A PNEUMATIC TIRE

Abstract: A pneumatic tire has a block pattern which is divided by plural circumferential grooves (G) extending in the circumferential direction of a tire and a number of lateral grooves (g) crossing the circumferential grooves. In the block pattern, the block circum­ ferential/lateral ratio Lm/Wm of the average block circumferen­ tial length Lm to the average block width Wm is 0.38 or more and 0.55 or less, and the block circumferential length ratio Lm/L of the average block circumferential length Lm to the circumferen­ tial length L of the tread part is 1/120 or more and 1/85 or less.

Description: The present invention relates to a pneumatic tire having tread patterns that can improve both the riding comfort and steering stability. In addition to the riding comfort and steering stability, improvements of the performances such as low noise characteristic and hydroplaning characteristic are demanded in a pneumatic tire. And it is known that a tread pattern formed in the tread part is one of the factors that affect such performances. There are such tread patterns as the lug pattern mainly composed of lateral grooves, the rib pattern mainly composed of circumferential grooves, the rib and lug pattern which intermediates the former two and the block pattern having blocks by dividing the tread part by circumferential and lateral grooves. And it is known that although, specifically, a tire having the block pattern is generally superior in characteristics against the road surface such as driving performance and hydroplaning characteristic, it is generally inferior in the cornering power and wear resistance due to the relatively low rigidity of the blocks. However, in radial tires which are widely used in these days, as the rigidity of the tread part is increased by a belt layer having superior hoop effect, and the wear resistance and cornering power are improved by employing harder tread rubbers. tires with the block pattern are being used in high-speed buses and passenger cars. As the application of such tires with the block pattern are widened, however, it is required to further improve the riding comfort and steering stability of a tire without affecting the low noise characteristic and hydroplaning characteristic. However, as the rigidity of the tread part should be generally reduced to improve the envelope performance in regard to convex run-over characteristics for the purpose of improving riding comfort, and the rigidity of the tread part should be generally increased to increase the cornering power for the purpose of improving steering stability, the riding comfort and steering stability are contradictory characteristics that could not be compatible, and there has never been any suggestion for improving the riding comfort and steering stability in conventional tread patterns that could compatibly realize the two characteristics. According to GB-A-2014520, a known pneumatic tire has such tread pattern that a tread part is divided into blocks aligned parallel by plural vertical grooves which are extended in the circumferential direction and a number of lateral grooves which cross the circumferential grooves, wherein the block circumferential/lateral ratio Lm/Wm of the average block length Lm, which is the quotient of the actual length Le in the circumferential direction, that is the difference between the circumferential length L of the tread part and the total width Lg of the lateral grooves in the circumferential direction divided by the number Ng of the lateral grooves to the average block width Wm, which is the quotient of the actual length We in the direction of the tire's width, that is a difference between the ground-contact width W of the tire and the total width WG of the circumferential grooves in the direction of tire's width, divided by the added number of circumferential grooves NG + 1, that is the number NG of the circumferential grooves added by 1, and the block circumferential length ratio Lm/L, which is the ratio of the average block circumferential length Lm to the circumferential length L of the tread part, is about 1/113 or more and 1/31 or less. It is hence a primary object of the invention to provide a pneumatic tire that can improve the riding comfort and steering stability without affecting the hydroplaning and low noise characteristics basically by limiting mainly the circumferential/lateral ratio of the blocks within a specific range. This object is achieved by the features of claim 1. In this invention, block circumferential/lateral ratio Lm/Wm is set to be 0.38 or more and 0.55 or less as mentioned hereinbefore. It has been obtained as the rate of the average block circumferential length Lm at which the envelope power EP can be reduced within a specific range so as to improve the riding comfort while controlling the pattern noise to the average block width Wm at which the cornering power CP can be increased so as to improve the steering stability while controlling the reduction of hydroplaning characteristic. Therefore, by specifying the block circumferential/lateral ratio Lm/Wm within the