Heavy duty tyre

Abstract: A heavy duty tyre comprises a pair of axially outermost circumferential rows (R3) of shoulder blocks (5c) disposed in a tread (2), characterised in that each of the shoulder blocks (5c) is provided with a circumferentially extending narrow groove (9), the narrow groove (9) subdividing the shoulder block (5c) into an axially inner wide main part (6) and an axially outer narrow lateral part (7), and the narrow groove (9) being curved convexly so that said narrow lateral part (7) is narrower in a middle region (10) than both circumferential end regions (11) in the circumferential direction.

Description: The present invention relates to a heavy duty tyre having an improved tread structure capable of preventing uneven wear. In recent years, tread patterns comprising blocks such as block pattern, rib-block pattern and the like are widely used in not only passenger car tyres but also heavy duty tyres such as pneumatic tyres for trucks, buses and the like. In the heavy duty tyres, the load of each block is relatively heavy, and accordingly uneven wear is very liable to occur. For example, uneven wear between the axially outer part and the inner part of a block caused by a difference in the rolling diameter of the tyre is liable to occur in the axially outer blocks such as the shoulder blocks. This type of uneven wear is called shoulder wear. Uneven wear at the circumferential edges (heel and toe) of a block is called heel and toe wear, and this type of uneven wear occurs in every block, but the degree is higher in the axially outer blocks such as shoulder blocks. In order to counter the shoulder wear, it has been proposed to subdivide each shoulder block into an axially outer narrow width part and an axially inner wide part by a narrow groove extending straight in the tyre circumferential direction so as to concentrate the wear on the axially outer narrow width part. In this proposition, however, the heel and toe wear can not be improved, and as the rigidity of the axially outer narrow width part is low, this part is liable to be torn off. Therefore, it is an object of the present invention to provide a heavy duty tyre in which the heel and toe wear as well as shoulder wear is effectively improved and the resistance to tear-off is increased. According to the present invention, a heavy duty tyre comprises a pair of axially outermost circumferential rows of shoulder blocks, each of the shoulder blocks provided with a circumferentially extending narrow groove, the narrow groove subdividing the shoulder block into an axially inner wide main part and an axially outer narrow lateral part, wherein the narrow groove is curved convexly so that the narrow lateral part is narrower in a middle region than in both circumferential end regions in the circumferential direction. Preferably, the narrow groove has a width in the range of from 1.5 to 3.0 mm. The narrow lateral part has a minimum axial width in the range of from 5 to 8 % of the average axial width of the shoulder block, and a maximum axial width in the range of from 7 to 12 % of the average axial width. Embodiments of the present invention will now be described in detail in conjunction with the accompanying drawings: Fig.1 is a developed view showing a tread pattern according to the present invention; Fig.2 is a cross sectional view taken along a line A-A in Fig.1; Fig.3 is a plan view of a shoulder block showing an example of the narrow groove; Fig.4 is a plan view of a shoulder block showing another example of the narrow groove; and Fig.5(A) and 5(B) to Fig.7(A) and 7(B) are graphs showing test results; According to the present invention, a heavy duty tyre 1 is provided in the tread 2 with at least two circumferential grooves 3 extending continuously in the tyre circumferential direction and axial grooves 4 extending therefrom to the tread edges E. In this embodiment, the tyre is a radial ply tyre of size 11R22.5 for trucks and buses. In a meridian section of the tyre, the tread 2 has a single radius of curvature TR as shown in Fig.2. In Fig.1, The circumferential grooves 3 include a pair of axially outermost grooves 3c, a pair of axially inner grooves 3b, and a central groove 3a on the tyre equator C. Each circumferential groove 3 is a zigzag groove. But various groove shapes, e.g. straight grooves, wavy grooves and the like can be used. The axial grooves 4 include axially outer grooves 4c extending between the outermost circumferential grooves 3c and the tread edges E, axially inner grooves 4a extending between the central circumferential groove 3a and the inner circumferential grooves 3b, and middle grooves 4b extending between the inner circumferential grooves 3b and the outermost circumferential