Radial passenger tire with improved tread contour

The invention relates to a tire and more specifically a passenger tire having a specific contour of the radially outer tread area.

It is usual to design a passenger tire mold cavity with a profile giving the cured tire a tri-radius crown area. Starting from center line, the first radius has the highest value so as to result in the flattest radially outer tread area in the cured tire. The second radius would permit to reach the necessary mold contour drop at the beginning of the cavity shoulder radius. This second radius has an intermediate value between the first radius and the third radius which is the shoulder radius and which permits a progressive footprint edge and a crown area junction with the tire sidewalls.

When a tire built in such a cavity is loaded on the ground, it is not difficult to flatten the first radius area. The tread portion having the second radius area which is of smaller value will resist more to be flattened to the ground. It will rather have tendency to have a hinge effect by rolling on its curvature like in a rocking chair. The “rocking chair effect” induces a lift-off of the center part and an increase in pressure in the shoulder portions as well as a slippage, which is in fact a lateral displacement in the footprint. The presence of the tire grooves in the circumferential direction will permit to avoid this rocking chair effect; however the proximity of the shoulders which are usually designed with a very low radius and have a high stiffness, will result in shifting the lift-off and the slippage to an area closer to the shoulder.

However, in the current trend to design a tread pattern for ultra high performance (UHP) tires, such circumferential grooves are normally not located in the above mentioned second radius area nor in the low meridional bending stiffness area which is in the center area. The result is an even more pronounced rocking chair effect. If the tire shoulder construction is not carefully designed (high bending stiffness), this rocking chair effect results in a poor pressure distribution. One of the effects will be a tire shoulder slippage which results in irregular wear.

U.S. Pat. No. 5,803,999 discloses a tire with a tread profile comprising a curved crown part having a radius of curvature R 1 for its center portion, a pair of curbed middle portions having a radius of curvature R 2 and a pair of curved shoulder portions having a radius of curvature R 3 which extend continuously without forming any inflection point, wherein R 1 ranges between 2 and 3.5 times the tread width TW, and is less than the radius R 2 and more than the radius R 3 . The tread shoulder drop ranges between 0.038 and 0.050 times the tread width.

U.S. Pat. No. 5,616,195 discloses a truck tire wherein the radius R 2 equals 1.3 to 2 times R 1 . Radius R 3 equals 0.7 to 2.5 times R 1 . Radii R 1 and R 2 are convex whereas R 3 is concave.

U.S. Pat. No. 5,360,044 relates to a tire with a tread surface extending along a curvature plane including a first arc with a radius of curvature R 1 , a second arc R 2 intersecting the first arc at an intersection distance from the tire's equatorial plane by 0,2 to 0,25 times the tire width and a third arc with the radius R 3 passing through a ground contact outer edge point of the ground contact surface. The curvature radius ratio R 1 /R 2 is set in a range from 1.2 to less than 1.6. The curvature radius ratio R 2 /R 3 is set in a range between 4 and 12.

U.S. Pat. No. 5,247,979 relates to a tire exhibiting improved handling and stability on wet roads where the net to gross is in a range comprised between 0.55 and 0.58. The radius or curvature of the shoulder of the tire R 2 and the ratio of curvature of the shoulder edge portion present a ratio R 2 /R 3 comprised between 14 and 20.

An object of the invention is to design a crown area of a tire so that the flattening on the ground can be done without a lift-off and/or a lateral displacement in the footprint. A further object of the invention is to reduce tire shoulder wear. A further object is to reduce heel and toe wear.

The invention provides a radial pneumatic passenger tire as defined in the claims.

More specifically the invention provides a tire wherein the center line area has a radius ranging from 400 to 900 mm, preferably from 500 to 800 mm. The second radius is greater than the first radius and ranges from 700 to 1300 mm, preferably from 800 to 1200 mm. In such a cavity shape wherein the tread includes circumferential grooves or grooves having a low inclination with respect to the equatorial plane, the tread flattening in the ground is easy in the centerline part due to its low meridional bending stiffness and the high lever effect. The meridional bending stiffness is primarily a function of the number and location of the circumferentially oriented grooves, the breaker layout and the tread thickness and stiffness. The second radius being high, the rocking chair effect is much reduced even under heavy cornering.

The tire shoulders are the crown area parts which have the highest deformation during the tire loading. The bending stiffness should be kept at a minimum level to avoid a tendency to a hinge effect with a high vertical loading and/or with a lateral force. In the invention, it may be considered to provide a third radius adjacent to the second and the shoulder radius which shoulder radius becomes the fourth radius. The third radius introduces a small drop compared to the second radius to attenuate the hinge effect linked to the shoulder deformation. This third radius will range in length between the second radius and the shoulder radius without changing the distribution of the first and the second radii.

