Heavy duty tyre

Abstract: A heavy duty tyre, in which a tyre noise is reduced without deteriorating the traction performance and tread durability, and which comprises a tread (12) provided with a plurality of circumferential grooves (3) and a plurality of axial grooves (4), each having a width (WL, WS) of from 1.0 to 15 % of the tread width (WT), the circumferential grooves (3) extending continuously in the circumferential direction of the tyre so as to axially divide the tread into rib portions, each axial groove (4) extending across one of the rib portions so as to have at least one end opened to one of the circumferential grooves (3), in a state where the tyre is mounted on its regular rim (J) and inflated to its regular inner pressure and then loaded by its regular load, the ratio (ΣVh/ΣVv) of the total volume (ΣVh) of the axial grooves (4) in the ground contacting part (S) of the tread to the total volume (ΣVv) of the circumferential grooves (3) in the ground contacting part (S) being in the range of from 0.3 to 0.6.

Description: The present invention relates to a pneumatic tyre, more particularly a heavy duty tyre in which tyre noise is reduced without deteriorating traction performance and tread durability. In heavy duty tyres, e.g. truck/bus tyres and the like, rib-type tread patterns are widely used because of the heavy service loads. However, in order to improve the traction performance, it is also known to use a tread pattern in which axial grooves are provided in addition to the circumferential grooves thereby defining blocks. During running, through such axial grooves, the air compressed between the tread surface and the road surface is discharged. As a result, a tyre noise which is so-called air-pumping noise is generated. On the other hand, tyre noise increases with increasing running speed. However, noise is important to improve the earth environment to reduce tyre noise. In order to reduce the noise from a tyre tread pattern, it has been proposed to decrease the number of axial and circumferential grooves, and to decrease the total volume of such grooves. These means can decrease the above-mentioned air-pumping noise, but other running performance characteristics, especially traction performance are generally decreased. Thus, noise reduction and improvement in traction performance are in an antinomic relationship. A tyre according to the preamble of claim 1 is known, for example, from EP-A-0422571. It is therefore, an object of the invention to provide a heavy duty tyre, in which tyre noise, especially air-pumping noise is reduced without deteriorating the traction performance, durability and the like. According to the present invention, a heavy duty tyre comprising a tread portion, said tread being provided with a plurality of circumferential grooves, and a plurality of axial grooves, said circumferential grooves extending continuously in the circumferential direction of the tyre so as to axially divide the tread into rib portions, each said axial groove extending across one of said rib portions so as to have at least one end opened to one of said circumferential grooves to divide the rib portion into tread blocks, wherein in the state where the tyre is mounted on its regular rim and inflated to its regular inner pressure and loaded with its regular load, the ratio (ΣVh/ΣVv) of the total volume (ΣVh) of said axial grooves in the ground contacting part of the tread to the total volume (ΣVv) of the said circumferential grooves in the ground contacting part is in the range of from 0.3 to 0.6, characterised in that the circumferential grooves have a width of from 1.0 to 15 % of the tread width measured between the edges of the tread and the axial grooves have a width of from 1.0 to 15 % of the tread width, said circumferential grooves and said axial grooves defining a plurality of crosses each surrounded by corners of tread blocks, all the corners of said crosses of at least one of the circumferential grooves being obtuse-angled corners, and all said obtuse-angled corners in each of the crosses are disposed at different positions with respect to both the circumferential direction and axial direction of the tyre. An embodiment 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 a tyre according to the present invention; Fig.2 is a developed plan view showing part of the tread pattern thereof; Fig.3 is a view for explaining the groove volume ratio; Fig.4 is an enlarged plan view showing a cross of one circumferential groove and two axial grooves; Fig.5 is an