Pneumatic tyre

Abstract: A pneumatic tyre comprises a tread portion (T) provided with three straight main grooves (2,3,4) extending continuously in the tyre circumferential direction, the three main grooves being arranged asymmetrically about the tyre equator (C) such that: an exterior main groove (2) is disposed in an exterior half (O) which is one half of the tread portion on one side of the tyre equator (C); and an axially inner main groove (3) and an axially outer main groove (4) disposed in an interior half (I) which is the other half of the tread portion, exterior oblique grooves (9) disposed axially outside the exterior main groove (2) and inclined at an angle β1 of 45 to 70 degrees with respect to the circumferential direction of the tyre, interior oblique grooves (10) disposed axially outside the axially outer main groove (4) and inclined reversely to the exterior oblique grooves (9) at an angle β3 of 60 to 80 degrees with respect to the circumferential direction, central oblique grooves (11) disposed between the exterior main groove (2) and the axially inner main groove (4) and inclined to the same direction as the exterior oblique grooves (9), and between the exterior main groove (2) and the axially inner main groove (3), the angle β2 of the central oblique grooves (11) being in the range of 20 to 45 degrees with respect to the circumferential direction.

Description: The present invention relates to a pneumatic tyre having circumferentially extending straight main grooves which are arranged asymmetrically about the tyre equator to improve wet performance, uneven wear resistance and tyre noise. In recent years, in order to improve wet performance especially during high speed running, tread patterns which are asymmetrical about the tyre equator are widely used in passenger car tyres especially for high performance sport car tyres. Such asymmetrical tread patterns are disclosed in JP-A-1-115705, JP-A-3-32906, JP-A-7-32822, JP-A-3-121912, JP-B-5-15562 and JP-B-6-57485. In general, asymmetrical tread patterns are directionally bound. In other words, the tyre has a designed rotational direction. If such tyres are incorrectly installed to rotate in the reverse direction, not only the expected performance such as drainage, wet grip and the like are not be obtained but also there is a danger of accident due to skidding. Further, there are actual problems in that two types of tyres for used on the right side and left side of a car have to be made in every tyre size, and thus two different tyre moulds are required, which increase tyre manufacturing costs, storage costs and the like. It is therefore, an object of the present invention to provide a pneumatic tyre in which an asymmetrical tread pattern is provided to improve wet performance, which pattern is however not bound directionally. A tyre according to the pre-characterising part of the claim of the present invention is disclosed in DE 38 15 829 A1. The present invention provides such a tyre as set out in the characterising part of claim 1. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings. Fig.1 is a developed plan view of an embodiment of the present invention showing an example of the tread pattern; Figs.2 and 3 are developed plan views each showing a prior art tread pattern; Fig.4 is a graph showing relationships between the angle of the exterior oblique grooves and the wear resistance and noise performance; Fig.5 is a graph showing relationships between the angle of the interior oblique grooves and the wear resistance and noise performance; Fig.6 is a graph showing relationships between the angle of the central oblique grooves and the wear resistance and noise performance; Fig.7 is a diagram showing the angles of the oblique grooves; and Fig.8 is a view showing paired oblique grooves. The tyre of Fig.1 is a radial car tyre and has a tread portion, a pair of axially spaced bead portions, and a pair of sidewall portions extending between the tread edges and the bead portions to provide a toroidal shape. The tyre is reinforced by a pair of bead cores disposed one in each of the bead portions, at least one radial carcass ply extending between the bead portions, and a belt disposed radially outside the carcass in the tread portion T. The tread portion T is provided with a tread pattern which is not bound directionally but positionally. In this application, one half of the tread portion which is disposed on the outside of the vehicle (in Fig.1 the right-hand side of the tyre equator C) is called simply the "exterior half", and the other half which is disposed on the inside of the vehicle (in Fig.1 the left-hand side) is called the "interior half". In the exterior half O, a main groove 2 (exterior main groove) is disposed. This groove 2 is straight and extends continuously in the tyre circumferential direction. In the interior half I, an axially inner main groove 3 (first interior main groove) and an axially outer