Pneumatic tyre

Abstract: A pneumatic tyre comprising a tread portion (2) provided with a pair of circumferential grooves(9) and axial grooves (10) extending from the circumferential grooves to tread edges (Te), each of the axial grooves (10) extending along a parabola (12) having an axis extending parallel to the tyre axial direction and a vertex disposed on the axially outer edge of the circumferential groove and a focus disposed axially outwards of the parabola vertex and the axially outer edge of the axial groove extends on the parabola from the parabola vertex to the tread edge (Te), whereby flow of water is improved and flow of air is also improved to improve wet performances without increasing tyre noise.

Description: The present invention relates to a pneumatic tyre having an improved tread portion provided with parabolic grooves to improve wet performance of the tyre. Recently, as the silence of the car mechanics has been remarkably improved, the request for tyres to generate less noise has become stronger. On the other hand, in pneumatic tyres, tread patterns comprising circumferential and axial grooves to define blocks are widely used to provide good wet performance. Hitherto, in order to improve wet performance, usually the number and/or width of the circumferential and axial grooves has increased and/or the land/sea ratio of the tread has been decreased. Those countermeasures are however liable to increase the so called air pumping sound and air resonance sound. Thus the tyre running noise is liable to increase. Hitherto, therefore, it has been inevitable to adjust such contradictory wet performance and tyre noise by sacrificing one of them. It is therefore, an object of the present invention to provide a pneumatic tyre in which the flow of water is improved and at the same time the flow of air is improved to improve the wet performance without increasing tyre noise. A tire according to the preamble of claim 1 is known by e.g. GB-A-1 499 365. According to the present invention, a pneumatic tyre comprises, a tread portion having a tread surface having tread edges, a pair of circumferential grooves one disposed on each side of the tyre equator and near the tyre equator and extending continuously in the circumferential direction of the tyre so as to define a central part between the circumferential grooves and a pair of axially outer parts axially outward of the circumferential grooves, and axial grooves disposed in each of the axially outer parts at pitches P in the tyre circumferential direction, and each of the axial grooves extending from one of the circumferential grooves to one of the tread edges along a parabola, and the parabola axis extending parallel to the tyre axial direction, and the parabola focus disposed axially outward of the parabola vertex, wherein the parabola has a vertex disposed on the axially outer edge of the circumferential groove, and the axial groove has an axially outer edge extending on the parabola from the parabola vertex to the tread edge. Accordingly, flow of water along the parabola from the circumferential groove to the axial grooves or from the axial grooves to the circumferential groove is made very smooth and thus drainage is effectively improved to improve wet performances. Further, the flow of air is also smooth and tyre noise especially pumping noise is effectively decreased, and as a result resonance noise is also decreased. Furthermore, as the groove edges are curved along a parabola, the edges are prevented from simultaneously contacting the ground, and as a result the so called impact sound is decreased, which also reduces the tyre noise. Embodiments of the present invention will now be described by way of example only in conjunction with the accompanying drawings in which:- Fig. 1 is a cross sectional view of an embodiment of the present invention; Fig. 2 is a developed view showing an example of the tread pattern thereof; Fig. 3 is a diagram for explaining the parabola and axial grooves; Fig. 4 shows a flow of water in the above-mentioned tread pattern; Fig. 5 is a developed view showing another example of the tread pattern; and Fig. 6 is a developed view showing a tread pattern used in comparison tests as in the reference tyre. Fig. 1 shows a pneumatic tyre 1 according to the invention which is a low aspect ratio radial tyre for passenger cars, and an example of the internal tyre structure is shown. The tyre 1 comprises a tread portion 2, a pair of sidewall portions 3, a pair of axially spaced bead portions 4 each provided with a bead core 5 