Pneumatic tyre

Abstract: A pneumatic tyre comprising a tread portion (2) with tread edges, a pair of sidewall portions (3), a pair of bead portions (4), a carcass (6) extending between the bead portions (4), a belt (7) disposed radially outside the carcass (6) in the tread portion (2), characterised in that said tread portion (2) is provided on each side of the tyre equator with axial grooves (11) extending axially inwardly from the tread edges, said axial grooves (11) are inclined to the same direction with respect to the tyre axial direction to form a nondirectional tread pattern, and the inclination angle of the axial grooves (11) with respect to the tyre circumferential direction is decreased from the axially outer end to the axially inner end thereof, so as to have a minimum inclination angle of less than 45 degrees, and a maximum inclination angle of from 60 to 85 degrees.

Description: The present invention relates to a pneumatic tyre having an improved non-directional tread pattern. Recently, a non-directional tread pattern which is generally symmetrical about a point on the tyre equator is widely used in passenger car tyres. In non-directional tread patterns, axial grooves extending axially inwardly from the tread edges (E) are inclined to the same direction with a left side upward inclination as shown in Fig.2 or a right side upward inclination as shown in Figs.4(a)-(b). In general, the inclination angle (b) of the axial grooves (a) and (b) to the tyre circumferential direction is set in the range of more than 50 degrees as shown in Figs.4(a)-(b). On the other hand, in passenger car tyres, a relatively low aspect ratio is widely employed nowadays. In other words, a relatively wide tread where drainage from the central part thereof is difficult is employed. If the number and/or width of the axial grooves (a) and (b) which extend from the tread central part to the tread edges (E) are increased, the drainage can be improved, but the pitch noise is inevitably increased. Thus, such measures can not be taken. The pitch noise is the noise generated by the blocks (c) when they periodically contact the road surface during running. The present inventors tried decreasing the inclination angle of the axial grooves to a value in the range of less than 45 degrees which is considerably smaller than the above-mentioned conventional range, and found that the drainage can be increased without increasing the axial grooves. However, the blocks between the axial grooves are liable to decrease in rigidity. Thus, the blocks are liable to slip against the road surface during running. Such slippage not only decreases the steering stability but also generates another type of noise (hereinafter slip noise) having a sharp peak (e) at about 1.6 KHz as shown in Fig.3 by a solid line. As inclination angle of the axial grooves is considerably decreased as explained above, another problem arises. In such a non-directional tread pattern, the residual cornering force (CF) is liable to increase, and the problem - drifting to one side of a vehicle arises. Furthermore, it was found that the steering stability can be improved by increasing the circumferential pitch lengths of the axial grooves. However, if the pitch lengths are increased, as the number of the axial grooves decreases, wet performance decreases. In general, on the other hand, the above-mentioned pitch noise can be reduced by employing variable pitches. However, if the pitch lengths are increased, the pitch noise reduction also becomes ineffectual. In the prior art document EP-A-0 588 781 teaches the use of included notches in the central circumferential rib of a tread pattern but discloses a central part having a width of 25-55%, more preferably 30-50%, for example 38% of the tread width and the distance w' between the inner end of the inclined grooves and the circumferential groove is 0.07 to 0.30 x more preferably 0.09 to 0.2 x the width w' of the part C but the axial grooves do not connect with the circumferential grooves in the central part thus affecting not only the wet performance but the noise characteristic and the pattern stability. Another prior art disclosure EP -A-0 782 936, regarded as closest prior art to the tire of claim 1 and according to Art. 54(3) EPC relevant for assessing novelty, discloses a tyre having a tread provided on each side of the tyre equator with axial grooves extending axially inwardly from the tread edges and inclined to the same direction with respect to the tyre axial direction to form a non-directional tread pattern. The inclination angle of the axial grooves with respect to the tyre circumferential direction is decreased from the axially outer end to the axially inner end having a minimum inclination of from 10 to 25° and a maximum angle of from 60 to 90°. The tread portion is provided in the central part with a pair of circumferential grooves one each side of the tyre equator and a circumferential centre groove. The two grooves are connected to the axial grooves so as to define circumferentially spaced blocks and the angles of the axial grooves gradually decrease from the tread edge to the circumferential grooves. The large centre circumferential groove is not connected to other grooves and so noise is kept therein. The objectives of the present invention therefore are to overcome the disadvantages of the above prior art providing a pneumatic tyre having a non-directional tread pattern, to control the residual cornering force to prevent the drifting to one side of a vehicle without sacrificing the wet and dry running performance and to improve the steering stability without sacrificing the wet performance and pitch noise by increasing the pitch lengths in the tread middle part but decreasing the pitch lengths in the tread shoulder part. According to the present invention, a pneumatic tyre is as set forth in claim 1. Further features of the invention are set out in the dependent claims. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings. Fig.1 is a cross sectional view of an embodiment of the present invention; Fig.2 is a developed plan view showing an example of the tread pattern thereof; Fig.3 is a graph showing the frequency spectrum of overall tyre noise; Figs.4(a)-(b) shows tread patterns according to prior art; Fig.5 is a cross sectional view of another embodiment of the present invention; Fig.6 is a schematic view showing an example of the tread pattern thereof; Fig.7 shows cross sectional views of the axial grooves thereof; Fig.8 is a diagram explaining the wording concerning the block; Fig.9 is a developed plan view showing an illustrative example of a tread pattern which does not form part of the invention; Figs.10 and 11 show the tread patterns used in comparison tests; Fig.12 is a cross sectional view of still another embodiment of the present invention; Fig.13 is a developed view of an example of the tread pattern thereof; Figs.14 and 15 are developed views each showing another example of the tread pattern; Fig.16 is a developed view showing a tread pattern according to prior art; and Fig.17 is a developed view showing a tread pattern used in comparison tests. In the figures showing the embodiments of the present invention, the pneumatic tyre 1 comprises a tread portion 2, a pair of sidewall portions 3, a pair of bead portions 4 with a bead core 5 therein , a carcass 6 extending between the bead portions 4, and a belt 7 disposed radially outside the carcass 6 and inside the tread portion 2. The invention is suitably applied to a radial tyre for passenger cars whose aspect ratio (section height/section width) is less than 70%, preferably 60 to 70 %. The carcass 6 comprises at least one ply of cords arranged radially at an angle of from 90 to 70 degrees with respect to the tyre equator (C), the ply extending between the bead portions through the tread portion 2 and sidewall portions 3 and being turned up around the bead cores 5 from the axially inside to the outside of the tyre to form a pair of turnup portions 6b and one main portion 6a therebetween. Further, each bead portion is provided between the main portion 6a and each turnup portion 6b of the carcass 6 with a bead apex 8 made of a hard rubber extending radially outwardly from the bead core 5. The belt 7 is disposed radially outside the carcass 6 and inside the tread portion 2 and comprises at least two crossed plies 7A and 7B of cords laid at an angle of from 10 to 35 degrees with respect to the tyre equator so that the cords of each ply cross the cords of the next ply. For the belt cords, high modulus metal cords such as steel cords are preferably used. Further, high modulus organic fibre cords such as aramid, rayon and the like can be used. As shown in Fig.1, it is possible to dispose a band or bandage 10 radially outside the belt 7 to prevent the belt edges from being lifted during high speed running and thereby to improve the high speed durability. The band is made of a low modulus organic fibre cord such as nylon and the cord angle is very small or substantially zero degrees to the tyre circumferential direction. The tread portion 2 is provided with a non-directional pattern which is generally point-symmetrical about a point on the tyre equator (C). In a tread central portion defined as having an axial width of 45% of the tread width, at least one circumferential groove 13 is disposed, and, on each side of the tyre equator (C), axial grooves 11 extending from the circumferential groove 13 to the tread edge (E) are disposed to define circumferentially spaced blocks 17. All the axial grooves 11 are inclined to the same direction with respect to the tyre axial direction. The inclination angle of the axial grooves 11 with respect to the tyre circumferential direction is decreased from the axially outer end to the axially inner end. As a result, each axial groove 11 is bent by two or more steps. The minimum inclination angle of each axial groove 11 is set to be less than 45 degrees, more preferably less than 40 degrees. Preferably, the maximum inclination angle is set in the range of from 60 to 85 degrees. EMBODIMENT 1 In order to improve the steering stability and slip noise by controlling the slippage of the blocks 17, the cord inclination of the radially outermost belt ply is specifically limited with relation to the inclination of the axial grooves. In Figs.1-3 concerning the first embodiment the tyre 1 is a radial tyre for passenger cars. The carcass 6 comprises a single ply. The turnup portions 6b thereof extend radially outwardly beyond one half of the tyre section height to form a so called (1-0) high ply turnup structure. The belt 7 comprises a radially inner ply 7B and a radially