Pneumatic tires

Abstract: A pneumatic tire (1) comprising a radial carcass (3), a belt (4) and a tread portion (5) is provided at its side end region (T S ) outside of a ground contact area (T M ) of the tread portion with a plurality of fine grooves (13), each being a groove in which each of opposed groove walls (13a) extends slantly in substantially a direction opposite to the rotating direction (L) of the tire toward a groove bottom of the fine groove, which considerably improves straight running stability while controlling wandering phenomenon during running on inclined road surfaces.

Description: This invention relates to a pneumatic tire, and in particular to a pneumatic tire capable of controlling complicated movements of the tire or so-called wandering phenomenon not predicted by a driver, which is produced when the tire is run at a high speed on an inclined road surface or a road surface having irregular portions such as a rutted road surface, to improve straight running stability. A high performance tire of this type is required to have a large lateral rigidity against a large lateral force produced in the tire during cornering and excellent traction and braking performances during high-speed running, steering stability and so on with a high powered vehicle. For this end, tires having a wider ground contact width by making the aspect ratio thereof small have been developed. A tire having a small aspect ratio exhibits an excellent steering stability on a flat road surface. However, when the aspect ratio is in particular not more than 60%, if a tire having such an aspect ratio is run at a high speed on an inclined road surface or a road surface having irregular portions such as a rutted road surface, non-uniform force is locally applied to the tire due to the inclined or irregular road surface to produce complicated movements of the tire or so-called wandering phenomenon, whereby the vehicle is moved in a direction not predicted by a driver. Such a wandering phenomenon is a risky factor damaging the straight running property of the vehicle, which becomes a serious problem with the development of high running performance tires. Attention is also drawn to the disclosures of each of GB-A-2093777, GB-A-2053817, FR-A-2418719, GB-A-2190048, JP-A-62-261508 and JP-A-62-261507. It is, therefore, an object of the present invention to provide high-performance pneumatic tires having a small aspect ratio and wherein the wandering phenomenon is controlled by improving the straight running stability on an inclined or irregular road surface. The present invention in one aspect provides a pneumatic tire comprising: a carcass toroidally extending between a pair of bead portions, a belt superimposed about a crown portion of the carcass and a tread portion arranged on the belt, the tire having an equatorial plane, said tread portion having a ground contact area and side end regions when the tire is inflated under a normal internal pressure and loaded under a normal load on a flat road, said tread portion having a plurality of fine grooves which extend outward in a widthwise direction of the tread, said fine grooves being arranged in each of said side end regions outside said ground contact area of the tread portion, and each groove wall of said fine grooves extends in substantially a radial direction and slants in substantially a direction opposite to a rotating direction of the tire toward a groove bottom thereof. The tire of the invention preferably further comprises lateral grooves extending in the widthwise direction of the tread to define land portions at the side end regions of the tread, said lateral grooves having opposed walls each inclined at an inclination angle with respect to the radial direction of the tire, the direction of inclination of the walls of the lateral grooves being in substantially a direction opposite to the rotating direction of the tire toward the groove bottom thereof. More preferably the aforesaid lateral grooves are arranged in a ground contact area and side end regions of the tread, and an inclination angle of a groove wall of the lateral grooves arranged in the ground contact area extends slantly in a direction opposite to the rotating direction of the tire toward the groove bottom thereof and is made smaller than that of the lateral groove arranged in the side end region that extends slantly in the direction opposite to the rotating direction of the tire toward the groove bottom thereof. It is also preferred that the groove walls of the fine grooves and/or the lateral grooves at a side end region of the tread have an inclination angle of 5-15° with respect to the radial direction of the tire. The invention in another aspect provides a pneumatic tire comprising: a carcass toroidally extending between a pair of bead portions, a belt superimposed about a crown portion of the carcass and a tread portion arranged on the belt, the tire having an equatorial plane, said tread portion having a ground contact area and side end regions when the tire is inflated under a normal internal pressure and loaded under a normal load on a flat road, said tread having a plurality of fine grooves which extend outward in a widthwise direction of the tread, said fine grooves being arranged in each of said side end regions outside said ground contact area of the tread portion, such that each fine groove includes a first portion located within a side end region and a second portion located within the ground contact area, each fine groove having a pair of