main groove 4 (second interior main groove) are disposed. These grooves 3 and 4 are straight and extend continuously in the tyre circumferential direction. The exterior main groove 2 has a groove width of from 6 to 20 mm preferably not less than 7 mm at the tread surface, and a groove depth of from 5 to 15 mm preferably not less than 7 mm for good drainage. In this example, the groove width is 8.6 mm, and the groove depth is 9 mm. The interior main grooves 3 and 4 have the same groove depth as the exterior main groove 2, but the groove width is slightly smaller than that of the exterior main groove 2. In this example, the width is 7.2 mm. It is however possible to have the same width. The above-mentioned three grooves 2, 3 and 4 are the only grooves which extend continuously in the tyre circumferential direction, and no circumferential groove is disposed on the tyre equator C. If the number of all the circumferential main grooves is two, wet performance becomes poor. If the main groove number is four or more, tyre noise (air column resonance noise) increases. The exterior main groove 2 is disposed within a region (c) always maintains a definite distance from the tyre equator C. As shown in Fig.1, six regions (a, b, c, d, e and f) having a width of one sixth of the tread width TW are defined in that order from the edge Eo of the exterior half portion to the edge Ei of the interior half portion. Thus, the region (c) is defined between the tyre equator C and an axial position at one sixth of the tread width TW from the tyre equator C. The regions (a and b) where a relatively large wear energy is produced during cornering are provided with no circumferential groove. Therefore, it becomes possible to improve the steering stability and uneven wear. In order to provide an adequate spacing between the axially inner and outer main grooves 3 and 4 and thereby to decrease the rigidity of the interior half portion I uniformly, the inner main groove 3 is disposed within the region (d) between the tyre equator C and a position at one sixth of the tread width TW from the tyre equator C, and the outer main groove 4 is disposed within the region (e) between the positions at one sixth and two sixth of the tread width TW from the tyre equator C. Such an arrangement is also useful in improving the wet performance. The axial distances between the main grooves 2, 3 and 4 are set in the range of not less than 8 %, preferably not less than 11 %, more preferably 11 to 17 % of the tread width TW so as not to excessively decrease the rigidity of the land part therebetween. With regard to uneven wear resistance and wet performance, a tread pattern comprising three main grooves (g, h and i) arranged symmetrically about the tyre equator C as shown in Fig.2 and a tread pattern comprising three main grooves arranged asymmetrically as shown in Fig.1 were compared, and it was found that the wet performance of each has no substantial difference. However, the symmetrical arrangement is poor in wear resistance as tread wear is larger in the exterior shoulder portion (j) than the interior shoulder portion (k). In general, wear energy produced during cornering is greater in the exterior half O than the interior half I. Accordingly, if there is no substantial difference in tread pattern rigidity between the interior half and the exterior half, the exterior half wears faster than the interior half. Therefore, in the present invention, in order to decrease uneven wear, the tread pattern rigidity is increased to be more in the exterior half O than in the interior half I by employing the asymmetrical arrangement of the main grooves. If the three main grooves 2, 3 and 4 are disposed in the interior half I, wet performance becomes poor. According to the present invention, oblique grooves 9 (exterior oblique grooves) are disposed in a tread shoulder portion 5 (exterior shoulder portion) between the exterior main groove 2 and the exterior tread edge Eo. The exterior oblique grooves 9 are inclined at an angle 1 of 45 to 70 degrees to the circumferential direction. Further, oblique grooves 10 (interior oblique grooves) are disposed in a tread shoulder portion 7 (interior shoulder portion) between the axially outer main groove 4 and the interior tread edge Ei. The interior oblique grooves 10 are inclined reversely to the exterior oblique grooves 9 at an angle 3 of 60 to 80 degrees to the circumferential direction. Furthermore, central oblique grooves 11 are disposed in a tread crown portion 6 between the exterior main groove 2 and the axially outer main groove 4. The central oblique grooves 11 are inclined to the same direction as the exterior oblique grooves 9. The angle 2 of the central oblique grooves 11 is 20 to 45 degrees to the circumferential direction in a part P between the main grooves 2 and 3. The angle 2 increases to more than 45 degrees near the main groove 4 and thus the intersecting angle becomes more than 45 