outer ply 7A. In this example, on the radially outside of the belt 7, a band 10 is disposed, which band comprises a pair of narrow-width edge plies 10A covering only the axial edge portions of the belt 7, and a full-width ply 10B disposed radially outside the edge plies 10A to cover substantially all over the width of the belt 7. The band 10 is preferably formed in the so called jointless structure by spirally winding a ribbon-like narrow ply or a strip of rubber in which one or more cords are embedded. The band 10 is not always necessary, and it may be omitted. The tread portion 2 in this embodiment is provided on each side of the tyre equator (C) with a circumferential groove 13 disposed near the tyre equator (C) and extending straight in the tyre circumferential direction. Further, the tread portion 2 is provided on the right side and left side of the tyre equator (C) with right and left axial grooves 11 which extend from the right and left tread edges (E) toward the tyre equator (C). Each of the axial grooves 11 is connected to one of the straight circumferential grooves 13 and terminates at the circumferential groove 13. Therefore, a continuously extending straight rib 14 is formed between the circumferential grooves 13. The straight circumferential grooves 13 preferably have a groove width of 4 to 20 mm, and a groove depth of 5 to 10 mm. Each of the axial grooves 11 is inclined such that the inclination angle b thereof to the tyre circumferential direction gradually decreases from the tread edge (E) toward the tyre equator (C) along its entire length L starting from the tread edge (E), that is, β1 > β2 > - - > β min as shown in Fig.2. In this embodiment, the axial grooves 11 have a left side upward inclination. The minimum angle β min of the axial groove 11 is set in the range of not more than 45 degrees, preferably 15 to 40 degrees, more preferably 20 to 35 degrees. The axial length L of the axial groove 11 measured from the tread edge (E) to the axial inner end is set in the range of 55 to 100 %, preferably 60 to 90 %, more preferably 70 to 85 % of half the tread width tW. Therefore, in the tread shoulder region which affects most greatly the cornering performance, the blocks 17 between the axial grooves 11 are increased in rigidity in the axial direction of the tyre, and the steering stability is improved. On the other hand, in the central part of the tread, the drainage therefrom is increased. Further, each part or block between the circumferentially adjacent axial grooves 11 is subdivided by narrow circumferential grooves 15 and slits 16 extending between the axial grooves 11. With respect to the circumferential direction, the axial grooves 11 are inclined to the reverse direction to the cords 7C of the outermost belt ply 7B so as to cross the cords 7C. The difference between the minimum angle β min and the angle α of the cords 7C of the outermost belt ply 7B to the tyre circumferential direction is set in the range of not more than 25 degrees: |β min -α| ≤25 degrees. As a result, the tread rigidity can be increased since the longitudinal direction of the blocks between the axial grooves 11 crosses the cord direction, and the slip can be prevented. If the difference |β min -α| is more than 25 degrees, the effect of increasing the tread rigidity decreases, and accordingly the slip sound is not be reduced. Further, the steering stability is also deteriorated. Thus, such a range is not preferable. More preferably, the difference is set in the range of not more than 20 degrees: |β min -α| <20 degrees. The groove depth of the axial grooves 11 is preferably constant along the length thereof and in the range of from 4 to 10 mm (in this example about 8 mm). The groove width Ge of the axial grooves 11 measured at the tread edges (E) is preferably in the range of 4 to 25 mm (in this example about 6 mm), and the groove width is gradually decreased from the tread edges (E) towards the tyre equator (C). In this example, the groove width in the axially innermost end is decreased about 25 % in comparison with that in the axially outermost end, whereby a great decrease in the rigidity of the axially inner blocks can be prevented. It is possible to construct the axial grooves 11 to extend continuously from one of the tread edges (E) to the other. The shape of the blocks can be changed variously apart from the example shown in the figure. Comparison tests Test tyres (195/65R15 radial tyre for passenger cars) having the structure show in Fig.1 and the tread pattern shown in Fig.2 were made and tested for the following performances. A) Steering stability and Ride comfort test Running a test car (2500cc front-wheel-drive car provided on the four wheels with test tyres) on a tyre test course, the performances was evaluated into ten ranks by the driver's feeling, on a base of the prior art tyre being 6. The larger the index, the better the performance. Wheel rim size: 15X6-JJ Tyre air pressure: Front 200 kpa, Rear 200 kpa B) Tyre noise test According to the test procedure specified in JASO/Z101-83, a test car was coasted for a 50 metre distance at a speed of 60 km/h on a straight test course (ISO road surface) and the maximum noise sound level in dB(A) was measured at a frequency of 1.6 kHz and overall range with a microphone set at 1.2 metre height from the road surface and 7.5 metre