PNEUMATIC TYRE

Abstract: High performance tire, in which a plurality of narrow cuts are disposed on both shoulder parts where the heat generation is most remarkable, to control the heat generation by decreasing the pattern stiffness. The slippage of tread rubber is prevented due to improved grounding performance of both shoulder parts, so the high-speed durability and the stepped shoulder wear are improved in spite of the extra heavy tread with deepest longitudinal main grooves. At the same time, wear life of tread, wandering performance including wet grip performance, ride feeling and handling stability can effectively improved in well-balanced manner.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to heavy duty pneumatic radial ply tires suitable for use in heavy vehicles such as truck, bus and the like, and more particularly to an improvement of a pneumatic tire provided with a tread pattern divided by two main longitudinal grooves having deepest depth extending to the circumferential direction of said tread into a tread center portion and both shoulder portions in which rib or block pattern is provided thereon. 2. Prior art and problems thereof In general, it has been well known that the radial tire has several merits such as wear resistance of tread rubber, heat generation during high speed traveling, puncture and the like as compared with the conventional bias tires. In consequence of the remarkable improvement of highways networks, the demand for the heavy duty radial tires has been rapidly increased. On the other hand, the performance of vehicles has been improved, so that traveling distance of vehicles or cars have been considerably spread in conjunction with the development of the highways. Furthermore, when the length of highways grow longer, not only the chance of continuous traveling for a long period of time at high speed has been increased, but also the requirement for the tire having long wear life has been rapidly conspicuous from the economic reason. However, there has been fatal problems that the means according to the prior art to solve these problems are all antinomic to each other in that a tire designed to weigh heavily in durability to high speed running is inevitably followed by short life because of wear and, on the contrary, a tire weighted in resistance to wear and long life is insufficient in durability to high speed running. For instance, the means of the prior art for achieving improvement in durability to high speed running have been such means as the employment of a tread small in gauge for the need to decrease the heat generation. To explain it concretely, a tire in size of 10,00 R 20 having a maximum speed capability of 80 km/h is 20.6mm in maximum groove depth, on the contrary, a tire having a maximum speed capability of 100 km/h is 13.5 mm in maximum groove depth which is the category of "highway depth", thus, to compared with them, the difference in tread gauge is 7 mm at the difference of 20 km/h in maximum speed, the wear life of tread is reduced in keeping with the difference in tread gauge, accordingly the wear life of such tires having thin tread gauge is shortened as an inevitable consequence. However, the user of tires cannot be satisfied with such unbalanced tire performance for any length of time, the requirement for well balanced tire, which has a long life and an improved high speed durability, has become conspicuous of late. That is, it is required to increase the durability of such tire as the depth of longitudinal main grooves is deeper than that of the conventional tire and the thickness of tread gauge is thicker than that of the conventional one. Furthermore as the problem inherent to a tire having a carcass ply of radial construction, there is a problem of wear which causes generation of steps extending transversely in the cross sectional direction of the tire and particularly causes the tread rubber facing both the shoulder portions to wear more readily than the tread rubber facing the center portion which is called uneven shoulder wear. Many attempts such as means to increase the tread radius or to decrease the tread width have been made to avoid uneven shoulder wear of the tire, but hitherto none has let to fully satisfactory results which can effectively avoid uneven shoulder wear of the tire, by which the wear life of the tire is caused to be greatly shortened. A tyre according to the preamble of claims is disclosed in the Applicant's EP-A-0197735 (published 15.10.86 and cited under Art. 54(3) EPC). BRIEF SUMMARY OF INVENTION It is an object of the present invention to provide a pneumatic radial tire improved in both the resistance to wear and the high speed durability at the same time together with the improvement in the maneuverability and the ride feeling in a pneumatic radial ply tire suitable for use in heavy duty vehicles such as truck, bus and the like. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a developed plane view of a tread pattern having a series of narrow cut grooves at shoulder parts according to the present invention; Fig. 2 is a view showing a cross sectional structure of the steel radial tire to be used for truck and bus; Fig. 3 (a) is a developed plane view of a tread pattern in which the index of circumferential stiffness satisfies a value specified by the present invention but the index of groove volume is out of the scope of this claim; Fig. 3 (b) is a developed plane view of a tread pattern having a series of lateral grooves at shoulder parts according to the prior art: Fig.3(c) is a developed plan view of a tread pattern in which the index of transverse stiffness at shoulder parts is lower than that of circumferential stiffness; Fig.4 is a graph showing the relation of rubber gauge with heat generation at the shoulder part with respect to the pattern of Example 1 (Fig.1.) according to the present invention, that of Control Example (shown in Fig.3a and Fig.3b, and that of the conventional tire; Fig.5 is a graph showing the relation between pattern circumferential stiffness and temperature at the shoulder part; Fig.6 is a graph showing the relation between volumes of Cut grooves and temperature; Fig.7 is a graph showing the relation between circumferential stiffness and transverse one; Fig.8 is a graph showing the relation between transverse stiffness and temperature at the shoulder parts; Fig.9 is a graph showing the relation between pattern stiffness and volume of groove; Fig.10 a is a graph showing the relation between cornering power and the aspect ratio of a tire; Fig.10 b is a graph showing the relation between rolling resistance and the aspect ratio; Fig.11 is a view of block sample for describing the secondary moment of the pattern stiffness index at the section of block; and Fig. 12 is a developed plane view of another embodiment of tread pattern according to the present invention. As stated above, it could not be made by the prior art to get so well balanced improvement as to simultaneously increase the durability of high-speed running, resistance to wear (long life) and uneven shoulder wear. However, the inventors of the present invention made various investigations about tread patterns which can decrease the heat generation of such tires as being superior in resistance to tread wear. As the result, they found out the means as described in claim 1 to solve the said problem. That is, in a heavy duty pneumatic radial tire, which comprises at least one carcass ply of a radial construction or a semi-radial construction composed of one rubberized ply layer containing metal cords embedded therein and both edges thereof turned up around the bead cores, a bead apex rubber stock extending radially outwardly from its base adjacent to the bead core in a space between the main portion of the carcass ply and the turned up portion of the carcass ply is disposed, with a reinforcing layer composed of rubberized steel cords disposed outside said turned up portion of the carcass ply and, further, an protective layer composed of an organic fiber cord is disposed adjacent to the outside of said turned up portion, said reinforcing layer and said bead apex rubber stock, and a tread reinforced with a belt layer superimposed about said carcass ply for stiff reinforcement beneath a tread and composed of at least three rubberized ply layers each containing metal cords embedded therein, said metal cords of which being in parallel with each other in each ply and being crossed when grouped into plies with each other at a relatively small angle with respect to the circumferential direction of a tire, and further, sidewalls disposed between both edges of said tread and each beads, the said tread surface contacting with ground is partitioned by two main longitudinal grooves extending to the circumferential direction of a tire in the shape of zigzag or wavy line into a tread crown part having a width corresponding to about 30 to 65% of the tread width and centering at the equatorial plane of the tire and into tread shoulder parts lying on both sides outside said tread crown part; one or more central ribs or rows of central blocks comprising blocks in various shapes are provided on said crown part whereas outer ribs or outer rows of blocks on said both shoulder parts; and a plurality of narrow cut grooves extending to the axial direction of the tire are provided in the ribs or the blocks on said shoulder parts so that the circumferential stiffness index of the pattern and the volume index of the cut groove may be set at 30 to 70 and 2.5 to 15, respectively. Wherein said cut groove is not greater than 6 mm in width and is equal to or smaller than the longitudinal groove in depth and the center line of said cut grooves inclines at an angle in a range from 0 to 45 with respect to the axial direction of the tire and the circumferential pitch length between said cut grooves is within the range under 1.1% length of the circumference of the tire at the center of the tread under