walls, each of the walls of the first portion of the fine groove being inclined at an inclination angle with respect to the radial direction of the tire, the direction of inclination of said walls of the first portion being opposite to a rotating direction of the tire toward a groove bottom thereof, wherein either each of the walls of the second portion of the fine groove is inclined at an angle with respect to the radial direction of the tire which is smaller than said inclination angle and the direction of inclination of the walls of the second portion of the fine groove is inclined in a direction opposite to the rotating direction of the tire, or each of the walls of the second portion of the first groove is inclined at an angle with respect to the radial direction of the tire and the direction of inclination of the walls of the second portion of the fine groove is in the rotating direction of the tire. The tire of this further aspect of the invention preferably further comprises lateral grooves extending in the widthwise direction of the tread to define land portions at the side end regions of the tread, said lateral grooves having opposed walls each inclined at an inclination angle with respect to the radial direction of the tire, the direction of inclination of the walls of the lateral grooves being in a direction opposite to the rotating direction of the tire toward the groove bottom thereof. More preferably, the groove walls of the fine grooves and/or the lateral grooves at a side end region of the tread have an inclination angle of 5-15° with respect to the radial direction of the tire. It is also preferred that the number of fine grooves located in a side end region of the tread is larger than the number of fine grooves located in the ground contact area thereof. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a radial section view of an embodiment of a pneumatic tire according to the invention; Fig. 2 is a diagrammatic view of a tread pattern in a pneumatic tire according to the invention; Fig. 3 is a schematic sectional view taken along a line III-III of Fig. 2; Fig. 4 is a schematic sectional view of another embodiment taken along a line III-III of Fig. 2; Fig. 5 is a diagrammatic view illustrating a state of contacting a tire with an inclined road surface; Fig. 6 is a diagrammatic view illustrating the behaviour of a land portion including fine grooves; Fig. 7 is a diagrammatic view of a ground contact area of a tread portion at an inclined road surface; and Fig. 8 is another diagrammatic view illustrating the behaviour of a land portion including fine grooves. In Fig. 1 is sectionally shown a typical pneumatic tire 1 according to the invention in the widthwise direction of the tread thereof. The pneumatic tire 1 comprises a carcass 3 toroidally extending between a pair of bead portions 2, a belt 4 superimposed about a crown portion of the carcass and comprised of two belt layers, and a tread portion 5 arranged on the belt. The outer surface of the tread portion 5 has a tread pattern as shown in Fig. 2 wherein a pair of circumferential grooves 6 are arranged at both sides of an equatorial plane O, and a pair of auxiliary circumferential grooves 7 are arranged outside the circumferential grooves 6 in a region toward a side end of the tread, and land portions defined by these circumferential grooves 6 and auxiliary circumferential grooves 7 are respectively divided into a plurality of blocks 9 and 10 by a plurality of lateral grooves 8 each extending slantly with respect to the equatorial plane O and connecting the circumferential groove 6 to the auxiliary circumferential groove 7. Land portions defined by the auxiliary circumferential grooves 7 and the tread side ends are each divided into a plurality of blocks 12 by a plurality of lateral grooves 11 extending in the widthwise direction of the tread. This tread pattern is a so-called directional pattern in which the lateral grooves 8 extend so as to converge toward the equatorial plane in such a manner that the inclinations of the lateral grooves 8 located at both sides of the circumferential grooves 6 with respect to the equatorial plane are inclined opposite to each other. A tire having such a tread pattern is mounted on a vehicle so as to match a rotating direction L of the tire with a running direction of the vehicle. Furthermore, each of the blocks 12 is provided with one or more fine grooves 13, two fine grooves 13 in the illustrated embodiment. The fine groove 13 includes a so-called sipe if the width of the fine groove 13 is not more than a half of the width of the lateral groove 11 and the opposed groove walls of the fine groove are closed to each other during contact with the ground. In the invention, it is essential that the fine grooves 13 located at a side end region T S outside a ground contact area T M of the tread portion when the tire is inflated under a normal internal pressure and loaded under a normal load is a groove in which each of opposed groove walls extends slantly in substantially a direction opposite to the rotating direction L of the tire. In other words, the groove wall of the fine groove 13 slantly extends in a direction opposite to the converging direction of the lateral groove 8 at the equatorial plane of the tire. That is, as shown in Fig. 3 showing a section taken along a line III-III of Fig. 2, the groove wall 13a of the fine groove 13 is inclined in a direction opposite to the rotating direction L of the tire with respect to a radial direction of the tire, whereby