Heavy duty pneumatic tyre

Abstract: A heavy duty pneumatic tire improved in uneven tread wear comprises a tread portion provided with an axially outer longitudinal main groove (3) and an axially inner longitudinal narrow groove (6), each extending circumferentially of the tire to define a narrow rib (5) therebetween, and the axially inner edge of the narrow rib is chamfered by a slope (7). In the tire meridian section, the slope can be straight or concave. The axial width of the axially inner longitudinal narrow groove is in the range of from 0.5 to 2.5 mm. The axial width (W3) of the slope measured between the upper and lower edges is in the range of from 0.3 to 0.8 times the axial width (W2) of the narrow rib, and the radial height (H2) of the slope measured between the upper and lower edges is in the range of from 0.3 to 0.8 times the depth (H1) of the axially inner longitudinal narrow groove.

Description: The present invention relates to a heavy duty pneumatic tire for truck, bus and the like, improved in uneven tread wear. In heavy duty tires for truck, bus and the like, rib type tread patterns are usually used for resistance to wear. In such a rib pattern, however, so called shoulder wear in which the shoulder ribs wear earlier than the axially inner ribs is liable to occur, and the shoulder wear spreads axially inwards to cause uneven tread wear. In order to prevent the uneven wear, the laid-open Japanese patent application No. JP-A-5-246213 (corresponding to the preamble of claim 1) discloses a pneumatic tire, wherein, as shown in Fig.lO(A), a narrow width longitudinal groove (g) is disposed in the axially outer edge portion of the rib (f) adjacent to the shoulder rib (a). The narrow groove (g) extends continuously in the tire circumferential direction, and is intended to stop the axially inward spread of wear at the subdivided narrow rib (i), that is, to concentrate the wear on the narrow rib (i) and thereby prevent the main portion (h) of the rib from being worn unevenly. However, in this construction, when the narrow rib (i) is subjected to side force during cornering, the narrow rib (i) contacts the main portion (h) as shown in Fig.10(B), and the narrow rib (i) and the main portion (h) are liable to move together, thus functioning as a single body. Therefore, contrary to the intention, it is difficult to fully control the uneven wear. It is an object of the present invention to provide a heavy duty pneumatic tire in which uneven wear is fully prevented to improve the tire durability. According to the present invention, a heavy duty pneumatic tire comprises a tread portion provided with an axially outer longitudinal main groove and an axially inner longitudinal narrow groove, each extending circumferentially of the tire to define a narrow rib therebetween, the axially inner edge of said narrow rib being chamfered by a slope, the slope extending from the top surface to the axially inner surface of the narrow rib defining the upper and lower edges of the slope. Preferably, the axial width of the narrow groove is in the range of from 0.5 to 2.5 mm. The axial width of the slope measured between the upper and lower edges is preferably in the range of from 0.3 to 0.8 times the axial width of the narrow rib. The radial height of the slope measured between the upper and lower edges is also preferably in the range of from 0.3 to 0.8 times the depth of the narrow groove. Therefore, when the narrow rib is subjected to a strong side force during cornering, the top of the narrow rib does not contact the axially inwardly adjacent rib. Thus, wear remains concentrated on the narrow rib, and the axially inward spread of wear is stopped at the narrow rib to prevent uneven tread wear. Embodiments of the present invention will now be described in detail, by way of example only, in conjunction with the accompanying drawings in which:- Fig.1 is a developed plan view of an embodiment of the present invention showing an example of the tread pattern; Fig.2 is a cross sectional view of the tire taken along a line X-X in Fig.1; Fig.3 is a perspective view showing the tread shoulder portion thereof; Fig.4 is a cross sectional view of an example of the narrow rib; Fig.5 is a cross sectional view explaining the wear; Figs.6(A) and (B) are schematic cross sectional views explaining the deformation of the narrow rib when subjected to a side force; Figs.7, 8 and 9 are cross sectional views each showing another example of the narrow rib; Figs.10(A) and (B) are cross sectional views showing the prior art; and Fig.11 shows the cross sections of the narrow ribs used in the comparison tests the results of which are shown in Table 1. In the Figures, the heavy duty pneumatic tire 1 according to the present invention comprises a tread portion 12, a pair of axially spaced bead portions 14 and with a bead core 15 therein, a pair of sidewall portions 13 extending between the tread edges E and the bead portions 14, a carcass 16 extending between the bead portions 14, a belt 17 disposed radially outside the carcass 16 and inside a rubber tread. The tire 1 in this embodiment is a radial tire for truck and bus. The tire size is 