degrees. In this example, the oblique grooves 11 intersect the main groove 4 at about 50 degrees and the main groove 2 at about 23 degrees as shown in Fig.7. However, it is also possible to gradually decrease the inclination angle from the main groove 4 to the main groove 2. In this example, the oblique grooves 9, 10 and 11 are open to the axially adjacent main grooves 2, 3 and 4. Thus the exterior shoulder portion 5, crown portion 6 and interior shoulder portion 7 are divided into circumferential rows of blocks B by the oblique grooves 9, 10 and 11. The interior oblique grooves 10 and central oblique grooves 11 are inclined reversely and they intersect at the position of the main groove 4 in a V-shape formation. This formation further improves drainage in the interior half portion I. With regard to tyre noise, especially sound generated when the steering wheel is swung right and left, two type tread patterns: a tread pattern comprising oblique grooves (m) disposed in the exterior half 0 and oblique grooves (n) disposed in the interior half I are inclined to the same direction, as shown in Fig.3; and a tread pattern in which the inclining directions are reversed as shown in Fig.1, were compared. As a result, it was found that the tread pattern comprising the oblique grooves inclined to the same direction as shown in Fig.3 has a disagreeable difference in noise sound level between right turn and left turn. However, in the tread pattern comprising the reversely inclined oblique grooves 9, 10 as shown in Fig.1, not only the noise difference between right turn and left turn is very small but also the noise sound level is greatly decreased. In order to confirm that the inclination angle of the exterior oblique grooves 9 affects the exterior shoulder portion 5 in respect of its wear resistance, the wear resistance and also tyre noise was measured for changing inclination angles. As shown in Fig.4, when the inclination angle was less than 40 degrees, the rigidity of the exterior shoulder portion 5 greatly decreased and the wear resistance became poor. When the angle was more than 70 degrees, pumping noise originating from the oblique grooves increased. Therefore, the angle 1 of the exterior oblique grooves 9 is preferably set in the range of 45 to 70 degrees, more preferably 50 to 70 degrees. The uneven wear in the exterior shoulder portion 5 was measured after running 3000 km on dry roads. In the graph, the wear is indicated by an index where a pass mark was not less than 100. The tyre noise was evaluated by the driver's feeling into ten ranks where the pass mark was not less than five. The inclination angle of the interior oblique grooves 10 affected wear resistance of the interior shoulder portion 7 less, because the wear energy was relatively small. Fig.5 shows the results of tests in which the wear resistance of the interior shoulder portion 7 and tyre noise were measured changing the inclination angle of the interior oblique grooves 10. When the inclination angle was less than 50 degrees, the rigidity of the interior shoulder portion 7 decreased and the wear resistance became poor. When the angle was more than 80 degrees, the noise performance became worse. Therefore, the angle 3 of the interior oblique grooves 10 is set in the range of 60 to 80 degrees, more preferably 60 to 75 degrees. Similarly, to confirm that the inclination angle of the central oblique grooves 11 affects the wear resistance of the crown portion 6, the wear resistance of the crown portion and also tyre noise performance were measured for different inclination angles. As shown in Fig.6, when the angle was less than 20 degrees, so-called heel/toe wear increased in the crown portion 6 and the wear resistance became poor. When the angle was more than 45 degrees, the noise performance was greatly deteriorated. Therefore, the angle 2 is set in the range of from 20 to 45 degrees, more preferably 20 to 35 degrees. The inclination angles of the oblique grooves 9, 10 and 11 may be constant along the length, but they preferably vary. In this example, from the ground contact edge Ei of the interior half portion to the ground contact edge Eo of the exterior half portion, the oblique grooves increase in inclination angles as shown in Fig.7. The oblique grooves 9, 10 and 11 preferably have a groove width of 2 to 6 mm, and a groove depth not less than 5 mm. In this example, as shown in Fig.8, the central oblique grooves 11 comprise grooves 11i and grooves llo alternately arranged in the tyre circumferential direction, wherein each of the grooves 11i has a wide part 11A on the side of the interior half portion I and a narrow part 11B on the side of the exterior half portion O, and each of the grooves 11o has a wide part 11A on the side of the exterior half portion O and a narrow part 11B on the side of the interior half portion I. Further, the exterior oblique grooves 9 comprise wide grooves 9A and narrow grooves 9B alternately