the ground contact pressure of the side end region T S of the tread is increased during contacting with a slant road surface. In this case, an inclination angle α of the groove wall 13a with respect to the radial direction of the tire is preferably within a range of 5-15°. As shown in Fig. 4 showing another section taken along-a line III-III of Fig. 2, the groove wall 11a of the lateral groove 11 is inclinated in a direction opposite to the rotating direction L of the tire with respect to the radial direction of the tire, whereby the ground contact pressure of the side end region T S of the tread is also increased during contacting with the slant road surface. In this case, an inclination angle β of the groove wall 11a with respect to the radial direction of the tire is preferably within a range of 5-15°. As mentioned above, the groove walls of the fine grooves 13 and further the lateral grooves 11 in the side end region are inclined to increase the ground contact pressure at the side end region T S , whereby control of the wandering phenomenon is realized. The control of the wandering phenomenon is described in detail below. The inventor has made studies with respect to a mechanism of generating the wandering phenomenon when the tire is run on an inclined road surface or an irregular road surface such as a rutted road surface or the like, and confirmed that (as shown in Fig. 5) a large lateral force F y is applied to the tire due to the increase of camber thrust F c received from the inclined road surface and particularly the straight running stability is adversely affected due to the increase of the lateral force F y in the case of a tire having a small aspect ratio. Therefore, in order to control the wandering phenomenon, it is effective to reduce the lateral force applied to the tire during the running on the inclined road surface or to reduce the camber thrust. Observing the behaviour of the tire during the running on the inclined road surface, as shown in Fig. 5, a region of the ground contact area facing the inclined road surface (mountain side) is forcedly pushed onto the road surface, while a region of the ground contact area opposed to the inclined road surface (valley side) rises away from the road surface. In the region forcedly pushed onto the road surface, the deflection deformation of the sidewall portion becomes large under loading W applied to the tire, whereby falling deformation T f of the carcass ply is caused and hence the side end region of the tread located outside the ground contact area in the vicinity of the falling deformed portion is newly contacted with the inclined road surface. At the same time, bending deformation T b1 is caused in the vicinity of the belt end accompanied with the falling deformation T f , whereby the ground contact pressure is increased in the vicinity of the end of the ground contact area. Moreover, the side end region of the tread changes in accordance with the inclination angle of the inclined road surface, but is a region located at each side of the ground contact area of the tread at a distance corresponding to 0.1 times the ground contact width. From the above examination, it is believed that the large camber thrust inducing the wandering phenomenon results from the increase of the ground contact pressure in the side end region of the tread. According to the invention, the increase of the ground contact pressure in the side end region during the running on the inclined road surface is inversely utilized to reduce the camber thrust to thereby control the wandering phenomenon. For this purpose, the fine grooves formed in the land portion at the side end region of the tread are formed as grooves in which each of opposed groove walls slants in substantially a direction opposite to the rotating direction of the tire as shown in Fig. 3. That is, when the ground contact pressure in the side end region is increased as mentioned above, since the groove walls of the fine grooves are inclined in a direction opposite to the rotating direction of the tire, the land portion defined by these fine grooves causes shearing deformation as shown by broken lines in Fig. 6 and hence a shearing force F xs in the running direction is applied from the road surface to the land portion as a reaction force. As a result, a shearing force F xs is applied to the side end region in the ground contact area of the tread on the inclined road surface as shown in Fig. 7, whereby moment M xs giving a slip angle γ xs is produced in the tire. The slip angle γ xs turns the rotating face of the tire toward a direction of falling downward on the inclined road surface and hence the camber thrust F y is reduced. When the inclination angle α of the groove wall in the fine groove shown in Fig. 3 is less than 5° with respect to the radial direction of the tire, the above function and effect are not so developed, while when said angle exceeds 15°, it is difficult to remove the tire from a mold after the tire building and vulcanization. Furthermore, when the lateral groove defining the block in the side end region is arranged so as to be inclined likewise the fine groove as shown in Fig. 4, the same function as shown in Fig. 6 can be expected as shown in Fig. 8, whereby the effect of reducing the camber thrust F y can be more enhanced. The inclination angle β of the groove wall in the lateral groove of Fig. 4 with respect to the radial direction of the tire is preferably within a range of 5-15° for the same reasons as described for the inclination angle