285/75R24.5. The tire has the so called square shoulder. The carcass 16 is composed of at least one ply of cords arranged radially at an angle of 90 to 70 degrees with respect to the tire equator C and extending between the bead portions 14 through the tread portion 12 and the sidewall portions 13 and turned up around the bead cores 15 in the bead portions to give a radial or semiradial structure. For the carcass cords, organic fibre cords, e.g. polyester, nylon, rayon, aromatic polyamide and the like and steel cords can be used. The belt 17 is composed of two to four plies of parallel cords (four plies 17A, 17B, 17C and 17D in the example) inclined at an angle with respect to the tire equator C, and at least two plies (17B and 17C) thereof are oriented to cross each other. For the belt cords, organic fibre cords, e.g. nylon, aromatic polyamide, rayon, polyester and the like and steel cords can be used. In Fig.1 showing an example of the tread pattern, the tread portion 12 is provided on each side of the tire equator C with an axially outer longitudinal main groove 3, an axially inner longitudinal main groove 8, and a longitudinal narrow groove 6 disposed between the wide longitudinal grooves 3 and 8. Each of the longitudinal grooves 3, 8 and 6 extends continuously in the tire circumferential direction. Accordingly, the tread 2 is provided on each side of the tire equator C with a shoulder rib 4 between the axially outer longitudinal main groove 3 and the adjacent tread edge E, a narrow rib 5 between the axially outer longitudinal main groove 3 and the longitudinal narrow groove 6, an axially inner rib 10 between the longitudinal narrow groove 6 and the inner longitudinal main groove 8, and a central rib 11 between the two inner longitudinal main grooves 8. The axial distance SW of the axially outer longitudinal main groove 3 from the tire equator C to the groove centre is preferably set in the range of from 22.5 to 37.5 % of the ground contacting width TW. If the distance SW is more than 37.5% of the ground contacting width TW, the rigidity of the shoulder rib 4 decreases to deteriorate the handling performance, and further the ground pressure becomes uneven and uneven wear is liable to occur. Here, the ground contacting width TW is the axial width of the ground contacting area S with the tyre under a standard condition in which it is mounted on a standard rim and inflated to standard load and then loaded with its standard load. The standard rim is the standard rim specified in JATMA, the "Measuring Rim" in ETRTO, the "Design Rim" in TRA or the like. The standard pressure is the maximum air pressure in JATMA, the "Inflation Pressure" in ETRTO, the maximum pressure given in the "Tire Load Limits at Various Cold Inflation Pressures" table in TRA or the like. The standard load is the maximum load capacity in JATMA, the "Load Capacity" in ETRTO, the maximum value given in the above-mentioned table in TRA or the like. The groove-top width GW of the axially outer longitudinal main groove 3 is preferably in the range of from 4 to 12 % of the ground contacting width TW. The groove depth GH of the axially outer longitudinal main groove 3 is in the range of from 5 to 18 % of the ground contacting width TW. (For example, TW = 190 mm, GW = 10 mm, GH = 14.6 mm) The inner longitudinal main groove 8 is wider than the axially outer longitudinal main groove 3 to increase the drainage. In the groove 8, projections 21 standing out from the groove bottom by a small height are disposed in a circumferential row to prevent stone trapping. The axially outer longitudinal main groove 3 and the inner longitudinal main groove 8 are straight grooves in this example, but a zigzag groove may be used for one or both grooves. The longitudinal narrow groove 6 has a groove width gw in the range of from 0.5 to 2.5 mm, and a groove depth H1 in the range of from 50 to 100 % of the groove depth GH of the axially outer longitudinal main groove 3. (For example, gw = 2 mm, H1 = 10mm) The longitudinal narrow groove 6 extends circumferentially substantially parallel to the axially outer longitudinal main groove 3, and the above-mentioned narrow rib is defined between the longitudinal narrow groove 6 and the axially outer longitudinal main groove 3. If the groove width gw is less than 0.5 mm, it becomes to hard to make the tire vulcanising mould. If the groove width gw is more than 2.5 mm, the axial deformation of the narrow rib 5 increases and the rubber is liable to be torn off. Further, it becomes hard to concentrate the wear energy on the top of the narrow rib 5. If the groove depth H1 is less than 50 % of the groove depth GH, the rigidity of the narrow rib 5 excessively increases and the rigidity difference from the inner rib 10 decreases. As a result, it becomes difficult to concentrate the wear on the narrow rib 5. If more than 100%, the thickness of the tread rubber excessively increases, the heat generation during running increased and also the tire weight increases. More preferably, the groove depth H1 is set in the range of from 50 to 80 % of the groove depth GH, whereby the narrow rib rubber can be effectively prevented from being torn