grooves 3c. By the circumferential grooves 3 and axial grooves 4, a block pattern made up of a plurality of blocks 5 is formed in the tread 2. These blocks 5 are arranged in a plurality of circumferential rows comprising two rows R1 of inner blocks 5a between the axially inner grooves 3b, two rows R2 of middle blocks 5b between the axially outermost grooves 3c and the axially inner grooves 3b, and two rows R3 of shoulder blocks 5c axially outside the axially outermost grooves 3c. Preferably, the circumferential grooves 3 and axial grooves 4 have a width of at least 2 % of the tread width TW between the tread edges E. Here, the tread width TW is the maximum axial width of the ground contacting area with the tyre in a standard condition in which it is mounted on a standard rim, inflated to standard load and then loaded with 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 "Tyre 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. Each of the shoulder blocks 5c is provided with a narrow groove 9, whereby the shoulder block 5c is subdivided into an axially inner wide main part 6 and an axially outer narrow lateral part 7. The narrow groove 9 has a width in the range of from 1.5 to 3.0 mm (in this embodiment about 2.0 mm) and a depth GD1 in the range of not less than 30 %, preferably 50 to 100 %, more preferably 55 to 80 % of the depth GD2 of the axially outermost circumferential grooves 3c. Both the circumferential ends thereof are opened to the outer axial grooves 4c circumferentially adjacent to the shoulder blocks 5c. Fig.3 shows enlarged an example of the narrow groove 9, wherein the narrow groove 9 comprises a central convex part 9A and a pair of circumferential end parts 9B. The convex part 9A is curved convexly towards the axially outside of the tyre tread, and the axially outermost point T (defined on the groove centre line) lies in the middle range X of the circumferential length L of the block. Therefore, the narrow lateral part 7 has a narrow width part 10 in the middle range X and wide parts (hereinafter heel part 11 and toe part 12) in the circumferential ends. Here, the middle range X is, as shown in Fig.3, a 30 % length range centred on the circumferential midpoint P. Preferably, the convex part 9A has a radius of curvature Rw (defined by the groove centre line) in the range of from 0.8 to 2.0 times the average width of the shoulder block. Further, the convex part 9A has a circumferential length Lw not less than 50 %, preferably not less than 60 % of the circumferential length L of the narrow groove 9 (in this embodiment about 80 %). Accordingly, the rigidity of the narrow lateral part 7 varies in the tyre circumferential direction. Therefore, even if a large tangential force is applied to the top of the shoulder block 5c when contacting or leaving the ground for example, excessive deformation and tear-off of the narrow lateral part 7 can be avoided because the wide heel part 11 and toe part 12 can resist such deformation and tear-off, and the narrow groove 9 can be closed to increase the apparent rigidity of the narrow lateral part 7. Further, wear energy concentrates on the narrow lateral part 7 and wear energy to the main part 6 decreases and uneven wear as a whole is improved. If the minimum axial width Wmin in the narrow part 10 is too small, such advantageous effects can not be obtained. Therefore, the minimum axial width Wmin is preferably set in the range of from 5 to 8 % of the average axial width of the shoulder block 5c. In Fig.3, the wide heel part 11 and toe part 12 have a constant axial width W because the circumferential end parts 9B of the narrow groove 9 and the axially outer edge E of the shoulder block 5c are substantially parallel with the tyre circumferential direction. Fig.4 shows another example of the narrow groove 9. In this example, the heel part 11 and toe part 12 have a variable axial width gradually increasing towards the respective circumferential ends. In this case, the circumferential end parts 9B of the narrow groove 9 are oblique and preferably formed in a linear shape or slightly concavely curved shape. Preferably, the axial width W of the narrow lateral part 7 measured at the circumferential ends (thus which is usually a maximum width) is set in the range of from 7 to 12 % of the above-mentioned average width of the shoulder block and more than the above-mentioned minimum width Wmin. If more than 12 %, the wear energy to the main part 6 increases. If lass than 7 %, the rigidity decreases and the above-mentioned effects can not be obtained. In the example shown in Fig.1, the above-mentioned narrow grooves 9 are provided on only the axially outermost shoulder blocks 5c. However, it is possible to further provide the