α of the fine groove wall. The land portions arranged side by side at the side end region in the circumferential direction creates slipping with respect to the road surface when leaving the road surface during the running of the tire, so that there is a tendency to unevenly wear the side of the land portion finally leaving the road surface or the so-called kicking-out side thereof. When the groove walls of the fine groove and lateral groove are inclined in the side end region, deformations as shown by broken lines in Figs. 6 and 8 are caused during contacting with the road surface, so that the ground contact pressure at the kicking-out side edge may increase to promote the occurrence of uneven wear. For this end, the inclination angle of the groove wall in the fine groove and the lateral groove located in the ground contact area of the tread running on a flat road surface is made small as compared with the inclination angle of the groove wall in the fine groove and the lateral groove located in the side end region running on the inclined road surface, or the fine groove and the lateral groove in the ground contact area are inclined in the rotating direction of the tire, whereby the straight running stability can be improved without causing uneven wear. Moreover, it is preferable that the number of fine grooves arranged in the side end region is set to be larger than that in the ground contact area of the tread. That is, it is desirable that the number of fine grooves is increased even in the end portion of the ground contact area for increasing the ground contact pressure on the inclined road surface, but the number of fine grooves is important in order to avoid uneven wear in the ground contact area. For this end, it is-desired that the number of fine grooves in the side end region not depending upon this restriction is made larger than that in the ground contact area to simultaneously prevent the occurrences of uneven wear and wandering phenomenon. The following examples are given in illustration of the invention and are not intended as limitations thereof. A pneumatic radial tire having a tire size of 235/45ZR17 is prepared according to a structure and a shape as shown in Figs. 1 and 2. In this tire, the belt is comprised of two rubberized belt layers each containing steel cords arranged at a cord angle of 22° with respect to the equatorial plane of the tire, the cords of which layers being crossed with each other, and has a width of about 220 mm. Furthermore, the width of the ground contact area T M of the tread is 190 mm. In a portion of the block 12 located in the ground contact area T M and outside a region corresponding to 0.7 times the ground contact width around the equatorial plane of the tire, the inclination angle α of the fine groove 13 having a width of 0.5 mm and a depth of 8 mm and the inclination angle β of the lateral groove 11 having a width of 3 mm and a depth of 8 mm are changed as shown in Table 1. In the remaining portion of the block 12 located in the side end region T S outside the ground contact area at a distance corresponding to 0.13 times the ground contact width, the inclination angles α and β are changed as shown in Table 1, while the depths of the fine groove 13 and the lateral groove 11 are gradually reduced from 8 mm to 3 mm. A comparative tire is prepared having the same shape and structure as the above tire except that the inclination angles α and β of the fine groove and the lateral groove are 0°, respectively. Each of these tires is inflated under a normal internal pressure of 2.4 kgf/cm 2 and then run on a flat belt type testing machine lined with a safety walk having an inclination angle of 5° on its surface at a speed of 50 km/h under a normal load of 650 kgf (JATMA), during which the lateral force F y in a direction of raising on the inclined surface is measured. Furthermore, the tire is mounted onto a passenger car of FR driving system having a displacement of 3000 cc and run on a rutted road surface at a high speed (80 km/h) under a loading of two persons, during which the straight running stability is evaluated on a ten point scale by a feeling test of a driver. Moreover, the tire is run on a course consisting of expressway and general-purpose roads at a ratio of 4:6 at a speed of about 100 km/h on the express way and a speed of about 50 km/h on the general-purpose road over a distance of 10,000 km, and thereafter a worn difference between a central part in the widthwise direction and each edge part in the circumferential direction of the blocks located at the end of the ground contact area is measured to evaluate the resistance to uneven wear. The measured results are also shown in Table 1. As seen from the data of Table 1, in the tires according to the invention, the lateral force F y is considerably controlled to improve the straight running stability and also the wear resistance is good. Inclination angles α, β in end portion of ground contact area Inclination angles α, β in side and region Lateral force Fy Straight running stability Resistance to uneven wear as worn difference (mm) Remarks 0° 0° 100 5 1.2 Comparative Example 5° 8° 87 6.5 1.4 Acceptable Example -2° 8° 93 5.7 1.1 Acceptable Example 5° 5° 92 5.8 1.3 Acceptable Example As mentioned above, the straight running stability on an inclined road surface or an irregular road surface such as a rutted road or the like can be improved in the pneumatic tires according to the invention, particularly high-performance radial tires having a small aspect ratio.