inflation at normal maximum internal pressure. The definition of "circumferential pattern stiffness" and "volume index" of the cut grooves will be explained hereinafter. (Circumferential pattern stiffness index) In said shoulder parts, circumferential pattern stiffness, which is in contact length with ground under specified load and inner pressure, is defined by below equations: kp: Pattern stiffness (kg/mm) F: Tangential stiffness (kg/mm) Y: Displacement of pattern (mm) h: Pattern depth (mm) E: Elastic modulus at elongation of tread rubber (kg/mm²) G: Shearing modulus (=E/3) (kg/mm²) I: secondary moment at block section (mm⁴) I = a b 3/12 in the block shown in Fig. 11 A: Sectional area of block (mm²) In the case of actual pattern, the pattern depth is different from the depth h1 of cut groove in the shoulder parts, and several number of blocks are adopted, so it is treated as follow: the pattern stiffness at tread surface, Kps = Kps1 + Kps2 . . . + Kpsn to use h = h1 in equation 1 ○ the pattern stiffness (Kpβ) at base tread part is to use h = ho - h1 (Fig.1) in equation 1 ○ the total pattern stiffness Kpτ the pattern stiffness Kpo in the case of no subgroove is to use h = ho in equation 1 ○. (Volume index of the cut groove) the ratio of the volume of groove to the total volume of shoulder part. Volume index of the cut groove = (Example) An example according to the present invention will be explained with reference to the drawings. A radial tire T for heavy duty vehicles according to the present invention as shown in Fig.2, for reinforcing the region of the turned up portion 2' of the carcass ply 2 composed of steel cords which is turned up around a bead core 7 from inside towards outside, a steel cord reinforcing layer 4 has been arranged in such a way as to extend from outside to inside of the bead region in the axial direction through the bead base, thereby surrounding the bead region. The steel cord reinforcing layer 4 is provided for an exclusive purpose of mitigating the stress concentration on the upper end 2a of the turned up portion 2' of the carcass ply 2 and, accordingly, a structure having the upper end part 4a of the steel cord reinforcing layer 4 placed higher than the upper end 2 of the turned up portion of the carcass ply 2 has been employed. On the other hand, in order to more effectively prevent withdrawal of the carcass ply 2, a structure has also been employed in which the upper end 4a of the steel cord reinforcing layer 4 is kept lower, while relatively raising the upper end 2a of the turned up portion of the carcass ply 2 (Fig. 2), as to the prevention of the withdrawal of the carcass ply, this arrangement is effective, but the crack is liable to occur at the end, this is because the end of the turned up portion of the carcass ply is higher and the stress concentration is liable to occurs at the upper end 2a of the turned up portion of the carcass ply, in order to alleviate the stress concentration, a fiber cord reinforcing layer 6 can be arranged further outside and adjacent to the steel cord reinforcing layer 4. A apex rubber E composed of a single rubber stock or two kinds of rubber stock is disposed between the main portion 2 of the carcass ply and the turned up portion 2', extends outwardly radially from the bead core 7 and decreases in thickness towards the sidewall of the tire to form a substantially triangular section. And a belt 3 superimposed about said carcass 2 beneath a tread 5 and composed of a plurality of rubberized ply layer each containing steel cords embedded therein. It is preferable that a width BW of the belt structure 3 reinforcing the tire tread ranges from 80 to 95% of a width TW of the tread 5 so that the tread may be widely and surely supported and reinforced. Further, the formation of a belt layer with cords intersecting with each other in triangular pattern is advantageous for the increase in stiffness thereof and reinforcement of the tread. That is to say, it is preferable for increasing stiffness of the belt structure that at least three plies each composed of low-stretchable or non-stretchable cords are stacked so as to be in direct contact with each other, wherein the adjacent two plies are arranged at bias angles equal in degree but directionally opposite to each other. Preferably as small as ranging from 10° to 25°, with respect to the equatorial plane of the tire, and cords of the third ply provide an angle larger than that provided by cords of the former two plies, for example, within the range from 40° to 70° with respect to the above said equatorial plane. On the assumption that the belt layer is of highly stiff structure as above, the stiffness of the tread region become high at the same time, owing to the contribution of said high stiffness, resistance of tread rubber to wear is increased. In order to increase the wear life of tread rubber, it is necessary to improve the resistance of tread rubber to wear together with increasing of tread rubber in thickness at (A), (B) and (C). However, the heat generation of tire tends to increase with increasing of rubber gauge of (A), (B) and (C), as the result. There was no other means of designing a tire for high speed use but to thin the tread rubber gauge so as to control the heat generation, that is at the sacrifice of the wear life of tire. Generally, it is known that the temperature generated while a tire is in rotation, is highest at said both shoulder parts of the tire except special cases, then, the inventor of this invention, as to temperature rise at both shoulder parts, selected seven items such as a ratio between surface areas of cut grooves at the shoulder parts, shape index cross-sectional area of loaded tire/free surface area), pattern transverse stiffness, pattern circumferential stiffness, volume of cut groove, shoulder gauge, and tread radius as factors of temperature rise and examining a correlation between temperature and the above factors depending on multiple regression analysis, it has been found that "circumferential stiffness" and "volume of cut groove" greatly contribute to temperature rise. Fig. 5 graphically shows a relation between pattern circumferential stiffness at the shoulder part and temperature, wherein the lower the circumferential stiffness, the lower the temperature at the shoulder part. Fig.6 shows a relation between the volume of cut groove and temperature, wherein the larger the volume of cut groove, the lower the temperature, but, as regards an embodiment of this invention, it can be fully understood that, even when the volume of cut groove is equal to that of the conventional tire, a temperature is nearly 20° lower than that in the conventional tire. A comparison of transverse stiffness with circumferential one of a pattern indicates that, as shown in Fig.8. a temperature in the pattern No.4 is a developed plan view of a tread pattern in which the index of transverse stiffness at shoulder parts is lower than that of circumferential stiffness; having transverse stiffness lower than circumferential as shown in Fig.7 is 111° which is not low at all. Incidentally, circumferential stiffness per pitch of the pattern No.4 was 44.0 whereas transverse stiffness 25.2. From the above fact, it has been found that lowering of temperature must depend on a reduction of circumferential stiffness rather than transverse one. Fig. 4 graphically shows that a relation between the tread gauge at the shoulder parts and temperature in the tire, wherein a temperature rises with the increase in tread gauge in conventional tire but, in the embodiment of this invention (Fig.1), despite the shoulder gauge of 23.8 mm, a temperature of the tire is as low as that of the conventional tire having the gauge of 18 to 19 mm. It is supposed that lowering of stiffness of the pattern depending on pattern effect leads to decrease in resistance of the tire to wear, however in this respect, Fig. 9 shows that values of pattern stiffness tyres shown in Fig. 1 and 3 are 15.6 and 7.7, respectively, that is, 1:0.5 in terms of ratio, and volumes of cut grooves are 4.16 cm³ and 14.4 cm³, respectively, that is, 1:3.5 in ratio. In the pattern shown in Fig. 1, the volume of groove is small in proportion to pattern stiffness (Fig. 9) as compared with that shown in Fig. 3a. That is to say, it has been found that, when reducing pattern stiffness, resistance to wear is reduced with the increase in volume of the cut groove and, therefore, by setting narrow the width t of cut at the shoulder part as in the pattern shown Fig. 1, control over temperature rise and prevention of decrease in resistance to wear are ensured. An appropriate width t of a cut s is under 6 mm. mm, more preferably from 1 to 3 mm. A volume index of the cut is preferably within the range from 2.5 to 15. An index exceeding 16 results in too large volume of the cut which accelerates wear of rubber and, further, unfavorably generates noise or bites pebbles. On the other hand, too small index not exceeding 2.4 degrades wet gripping and causes problems in safety of vehicles. An index of circumferential stiffness is preferable when ranging from 30 to 70 but, when being low as not greater than 29, causes the rubber block to tend to chip off and, when being 71 or higher, invites such problems as to generate the high temperature. And, a plurality of cut groove are arranged in such a way as to set the circumferential stiffness index to be 30 to 70, so the grounding performance of tread is improved and the slippage of tread did not occur, which lead to the great improvement of the so-called step shoulder wear. An example according to the present invention on the basis of the above described findings will be explained with reference to Fig.1. The tread 1 comprises a crown part CR defined by longitudinal main deepest grooves G1 and G2 extending to the circumferential direction of the tire and centered at the equatorial plane C of the tire so as to extend in width corresponding to 30 to 65% of the tread