aforementioned range, the riding comfort and steering stability can be improved without affecting the pattern noise and hydroplaning characteristics. In addition, the block circumferential length ratio Lm/L is set at 1/120 or more and 1/85 or less. The block circumferential length ratio Lm/L represents the rate of the average block circumferential length Lm that can reduce the envelope power EP so as to improve the riding comfort without affecting the pattern noise to the circumferential length L of the tread part, and the average block circumferential length Lm can be practically determined by specifying the value and limiting it within the block circumferential/lateral ratio Lm/Wm. An embodiment of the present invention will now be described, by way of example, referring to the attached drawings, in which; Fig. 1 is a plan view showing one of the embodiments of a tire's tread pattern the invention, Figs. 2 and 3 are plan views showing examples of tread patterns in conventional tires, Figs. 4(a) to (i) are plan views showing examples of tread patterns in the tires measured, respectively, Fig. 5 is a diagram showing the relation between the cornering power and the envelope power EP of the tires measured, Fig. 6 is a diagram showing the relation between the envelope power and the pattern noise when the average block circumferential length Lm is changed, and Fig. 7 is a diagram showing the relation among the average block width, hydroplaning characteristic and the cornering power CP. Fig. 1 shows a ground contact surface when a tire is mounted on a standard rim, inflated with a specified internal pressure and loaded with a specified load. In Fig.1 a tread part 2 has plural, in this embodiment, two circumferential grooves G and G extending in the circumferential direction, whereby a central rib 3 between the circumferential grooves G and G and ribs 4 and 4 between the circumferential groove G and a side edge s of the ground contact surface are provided. And by forming a number of lateral grooves g which are aligned parallel and cross the circumferential grooves G, each ribs 3 and 4 is divided into blocks B. The circumferential grooves G of the embodiment comprise straight grooves extending in the circumferential direction, and lateral grooves g are formed as inclined grooves having a narrow width and depth in comparison with the circumferential grooves G and crossing them at an angle. The circumferential grooves G can be formed as winding grooves such as zigzag and wavy forms in addition to straight grooves, and the lateral grooves can cross the circumferential grooves G orthogonally. Furthermore, the ratio of the average block circumferential length Lm, which is the length between the wall of the adjoining lateral grooves g and g, to the average block width Wm, which is the length between the walls of adjoining circumferential grooves G and G, or circumferential groove G and the side end s, that is the block circumferential/lateral ratio Lm/Wm, is set at 0.38 or more and 0.55 or less. And the block circumferential length rate Lm/L, which is the rate of the average block circumferential length Lm to the circumferential length L of the tread surface, is set at 1/120 or more and 1/85 or less. This is based on the result of studies about the effects of tread patterns to the riding comfort and steering stability. The riding comfort relates to the convex run-over performance that is the envelope performance, and the smaller the shock (envelope power EP) conducted to the axle in running over a convex, the more superior the riding comfort is. The steering stability relates to the side force (cornering power CP) occurring in cornering. Fig. 5 shows the measurement result of the cornering power CP and the envelope power EP by using tires having tread patterns shown in Figs. 4(a) to (i). A tire with a tire size of 195/60R19 mounted on a standard rim (15 x 5 1/2 JJ) and inflated with a specific internal pressure (1.9 kgf/cm²) was employed in the measurement. In regard to the cornering power CP, the side force occurring at 1 deg. of slip angle was measured by using a dynamic tire tester. In regard to the envelope power EP, the shock occurring in the axle was measured in running over a convex having a square sectional surface of 10 mm (H) x 10 mm (W) at a peripheral velocity equivalent to 40 km/h. The measured values are shown by index setting the allowance of the cornering power CP and envelope power EP of a tread pattern P5 at 100. In addition, the average block circumferential length Lm and the average block width Wm are 36.0 mm, 23.0 mm and 16.5 mm, and 41.3 mm, 29 mm and 21.6 mm, respectively, and the tread patterns P1 to P9 are settled as their combinations. In other words, the average block width Wm is 41.3 mm in the tread patterns P1 to P3, 29 mm in the tread patterns P4 to P6 and 21.6 mm in the tread patterns P7 to P9, and the average block circumferential length Lm is 36.0 mm in the tread patterns P1, P4 and P7, 23.0 mm in the tread patterns P2, P5 and P8, and 16.5 mm in the tread patterns P3, P6 and P9. As the smaller envelope power EP means superiority