disposed in the tyre circumferential direction. Also, the interior oblique grooves 10 comprise wide grooves 10A and narrow grooves 10B alternately disposed in the tyre circumferential direction. The ratio (GL/GS) of the large groove width GL and small groove width GS of the oblique grooves 9, 10 and 11 is preferably 1.3 to 3.5, more preferably 1.5 to 2.5. The wide exterior oblique grooves 9A are disposed such that their axially inner ends are each positioned on an extension of the wide parts 11A of the central oblique groove 11o. The narrow interior oblique grooves 10B are disposed such that the axially inner ends align with the axially outer ends of the narrow parts 11B in the tyre axial direction. The narrow exterior oblique grooves 9B are disposed such that the axially inner ends are positioned on an extension of the narrow parts 11B of the central oblique groove 11i. The wide interior oblique grooves 10A are disposed such that the axially inner ends align with the axially outer ends of the wide parts 11A in the tyre axial direction. By disposing the wide and narrow oblique grooves, the pattern noise generated by the oblique grooves is dispersed in a wide frequency range and the tyre noise is further improved. If oblique grooves having the same width are used, it is difficult to achieve both the wet performance and tyre noise. Further, in this example, second oblique grooves 13 are disposed in the exterior shoulder portion 5. Each of the second oblique grooves 13 extends between the same kind of oblique grooves 9, for example between the narrow grooves 9B, and they are inclined reversely to the oblique grooves 9 whereby the exterior half portion O can be improved in wet performance. The second oblique grooves 13 are inclined at an angle 4 of 15 to 35 degrees to the circumferential direction. The groove width is less than that of the circumferential main groove and 2 to 6 mm for example, and the groove depth is not less than 5 mm. If the second oblique grooves 13 are inclined in the same direction as the oblique grooves 9, the wheeling noise sound increases. In this example, each of the second oblique grooves 13 is composed of a wide groove 13A and a narrow groove 13B aligned with each other so as to continue from a narrow groove 9B to a narrow groove 9B. The wide grooves 13A and narrow grooves 13B are alternately disposed around the circumference of the tyre, which helps to improve dispersion of the pattern noise. The blocks B in the shoulder portions 5 and 7 and crown portion 6 can be provided with sipes S having substantially zero width to optimise the block rigidity. In this example, each sipe S is inclined to the same direction as the oblique grooves which define the block B provided with the sipe S. Comparison Tests Pneumatic tyres of size 205/65R15 were made, by way of test, and tested for wet performance, wear resistance and noise performance. Wet performance test A 3000cc rear-wheel-drive car provided on all wheels with test tyres was run on a wet asphalt road of a test course, and wet performance was evaluated by the driver's feeling. The results are indicated in Table 1 by an index based on the prior art tyre 1 being 100. The larger the value, the better the performance. Wheel Rim: 6.5JJ standard rim Inner pressure: front=rear=2.0 kgf/cm2 Wear resistance test The above-mentioned test car was run for 3000 km (including expressway), and the difference between the maximum wear in the exterior half and the maximum wear in the interior half was measured as wear resistance. The results are indicated in Table 1 by an index based on the prior art tyre 1 being 100. The larger the value, the higher the resistance. Noise performance test The test car was coasted on a smooth road surface at a speed of 50 km/h and the sound level in dB(A) of the overall noise was measured near the inside ear position of the driver and in the centre of the rear seat. The wheeling noise sound level was evaluated by the driver's feeling swinging the steering wheel right and left, and the noise difference between the right wheeling and left wheeling was also evaluated. The results are indicated by an index based on the prior art tyre 1 being 100. The larger the value, the better the noise performance. Tyre Prior Ex.1 Ex.2 Ex.3 Ex.4 Ex.5 Ref. Tread pattern Main Groove arrangement 1 Fig.2 sym. Fig.1 asym. Fig.1 asym. Fig.1 asym. Fig.1 asym. Fig.1 asym. Fig.3 asym. Exterior main groove (I) width (mm) 7.2 8.6 8.6 8.6 8.6 8.6 8.6 depth (mm) 10.0 10.0 10.0 10.0 10.0 10.0 10.0 distance from equator (mm) 40.0 10.0 10.0 10.0 10.0 10.0 10.0 Axially inner interior main groove (g) width (mm) 8.6 7.2 7.2 7.2 7.2 7.2 7.2 depth (mm) 10.0 10.0 10.0 10.0 10.0 10.0 10.0 distance from equator (mm) - 15.0 15.0 15.0 15.0 15.0 15.0 Axially outer interior main groove (h) width (mm) 7.2 7.2 7.2 7.2 7.2 7.2 7.2 depth (mm) 10.0 10.0 10.0 10.0 10.0 10.0 10.0 distance from equator (mm) 40.0 47.0 47.0 47.0 47.0 47.0 47.0 Exterior