enlarged perspective view showing the cross of Figure 4; Fig.6 is a graph showing the relationship between the groove volume ratio and noise level; and Fig.7 is a plan view showing a tread pattern used in comparison tests. In Figs.1 to 4, the heavy duty tyre 1 according to the invention is a truck/bus radial tyre of size 11R22.5, which has a tread portion 12, a pair of axially spaced bead portions 14, and a pair of sidewalls 13 extending between the tread edges and the bead portions 14. The tyre 1 further comprises a pair of bead cores 15 one disposed in each of the bead portions 14, a carcass 16 extending between the bead portions 14 and turned up around the bead cores 15 from the axially inside to outside of the tyre, and a belt 17 disposed radially outside the carcass 16 and inside the tread portion 12. The carcass 16 comprises at least one ply, in this embodiment one ply, of cords arranged at an angle of 70 to 90 degrees with respect to the tyre equator C to provide a radial or semiradial carcass arrangement. For the carcass cord, organic fibre cords, e.g. nylon, polyester, aromatic polyamide or the like, or steel cords can be used. The belt 17 in this embodiment comprises four plies, each composed of parallel cords inclined with respect to the tyre equator C. For the belt cords, steel cords and organic fibre cords, e.g. nylon, polyester, aromatic polyamide or the like can be used. The tread portion 12 has a tread surface 2 having a plurality of circumferential grooves 3 extending circumferentially of the tyre and a plurality of axial grooves 4 extending in a direction crosswise to the circumferential grooves 3. In the present invention, each circumferential groove 3 is a groove extending continuously in the circumferential direction of the tyre, and each axial groove 4 is a groove extending between the adjacent circumferential grooves or between the axially outermost circumferential groove and the adjacent tread edge E, at least one end of which is opened to one of the circumferential grooves 3. In this embodiment, as shown in Fig.2, the circumferential grooves 3 comprise a pair of axially inner circumferential grooves 3A each disposed on each side of the tyre equator C; a pair of middle circumferential grooves 3B each disposed axially outside each of the inner circumferential grooves 3A; and a pair of outer circumferential grooves 3C each disposed between each of the middle circumferential grooves 3B and the adjacent tread edge E. The axial grooves 4 in this embodiment comprise axially inner axial grooves 4A extending from the inner circumferential groove 3A to the middle circumferential groove 3B, both the ends of which are opened to the circumferential grooves 3A and 3B; middle axial grooves 4B extending from the middle circumferential groove 3B to the outer circumferential groove 3C, both the ends of which are opened to the circumferential grooves 3B and 3C; and outer axial grooves 4C extending from the outer circumferential groove 3C to the tread edge E, both the ends of which are opened to the circumferential groove 3C and the tread edge E. The centre rib 5 defined between the inner circumferential grooves 3A is not provided with such axial grooves extending across the entyre width of the centre rib, but indent portions 6 or short axial grooves each of which has an axially inner end terminated in the centre rib and an axially outer end opened to the inner circumferential groove. Grooves 3A are provided on both sides of the centre rib. The widths WL of the circumferential grooves 3A, 3B and 3C are in the range of from 1.0 to 15 % of the tread width WT. In this embodiment, the middle circumferential grooves 3B are wider than the inner and shoulder circumferential grooves 3A and 3C. The depths H1 of the circumferential grooves 3A, 3B and 3C are in the range of from 6 to 16 % of the tread width WT. In this embodiment, all the circumferential grooves 3A, 3B and 3C have substantially the same depth. The widths WS of the axial grooves 4A, 4B and 4C are in the range of from 1.0 to 15 % of the tread width WT. The depths H2 of the inner and middle axial grooves 4A and 4B are in the range of from 6 to 16 % the tread width WT. In this embodiment, the depths H2 are not larger than the depth H1 of the circumferential grooves 3. The depth H3 of the outer axial grooves 4C are less than 25% and not less than 10% of the depth H2 of the axial grooves 4A and 4B, (10%≤H3<25%) so that the discharge of the compressed air towards both sides of the tyre is decreased to reduce the noise level, and