sideways from the running centre line in the midpoint of the course. The results are indicated by a relative value based on that the prior art tyre is 0. Tire Prior Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ref. 1 Ref. 2 Ref. 3 Carcass Number of ply 1 Cord 1500d/2 polyester Cord count 52 ends Cord angle 88 degrees Belt Number of ply Code Cord count Belt cord angle (deg) Inner ply Outer ply α 2 steel 40 ends 22 22 22 22 22 22 22 19 22 19 22 22 22 22 22 22 22 22 19 22 19 22 Groove angle β (deg) 20 20 25 30 35 40 45 35 50 40 50 | β-α | 2 2 3 8 13 18 23 16 28 21 28 Groove inclination S R R R R R R R R S S Tire noise dB(A) Overall 0 -0.7 -0.6 -0.4 -0.2 -0.1 -0.1 -0.2 0 0.1 0 1.6 kHz 0 -1 -0.8 -0.5 -0.3 -0.1 -0.1 -0.2 -0.1 0.2 0.1 Steering stability 6 7 7 7 6 6 6 6 6 6 6 Ride comfort 6 6 6 6 6 6 6 6 6 6 6 From the test results, it was confirmed that the Example tyres were superior to the prior art tyre in tyre noise. Especially, the Example tyres 1 to 3 of which β min -α| was not more than 10 degrees, were further increased in the tread rigidity and as a result, the steering stability and the 1.6 kHz noise or the slip sound were greatly improved as shown in Fig.3 where the 1.6 kHz peak disappeared in the Example tyre. Additionally, the overall noise energy was effectively decreased as the slip sound is decreased. Thus, it is preferable that |β min -α| is not more than 10 degrees. As described above, as the axial grooves are inclined reversely to the cords of the radially outermost belt ply and |β min -α| is set to be not more than 25 degrees, the tread rigidity is increased to improve the steering stability and prevent the occurrence of the slip noise. EMBODIMENT 2 In order to prevent the drifting to one side of a vehicle by controlling the residual cornering force, the inclinations of the front and rear sides of the blocks 17 are varied. In Figs.5-11, concerning the second embodiment, the tyre 1 is also a radial tyre for passenger cars. Similar to the above-mentioned first embodiment, the tread portion 2 is provided with two circumferential grooves 13. Between the circumferential grooves 13, a circumferentially continuously extending rib 14 is formed. The number of the circumferential grooves 13 is preferably 1 to 3. All the circumferential grooves 13 are disposed in only a central part M of the tread to increase the pattern rigidity and thereby improve the dry running performance. Thus, no circumferential groove is disposed outside the central part M. Here, the central part M is centred on the tyre equator (C), and the axial width thereof is 25% of the ground contacting width TW. The ground contacting width TW is the axial distance between the tread edges (E) or the axial edges of the ground contacting area (S) of the tread portion 2 under a standard condition in which the tyre 1 is mounted on a standard rim and inflated to a standard pressure and then loaded with a standard tyre load. The axial grooves 11 extend from the tread edges (E) to the axially outermost circumferential grooves 13A to form blocks 17 each defined by the circumferentially adjacent axial grooves 11, one of the tread edges (E) and one of the axially outermost circumferential grooves 13A. Further, each of the blocks 17 is provided with a blind groove 19 which extends axially inwardly from the tread edge (E) and terminates within the block 17 in the middle of the lateral part N. In the example in Fig.6, all the axial grooves 11 and blind grooves 19 have a right side upward inclination. However, the inclination can be reversed as shown in Fig.9. The stream angle () of each of the axial grooves 11 is set in the range of from 10 to 40 degrees. The stream angle () is, as shown in Fig.6, the angle between the tyre equator (C) and a straight line AB, the straight line AB drawn from the intersecting point (A) of the axial groove 11 with the axial edge line (K) of the central part (M) that is a circumferential line (K) drawn at a distance of TW/8 from the equator (C), to the intersecting point (B) of the axial groove 11 with the tread edge line (L) that is the edge line of the ground contacting area (S). The stream angle () has a great influence on the steering stability on wet roads. If the angle exceeds 40 degrees, the water drainage becomes insufficient, and aquaplaning is liable to occur, decreasing the wet performance. If the angle is less than 10 degrees, road grip on dry roads becomes insufficient. For better understanding of this embodiment, the following are noted together with Fig.8 in which a simplified model of the block 17 is shown. Looking at each block 17, the rear edge (hereinafter heel edge) first contacts with the road surface and then the front edge (hereinafter toe edge). Next turning to the above-mentioned heel edge, the rear end (hereinafter far end Fr) first contacts with the road surface and then the front end (hereinafter near end Re). Further, giving an eye to the toe edge, the rear end (i.e. near end Re) first contacts with the road surface and then the front end (i.e. far end Fr). Again turning back to Fig.6, each block 17 has a toe edge and a heel edge both inclined to the same direction with respect to the tyre axial direction, and each of the edges has a far end Fr and a near end Re. The block side face 9 along each of the toe and heel edges is gradually increased in the