As mentioned above, to avoid the “rocking chair effect”, the second radius has a high value. However, to reach a certain required drop at the shoulder, the center part must have a low equivalent radius. This equivalent radius is smaller than R 2 . An equivalent radius is by definition the radius of a circle tangent to R 2 at the R 2 starting edges. According to the invention, the center part must not be greater than ½ of the tread width (TW) and the equivalent radius must not be lower than 300 mm. However, this low radius parts have to be flattened on the ground. For this reason, it is preferable that this part is decoupled with circumferential grooves. Put in other words, the tread portion defined by radius R 2 should preferably be separated from the portion defined by radius R 1 by circumferential grooves. If that case, for the center part area, the true R 1 doesn't need to be tangent to R 2 and R 1 may have any value (lower, equal or greater than R 2 ). The center part may be composed of one or two radii (e.g. a flat center rib with two radii both sides to join R 2 and separated from the center rib by circumferential grooves).

“Apex” means an elastomeric filler located radially above the bead core and between the plies and the turnup ply.

“Aspect Ratio” means the ratio of its section height to its section width.

“Bead” means that part of the tire comprising an annular tensile member wrapped by ply cords and shaped, with or without other reinforcement elements such as flippers, chippers, apexes, toe guards and chafers, to fit the design rim.

“Belt Structure” or “Reinforcing Belts” means at least two annular layers or plies of parallel cords, woven or unwoven, underlying the tread, unanchored to the bead, and having both left and right cord angles in the range from 17 degrees to 27 degrees with respect to the equatorial plane of the tire.

“Casing” means the carcass, belt structure, beads, sidewalls, and all other components of the tire excepting the tread and undertread. The casing may be unvulcanized rubber or previously vulcanized rubber to be fitted with a new tread.

“Chafers” refers to narrow strips of material placed around the outside of the bead to protect cord plies from the rim, distribute flexing above the rim, and to seal the tire.

“Circumferential” means lines or directions extending along the perimeter of the surface of the annular tread perpendicular to the axial direction.

“Cord” means one of the reinforcement strands of which the plies in the tire are comprised.

“Lateral” means an axial direction.

“Ply” means a continuous layer of rubber-coated parallel cords.

“Radial” and “radially” mean directions radially toward or away from the axis of rotation of the tire.

“Radial Ply Tire” means a belted or circumferentially-restricted pneumatic tire in which the ply cords which extend from bead to bead are laid at cord angles between 65 degrees and 90 degrees with respect to the equatorial plane of the tire.

“Section Height” means the radial distance from the nominal rim diameter to the outer diameter of the tire at its equatorial plane.

“Section Width” means the maximum linear distance parallel to the axis of the tire and between the exterior of its sidewalls when and after it has been inflated at normal pressure for 24 hours, but unloaded, excluding elevations of the sidewalls due to labeling, decoration or protective bands.

“Shoulder” means the upper portion of sidewall just below the tread edge, tread shoulder or shoulder rib means that portion of the tread near the shoulder.

“Sidewall” means that portion of a tire between the tread and the bead.

“Tread” means that portion of the tire that comes into contact with the road under normal inflation and load.

“Tread width (TW)” means the arc length of the tread surface in the axial direction, that is, in a plane passing through the axis of rotation of the tire.

Similar elements throughout the drawings may be referred to by similar references numerals where an “′” has been added. For example, the element 3 B in a figure (or embodiment) may be similar in many respects to the element 3 B′ in an other figure (or embodiment). Such a relationship, if any, between similar elements in different figures or embodiments will become apparent throughout the specification.

FIG. 1 is a cross sectional view of the preferred embodiment tire according to the invention;

FIG. 2 is a schematic cross section of a tire mold according to the invention;

FIG. 3 shows an outline of an enlarged portion of a further embodiment of the invention showing in detail the cross section of a tread contour close to the tire shoulder;

FIG. 4 is an exemplary illustration of a tire footprint of a prior art radial tire exhibiting a square shape with a tendency to a butterfly shape; and

FIG. 5 is an exemplary illustration of the footprint of the preferred embodiment tire as represented in FIG. 1 .