further the circumferential rigidity of the tread shoulder portion increases to prevent the occurrence of uneven wear such as heel and toe wear. If the depth H3 is less than 10% of the depth H2, the water drainage is insufficient, and wet road grip is liable to be lost. Here, the tread width WT is defined as the axial distance between the tread edges E. If the groove widths WL and WS are less than 1.0 % of the tread width WT, the drainage and the wet grip performance of the tyre are insufficient. If the widths are more than 15%, the ground contact area is decreased, and as a result, tread wear is increased and uneven wear is liable to occur along the groove edges. For a tyre size of 11R22.5, the widths WL and WS are at least 2 mm. In the ground contact part S (shown in Fig.3 by the chain line) of the tread surface 2 with the tyre in its normal loaded state, the ratio between the total volume of the circumferential grooves 3 and the total volume of the axial grooves 4 is specifically defined by this invention. Here, the normal loaded state is such that the tyre 1 is mounted on its regular rim J and inflated to its regular pressure and then loaded by its regular load. The regular rim is the rim officially approved for the tyre by ETRTO (Europe), JATMA (Japan), TRA (USA) and the like, and the regular inner pressure and the regular load are the maximum air pressure and the maximum tyre load for the tyre officially specified in the Air-pressure/Maximum-load Table by the same associations. According to the present invention, the ratio ΣVh/ΣVv of the total volume ΣVh of the axial grooves 4 in the ground contact part S to the total volume ΣVv of the circumferential grooves 3 in the ground contact part S is set in the range of from 0.3 to 0.6, and as a result the noise is reduced without sacrificing traction performance. If the ratio ΣVh/ΣVv is more than 0.6, the noise level is increased. If the ratio ΣVh/Vv is less than 0.3, the grip performance is insufficient. In Fig.3, for example, the total circumferential groove volume ΣVv is the total of the volume (V3A) of the shaded part of the inner circumferential grooves 3A, the volume (V3B) of the shaded part of the middle circumferential grooves 3B, and the volume (V3C) of the shaded part of the outer circumferential grooves 3C. The total axial groove volume ΣVh is the total of the volume (V4A) of the shaded part of the inner axial grooves 4A, the volume (V4B) of the shaded part of the middle axial grooves 4B, the volume (V4C) of the shaded part of the outer axial grooves 4C, and the volume (V4D) of the shaded part of the indent portions 6. In order to set the groove volume ratio ΣVh/ΣVv in the above-mentioned range, the depth H2 and/or depth H3 of the axial grooves 4 are preferably selected, so then whereby noise generation is most effectively prevented without loss of the running performance properties of the tyre. In this embodiment, the tread pattern has point symmetry about a point on the tyre equator. All the circumferential grooves 3 are slightly zigzagged, and each circumferential groove 3 comprises alternately arranged long straight segments and sort straight segments. The long segments of all the circumferential grooves 3 have the same length, and they are inclined at a small angle Θ of 2 to 10 degrees with respect to the tyre circumferential direction. The long segments of the inner circumferential groove 3A are inclined in the same direction as the outer circumferential groove 3C, but in the middle circumferential groove 3B they are inclined in the reverse direction. All the axial grooves 4 are generally straight, and inclined at a small angle of 0 to 30 degrees with respect to the tyre axis. All the inner and middle axial grooves 4A and 4B are inclined in the same direction, but all the shoulder axial grooves 4C are inclined in the reverse direction. The inner and middle axial grooves 4A and 4B extend completely across the respective ribs defined between the inner, middle and outer circumferential grooves 3A, 3B and 3C, whereby the above-mentioned two ribs on each side of the tyre equator are circumferentially subdivided into a row of inner blocks 10A and a row of middle blocks 10B, respectively. The outer axial grooves 4C also extend completely across the ribs defined between the outer circumferential grooves 3D and the tread edges E, whereby each of the ribs is circumferentially subdivided into a row of shoulder blocks 10C. The axially outer ends of the indent portions 6, the axially inner ends of the shoulder axial grooves 