oblique groove width (mm) 4.0 4.0 4.0 3.0 6.0 3.0/6.0 4.0 inclination angle 1 (deg) 60 60 45 60 60 60 60 Central oblique groove width (mm) 2.0 2.0 2.0 1.0 3.0 1.0/3.0 2.0 inclination angle 2 (deg) 23 40 25 23 23 23 23 Interior oblique groove width (mm) 4.0 4.0 4.0 3.0 6.0 3.0/6.0 4.0 inclination angle 3 (mm) -67 -75 -65 -67 -67 -67 -67 Exterior second oblique groove width (mm) -- -- -- 3.0 3.0 1.0/3.0 -- inclination angle 4 (mm) -20 -20 -20 Wet performance 100 106 107 107 111 110 105 Wear resistance 100 107 105 106 104 107 105 Pass-by Noise 100 101 104 105 102 106 100 Wheeling Noise 100 104 104 104 104 104 94 The distance from equator was measured axially to the axially inner edge of the main grove. In Ex.5, wide and narrow oblique grooves were alternately disposed in the tyre circumferential direction as shown in Fig.8. As described above, the three main grooves are disposed asymmetrically. Accordingly, the tread rigidity is higher in the exterior half than in the interior half. Therefore, although the wear energy is larger in the exterior half than the interior half, uneven wear is effectively prevented while improving the steering stability. Further, as the central oblique grooves and the exterior oblique grooves are inclined in the same direction, dewatering from the crown portion and exterior shoulder portion is improved. Furthermore, as the interior oblique grooves are inclined reversely to the exterior oblique grooves, the difference in wheeling noise sound between the right wheeling and left wheeling is reduced. As the exterior shoulder portion, crown portion and interior shoulder portion are divided into blocks, and no circumferential extending main groove is disposed in the region extending axially inwardly from the ground contact edge of the exterior half for a distance of 1/3 of the tread width, the wear resistance is improved by the pattern rigidity increased in the exterior half, and at the same time, the excellent wet performance of a block pattern can be obtained The exterior second oblique grooves, which are inclined reversely to the exterior oblique grooves, improve the wet performance without decreasing the pattern rigidity in the exterior half. The present invention can be suitably applied to belted radial tyres for passenger cars.

Claims: A pneumatic tyre comprising a tread portion (T) provided with three straight main grooves (2,3,4) extending continuously in the tyre circumferential direction, the three main grooves being arranged asymmetrically about the tyre equator (C), an exterior main groove (2) which is one of the main grooves is disposed in an exterior half (O) which is one half of the tread portion on one side of the tyre equator (C), and an axially inner main groove (3) and an axially outer main groove (4) which are the remaining two main grooves are disposed in an interior half (I) which is the other half of the tread portion, exterior oblique grooves (9) are disposed axially outside the exterior main groove (2) and inclined at an angle (1) of 45 to 70 degrees with respect to the circumferential direction of the tyre, interior oblique grooves (10) are disposed axially outside the axially outer main groove (4) and inclined reversely to the exterior oblique grooves (9) at an angle (3) of 60 to 80 degrees with respect to the circumferential direction, central oblique grooves (11) are disposed between the exterior main groove (2) and the axially inner main groove (4) and inclined in the same direction as the exterior oblique grooves (9), and between the exterior main groove (21) and the axially inner main groove (3), the angle (2) of the central oblique grooves (11) being in the range of 20 to 45 degrees with respect to the circumferential direction, characterised in that circumferential grooves are disposed only in a region of 2/3 of the tread width from the ground contact edge (Ei) of the interior half. A pneumatic tyre according to claim 1, characterised in that the interior oblique grooves (10) comprise wide grooves and narrow grooves alternately disposed in the circumferential direction of the tyre. A pneumatic tyre according to claim 1 or 2, characterised in that the central oblique grooves (11) comprise wide grooves and narrow grooves alternately disposed in the circumferential direction of the tyre. A pneumatic tyre according to claim 1, 2 or 3 characterised in that the exterior oblique grooves (9) comprise wide grooves and narrow grooves alternately disposed in the circumferential direction of the tyre. A pneumatic tyre according to any of claims 1 to 4, characterised in that the tread portion (T) is provided axially outside the exterior main groove (21) with second exterior oblique grooves (13) inclined reversely to said exterior oblique grooves (9) and extending between the exterior oblique grooves.

Inventors: IKEDA AKIO, IKEDA, AKIO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1998
Main classification: B60C11
IPC codes: B60C11