narrow grooves 9 on the second outermost blocks (in this example middle blocks 5b) such that that the narrow lateral part 7 faces the outermost circumferential groove 3c. Comparison Test Truck/bus radial tyres of size 11R22.5 (Wheel rim: 22.5X7.50) having the same tread pattern shown in Fig.1 except for the narrow grooves were made as test tyres and tested for wear energy. The example tyre was provided with narrow grooves 9 as shown in Fig.1. Reference tyre 1 was provided with straight narrow grooves instead of the curved narrow grooves 9. Reference tyre 2 was not provided with the narrow groove. Using a combination sensor capable of measuring a shear stress and slip and their directions, the shear stress and slip were measured at the heel and toe of various blocks (measuring points are indicated in Fig.1 as 1 ○ to ), and the wear energy was computed by multiplying these values. The test conditions are as follows: Slip angle: 0 and 1 degrees Inner pressure: 850 kPa The results are shown in Fig.5(A)-5(B) to Fig.7(A)-7(B). From the test results, it was confirmed that the tyre according to the present invention was decreased in the difference in wear energy between the heel and toe and thus the wear energy was made more even in the circumferential direction. Further, the average of the wear energies at the above-mentioned twelve measuring points, and the ratio of the average of the wear energies at the six toe-side measuring points to the average of the wear energies at the six heel-side measuring points were computed. The results are shown in the following Table 1. Tyre Ex. Ref.1 Ref.2 Average wear energy (J/sq.m) Slip angle 0 deg. 295.9 278.7 326.2 Slip angle 1 deg. 1011.4 1180.8 1081.6 Wear energy ratio 1.5 1.56 1.8 The Example tyre was slightly increased in average wear energy in comparison with Reference tyre 1, but the wear energy ratio was effectively reduced. Therefore, the Example tyre was improved in wear resistance in its entirety.

Claims: A heavy duty tyre comprising a pair of axially outermost circumferential rows (R3) of shoulder blocks (5c) disposed in a tread (2), characterised in that each of the shoulder blocks (5c) is provided with a circumferentially extending narrow groove (9), the narrow groove (9) subdividing the shoulder block (5c) into an axially inner wide main part (6) and an axially outer narrow lateral part (7), and the narrow groove (9) being curved convexly so that said narrow lateral part (7) is narrower in a middle region (10) than both circumferential end regions (11,12) in the circumferential direction. A heavy duty tyre according to claim 1, characterised in that the minimum axial width (Wmin) of said narrow lateral part (7) in the middle region (10) is in the range of from 5 to 8 % of the average axial width of the shoulder block (5c). A heavy duty tyre according to claim 1, characterised in that said narrow lateral part (7) has a minimum axial width (Wmin) in the range of from 5 to 8 % of the average axial width of the shoulder block, and a maximum axial width in the range of from 7 to 12 % of the average axial width. A heavy duty tyre according to claim 1, 2 or 3, characterised in that the narrow groove (9) comprises a central convexly curved part (9A) and a pair of circumferential end parts (9B), and the circumferential end parts (9B) are substantially straight. A heavy duty tyre according to claim 1, 2 or 3, characterised in that the narrow groove (9) comprises a central convexly curved part (9A) and a pair of circumferential end parts (9B), and the circumferential end parts (9B) are concavely curved. A heavy duty tyre according to claim 4 or 5, characterised in that the circumferential end parts (9B) are inclined so that the axial width of the narrow lateral part gradually increases towards both the circumferential ends thereof. A heavy duty tyre according to claim 4 or 5, characterised in that the circumferential end parts (9B) extend substantially parallel with the circumferential direction. A heavy duty tyre according to claim 4 or 5, characterised in that the circumferential length (LW) of the convexly curved part is not less than 50% of the circumferential length (L) of the narrow groove (9). A heavy duty tyre according to claim 4 or 5, characterised in that the convexly curved part (9A) has a radius of curvature (Rw) in the range of from 0.8 to 2.0 times the average axial width of the shoulder block. A heavy duty tyre according to any of claims 1 to 9, characterised in that the narrow groove (9) has a width in the range of from 1.5 to 3.0 mm.

Inventors: MATSUURA SHINICHI, TSUDA SATOSHI, MATSUURA, SHINICHI, TSUDA, SATOSHI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1999
Main classification: B60C11
IPC codes: B60C11