therein, a radial carcass 6 extending between the bead portions 4 through the tread portion 2 and sidewall portions 3 and turned up around the bead cores 5 to be secured thereto, and a stiff belt 7 disposed, as a hoop, radially outside the carcass 6 and inside the tread portion 2. In the tread portion 2, a pair of circumferential grooves 9L and 9R (generically 9) and axial grooves 10L and 10R (generically 10) are disposed to form novel tread patterns shown in Figs. 2-5. The circumferential grooves 9L and 9R are disposed one on each side (left side and right side) of the tyre equator C and near the tyre equator C. They extend continuously in the circumferential direction of the tyre so as to define a central part of the tread between the circumferential grooves 9L and 9R and a pair of axially outer parts 2L and 2R of the tread each extending from the axially outer edge 9e of one of the circumferential grooves 9L, 9R to one of the tread edges Te. The axial grooves 10L and 10R are arranged circumferentially at regular or irregular pitches and connected with the circumferential grooves 9L and 9R, respectively. Each axial groove 10 extends as a parabola 12 from one of the circumferential grooves 9 to beyond one of the tread edges Te so that the axially inner end is opened to the circumferential groove 9 and the axially outer end is opened in a buttress portion 15. Here, the tread edges Te are each defined as the axial outermost edge of the ground contacting region of the tread portion 2 under the condition when the tyre is mounted on its standard rim, inflated to its normal pressure and then loaded with 80 % of the maximum load. The standard rim is a rim officially approved for the tyre by, for example JATMA (Japan), TRA (USA), ETRTO (Europe) and the like, and the normal pressure and maximum load are officially specified by the same associations. The tread width TW is defined between the tread edges Te. As shown in Fig. 3, the parabola 12 is described according to the equation Y = A X 2 where A is a constant value in the range of from 0.030 to 0.060. This equation is expressed, based on X-Y coordinates in which the X-axis extends parallel to the tyre circumferential direction, the Y-axis extends parallel to the tyre axial direction, the origin O is set at a small axial distance H1 from the tyre equator C, the unit length H2 in the Y-axis is equal to one tenth of the axial distance between the origin O and tread edge Te, the unit length H3 in the X-axis is equal to one tenth of a pitch P. Accordingly, the vertex of the parabola is at the origin O, and the parabola axis extends parallel to the tyre axial direction. The direction of the parabola is such that the focus is axially outside the origin O. As shown in Figs. 2 and 3, a plurality of parabolas 12 are described on each side of the tyre equator C, setting the origins O thereof on a circumferential line 14 at pitches P. For the pitches P, it is possible to use regular pitches or variable pitches. In the case of variable pitches, accordingly, the above-mentioned unit length H3 on one side of the parabola axis may be different to the other side. Accordingly, the parabola 12 always intersects the circumferentially adjacent parabolas 12 at points K which are (+5, 25A) and (-5, 25A). In this specification, the pitches of the axial grooves 10 are defined by the pitches P. Preferably, the number of pitches P per circumference is in the range of from 40 to 80 in case of passenger car tyres. A part of the tread which extends axially inwardly from the above-mentioned circumferential straight line 14 by an axial distance W1 is engraved as the circumferential groove 9. Further, a generally triangular part (O-K-O) surrounded by the circumferential straight line 14 and two adjacent parabolas 12 is engraved as a flaring junction part 17 between the circumferential groove 9 and axial groove 10. No step difference is formed in the depth of the channel from the circumferential groove 9 to the axial groove 10 through the flaring junction part 17, but a gradual change may be possible. Therefore, a plurality of flaring junction parts 17 are formed repeatedly in the tyre circumferential direction along the axially outside of the circumferential groove 9. Thus, the circumferential groove 9 is regarded as having an axially outer edge which has a wavy configuration. On the other hand, the axially inner edge of the circumferential groove 9 preferably has a generally straight configuration. In this example, therefore, the central part of the tread is formed as a circumferentially continuously