inclination angle from the near end Re to the far end Fr as shown in Figs.6-8. The inclination angle is measured in a tyre cross section parallel with the tyre equatorial plane, with respect to the tyre radial direction. The angle α in the far end Fr is in the range of from 20 to 40 degrees. The angle β in the near end Re is in the range of from 0 to 20 degrees. Preferably, the block side face 9 extends smoothly between the ends Fr and Re. Therefore, the moment caused on the block by the decreased stream angle () can be cancelled or. decreased, and the drifting to one side of a vehicle can be controlled. If the angle α is less than 20 degrees and/or the angle β is not less than 20 degrees, as the difference therebetween is very small, the drifting to one side of a vehicle can not be controlled. If the angle α exceeds 40 degrees, the road grip becomes unbalanced between the right and left of the tyre and the steering stability decreases. Further, as the groove volume decreases, wet performance also decreases. Incidentally, for the cords of the belt 7, organic fibre cords, e.g. nylon, polyester, rayon, aromatic polyamide and the like and steel cords can be used. Comparison tests Test tyres (195/65R15 radial tyre for passenger cars) having the structure show in Fig.5 were made and tested for the following performances. 1) Dry road steering stability test Running a test car provided on the four wheels with test tyres on an asphalt-surfaced road, the steering stability was evaluated into five ranks by the driver's feeling, based on that 3 is standard. The larger the index, the better the performance. Wheel rim size: 6JJ Inner pressure: 2.0 kgf/sq.cm Tyre load: 400 kgf 2) Wet performance test Running the same test car on the wet road surface with a 5 mm depth water and the speed causing aquaplaning was measured. In the table, the results are indicated by an index based on that Ex.21 tyre is 100. The larger the index, the better the wet performance. 3) Residual cornering force test The cornering force was measured with using a flat belt cornering tester. Tire Ex. 21 Ex. 22 Ref. 21 Ref. 22 Ref. 23 Ref. 24 Tread pattern Fig. 6 Fig. 9 Fig. 6 Fig. 10 Fig. 11 No. of circum. grooves 2 2 2 2 4 2 Stream angle () (deg) 40 40 40 50 50 -- Inclination RSU LSU RSU RSU RSU Type of angle in Table 3 1 2 3 3 3 - Steering stability (dry) 3 3 3 3 2.5 3 Wet performance 100 102 98 90 104 85 Residual CF (kgf) 7.2 4.5 12.6 7 7.4 5.6 Type Tread pattern Angle (deg) a-a b-b c-c d-d' e-e' f-f' a a' b b' c c' d d' e e' f f' 1 Fig. 6 α 25 β 5 γ 15 γ 15 β 5 α 25 α 25 β 5 γ 15 γ 15 β 5 α 25 2 Fig. 9 β 5 α 25 γ 15 γ 15 α 25 β 5 β 5 α 25 γ 15 γ 15 α 25 β 5 3 Fig. 10 Fig. 11 10 10 10 10 10 10 10 10 10 10 10 10 From the test results, it was confirmed that the Example tyres were improved in residual cornering force and the dry and wet running performance in comparison with the reference tyres. Thus the drifting to one side of a vehicle can be controlled. EMBODIMENT 3 In order to improve the steering stability without sacrificing wet performance and pitch noise, the circumferential pitch lengths of the axial grooves 11 are increased in the tread middle part but decreased in the tread shoulder part. In Figs.12-17, concerning the third embodiment, the tyre 1 is also a radial tyre for passenger cars. The tread portion 2 is provided with a pair of axially inner circumferential grooves 13 each disposed on each side of the tyre equator (C), and a pair of axially outer circumferential grooves 15 each disposed between one of the axially inner circumferential grooves 13 and the adjacent tread edge (E). As a result, the tread portion 2 is divided into a central part Cr between the axially inner circumferential grooves 13, a pair of middle parts Md between the axially inner and outer circumferential grooves 13 and 15, and a pair of shoulder parts Sh between the axially outer circumferential grooves 15 and the tread edges (E). Between the axially inner circumferential grooves 13, a straight rib 14 is formed. The rib is devoid of any axial groove. Thus, the rib extends continuously in the circumferential direction. However, it is possible to provide axial grooves which do not extend across the whole width of the rib or the crown region Cr. As far as the rib substantially does not lose circumferential continuity, it may be possible that a sipe or slit which has not a substantial groove width extends across the rib. In this example, the axially inner circumferential grooves 13 are a straight groove extending continuously in the tyre circumferential direction. A zigzag groove can be used, but a straight groove is used for better drainage. The distance L1 of the axially inner circumferential grooves 13 measured from the groove centre line to the tyre equator (C) is not more than 15 % of the ground contacting width TW, and preferably not less than 10 %. If the distance L1 is more than 15%, although it is somewhat effective for noise reduction, the drainage from the crown region Cr decrease. If the distance L1 is less than 10%, the rigidity of the crown region Cr is liable to decrease, thus the steering stability on dry roads and noise are liable to deteriorate. If the groove width of the axially inner circumferential grooves 13 is too small, the drainage is liable to become insufficient. Contrary, if the groove width is too large, noise