially circumferential groove extending around the entire circumference of the tread.

13. The tire according to claim 12, wherein the mean width W of the single, substantially circumferential groove has variations (v) at least equal to 1 mm around the entire circumference of the tire.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features and other advantages of the invention will be understood better with the aid of FIGS. 1 to 8, said figures not being drawn to scale but in a simplified manner so as to make it easier to understand the invention:

(2) FIG. 1 is an overall view of the tire 1 according to an embodiment of the invention, in particular the tread 2 thereof and the tread surface 21 thereof. The tire in question is provided with a mounting direction SM indicated for example on the sidewall, but able to be indicated as desired by the manufacturers.

(3) FIG. 2 shows a meridian section through the crown of a tire according to an embodiment of the invention and illustrates the central part Pc, the lateral parts PI, the width LBDR of the tread, the grooves 22 and the two main lateral faces 221 and 222 thereof that are connected by a bottom face 223.

(4) FIGS. 3A and 3B show two types of radially exterior meridian profile of the tread 2 of a passenger vehicle tire, for which the measurement of the width of the tread is specified.

(5) FIG. 4 defines the “inner axial edge” 46 and “outer axial edge” 45 of a tread.

(6) FIG. 5 illustrates a disposition of grooves 22 according to an embodiment of the invention.

(7) FIG. 6 illustrates a first embodiment of a tread according to an embodiment of the invention with a single circumferential groove 22, or circumferential furrow, the lateral faces 221 and 222 of which have variations in axial position (v) in the circumferential direction at least equal to 1 mm with respect to their mean axial position in order to create local Venturi effects.

(8) FIG. 7 illustrates a second embodiment of a tread according to an embodiment of the invention with a single circumferential groove 22, or circumferential furrow, the lateral faces 221 and 222 of which have variations in axial position (v) in the circumferential direction at least equal to 1 mm with respect to their mean axial position in order to stiffen the tire around the lateral faces of the groove.

(9) FIG. 8 illustrates the mean taper angle AI of the inner lateral face of a groove 22, axially closest to the inner axial edge 46 intended to be positioned on the inner side of the vehicle, and the mean taper angle AE of the outer lateral face of a groove 22, axially furthest away from the inner axial edge 46.

DETAILED DESCRIPTION OF THE DRAWINGS

(10) FIG. 1 shows a perspective view of a tire 1 having a tread 2 and a optionally a mounting direction SM recommended by the manufacturer. The tread has a tread surface 21. FIG. 1 also shows the frame of reference (O, X, Y, Z) used.

(11) FIG. 2 schematically shows a meridian section, in a meridian plane YZ, through the crown of a tire according to the invention. It illustrates in particular the width LBDR and the thickness E of the tread 2, and also the central part Pc and lateral parts PI thereof. Also shown are a groove 22 and the lateral faces 221 and 222 and the bottom face 223 thereof. The depth H of a groove 22 is the radial distance between the radially innermost point of the bottom surface 223 and the point of the tread 21 that is closest to said point. FIG. 2 also shows the mean width W of the groove 22.

(12) In FIGS. 3A and 3B, the axial ends 7 of the tread, which make it possible to measure the tread width in this meridian plane, are determined in a meridian plane. In FIG. 3A, in which the tread surface 21 intersects the outer axial surface of the tire 8, the axial border 7 is determined trivially by a person skilled in the art. In FIG. 3B, in which the tread surface 21 is continuous with the outer axial surface of the tire 8, the tangent to the tread surface at any point on said tread surface in the region of transition towards the sidewall is plotted on a meridian section of the tire. The first axial border 7 passes through the point for which the angle β (beta) between said tangent and an axial direction YY′ is equal to 30°. When, in a meridian plane, there are several points for which the angle β between said tangent and an axial direction YY′ is equal to 30°, it is the radially outermost point that is taken into account. The same method is followed to determine the second axial end of the tread surface. The width of the tread, in the meridian plane, is the axial distance between the two points of the meridian plane of the two axial ends of the tread surface. The width of the tread of the tire is the maximum value of the widths of the tread over all the meridians.

(13) FIG. 4 schematically shows tires mounted on mounting rims of wheels of a vehicle 200 and having a predetermined direction of mounting on the vehicle. Each tire has an outer axial edge 45 and an inner axial edge 46, the inner axial edge 46 being the edge mounted on the bodyshell side of the vehicle when the tire is mounted on the vehicle in said predetermined direction of mounting, the outer axial edge 45 being the opposite of that. In the document, the expression “outboard side of the vehicle” denotes the outer axial edge 45.