off. The provision of the narrow rib 5 is to concentrate wear thereon. For this purpose, it is preferable that the axial width W2 of the narrow rib 5 is in the range of from 3.0 to 7.0 mm and/or in the range of from 0.1 to 0.2 times the axial distance Wl between the axially inner edge of the axially outer longitudinal main groove 3 and the axially outer edge of the axially inner longitudinal main groove 8. If the width W2 is less than 0.1 times and 3.0 mm, the narrow rib rigidity excessively decreases and the narrow rib can easily move rather than resist movement. Therefore, wear is caused on the other rib than the narrow rib 5. If the width W2 is more than 0.2 times and 7.0 mm, the narrow rib rigidity excessively increases and the wear is not concentrated on the narrow rib 5. More preferably, the narrow rib width W2 is in the range of from 4.0 to 6.0 mm. The axially inside corner or edge of the narrow rib 5 is chamfered by a slope 7 along the entire length of the narrow rib 5. In a cross section including the tire axis, the slope 7 is not convex between the upper edge Pu and lower edge Pd thereof. That is, the slope 7 is a straight line L or a concave line running on the opposite side of the straight line L to the longitudinal narrow groove 6. In the example shown in Figs.3 and 4, the slope 7 is straight from the upper edge Pu to the lower edge Pd. Accordingly, the inclination angle thereof is constant. The axial width W3 of the slope 7 measured from the lower edge Pd to upper edge Pu is in the range of from 0.3 to 0.8 times the narrow rib width W2. The slope height H2 measured radially from the tread 2 to the lower edge Pd is in the range of from 0.3 to 0.8 times the groove depth H1 of the longitudinal narrow groove 6. As the narrow rib 5 is provided with the slope 7, as shown in Fig.6 even if the tire is subjected to a strong side force from the road surface during cornering, the upper part of the narrow rib 5 is kept out of contact with the inner rib 10. Thus, the inner rib 10 is not moved by the narrow rib 5. If the axial width W3 is less than 0.3 times the narrow rib width W2, the inner rib 10 is liable to move together with the narrow rib 5, the wear is liable to spread over the inner rib 10. If the axial width W3 is more than 0.8 times the narrow rib width W2, the top width of the narrow rib 5 excessively decreases, and the upper part of the narrow rib 5 is liable to be damaged. For example, cracks, tearing and the like occur, and uneven wear is liable to occur on the inner rib 10. More preferably, the narrow rib width W2 is in the range of from 0.4 to 0.6 times the slope axial width W3. If the slope height H2 is less than 0.3 times the groove depth H1 of the longitudinal narrow groove 6, the narrow rib 5 and inner rib 10 are moved together and the wear does not concentrate on the narrow rib 5. If more than 0.8, as the rigidity of the narrow rib 5 decreases, the lower edge portion contact with the inner rib 10 when deformed, and the wear does not concentrate on the narrow rib 5. More preferably, the slope height H2 is in the range of from 0.4 to 0.6 times the groove depth H1. In this example, the narrow rib 5 is provided with sipes 22 extending substantially parallel to the tire axial direction across the whole width of the narrow rib 5. Further, to improve the grip performance, the shoulder ribs 4, inner ribs 10 and central rib 11 are each provided with narrow axial grooves 23 and/or sipes 24, at least one end of which reaches to one of the longitudinal grooves 3, 8 and 6 to open thereto. Figs.7, 8 and 9 show other examples of the slope 7S, wherein the slope 7S is curved partially or wholly, and the slope 7S is concave on the whole. In Fig.7, the slope 7S consists of a straight upper part 30 and a concave lower part 29. The upper part 30 extends straight radially inwardly from the upper edge Pu, inclining towards the longitudinal narrow groove 6 at a small angle of (α 1). The concave part 29 extends radially and axially inwardly from the radially inner end of the upper part 30 to the lower edge Pd. In Fig.8, the slope 7S consists of a straight upper part 30, a concave part 29, and a straight lower part 31. The upper part 30 extends straight radially inwardly from the upper edge Pu, inclining towards the longitudinal narrow groove 6 at a small angle. The concave part 29 extends radially and axially inwardly from the radially inner end of the upper part 30. The lower part 31 extends straight axially inwardly from the radially inner end of the concave part 29 to the lower edge Pd, inclining at a large angle (α 2). In Fig.9, the slope 7S consists of a concave part 29. That is, the slope 7S is curved from the upper edge Pu to the lower edge Pd. In Figs.7-9, the inclination angle of the slope 7S with respect to the normal to the tread decreases from the upper edge Pu to the lower edge Pd on the whole. The inclination angle (α 1) of the upper straight part 30 is not less than 0 degree and less than the inclination angle (β) of the straight L drawn between the upper and lower edges Pu and Pd. The inclination angle (α 2) of the lower straight part 31 is more than the inclination angle (β) and