is liable to increase. Thus, the groove width is set in the range of from 2 to 9 %, preferably 3 to 7 %, more preferably 5 to 7% of the ground contacting width TW, (in this embodiment 5.3%) The axial distance L2 of the axially outer circumferential grooves 15 measure from the groove centre line to the tread edges (E) is set in the range of from 9 to 21 % of the ground contacting width TW. If the distance L2 is more than 21%, the noise is liable to increase. If the distance L2 is less than 9%, the noise decreases, but the rigidity of the shoulder part Sh decreases and the steering stability decreases. If the groove width of the axially outer circumferential grooves 15 is too small, the drainage is liable to decrease. Contrary, if the width is too large, the noise is liable to increase. Thus, the groove width of the axially outer circumferential grooves 15 is preferably 1.8 to 2.6%, more preferably 2.0 to 2.4% of the ground contacting width TW. The groove depth of the axially inner and outer circumferential grooves 9 and 10 is preferably not less than 5 mm, preferably not less than 7 mm. Each shoulder part Sh is divided by first and second axial grooves 11a and 11b into a circumferential row of blocks 17a. The first axial grooves 11a extend from the tread edges (E) to the axially inner circumferential grooves 13. The second axial grooves 11b are disposed between the first axial grooves 11a, and extend from the tread edges (E) to the axially outer circumferential grooves 15 but not reached to the axially inner circumferential grooves 13. The middle part Md is therefore divided by only the first axial grooves 11a into a circumferential row of blocks 17b. Accordingly, the number of the blocks 17b in the middle part Md is 1/2 of the number of the blocks 17a in the shoulder part Sh. If the block number of the middle part Md is less than 1/2 of the block number on the shoulder part Sh, the wet performance greatly decreases and noise increases. If more than 1/2, the steering stability can not be improved. If the groove width of the first and second axial grooves 11a and 11b is too small, the wet performance is liable to decrease. .Contrary, if too large, noise is liable to increase. Thus, the groove width of the first and second axial grooves 11a and 11b is 2 to 4%, preferably 2.4 to 3.4%, more preferably 2.6 to 3.2% of the ground contacting width TW. The groove depth is preferably not less than 5 mm. The percentage of the grooved area to the whole tread area between the tread edges (E) is preferably not more than 28%, whereby the steering stability and wear resistance can be further improved. Further, it is preferable that the middle part Md is not provided with a groove which has a groove width of not less than 2 mm and a depth of not less than 3 mm and crosses the first axial grooves. If such groove is disposed, the improvement in the block rigidity in the middle part Md is liable to be hindered. However, it is possible to dispose a sipe or slit which has a groove width of less than 1.0 mm, in other words, a narrow groove which has not a substantial width. For reducing room noise, the sipe is preferably disposed in the block 17b to extend substantially parallel to the first axial groove 11a. In this example, the axially outer circumferential grooves 15 extend zigzag continuously in the circumferential direction to improve the wet performance, and, the segments 15a of the zigzag circumferential groove 15 are held in common with the first axial grooves 11a. In the middle part Md and the shoulder part Sh, variable pitches are employed in the circumferential arrangement of the first and second axial grooves 11a and 11b. Further, the same variable pitches are employed in the zigzag pitches PG of the axially outer circumferential grooves 15, whereby the pitch noise can be improved. Thus, the number of the zigzag is equal to the number of the blocks 17a in the shoulder part Sh and 2 times the number of the blocks 17b in the middle part Md. In the shoulder part Sh, the first and second axial grooves 11a and 11b preferably extend at an angle q1 of 60 to 85 degrees with respect to the circumferential direction of the tyre, whereby both the wet performance and noise can be maintained at a good level. If the angle 1 is less than 60 degrees, the steering stability has a tendency to decrease. If the angle 1 is more than 85 degrees, the wet performance has a tendency to decrease. Further, in the middle part Md, the first axial grooves 11a preferably extend at an angle 2 of 20 to 40 degrees with respect to the circumferential direction of the tyre, whereby both the wet performance and noise can be maintained at a good level. If the angle 2 is less than 60 degrees, the steering stability has a tendency to decrease. If the angle 2 is more than 85 degrees, the wet performance has a tendency to decrease. In the middle part Md, the inclination angle of the first axial grooves 11a with respect to the circumferential direction decrease from the axially outside to the inside to improve the drainage from the neighbourhood of the tyre equator. In this example, the angle changes from a certain value 2a to a certain value 2b in the middle of the length. Further, the belt 7 in this example is composed of two cross plies 7A and 7B of steel cords laid at an angle of from 10 to 35 degrees with respect to the