4C, and the axially inner and outer ends of the inner and middle axial grooves 4A and 4B are positioned at the short segments of the circumferential grooves 3A, 3B and 3C. The inner axial groove line and the middle axial groove line extend in a line, but the shoulder axial groove line is bent, so that transmission of the noise generated from the axially inward part is hindered to reduce the noise level in use of the tyre. The indent portion 6 is located as an axially inward extension of the inner axial groove line, and the axially inner end thereof is terminated before the tyre equator C. The axial distance L1 therebetween is in the range from 0.02 to 0.04 times the tread width WT. If the distance L1 is less than 0.02 times the tread width WT, the volume of the indent portions 6 is too large which increases the air-pumping noise level. If the distance L1 is more than 0.04 times the tread width WT, the road grip of the centre rib 6 is decreased, and the traction performance, especially that on snow covered roads is decreased. When the axial grooves 4 cross diagonally the circumferential groove 3, acute-angled corners DA and obtuse angled corners DB are formed as a cross (7A, 7B, 7C). The formation of acute angled corners is not preferable since uneven wear is liable to start spreading therefrom. In this embodiment, therefore, such acute angled corners are rounded off, as shown in Figs.4 and 5, by providing straight sides 9L intersecting the adjacent straight sides 3L and 4L with obtuse angles. Accordingly, all the resultant corners P1, P2 ---P5 and P6 in each cross (7A, 7B, 7C) are obtuse corners, whereby the corners are increased in rigidity and the spreading of uneven wear is suppressed. Further, such rigid corners effectively improve the traction performance. Further, in this embodiment, all the corners P1, P2 --- P5 and P6 in each cross (7A, 7B, 7C) are disposed at different positions with respect to both the circumferential and axial directions of the tyre. As shown in Fig.4, between the corners P1 -- P6, spaces m1, m2, m3, m4, m5 are provided in the circumferential direction, and spaces n1, n2, n3, n4, n5 are provided in the axial direction. As a result, the traction performance is improved. Furthermore, as the above-mentioned straight sides 3L are inclined at an angle of from 2 to 10 degrees with respect to the tyre equator, the traction performance is further improved. In the present invention, in addition to the above-mentioned circumferential grooves 3 and axial grooves 4, the tread surface 2 may be provided with narrow grooves or slits so-called sipes whose width is about 1-2 mm or less and at least one end is opened. If the total volume of such sipes in the ground contacting part is less than 5% of the volume of the land part of the tread, such sipes do not affect the air-pumping noise, so the sipe volume is disregarded in calculating the volume ratio ΣVh/ΣVv. Tyres of size 11R22.5 having the construction shown in Fig.1 were made by way of test and the following tests were carried out. The test results are shown in Table 1. A) Noise test The noise test was carried out according to JIS C606 (Japanese Industrial Standard). Coasting a test vehicle provided with the test tyre on a straight test course at a speed of 70 km/H for 50 metres, the maximum level in dB(A) of the passing noise sound was measured with a microphone set in the middle of the course at a position 7.5 metres from the centre line of the course at a height of 1.2 meter from the road surface. The test tyre was mounted on its regular rim of size 7.5X22.5 and inflated to an inner pressure of 7.25kg/cm 2 . The tyre load was 2000kgf per a tyre. B) Traction performance test A gradeability test was carried out as a traction performance test. The results are indicated in an index based on that the reference tyre is 100. The larger the value, the better the traction. C) Wear resistance test The amount of tread wear after running 100,000 kilometres was measured. The results are indicated by an index based on the reference tyre being 100. The larger the value, the better the resistance. Ex.1 Ref.1 Tread pattern Fig.2 Fig.7 Ground contacting area (cm 2 ) 450 450 Total circumferential groove volume ΣVv (cm 3 ) 210 221 Total axial groove volume ΣVh (cm 3 ) 113 181 ΣVh/ΣVv 0.44 Noise dB(A) 77.1 78.5 Traction performance (index) 100 100 Wear resistance (index) 125 100 As described above, the heavy duty tyre of the present invention can be reduced in running noise without sacrificing traction performance and durability.