width TW, and also shoulder parts extending outside both sides of the said crown part CR, in which circumferentially extending longitudinal main grooves g1 and g2 narrower than the said longitudinal main grooves G1 and G2 are arranged on the crown part, sub-groove m and n extend between longitudinal main grooves and between the main groove and the narrow groove to communicate with each other so as to form rows of blocks B1, B2 and B3 on the crown part, each row of block comprises approximately parallelogrammatic blocks B. In the shoulder blocks SH lying outside longitudinal main groove G1 and G2, a plurality of cut s are provided in the circumferential direction of the tire at equal pitches of 15 mm and at an inclination angle α of 30° with respect to the axial direction of the tire, the width t and depth h of the cut s being 2 mm and 13 mm, respectively. A depth ho of the longitudinal main grooves G1 and G2 is to be of extra heavy tread type. For example, a depth ranging from 16.6 mm to 20.6 mm is adopted to the tire of 10.00 R 20 in size. In the present example, a depth is set at 18.00 mm. A width GW of the groove G in the shape of U at cross section A2-A2 is 14.5 mm. A longitudinal narrow groove g is 18 mm depth and 9.5 mm width; a sub-groove m is 5 mm width and 11 mm depth; and another sub-groove n is 2 mm width and 9 mm depth. In the example No. 1 shown in Fig. 1, a pattern stiffness index and cut groove volume index are 64 and 10.5, respectively. On the other hand, control example 1, 2 according to the prior art were made on trial. Specifications of test samples are listed in Table 4. Above said test samples are 10.00 R 20 14 PR. in size and tire construction except tread pattern and component materials are of all same specification. The test results of these three kind of tires are as follow. (High-speed durability) The test of durability to high speed running about said sample tires was performed according to the following method, results thereof being shown in Table 5. Tires were driven on the drum type running tester according to the stepped speed running method on conditions that: wherein results were evaluated by the level of speed at which the tire failed due to heat generation and the length of time spent for running at the above speed. A tire of Example 1 according to the present invention cleared a speed of 130 km/h but a tire of Control Example 1 which attached much importance to the resistance to wear, could clear only 100 km/h, lower by two step than 130 km/h. On the contrary, a tire of Control Example 2 which sacrificed the resistance to wear, cleared a speed of 130 km/h because of lower heat generation due to reduced pattern circumferential stiffness. A tire according to the present invention designed to be provided with "cut grooves" on the shoulder zones for reducing pattern circumferential stiffness, controlled heat generation in the shoulder parts, and provided high degree of durability. (Resistance to wear and stepped shoulder wear) As regards resistance of the tread to wear, comparison tests using the actual truck were performed and amounts of wear of tread rubber per 1, 000 km running were compared by measurements of remaining groove depth after 50,000 km running. As shown in Table 6, a tire (Example 1) according to the present invention demonstrated an excellent resistance to wear by the contribution of heat generation to a low degree, more uniform distribution of grounding pressure and high gripping performance. At the same time, the stepped shoulder wear is greatly improved, too. (Wet grip performance) Values obtained from comparison of wet grip performances are shown in Table 7. Wet grip performances were observed by confirming breaking distances traveled by the car driven at a speed of 80 km/h on the asphalt-paved wet road, and indicated in terms of index on the assumption that the index of breaking distance of Control Example 1 is 100. In this case, too, the larger the index number, the better the performance, and a tire (Example 1) according to the present invention which exhibited a strong breaking force is verified to be excellent in wet grip performance as one of important requirements for safety of a vehicle. (Effect of the Invention) As has been described, this invention provides a well-balanced tire in which a plurality of narrow cuts are disposed on both shoulder parts, where the heat generation is most remarkable in a conventional tire, to control the heat generation by decreasing the pattern stiffness and preventing the slippage of tread rubber due to improved grounding performance of both shoulder parts, so the high-speed durability and the stepped shoulder wear are improved in spite of the extra heavy tread with deepest longitudinal main grooves, and, at the same time, wear life of tread, wandering performance including wet grip performance, ride feeling and handling stability can simultaneously and effectively improved in well-balanced manner, which provide a high-performance tire.