in riding comfort, and the stronger cornering power CP means superior steering stability, it was found that as the cornering power CP of the tires with the tread patterns P1, P2 and P3 where the average block width Wm is 41.3 mm is stronger, they are superior in steering stability, and as the envelope power EP of the tires with the tread patterns P3, P6 and P9 where the average block circumferential length Lm is 16.5 mm is smaller, they are superior in riding comfort. Thus, as the riding comfort and the steering stability depend on the average block circumferential length Lm and the average block width Wm, respectively, the riding comfort and the steering stability depend on different dimensions of the tread pattern, and it was found, therefore, that they can be compatibly realized from the measurement result. Fig. 6, additionally, shows a result of measuring the relation between the average block circumferential length Lm and the noise characteristic. The noise characteristic was measured at a distance of 1 m in the direction of tire's axis by loading the tire with a specific load and rotating it at a speed equivalent to 60 km/h. It was found, as recognized in Fig. 6, that the envelope power EP is reversely related to the pattern noise in regard to the average block circumferential length Lm, and a preferable range of the average block circumferential length Lm to control the pattern noise at 75 dB(A) or less and the index of envelope power EP at 100 or less is 16 mm or more and 23 mm or less in the average block circumferential length. When obtaining the average block circumferential length Lm in the block circumferential length ratio Lm/L that is the ratio to the circumferential length L of the tread surface, the ratio Lm/L comes to be within a range between 1/120 and 1/85. Fig. 7, furthermore, shows a result of measuring the relation between the hydroplaning characteristic and the average block width Wm. In regard to the hydroplaning characteristic. a speed at which hydroplaning occurs was measured by a dynamic tester. In the average block width Wm, the hydroplaning characteristic is reversely related to the cornering power CP. And although the cornering power CP is reduced by the reduction of the average block width Wm, the hydroplaning characteristic is improved, on the contrary. Therefore, it is recognized that an optimum range of the average block width Wm to obtain the hydroplaning characteristic of 75 km/h or higher speed and the index of cornering power CP of 100 or larger number is 28 mm or more and 56 mm or less. When the average block width Wm is obtained as the block width ratio Wm/W that is the ratio to the ground-contact width W of the tread in the direction of tire's width, the range of the ratio Wm/W comes to be 1/5 or more and 2/5 or less. From the measurement results shown in Figs. 4 to 7, the range to improve both the riding comfort and the steering stability without affecting the noise characteristic and hydroplaning characteristic is as follows: 16 mm ≦ Lm ≦ 23 mm 28 mm ≦ Wm ≦ 56 mm 1/120 ≦ Lm / L ≦ 1/85 1/5 ≦ Wm / W ≦ 2/5 Here, the block circumferential/lateral ratio Lm/Wm, that is the ratio of the average block circumferential length Lm to the average block width Wm, can be obtained to be 0.38 or more and 0.55 or less by formulas 1 and 2. That is, by treating the deviation of the maximum and minimum values from the center value ( Lm / Wm = 19.5/42 =0.46) in the block circumferential/lateral ratio Lm/Wm as well as the deviation of the maximum and minimum values from the center value ( Lm = 19.5) in the average block circumferential length Lm, the maximum and minimum values of the ratio Lm/Wm are obtained by follows: Lm/Wm ; (max) = 23/19.5 x Lm / Wm = 0.55 Lm/Wm ; (min) = 16/19.5 x Lm / Wm = 0.32 It is obviously recognized that the form ratio of a block can compatibly realize the riding comfort and the steering stability without negatively affecting the other performances by regulating the block circumferential/lateral ratio Lm/Wm within the range. Considering that in the average block circumferential length Lm so-called pitch variation is to be employed where the block length is changed in plural types to avoid a periodical noise, the average block circumferential length Lm can be obtained as an average quotient of the actual length Le of the tread surface in the circumferential direction that is the difference between the circumferential length L of the tread surface and the total width Lg of the lateral grooves g in the circumferential direction divided by the number Ng of lateral grooves g. Furthermore, considering that the circumferential grooves G may not be formed at a regular interval, the average block width Wm can be defined as an average quotient of the actual length We in the direction of tire's width that is the difference between the ground-contact width W of the tread and the total width WG of the circumferential grooves G in the direction of tire's width divided by the added number NG + 1 of the circumferential grooves where 1 is added to the number NG of circumferential grooves G. Moreover, the