With reference to FIG. 1 , a cross section of the pneumatic radial tire 10 for use in passenger vehicles is illustrated, showing the invention. The tire 10 has a tread 20 and a casing 12 . The distance halfway between the axially outer edges of the tread 20 , as shown in FIG. 1 , defines the centerline of the tread 20 or equatorial plane EP of the tire. The casing 12 has two sidewalls 14 , 16 , one or more radial plies 18 extending from and wrapped about two annular beads 13 , and belt reinforcement structure 15 located radially between the tread 20 and the plies 18 . The belt reinforcing structure is as represented in FIG. 1 , comprised of two belt plies 24 having laterally extending ends 24 A close to the shoulders of the tire 10 and an overlay 23 . As well known to the man skilled in the art such an overlay is obtained by helically winding during tire building a single flat strip of cord reinforced material radially outward on of the belt plies 24 . The carcass plies 18 and the belt reinforcement structure 15 are made of cord-reinforced elastomeric material; the cords are preferably steel cord for the belt plies 24 and textile cord for the carcass ply or plies 18 and for the overlay 23 . Similarly, the annular beads 13 have steel wires wrapped into a bundle also known as the bead core.

The carcass 30 includes the inner liner 19 , the plies 18 , sidewalls 14 , 16 , the beads 13 , and a pair of apexes 40 and 41 located over the beads 13 and between the plies 18 that are folded around the beads 13 . This carcass structure 30 is the foundation upon which, and contiguous with, the tread 20 resides.

The liner component 19 of preferably halobutyl rubber is air-impervious and seals the pressurized air in tire 10 . A pair of annular shaped wheel-rim retainer lips 37 retain the tire on the rim and protect the rim against damage. The chafers 38 protect the cord plies from the rim and distribute flexing above the rim.

Several circumferentially extending grooves are located on each side of the equatorial plane EP and define with laterally extending grooves a multiplicity of ground contacting elastomeric blocks (not represented). More or less grooves may be used depending on the width of the tread and other design considerations.

The radially outer tread surface has a maximum diameter D at the equatorial plane and is defined by a substantially constant radius of curvature R 1 extending laterally outwardly from the equatorial plane when the tire is inflated to its nominal pressure. The radius of curvature R 1 has according to the invention a length comprised between 400 to 900 mm, preferably from 500 to 800 mm. Its center C 1 is substantially on the equatorial plane and extends from a point P 0 on the equatorial plane of the tread to a point P 1 on each tread half located in the range of between 25% and 50% of half the treadwidth TW from P 0 defining thereby a first radially outer convex tread surface 3 A.

As further shown in FIG. 1 , at the point P 1 located on each half of the tread 20 , the curvature of the radially outer tread surface changes wherein the tread surface is defined by a second radius of curvature R 2 , also originating internally to the tire 10 . The second radius of curvature R 2 , having its center C 2 on the equatorial plane or not, extends from the point P 1 to a point P 2 located in the range of between 70% and 90% of half the treadwidth from P 0 defining thereby a second radially outer convex tread surface 3 B when the tire is inflated to its nominal pressure. The second radius is greater than the first radius and ranges from 700 to 1300 mm, preferably from 800 to 1200 mm about 1000 mm being most preferred.

A third radius of curvature R 3 has its center C 3 located also internally to the tire but not on the equatorial plane and extends from the point P 2 towards the tread edge, cutting the curve defining the shoulder of the tire and connecting the sidewall to the tread. This shoulder defining curve may have a single radius of curvature comprised between 20 and 30 mm and is not critical for the instant invention.

The third radius of curvature R 3 defines a third radially outer convex tread surface 3 C and has a radius comprised between 130 and 200 mm and preferably between 150 and 180 mm.

The combination of the radially outer surfaces 3 A, 3 B, and 3 C define a contour as shown in FIG. 1 wherein the contour is represented as a line 3 extending continuously across the crown portion of the tire. As said above, the tread comprises grooves which are not represented on FIG. 1 which grooves define elastomeric blocks. The grooves have a radial depth extending from the surface 3 to the base of the grooves.

The tire 10 preferably has a relationship between the first, second and third radii of curvatures such that the second radius of curvature R 2 is in the range of 1.5 to 2.5 times preferably about 2 times the first radius of curvature R 1 and the third radius of curvature R 3 is in the range of 0.1 to 0.5 times the second radius of curvature R 2 , preferably 0.2 to 0.4 times R 2 . Preferably the third radius of curvature R 3 is in the range of 0.25 to 0.35 times the second radius of curvature R 2 . The radii are preferably oriented such that the adjacent portions of the tread contour are tangent. Should this not be the case for R 1 and R 2 , the two tread portions 3 A and 3 B are preferably separated by a groove. The second radius of curvature and the third radius of curvature must be tangent, the tread portions 3 B and 3 C may not be separated by a groove.

