Pneumatic tires

Abstract: A Pneumatic tire (1) comprises a toroidal carcass (3), a belt (4) composed of at least two rubberized cord layers (6) arranged radially outside the carcass, and a tread (5) surrounding the periphery of the belt. The tread rubber comprises a base rubber (8) made of a foamed rubber, and a cap rubber (9) made of a non-foam rubber, the cap rubber being arranged radially outside the base rubber.

Description: The present invention relates to pneumatic tires having an improved tread structure. For example, pneumatic tires as described in Japanese patent application Laid-open No. 63-93,604 are known, in which their tread structure is improved. Such tires are pneumatic tires having a cap/base structure in which a tread is constituted by an outer tread portion which is arranged on a radially outer side of the tire and made of a single kind of rubber and by an inner tread portion which is arranged radially inside a surface of a wearing indicator of the tread and also on a radially inner side of the tire and made of plural kinds of rubbers. The inner tread portion is divided into at least three zones in an axial direction of the tire, that is, a central zone extending over the tire equator, and side zones continuously positioned on the opposite sides of the central zone. The hardness of the tread rubber in the central zone of the inner tread portion is made smaller than that of the tread rubber in the side zones and that of the tread rubber in the outer tread portion, and the hardness of the tread rubber in the inner tread portion is increased from the central zone to the side zones. Further, the hardness of the tread rubber of that portion of the inner tread which is located on the axially outermost portion of the tire is not greater than that of the tread rubber in the outer tread portion. When the hardness of the rubber of the tread is decreased as it goes to the radially inner side and to the axially inner side in the above manner, anti-vibration riding comfortability can be improved without adversely affecting wear resistance. The tread rubber, that is, the rubber of the inner and outer tread portions, is made entirely of a non-foam rubber in the above-mentioned tire. When compression forces are applied to such a non-foam rubber, its volume is not substantially reduced. Thus, even when the hardness of the rubber of a part of the tread, that is, the inner tread portion, is decreased as mentioned above, the compression modulus of the entire tread does not substantially decrease. As a result, anti-vibration riding comfort-ability can merely be improved to some extent, but a sufficient effect cannot be obtained during actual use. In addition, the tensile modulus of an ordinary non-foam rubber is reduced corresponding to reduction in the rubber hardness as mentioned above. Attention is drawn to DE-A-1 016 583 which discloses a tire having a tread rubber comprised of a base rubber made of foamed rubber and a cap rubber made of non-foamed rubber, wherein the cap rubber is arranged radially outside the base rubber (Figs. 3 and 4). Attention is also drawn to GB-A-2 044 191 which is aimed at maintaining good adhesion on a wet paved surface and obviating the requirement for an intricately patterned mold, and for this purpose the cap rubber of the tread is made of a porous (void-contained) rubber. The present invention aims to provide pneumatic tires of which anti-vibration riding comfortability can be greatly improved without adversely affecting wear resistance. The present invention provides a pneumatic tire comprising a toroidal carcass, a belt composed of at least two rubberized cord layers arranged radially outside the carcass, and a tread surrounding the periphery of the belt, the tread rubber comprising a base rubber made of a foamed rubber and a cap rubber made of a non-foam rubber, said cap rubber being arranged radially outside the base rubber, characterized in that the width of the base rubber is less than that of the belt, and in that b/W is in a range from 0.15 to 0.65, in which b and W are respectively the width of the base rubber and the ground contact width of the tread when a normal internal pressure is applied to the tire. In the above tire, it is preferable that the widthwise center of the base rubber is substantially in conformity with the equatorial plane of the tire; the width of the base rubber is made less than that of the belt so that the cap rubber may be laid on widthwise opposite sides of the base rubber. Further, the expansion ratio (Vs) of the base rubber is preferably in a range from 10% to 50%. Further, the ratio t/T in which t and T are the maximum gauge of the base rubber and that of the tread rubber, respectively, preferably falls in a range from 0.05 to 0.50. The tread rubber of the pneumatic tire according to the present invention has the base rubber made of a foamed rubber. When compression forces are applied to such a foamed rubber, bubbles inside the rubber are collapsed to decrease the volume by compression. Consequently, its compression modulus becomes smaller as compared with that of the non-foam rubber. Thus, as compared with a tire in which the entire tread is constituted by non-foam rubber, the rigidity of the above tread rubber against compression forces becomes lower. When such a pneumatic tire is