circumferential length L of the tread surface is defined as the total circumferential length of the tread part passing the tire's equator in a tire inflated with a specific internal pressure. And the ground-contact width W is defined as the maximum ground-contact width in the direction of tire's axis where the tire contacts the ground in such state that it is inflated with a specific internal pressure and loaded with a specific load. Even when a circumferential groove G is thin, it should be counted in the number NG. Although the measurement was performed in a tire with a tire size of 195/60R15, it was confirmed that the same results were obtained in those with different tire sizes such as 205/60R15. Thus, the block circumferential/lateral ratio Lm/Wm is the ratio of the value of the average block circumferential length Lm which reduces the envelope power EP within a specific range and improves the riding comfort while controlling the pattern noise to the average block width Wm that increases the cornering power and improves the steering stability while controlling the reduction of hydroplaning performance. Therefore, by setting the block circumferential/lateral ratio Lm/Wm within the range, a form ratio circumferential to lateral of the block B that can improve the riding comfort and steering stability without affecting the pattern noise and hydroplaning performance can be obtained. The block circumferential length ratio Lm/L is a ratio of the block length in the circumferential length L of the tread part, that can reduce the envelope power EP and improve the riding comfort without affecting the pattern noise, and the average block circumferential length Lm in a specified tire can be determined by this range. The block width ratio Wm/W is a ratio of the block width in the ground-contact width W, which maintains the hydroplaning performance and the cornering power CP at specified levels or higher levels, respectively. And a preferable width of the block B can be determined by this ratio. Examples Prototypes of tires with a tire size of 195/60R15 was produced according to the specifications shown in Table 1. After mounting the tires on a standard rim, inflating with a specific internal pressure and loading a specific load, the envelope power EP, cornering power CP, pattern noise and hydroplaning characteristic were measured in the same manner as mentioned hereinbefore, and the results are simultaneously shown in Table 1. In addition, the tires were employed on all wheels of a passenger car (2000 cc), and the evaluation result of riding comfort and steering stability by the driver's feeling was shown as well. Moreover, as comparison examples, conventional tires with tire patterns shown in Figs. 2 and 3 were produced, and the measurement result in the same manner is also presented in Table 1. The tire of the invention, as shown in Table 1, improves the riding comfort and the steering stability by reducing the envelope power EP and increasing the cornering power CP without negatively affecting the noise and hydroplaning characteristics.

Claims: A pneumatic tire having such a tread pattern that a tread part is divided into blocks B aligned parallel by plural circumferential grooves G which are extended in the circumferential direction and a number of lateral grooves g which cross the circumferential grooves G, a block circumferential/lateral ratio Lm/Wm, which is the ratio of the average block circumferential length Lm to the average block width Wm, said average block circumferential length Lm being defined as the quotient of the actual length Le of the tread surface in the circumferential direction, that is the difference between the circumferential length L of the tread surface and the total width Lg of the lateral grooves g in the circumferential direction, divided by the number Ng of the lateral grooves g; said average block width Wm being defined as the quotient of the actual length We of the tread surface in the direction of the tire's width, that is the difference between the ground-contact width W of the tire's tread and the total width WG of the circumferential grooves G in the direction of tire's width, divided by the added number of circumferential grooves NG + 1, that is the number NG of the circumferential grooves G added by 1; and a block circumferential length ratio Lm/L, which is the ratio of said average block circumferential length Lm to said circumferential length L of the tread part, characterized in that said ratio Lm/Wm is 0,38 or more and 0,55 or less and said ratio Lm/L is 1/120 or more and 1/85 or less. A pneumatic tire according to claim 1, wherein the block width ratio Wm/W of said average block width Wm to said ground-contact width W is 1/5 or more and 2/5 or less.

Inventors: IWAHASHI, YOSHIICHIRO C/O KABUSHIKI KAISHA, KAKUMU, KIICHIRO, NAKAGAWA, HIROYUKI, SHIRAISHI, MASAKI, 17-19, HAINESU-OOSHIBA 304, SUZUKI, SHIGEAKI, TANAKA, MASATOSHI, SUZUKI, SHIGEAKI, C/O KABUSHIKI KAISHA
Applicants: HONDA GIKEN KOGYO KABUSHIKI KAISHA, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1990
Main classification: B60C11
IPC codes: B60C11