Claims: A heavy duty tyre comprising a tread portion (12), said tread (12) being provided with a plurality of circumferential grooves (3), and a plurality of axial grooves (4), said circumferential grooves (3) extending continuously in the circumferential direction of the tyre so as to axially divide the tread into rib portions, each said axial groove (4) extending across one of said rib portions so as to have at least one end opened to one of said circumferential grooves to divide the rib portion into tread blocks, wherein in the state where the tyre is mounted on its regular rim and inflated to its regular inner pressure and loaded with its regular load, the ratio (ΣVh/ΣVv) of the total volume (ΣVh) of said axial grooves (4) in the ground contacting part of the tread to the total volume (ΣVv) of the said circumferential grooves (3) in the ground contacting part (S) is in the range of from 0.3 to 0.6, characterised in that the circumferential grooves (3) have a width of from 1.0 to 15 % of the tread width (WT) measured between the edges of the tread and the axial grooves (4) have a width of from 1.0 to 15 % of the tread width (WT), said circumferential grooves (3) and said axial grooves (4) defining a plurality of crosses (7A,7B,7C) each surrounded by corners of tread blocks, all the corners (P1,P2,P3,P4,P5,P6) of said crosses of at least one of the circumferential grooves being obtuse-angled corners, and all said corners (P1,P2,P3,P4,P5,P6) in each of the crosses (7A,7B,7C) are disposed at different positions with respect to both the circumferential and axial directions of the tyre. A heavy duty tyre according to claim 1, characterised in that said circumferential grooves (3) include a pair of inner circumferential grooves (3A) each disposed on each side of the tyre equator, a pair of middle circumferential grooves (3B) each disposed axially outward of each said inner circumferential groove, and a pair of outer circumferential grooves (3C) each disposed between each said middle circumferential groove (3B) and the adjacent tread edge (E), said axial grooves (4) include inner axial grooves (4A) extending from the inner circumferential grooves (3A) to the middle circumferential grooves (3B), middle axial grooves (4B) extending from the middle circumferential grooves (3B) to the outer circumferential grooves (3C), and outer axial grooves (4C) extending from the outer circumferential grooves (3C) to the tread edges (E). A heavy duty tyre according to claim 2, characterised in that a centre rib (5) defined between the inner circumferential grooves (3A) is continuous in the circumferential direction of the tyre, said centre rib (5) is provided on both sides thereof with indent portions (6) aligned with the axial grooves (4A) and each defining the axial groove (4) of which volume (V4D) is to be included in said total volume (ΣVh), each of the indent portions (6) having an axially inner end terminated before the tyre equator by a distance L1 of 0.02 to 0.04 times the tread width (WT). A heavy duty tyre according to claim 1 or 2, characterised in that the axial grooves (4) which comprise axially outward axial grooves (4C) extending from the tread edges (E) to the adjacent circumferential grooves (3C) and axially inward axial grooves (4A) extending axially inwardly from said adjacent circumferential grooves, and the depth (H3) of said axially outward axial grooves (4C) is not less than 10% and less than 25% of the depth of said axially inward axial grooves (4A). A heavy duty tyre according to claim 2, characterised in that the depth (H3) of the outer axial grooves (4C) is not less than 10% and less than 25% of the depth of the middle axial grooves (4B). A heavy duty tyre according to any one of the preceding claims, characterised in that each of said circumferential grooves (3) comprises alternate first inclined segments and second segments so as to provide a zigzag configuration, and said first segments are inclined at an angle of from 2 to 10 degrees with respect to the tyre equator. A tyre according to any of claims 1 to 6, characterised in that each of said plurality of crosses (7A,7B,7C) are defined by two axial grooves each having one end opened to the same circumferential groove such that the plurality of crosses (7A,7B,7C) are each surrounded by the corners of four tread blocks. A tyre according to claim 7, characterised in that the two axial grooves defining the cross are substantially straight grooves. A tyre according to claim 8, characterised in that the two axial grooves are substantially arranged in line with each other.

Inventors: FUJIWARA KENICHI, FUJIWARA, KENICHI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1993
Main classification: B60C11
IPC codes: B60C11