Claims: A heavy duty pneumatic radial tyre (T) comprising a pair of bead cores (7) disposed one in each bead of the tyre, a carcass ply (2) of a radial or semi-radial construction composed of one rubberised ply layer containing metal cords embedded therein and turned up at the edges around said bead cores, a tread (5) disposed radially outside said carcass ply, a belt layer (3) disposed between the carcass ply (2) and the tread (5) and composed of at least three plies of rubberised parallel metal cords laid at a relatively small angle with respect to the circumferential direction of tyre so that the cords in each ply cross the cords of the other plies, and sidewalls disposed between both edges of said tread (5) and each bead, said tread (5) provided with two zigzag or wavy circumferential grooves (6₁,6₂) extending in the circumferential direction of the tyre to divide the tread (5) into a central crown part (CR) and two shoulder parts (SH) one on each side of the crown part, said crown part of the tread having a width of from 30 to 65% of the width (TW) of the tread, the crown part being provided with one or more central ribs or rows of central blocks (B) and each of said shoulder parts (SH) being divided into blocks substantially separated in the circumferential direction of the tyre by a plurality of axially extending narrow grooves (S) having a width (t) of not more than 6mm so that the groove volume index and circumferential stiffness index of the shoulder part (SH) are respectively in a range of 2.5 to 15 and in a range of 30 to 70, said groove volume index and said circumferential stiffness index are defined as follows: wherein V = Total volume of the narrow grooves formed in each shoulder part S = Surface area of the shoulder part d = Depth of the circumferential groove Kpo = Circumferential stiffness of each shoulder part when no groove is formed Kpt = Circumferential stiffness of the same shoulder part when said narrow grooves are formed, and further said circumferential stiffness is defined as follows: Circumferential stiffness = F/y wherein F = Tangential force in the circumferential direction of the tyre at the ground contacting surface of the shoulder part y = Variation of the ground contacting surface in the circumferential direction of the tyre, characterized by a rubber bead apex disposed between the turned up portion (2') of the carcass ply (2)and the main portion of the carcass ply and extending radially outwardly from the base thereof adjacent to the bead core, a reinforcing layer (4) composed of rubberised steel cords and disposed outside each of the turned up portions of the carcass ply, a protective layer (6) composed of organic fibre cords and disposed adjacently outside the carcass ply turned up portion and the reinforcing layer (4) and the bead apex (E) A radial ply tyre according to claim 1 characterized in that the depth (h₁) of each narrow groove (S) is not more than the circumferential groove depth (h₀). A radial ply tyre according to claim 1 or 2 characterised in that the centre line of each narrow groove (S) inclines at an angle of 0° to 45° with respect to the axial direction of the tyre. A radial tyre according to claim 1, 2 or 3 characterised in that the circumferential pitches of the narrow grooves (S) are less than 1.1% of the circumference of the tyre measured at the centre of the tread when the tyre is inflated to a maximum inner pressure for the tyre.

Inventors: FUJIWARA, KENICHI
Applicants: SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1986
Main classification: B60C11
IPC codes: B60C15, B60C11