tyre equator. The axially inner circumferential grooves 13 may be formed in a zigzag form as shown in Fig.14. The axially outer circumferential grooves 15 may be formed in a straight form as shown in Fig.15. Comparison tests Test tyres (195/65R15 91H radial tyres for passenger car) having the structure shown in Fig.12 were made by way of test and tested for the following performances. A) Pitch noise and Room noise Test Running a 2500cc passenger car on a smooth road surface at a speed of 60 km/h, the pitch noise was evaluated into ten ranks by the test driver's feeling. Also the Room noise at 40 km/h was evaluated. B) Wet performance (Resistance to aquaplaning) Test A test car provided on all four wheels with test tyres was run on an asphalt road with a 5 mm depth 20 metre long water pool along a 100 metre radius circle. Increasing the speed, the maximum lateral-G of the front wheel was measured in the speed range of 50 to 80 km/h. The test results are shown in Table 4, wherein the results are indicated by an index based on that the prior art tyre 1 is 100. The larger the index, the higher the resistance to aquaplane. C) Tyre noise test According to the test procedure specified in JASO/Z101-83, a test car was coasted for 50 metre distance at a speed of 60 km/h in a straight test course (ISO road surface) and the maximum noise sound level in dB(A) was measured with a microphone set at 1.2 metre height from the road surface and 7.5 metre sideways from the running centre line in the midpoint of the course. D) Steering stability test Running a 2500cc test car provided on four wheels with test tyres in a test course, the steering stability was evaluated into ten ranks by the driver's feeling the larger the index, the better the stability. (10: excellent, 9: very good, 8: fairly good, 7: good, 6: allowable, 5: allowable limit) Wheel rim: 15X6-JJ Tyre pressure: 200 kpa (front and rear) From the test results, it was confirmed that the Example tyres were improved in steering stability without sacrificing wet performance and noise performance. Among the tread patterns shown in Figs.13, 14 and 15, the patterns having the straight inner circumferential grooves were superior in steering stability. Further, in the case of the straight inner circumferential grooves, the noise performance was improved when the axially outer circumferential grooves were zigzag, but when the axially outer circumferential grooves were straight, the wet performance was improved. As explained above, as the number of blocks is increased in the shoulder part Sh, the pitch noise and room noise are effectively improved without sacrificing the wet performance. As the number of the blocks in the middle part Md is decreased, the block rigidity is increased and the steering stability can be improved.

Claims: A pneumatic tyre comprising a tread portion (2) with tread edges, a pair of sidewall portions (3), a pair of bead portions (4), a carcass (6) extending between the bead portions (4), a belt (7) disposed radially outside the carcass (6) in the tread portion (2), said tread portion (2) being provided on each side of the tyre equator with axial grooves (11) extending axially inwardly from the tread edges, said axial grooves (11) being inclined to the same direction with respect to the tyre axial direction to form a non-directional tread pattern, wherein the inclination angle (β) of the axial grooves (11) with respect to the tyre circumferential direction is decreased from the axially outer end to the axially inner end thereof, so as to have a minimum inclination angle of less than 45 degrees, and a maximum inclination angle of from 60 to 85 degrees, the tread portion is provided in a central part with two circumferential grooves (13) one on each side of the tyre equator (C) defining a single rib (14) disposed on the tyre equator and extending continuously in the circumferential direction, the axial grooves (11) extending to the circumferential grooves (13) from the tread edges (E) so as to define circumferentially spaced blocks (17) between the axial grooves and the circumferentially extending rib (14) between the circumferential grooves (13) and the inclination angle β of each of the axial grooves (11) with respect to the tyre circumferential direction C gradually decreases from the tread edge to the circumferential groove. A pneumatic tyre according to claim 1, characterised in that the circumferential grooves (13) are straight grooves having a width of 4-20mm and a depth of 5-10mm. A pneumatic tyre according to claim 1 or 2, characterised in that a band (10A) is disposed on the radially outside of the belt, the band comprising a pair of narrow width edge plies covering only the axial edge portions of the belt and a full width ply (10B) disposed radially outside the edge plies to cover substantially all over the width of the belt. A pneumatic tyre according to claim 1, 2 or 3, characterised in that said belt (7) comprises at least two plies (7a,7b) of cords laid at an inclination angle (α) of from 10 to 35 degrees with respect to the tyre equator so that the cords of each ply cross the cords of other ply, said axial grooves (11) being inclined in the reverse direction to the cords of the radially outermost ply of the belt, with respect to the tyre circumferential direction, the angle α of the cords of the outermost