(14) FIGS. 5 to 7 show various embodiments of a tread according to the invention. FIG. 5 shows a tread, the central part Pc of which has several grooves 22 such that any portion of the central part Pc of the tread contained between two meridian planes (M1, M2), the circumferential distance dc of which is at most equal to one tenth of the outer perimeter of the tire P, comprises at least one and at most three grooves 22 that open onto the meridian planes (M1, M2) delimiting the portion in question, and are closed on the circumferential planes (C1, C2) delimiting the central part of the tread.

(15) FIGS. 6 and 7 show two embodiments of a tread according to the invention with a single, substantially circumferential groove, or circumferential furrow, in the central part Pc. FIG. 6 presents variations (v) in the axial positions of the lateral faces 221 and 222 of the groove 22, creating variations in the width W of the groove that are likely to create Venturi effects in order to accentuate the suction effect of the invention, according to a first embodiment of the invention. This also increases the shear stiffness of the tread around the groove.

(16) FIG. 7 presents variations (v) in the axial positions of the lateral faces 221 and 222 of the groove 22, without any variation in the width W of the groove, according to a second embodiment of the invention. This makes it possible to increase the shear stiffness of the tread around the groove without creating a Venturi effect, since the width W of the groove remains constant around the circumference. Stiffening the tread pattern in this way makes it possible to improve the performance in terms of behaviour and of wear of the tire.

(17) FIG. 8 shows, according to a third embodiment of the invention, the angles AI and AE, referred to as taper angles, of the inner lateral face 221 and of the outer lateral face 222, respectively, depending on their respective positions with respect to the inner axial edge 46 and outer axial edge 45 of the tire. The inner lateral face 221 is axially closer to the inner axial edge 46 of the tire. The taper angle of a lateral face of a groove is measured in the meridian plane between the radial axis and the tangent to the lateral face of the groove at the median radial coordinate point, between the tread surface and the radially innermost point of the bottom face of the groove in the meridian plane in question. The figure is in accordance with the invention; specifically, the taper angle AI of the inner lateral face 221 is at least equal to the taper angle AE of the outer lateral face 222. According to this embodiment of the invention, the taper angle AE is at most equal to the taper angle AI.

(18) The inventors made calculations and carried out tests on the basis of the invention for a passenger vehicle tire of size 245/40 R18, inflated to a pressure of 1.7 bar in the cold state, with an axial width of 245 mm. They compared the tire A according to the prior art, with no grooves at all in its tread, with the tire B according to the invention, in the tread of which there are, in the central part Pc thereof, two circumferential grooves with a width of 10 mm and a depth of 4 mm, for a void ratio of 11%. The tread has a thickness of 5.7 mm. There are no other grooves or cuts in the lateral parts PI of the tread, the void ratio of which is zero.

(19) The tread comprises a rubber material, the loss factor of which, measured at 60° C. and 10 Hz, is equal to 54%. The taper angles of the lateral faces of the grooves are zero.

(20) The grooves were obtained by cutting a control tire A.

(21) The calculations showed a change in pressure in the contact patch that is able to be observed in a high-speed test. Positioning the grooves in the central part of the tread makes it possible, from the calculations, to double the pressure difference effect compared with positioning the grooves in the lateral parts.

(22) The tires were tested during braking on one and the same motor racing vehicle. Said racing vehicle was tested with a set of tires A, referred to as the control set, and then with a set of tires B according to the invention. The result given is the average of three tests.

(23) For braking at low speed of between 100 km/h and 5 km/h, the reduction in the contact area on account of the presence of the grooves is visible and the braking distance for the set of tires B according to the invention increases by more than 2.5% compared with the control set of tires A, this corresponding to a deterioration in the grip performance, but in a range of speeds that is of little interest on a motor racing circuit.

(24) By contrast, in a braking test at a high speed of between 250 km/h and 150 km/h, the braking distance for the set of tires B according to the invention decreases by more than 0.8% compared with the control set of tires A, this corresponding to an improvement in the grip performance, in a range of speeds that is of interest on a motor racing circuit.

(25) The scope of protection of the invention is not limited to the examples given hereinabove. The invention is embodied in each novel characteristic and each combination of characteristics, which includes every combination of any features which are stated in the claims, even if this feature or combination of features is not explicitly stated in the examples.