extending straight rib. In order to further provide edge effect, it is possible to dispose small size sipes, cuts and the like in this rib, which for example extend from the edges of the rib towards the tyre equator but terminate before the equator as shown in the figures. The axial distance H1 from the tyre equator C to the line 14 is set in the range of from 0.05 to 0.20 times the tread width TW. The axial distance W1 is set in the range of from 0.03 to 0.10 times the tread width TW. With respect to each of the parabolas 12, one axial groove 10 is formed on one side of the parabola axis, and on the other side of the parabola axis one sipe 16 is formed along the parabola. Further, on each side of the tyre equator C, the axial grooves 10 are formed on the same side of the parabola axes. Here, a sipe is a cut or narrow groove having a width of from 0 to 2 mm. As a result, the blocks 18 defined between the axial grooves 10 increase the pattern rigidity in the tyre axial direction to increase the cornering force and thus cornering performance in dry conditions can be improved. On the other hand, the sipes 16 can display their edge effect to improve wet performance especially wet road grip without decreasing the pattern rigidity. Preferably, the axially outer end of each sipe 16 is terminated in the block 18 to maintain the pattern rigidity at a high level. Each of the axial grooves 10 is formed on the other side of the parabola 12 than the same side as the focus, placing an axially outer groove wall 13A on the parabola 12 and an axially inner groove wall 13B extending substantially parallel thereto on the outside of the parabola. Accordingly, a convexly curved groove edge or groove wall which extends from a parabola vertex to the tread edge is formed. The groove width W2 of the axial grooves 10 is set to be not more than the axial distance W1 but more than the width of the sipe. In the example shown in Fig. 2, the axial grooves 10R on the right side of the tyre equator C are formed on the lower side Re of the parabola axes, but the axial grooves 10L on the left side are formed on the upper side Fr to form a tread pattern which has substantially point symmetry. Further, the tread portion 2 in this example is provided with two circumferential grooves 11L and 11R each disposed in the middle of each of the axially outer parts 2L and 2R and extending continuously in the tyre circumferential direction in addition to the above-mentioned pair of circumferential grooves 9L and 9R. Accordingly, as shown in Fig. 4. due to the existence of the flaring parts 17, the flow of water along the parabola 12 from the circumferential groove 9R to the axial grooves 10R and also from the axial grooves 10L to the circumferential groove 9L becomes very smooth and thus drainage is effectively improved. Further, as the axial groove 10 is along a parabola 12, there is no inflection point, and the resistance to water flow and air flow is very small. In the case of the point-symmetrical tread pattern shown in Fig. 2, discharge of water occurs mainly towards one side of the tyre as shown in Fig. 4. Therefore, the tyres having such a pattern are mounted on a car such that the direction of water discharge is to the outside of the car. As a result, the water discharged from the front tyres towards the inside of the car is decreased, and the water clearing burden on the rear tyres is decreased and the wet performances of the tyres as a whole is improved. Fig. 5 shows a modification of the above-mentioned tread pattern, wherein the axial grooves 10R on one (right) side of the tyre equator C are formed on the upper side Fr of the parabola axes, and the axial grooves 10L on the other (left) side are also formed on the same upper side Fr, thereby forming a tread pattern which is substantially a line symmetry about the tyre equator C, but the pattern is otherwise the same as the former example. If the above-mentioned constant A in the equation is less than 0.030, the flare of the flaring parts 17 decreases, and the effect of the flaring parts on guiding water from the circumferential grooves 9 to the axial grooves 10 becomes insufficient. Further, the corners formed between the axial grooves and the circumferential groove are liable to be torn off. On the other hand, if the constant A is more than 0.060, the intersecting angle of the axial groove with the circumferential groove becomes too large, and a smooth flow of water from the axial groove to the circumferential groove can not obtained. Comparison Tests Test tyres in size of 195/65R15 having