belt ply to the tyre circumferential direction satisfies |β min -α| ≤ 25 degrees βmin ≤ 45 degrees wherein β min is the minimum inclination angle of each said axial groove measured along the entire length of the axial groove with respect to the tyre circumferential direction. A pneumatic tyre according to claim 4, characterised in that the minimum angle β min and the cord angle α satisfy |β min -α| ≤ 20 degrees. A pneumatic tyre according to claim 4 or 5, characterised in that the inclination angle of each said axial groove (11) decreases from the axially outer end to the inner end by two or three steps. A pneumatic tyre according to claim 1, characterised in that said central part being centred on the tyre equator and having a width of 25% of the ground contacting width (TW), and in that the stream angle () of each said axial groove (11) is in the range of 10 to 40 degrees, wherein the stream angle () is the angle between the tyre equator and a straight line (AB) drawn between the intersecting point (A) of the axial groove with one of the edge lines of said central part and the intersecting point (B) of the axial groove with the edge line (L) of the ground contacting area; each of said circumferentially spaced blocks (17) has a toe edge and a heel edge both inclined to the same direction with respect to the tyre axial direction so that each of the edges has a far end and a near end, the inclination angle of the side face of each said block measured in a tyre cross section parallel with the tyre equatorial plane, with respect to the tyre radial direction, along each of the edges, is gradually increased from the near end to the far end, and the angle (α) in the far end is in the range of from 20 to 40 degrees and the angle (β) in the near end is in the range of from 0 to 20 degrees. A pneumatic tyre according to claim 1, characterised in that said circumferential grooves form axially inner circumferential grooves (13) of which centre line is spaced apart from the tyre equator by an axially distance (L1) of not more than 15% of the ground contacting width (TW), and in that the tread is further provided with a pair of axially outer circumferential grooves of which centre line is spaced apart from the adjacent tread edge (E) by an axially distance (L2) of 9 to 21% of the ground contacting width (TW), the middle part between the adjacent inner and outer circumferential grooves (13; 15) is divided into circumferentially spaced inner blocks (17b) by said axial grooves, and the shoulder part between the axially outer circumferential grooves and the adjacent tread edges is divided into circumferentially spaced outer blocks (17a) by said axial grooves and additional short axial grooves (11b) so that the number of the inner blocks (17b) in the middle part is 1/2 the number of the outer blocks (17a) the shoulder part. A pneumatic tyre according to claim 8, characterised in that the axially inner circumferential grooves extend straight in the circumferential direction, and the axially outer circumferential grooves extend zigzag in the circumferential direction. A pneumatic tyre according to claim 8 or 9, characterised in that , in the shoulder part, said axial grooves and said additional short axial grooves (11b) are inclined at an angle in the range of from 60 to 85 degrees with respect to the tyre circumferential direction, and in the middle part, said axial grooves are inclined at an angle in the range of from 20 to 40 degrees with respect to the tyre circumferential direction. A pneumatic tyre according to claim 1, characterised in that said belt (7) comprises at least two plies of cords laid at an inclination angle of from 10 to 35 degrees with respect to the tyre equator so that the cords of each ply cross the cords of other ply, said axial grooves (11) being inclined in the reverse direction to the cords of the radially outermost ply of the belt, with respect to the tyre circumferential direction, the angle α of the cords of the outermost belt ply to the tyre circumferential direction satisfies |β min -α|≤ 25 degrees β min ≤ 45 degrees wherein β min is the minimum inclination angle of each said axial groove measured along the entire length of the axial groove with respect to the tyre circumferential direction; said central part being centred on the tyre equator and having a width of 25% of the ground contacting width; each said of circumferentially spaced blocks (17) has a toe edge and a heel edge both inclined to the same direction with respect to the tyre axial direction so that each of the edges has a far end and a near end, the inclination angle of the side face of each said block measured in a tyre cross section parallel with the tyre equatorial plane, with respect to the tyre radial direction, along each of the edges, is gradually increased from the near end to the far end, and the angle (α) in the far end is in the range of from 20 to 40 degrees and the angle (β) in the near end is in the range of from 0 to 20 degrees, each said block is provided with a short axial groove extending axially inwardly from one of the tread edges and terminated within the block.

Inventors: HANYA MASAHIRO, WADA MITSUHIRO, YUKAWA NAOKI, HANYA, MASAHIRO, WADA, MITSUHIRO, YUKAWA, NAOKI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1997
Main classification: B60C11
IPC codes: B60C9, B60C11