Vehicle tyre

Abstract: A vehicle tyre having a unidirectional tread pattern, provided with two circumferential grooves (3) to divide the tread portion into two outer parts (5R,5L) and a central part (4) therebetween, the central part (4) is substantially continuous in the tyre circumferential direction and formed symmetrically about the tyre equator (CO), the outer parts (5R,5L) are provided with mirror-image patterns (PR,PL) , each formed by circumferentially repeating pattern units (7R1-7RN,7L1-7LN) having an average circumferential length (K), and the mirror-image patterns (PR,PL) are circumferentially shifted from each other by a distance S of from 0.15 to 0.25 times the above-mentioned average circumferential length (K).

Description: The present invention relates to a vehicle tyre having a unidirectional tread pattern having reduced pattern noise during running. Such tyre, corresponding to the preamble of claim 1, is known e.g. from DE-A-19548734. Attention is also known to the disclosure of EP-A-0 493 063 which shows a vehicle tyre having a tread with a tread central portion being provided with notches, which notches are staggered from the left to the right side with respect to the equatorial plane. In general, vehicle tyres such as pneumatic tyres are provided with various tread patterns to secure wet grip. Especially, unidirectional tread patterns symmetrical about the tyre equator are widely employed in passenger car tyres in recent years. In such unidirectional tread patterns, edges of tread grooves, especially those of axial or bias grooves formed on both sides of the tyre equator are liable to contact with the ground synchronously. As a result, running noise and so called pattern noise is liable to increase when compared with a bidirectional tread pattern. It is therefore an object of the present invention to provide a pneumatic tyre which has a unidirectional tread pattern having improved pattern noise. According to the present invention, a vehicle tyre comprises the features 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 view showing from above the tyre tread pattern according to the present invention; Fig.2 is a view for explaining the repetition of pattern units in the central tread part; and Fig.3 is a graph showing the relationship between the pattern noise and pattern shift. In Fig.1, a vehicle tyre 1 according to the present invention is a pneumatic tyre for passenger cars and is provided in a tread portion 2 with a unidirectional tread pattern. In the tread portion 2, a pair of circumferential grooves 3 are disposed one on each side of the tyre equator CO to divide the tread portion 2 into a central part 4 therebetween and a pair of right and left outer parts 5R and 5L axially outside the grooves 3. The central part 4 is formed symmetrically about the tyre equator CO and is substantially continuous in the tyre circumferential direction to form a rib. In this embodiment, the central part 4 is provided with a symmetrical pattern PC. As shown in Fig.2. the symmetrical pattern PC is formed by repeating pattern units 6 in the tyre circumferential direction. Each pattern unit 6 is also symmetrical about the tyre equator CO. The pattern units 6 are identical with each other except for the circumferential lengths or pitch lengths. The lengths can be determined according to known variable pitching methods. The right and left outer parts 5R and 5L are provided with right and left mirror-image patterns PR and PL, respectively. Each of the mirror-image patterns PR and PL is formed by repeating a plurality of kinds of pattern units 7 (7R, 7L) in the tyre circumferential direction. The sequence or order of the pattern units 7R in the right mirror-image pattern PR is the same as that of the pattern units 7L in the left mirror-image pattern PL. The pattern units 7R in the right mirror-image pattern PR include a number N of pattern units 7R1 to 7RN, and the pattern units 7L in the right mirror-image pattern PL include the same number N of pattern units 7L1 to 7LN, wherein the pattern units 7R1 to 7RN are mirror-images of the pattern units 7L1 to 7LN, respectively. The pattern units 7R, 7L are identical with each other except for the circumferential lengths or pitch lengths. The lengths and the above-mentioned sequence can be determined according to known variable pitching methods. The number N is preferably 3 to 7. In this embodiment, the number N is 5. In Fig.1, the above-mentioned circumferential grooves 3 are straight grooves. The central part 4 is provided with V-shaped sipes 10 and notches 11. The sipes 10 extend across the axial width of the central part 4. The width of the sipes is less than 2 mm, usually less than 1 mm, and the depth is less than that of the circumferential grooves 3. The notches 11 are formed on both sides of the central part 4. The sipes 10 and notches 11 are arranged alternately in the tyre circumferential direction. The sipes 10 are narrow and relatively shallow and the notches 11 do not enter to the tyre equator CO. Therefore, the central part 4 is substantially continuous in the tyre circumferential direction and forms a rib. Further, the outer parts 5R and 5L in Fig.1 are provided with bias grooves G1 extending from the tread edges E to the circumferential grooves 3 and bias grooves G2 extending from the tread edges E at positions between the bias grooves G1 towards the axially inside. Each of the bias grooves G2 is curved to cross the first adjacent bias groove G1, but terminates before the second adjacent bias groove G1 to form a closed end. The inclination of the bias grooves G2 becomes steeper from the axially inner closed end to the axially open end. The width of the central part is set in a range of from 6 to 20 % of the tread width TW. Each circumferential groove 3 is disposed in a range of from 4.5 to 19 % of the tread width TW from the tyre equator CO. The width of the circumferential grooves is in the range of from 5 to 20 mm. According to the present invention, the right mirror-image pattern PR must be circumferentially shifted from the left mirror-image pattern PL by a predetermined distance S. If the patterns are not shifted, the tread pattern as a whole will be completely symmetrical about the tyre equator CO. It is necessary to set the pattern shift distance S in the range of from 0.15 to 0.25 times the average of the different circumferential lengths K1 to Kn of the number N of the pattern units (7R1=7L1) to (7RN=7LN) . In this embodiment, the average circumferential length K is the average of the circumferential lengths K1, K2, K3, K4 and K5 of the pattern units (7R1, 7L1), (7R2, 7L2), (7R3, 7L3), (7R4, 7L4) and (7R5, 7L5). By setting the distance S in the above-mentioned range, noise frequency dispersion is promoted to improve the pattern noise making the tyre quieter in use but the wet grip performance is maintained at the same high level as the completely symmetrical tread pattern in which S = 0. Fig.3 shows a relationship between the shift distance S and pattern noise, which was obtained by an inventor's test. In this test, test tyres of size 235/45ZR17 (for 17X8JJ Rim) having the same tread pattern shown in Fig.1 except for shift length S were prepared. Then, a test car (2500cc FR passenger car) provided on all the wheels with the test tyres was run on a smooth asphalt road at a speed of 50 km/h and the noise in dB(A) was measured near the driver's ears as the pattern noise. In Fig.3, the measured pattern noise is indicated by a relative value based on a tyre whose S=0 being 0 dB(A). From the test, it was confirmed that when the ratio S/K is increased to about 0.1, the noise starts to reduce, and the reduction continues to about 0.25. If the ratio exceeds 0.25, the noise reduction stops and the tyre appearance is liable to become odd. The following Table 1 shows the results of further tests for steering stability and ride comfort. The pattern noise measured as above is also shown therein. (1) Steering stability test : The above-mentioned test car was run on an asphalt road in a tyre test course, and a test driver evaluated the steering response, tyre rigidity and road grip as the steering stability into ten ranks by his feelings, wherein the higher the rank, the better the steering stability. (2) Ride comfort test : During running the test car on a rough road in the tyre test course, harshness, thrust-up and the like were evaluated as the ride comfort into ten ranks by the test driver's feelings. The higher the rank, the better the ride comfort. Tyre Ref.1 Ref.2 Ex.1 Ex.2 Ex.3 Ref.3 Pattern shift S/K 0 0.1 0.15 0.2 0.25 0.3 Steering stability 6 6 6 6 6 6 Ride comfort 6 6 6 6 6 6 pattern noise 0 -0.3 -1.2 -1.6 -2.1 -2.3 Tread pattern: Fig.1 Air pressure: 230 kPa, From the tests, it was confirmed that the tyres according to the present invention were improved in pattern noise without deteriorating the steering stability and ride comfort. The present invention can be applied to various tyres, for example, heavy duty tyres, light-truck tyres and the like in addition to passenger car tyres. Further, apart from the tread pattern and pattern units shown in Figs.1 and 2, various patterns and units within the scope of the appended claims can be employed.