the internal structure shown in Fig. 1 were made and tested for wet performance and noise. The example tyre had the tread pattern shown in Fig. 2, but the reference tyre had the tread pattern shown in Fig. 6. Noise test: Using a 2000 cc passenger car provided on all wheels (15 X 6JJ normal rim) with the test tyres (Pressure: front 2.0 kgf/sq. cm, rear 1.9 kgf/sq. cm), the running noise was evaluated by the driver's feeling into ten ranks where 10 to 7 are excellent, 6 average, 5 on the allowable limit, 1 to 4 not allowable. Wet performance test: Using a 2000 cc passenger car provided on all wheels (15 X 6JJ normal rim) with the test tyres (Pressure: front 2.0 kgf/sq. cm, rear 2.0 kgf/sq. cm), lateral-G was measured on a wet track to measure the wet performance. The test car was run in a 100 m radius curved course on a paved road surface provided with a 5 mm depth 20 m long water pool, and the lateral-G was measured in the speed range of from 70 to 90 km/h to calculate the average lateral-G in this speed range. Further, the maximum lateral-G was measured. The larger the value, the better the wet performance. The results of those tests are as follows. Tyre Ex. Ref. Tread pattern Fig. 2 Fig. 6 Land/sea (%) 68.8 68.4 Noise under 30 km/h 6 6 30 to 60 km/h 6 6 60 to 90 km/h 7 6 over 90 km/h 6 6 Wet performance Maximum lateral G 0.34 0.306 Average lateral G 0.286 0.239 From the tests, it was confirmed that the example tyre was superior to the reference tyre in wet performance although the noise was improved or maintained at the same level as the reference tyre.

Claims: A pneumatic tyre (1) comprising, a tread portion (2) having a tread surface having tread edges (Te), a pair of circumferential grooves (9) one disposed on each side of the tyre equator (C) and near the tyre equator (C) and extending continuously in the circumferential direction of the tyre (1) so as to define a central part between the circumferential grooves and a pair of axially outer parts (2L and 2R) axially outward of the circumferential grooves, and axial grooves (10) disposed in each of the axially outer parts (2L and 2R) at pitches P in the tyre circumferential direction, each of the axial grooves (10) extending from one of the circumferential grooves (9L, 9R) to one of the tread edges (Te) along a parabola (12), and the parabola axis (Y) extending parallel to the tyre axial direction, and the parabola focus disposed axially outward of the parabola vertex (O), characterised in that the parabola (12) has a vertex (O) disposed on the axially outer edge (9e) of the circumferential groove, and the axial groove (10) has an axially outer edge (13A) extending on the parabola (12) from the parabola vertex (O) to the tread edge. A pneumatic tyre according to claim 1, characterised in that each said parabola (12) is described according to an equation Y = A X 2 where A is a constant value in the range of from 0.030 to 0.060, and the equation is expressed based on X-Y coordinates in which X-axis extends parallel with the tyre circumferential direction, Y-axis extends parallel with the tyre axial direction, the origin is set at the parabola vertex (O) at a small axial distance H1 from the tyre equator, a unit length H2 in Y-axis is equal to one tenth of the axial distance between the origin and tread edge, a unit length H3 in X-axis is equal to one tenth of a pitch P from the origin to a circumferentially adjacent origin. A pneumatic tyre according to claim 1 or 2, characterised in that a generally triangular part (O-K-O) surrounded by the circumferentially adjacent parabolas (12) and a circumferential straight line (14) drawn between the parabola vertexes thereof is engraved to form a flaring junction part (17) between the circumferential groove (9) and axial groove (10). A pneumatic tyre according to any of claims 1 to 3, characterised in that with respect to each said parabola (12), one axial groove (10) is formed on one side of the parabola axis (Y). A pneumatic tyre according to claim 4, characterised in that on the other side of the parabola axis (Y), a sipe (16) is disposed to extend along the parabola. A pneumatic tyre according to claim 4, characterised in that on one side of the tyre equator (C), the axial grooves (10) are formed on the same one side of the parabola axes (Y), but on the other side of the tyre equator, the axial grooves (10) are formed on the same other side of the parabola axes (Y).

Inventors: HAYASHI MOTOO, HAYASHI, MOTOO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1996
Main classification: B60C11
IPC codes: B60C11