Referring now to FIG. 2 there is represented schematically a cross section of a tire mold according to the invention. The different radii R 1 , R 2 , R 3 , and R 4 can be distinguished. The curve defining the shoulder of the tire and connecting the sidewall to the tread is in this case one constant radius R 4 . Reference 3 points to the outer contour of the tire which is ground contacting and reference 4 to outline of the maximum radial depth the different block defining grooves (not represented) may have.

In a further embodiment of the invention the dry braking performance and the cornering stability are improved with an increase of contact pressure of a part of the footprint, the braking force per surface unit being more or less proportional to the footprint pressure. The idea is based on the fact that a bending of a stiff and skew surface against a flat ground generates an increase of contact pressure. However, it is important that this pressure increase occurs only during the braking or lateral acceleration of the car. In normal running conditions, the high pressure in a part of the footprint may lead to irregular wear and to handling issues. The shoulder layout is tuned so that the skew surface will not generate a high footprint pressure in the normal running conditions. Only the overloading produced by braking or by lateral acceleration will generate a bending of this skew surface, leading to an increase of the contact footprint pressure with the ground resulting in an increased tire grip. In order to obtain this effect, radii R 2 and R 3 must have specific relationships and radius R 3 a specific length.

Referring to FIG. 3 there is represented an outline of an enlarged portion of this further embodiment of the invention showing in detail the cross section of a tread contour close to the tire shoulder. This shoulder tread contour can be implemented with advantage in a tire as represented in, and disclosed in connection with FIG. 1 , as well as in other tires which do not have the described ratios of the radii R 1 and R 2 . The skew surface is represented by the radially outer contour of the tire section. This contour must have a profile including two tread contour defining curves 3 B′ and 3 C′ which are tangent one with the other at point P 2 ′. The radius R 2 ′, determining second contour defining curve 3 B′ is superior or equal to 700 mm and preferably in the range of 800 to 1,200 mm. The third contour defining curve 3 C′ is tangent to the second curve 3 B′ and to a small circle A of which the center is on the extension of the second curve 3 B′ of radius R 2 ′ at a distance of 10 mm from the edge P 2 ′ which as said above is the tangent point between second curve 3 B′ and third curve 3 C′-see also circle RIO having its center on P 2 ′ and a radius of 10 mm-. The radius of the small circle A ranges from 0.1 mm to 0.8 mm and is preferably between 0.2 and 0.5 mm. Put in the words of the man skilled in the art, there is a drop of 0.1 mm to 0.8 mm at a distance of 10 mm laterally outward of tangent point P 2 ′ of second and third contour defining curves 3 B′ and 3 C′.

The third contour defining curve 3 C′ is determined by another arc of circle with radius R 3 ′ at least up to the tangent point with the small circle A. Alternatively the third contour defining curve may be continued laterally outward of said tangent point with the small circle A and outward to the tread edge P 3 ′, by a substantially flat portion or by another curve. The spacing of tangent point P 2 ′ to the neighboring tread edge P 3 ′ is at least 15 mm, preferably 20 to 40 mm.

References A, I and B point to the projections on an axial line OX of respectively the edges P 1 ′ and P 2 ′ of the second curve 3 B′, the edges P 2 ′ and P 3 ′ of the third curve 3 C′. Reference M points to the projection on the axial line OX of the lateral outer edge 24 A of the belt plies 24 . The distance between projected points A and I must be greater than 1.1 times the distance between projected points I and B, preferably about two 2 times the distance between points I and B. It is furthermore important that projected point I is located within 75 to 100% and preferably between 80 and 95% of the distance spacing the equatorial plane EP from the projected point M of the lateral outer belt edge 24 A.

In order to obtain the technical effect described in this embodiment, the tread portion within reach of the circle RIO and its close neighborhood may not be cut be a circumferentially extending groove; generally, the farther the circumferential grooves are spaced from the intersection point P 2 ′, the less their detrimental effect. Put in other words, it is believed that part of the technical effect may still be obtained if there are small narrow grooves or slots not too close to the intersection point P 2 ′. By groove is meant here an elongated void area having a width exceeding 1 mm.

FIG. 4 is an exemplary illustration of a tire footprint of the prior art radial tire exhibiting a tendency to the butterfly shape.

The tire 10 according to the invention, when statically loaded exhibits a footprint as illustrated in FIG. 5 . As shown the footprint is substantially rectangular with the leading and trailing edges being convex.