Claims: A pneumatic tire (1) comprising: a carcass (3) toroidally extending between a pair of bead portions (2), a belt (4) superimposed about a crown portion of the carcass and a tread portion (5) arranged on the belt, the tire having an equatorial plane (0), said tread portion having a ground contact area (T M ) and side end regions (T S ) when the tire is inflated under a normal internal pressure and loaded under a normal load on a flat road, said tread portion having a plurality of fine grooves (13) which extend outward in a widthwise direction of the tread, said fine grooves being arranged in each of said side end regions (T S ) outside said ground contact area (T M ) of the tread portion, and each groove wall of said fine grooves (13) extends in substantially a radial direction and slants in substantially a direction opposite to a rotating direction (L) of the tire toward a groove bottom thereof. A pneumatic tire as claimed in claim 1, characterized by further comprising lateral grooves (11) extending in the widthwise direction of the tread to define land portions (12) at the side end regions (T S ) of the tread, said lateral grooves having opposed walls each inclined at an inclination angle with respect to the radial direction of the tire, the direction of inclination of the walls of the lateral grooves being in substantially a direction opposite to the rotating direction (L) of the tire toward the groove bottom thereof. A pneumatic tire as claimed in claim 2, characterized in that the lateral grooves (11) are arranged in a ground contact area (T M ) and side end regions (T S ) of the tread, and an inclination angle of a groove wall of the lateral grooves arranged in the ground contact area extends slantly in a direction opposite to the rotating direction (L) of the tire toward the groove bottom thereof and is made smaller than that of the lateral groove arranged in the side end region that extends slantly in the direction opposite to the rotating direction of the tire toward the groove bottom thereof. A pneumatic tire as claimed in claim 2, characterized in that the groove walls of the fine grooves (13) and/or the lateral grooves (11) at a side end region (T S ) of the tread have an inclination angle of 5-15° with respect to the radial direction of the tire. A pneumatic tire (1) comprising: a carcass (3) toroidally extending between a pair of bead portions (2), a belt (4) superimposed about a crown portion of the carcass and a tread portion (5) arranged on the belt, the tire having an equatorial plane (0), said tread portion having a ground contact area (T M ) and side end regions (T S ) when the tire is inflated under a normal internal pressure and loaded under a normal load on a flat road, said tread having a plurality of fine grooves (13) which extend outward in a widthwise direction of the tread, said fine grooves being arranged in each of said side end regions (T S ) outside said ground contact area (T M ) of the tread portion, such that each fine groove includes a first portion located within a side end region (T S ) and a second portion located within the ground contact area (T M ), each fine groove having a pair of walls, each of the walls of the first portion of the fine groove being inclined at an inclination angle with respect to the radial direction of the tire, the direction of inclination of said walls of the first portion being opposite to a rotating direction (L) of the tire toward a groove bottom thereof, wherein either each of the walls of the second portion of the fine groove is inclined at an angle with respect to the radial direction of the tire which is smaller than said inclination angle and the direction of inclination of the walls of the second portion of the fine groove is inclined in a direction opposite to the rotating direction of the tire, or each of the walls of the second portion of the first groove is inclined at an angle with respect to the radial direction of the tire and the direction of inclination of the walls of the second portion of the fine groove is in the rotating direction of the tire. A pneumatic tire as claimed in claim 5, characterized by further comprising lateral grooves (11) extending in the widthwise direction of the tread to define land portions (12) at the side end regions (T S ) of the tread, said lateral grooves having opposed walls each inclined at an inclination angle with respect to the radial direction of the tire, the direction of inclination of the walls of the lateral grooves being in a direction opposite to the rotating direction (L) of the tire toward the groove bottom thereof. A pneumatic tire as claimed in claim 6, characterized in that the groove walls of the fine grooves (13) and/or the lateral grooves (11) at a side end region (T S ) of the tread have an inclination angle of 5-15° with respect to the radial direction of the tire. A pneumatic tire as claimed in any of claims 5 to 7, characterized in that the number of fine grooves (13) located in a side end region (T S ) of the tread is larger than the number of fine grooves located in the ground contact area (T M ) thereof.

Inventors: MATSUMOTO HIROYUKI, MATSUMOTO, HIROYUKI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1995
Main classification: B60C11
IPC codes: B60C11