Claims: A vehicle tyre comprising a tread portion (2) having a unidirectional pattern, the tread portion being provided with two circumferential grooves (3) to divide the tread portion into two outer parts (5R,5L) and a central part (4) therebetween, the central part (4) being substantially continuous in the tyre circumferential direction and formed symmetrically about the tyre equator, wherein the outer parts are provided with mirror-image patterns (PR,PL), respectively, each of the mirror-image patterns (PR,PL) formed by circumferentially repeating pattern units (7R1-7RN, 7L1-7LN) having an average circumferential length (K), and the mirror-image patterns are circumferentially shifted from each other by a distance S of from 0.15 to 0.25 times said average circumferential length (K), characterised in that the central part (4) is provided with a pattern symmetrical about the tyre equator which comprises notches (11) formed on both sides of the central part (4). A vehicle tyre according to claim 1, characterised in that each of the mirror-image patterns (PR,PC) of the outer parts is formed by repeating a number N of pattern units (7R1-7RN,7L1-7LN) having different circumferential lengths, and the number N is in the range of from 3 to 7. A vehicle tyre according to claim 1 or 2, characterised in that the outer parts (5R,5L) are provided with bias grooves (G1) extending from the tread edges to the circumferential grooves and bias grooves (G2) extending from the tread edges (E) at positions between the bias grooves (G1) towards the axially inside, and each of the bias grooves (G2) is curved to cross the first adjacent bias groove (G1), but terminates before the second adjacent bias groove (G1) to form a closed end. A vehicle tyre according to any of claims 1 to 3, characterised in that the central part (4) is provided with sipes (10) extending across the width thereof.

Inventors: HANYA MASAHIRO, HANYA, MASAHIRO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1999
Main classification: B60C11
IPC codes: B60C11