preferably substantially 90 degrees. The axial distance W5 between the lower edge Pd and the intersection of two tangential lines Tu and Td to the slope 7S drawn at the upper and lower edges Pu and Pd is not more than the above-mentioned slope axial width W3. The radius of curvature R1 of the curved part 29 is not more than 1.5 times the slope axial width W3. By using the concave slope rather than the straight slope, the rigidity is less decreased, and the space between the inner rib 10 and the narrow rib 5 can be maintained wider if the narrow rib 5 falls towards the inner rib 10. Thus the concave slope is superior in controlling uneven wear. In the case of the concave slope, for the same reasons as the straight slope, it is preferable that the slope axial width W3 is 0.3 to 0.8 times the narrow rib width W2, and the slope height H2 is 0.3 to 0.8 times the longitudinal narrow groove depth H1. But more preferably, the slope height H2 is 0.35 to 0.60 times the depth H1, and the slope axial width W3 is 0.4 to 0.6 times the width W2. In Fig.11(A), the narrow rib (i) is circumferentially divided by eighty slits (k) (width 5 mm, depth 9 mm). In Fig.11(B), the narrow rib (i) is circumferentially divided by eighty cuts (s) (depth 4.5 mm, width almost zero). In Fig.11(C), the axially outer edge of the narrow rib (i) chamfered along the entire circumference of the tire. Comparison tests Test tires having the same internal structure and tread pattern as shown in Figs.1 and 2 except for the narrow ribs were made and subjected to the following test. The specifications of the test tires and the test results are given in Tables 1 and 2. The test tires were mounted on the front wheels of a trailer head (2-DD tractor) and run on dry asphalt-surfaced roads for 20000 km. Then, as shown in Fig.5, the maximum (d) of the uneven wear in the inner rib was measured. Tire size: 285/75R24.5 Rim size: 8.25X24.5 Inner pressure: 7.65 kgf/sq.cm Carrying load: about 20 tons As described above, in the heavy duty pneumatic tires according to the present invention, the axially inside corner of the narrow rib is chamfered and the cross sectional shape is specifically limited. Therefore, the axially inner rib adjacent the narrow rib is effectively prevented from being worn unevenly. Especially, in the initial stage of tread wear life, rubber tears and cracks of the narrow rib on which the wear is to be concentrated, can be prevented and the prevention of the uneven wear can be further improved.

Claims: A heavy duty pneumatic tire comprising a tread portion (12) provided with an axially outer longitudinal main groove (3) and an axially inner longitudinal narrow groove (6), each extending circumferentially of the tire to define a narrow rib (5) therebetween, characterised in that the axially inner edge of said narrow rib is chamfered by a slope (7,7S), the slope extending from the top surface to the axially inner surface of the narrow rib defining the upper and lower edges (Pu,Pd) of the slope. A heavy duty pneumatic tire according to claim 1, characterised in that the axial width of the axially inner longitudinal narrow groove (6) is in the range of from 0.5 to 2.5 mm. A heavy duty pneumatic tire according to claim 1 or 2, characterised in that the axial width (W3) of the slope measured between the upper and lower edges (Pu,Pd) is in the range of from 0.3 to 0.8 times the axial width (W2) of the narrow rib, and the radial height (H2) of the slope measured between the upper and lower edges is in the range of from 0.3 to 0.8 times the depth (H1) of the axially inner longitudinal narrow groove. A heavy duty pneumatic tire according to claim 1, 2, or 3, characterised in that in the tire meridian section, said slope (7) extends straight from the upper edge to the lower edge. A heavy duty pneumatic tire according to claim 1, 2, or 3, characterised in that in the tire meridian section, said slope (7S) is concave. A heavy duty pneumatic tire according to claim 5, characterised in that in the tire meridian section, said slope (7S) consists of a radially outer straight part and a radially inner concave part. A heavy duty pneumatic tire according to claim 5, characterised in that in the tire meridian section, said slope (7S) consists of a radially outer straight part, a radially inner straight part, and a concave part therebetween. The heavy duty pneumatic tire according to claim 7, characterised in that in the tire meridian section, said slope (7S) consists of a single concave part. A heavy duty pneumatic tire according to claim 5, 6, 7, or 8, characterised in that in the tire meridian section, the axial distance (W5) between the lower edge and the intersection of two tangents (Tu and Td) drawn to the slope at the upper and lower edges is not more than the axial width (W3) between the upper and lower edges, and the concave part has a single radius (R1) of not more than 1.5 times the axial width (W3) between the upper and lower edges.

Inventors: FUKUMOTO TETSUHIRO, OUYA YUKIHIDE, UEYOKO KIYOSHI, FUKUMOTO, TETSUHIRO, OUYA, YUKIHIDE, UEYOKO, KIYOSHI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1997
Main classification: B60C11
IPC codes: B60C11