run, compression forces act upon the tread rubber at the ground contacting area due to loading, and vibrations are applied to the tire owing to the uneven ground surface. These vibrations are effectively attenuated with the base rubber having the volume reduced by compression under application of compression stresses, so that anti-vibration riding comfortability is greatly improved. On the other hand, since the cap rubber arranged radially outside the base rubber is made of the non-foam rubber, wear resistance will not lower. In this way, according to the present invention, anti-vibration riding comfortability can greatly be improved without adversely affecting wear resistance. Further, when the cap rubber is arranged not only radially outside the base rubber but also on the widthwise opposite sides thereof, rigidity of the opposite shoulder portions of the pneumatic tire and bending rigidity of the tread rubber are maintained at respective levels as conventionally possessed, while cornering stability is maintained. At the same time, the foamed rubber having a higher wearing rate, that is, the base rubber, is protected against wearing. Further, when the expansion ratio (Vs), the ratio b/W and the gauge ratio t/T fall within the above-specified respective ranges, anti-vibration riding comfortability and cornering stability can both be improved as compared with conventional tires, and reduction in entire wearing life can be prevented. For a better understanding of the invention, reference is made to the accompanying drawings, wherein: Fig. 1 is a sectional view of a first embodiment according to the present invention taken along a meridian line of a tire; Fig. 2 is a graph showing results in a first Test Example, that is, changes in anti-vibration riding comfortability with respect to the expansion ratio; Fig. 3 is a graph showing other results in the first Test Example, that is, changes in the anti-vibration riding comfortability with respect to the gauge ratio t/T; Fig. 4 is a graph showing still other results in the first Test Example, that is, changes in the complete wear life with respect to the expansion ratio; Fig. 5 is a graph showing further results in the first Test Example, that is, changes in the complete wear life with respect to the gauge ratio t/T; Fig. 6 is a graph showing results in a second Test Example, that is, changes in the anti-vibration riding comfortability with respect to the ratio b/W; Fig. 7 is a graph showing other results in the second Test Example, that is, changes in the anti-vibration riding comfortability with respect to the gauge ratio t/T; Fig. 8 is a graph showing still other results in the second Test Example, that is, changes in the anti-vibration riding comfortability with respect to the expansion ratio; Fig. 9 is a graph showing further results in the second Test Example, that is, changes in the cornering stability with respect to the ratio b/W; Fig. 10 is a graph showing still further results in the second Test Example, that is, changes in the cornering stability with respect to the gauge ratio t/T; Fig. 11 is a graph showing results in the second Test Example, that is, changes in the cornering stability with respect to the expansion ratio; Fig. 12 is a sectional view of a second embodiment according to the present invention taken along a meridian line of a tire; and Fig. 13 is a sectional view of a third embodiment according to the present invention taken along a meridian line of a tire. The first embodiment according to the present invention will now be explained with reference to the drawings. In Fig. 1, a pneumatic tire 1 comprises a toroidal carcass 3 having axially opposite end portions turned up around beads 2, a belt 4 radially outwardly arranged on the carcass 3, and a tread rubber 5 surrounding the belt 4. The belt 4 is constituted by piling at least two rubberized cord layers 6 one upon another. The tread rubber 5 is constituted by a base rubber 8 arranged on a radially inner side of the tire and made of a foamed rubber containing numerous closed cells, and a cap rubber 9 arranged on a radially outer side of the tire and made of a non-foam rubber. When compression forces act upon the foamed rubber constituting the base rubber 8, the closed cells inside the foamed rubber are collapsed to reduce their volume. Accordingly, compression modulus of the foamed rubber is smaller than that of the non-foam rubber constituting the cap rubber 9. Further, the base rubber 8 is so arranged that its widthwise center may be almost in conformity with the equatorial plane 10 of the tire. The width b of the base rubber is smaller than that of the belt 4. Thus, the cap rubber 9 made of the above non-foam rubber is arranged radially outside and on widthwise opposite sides of the base rubber 8. As a result, the outer and opposite sides of the base rubber 8 are surrounded by the cap rubber, so that cornering stability can be maintained, while rigidity of opposite shoulder portions 11 of the tire 1 and bending rigidity of the tread rubber 5 are maintained at the same levels as those of a tire in which the tread rubber is entirely made of the non-foam rubber. At the same time, the foamed rubber having a higher wearing rate, that is, the base rubber 8, is protected against wearing. Further, the ratio b/W falls in a range from 0.15 to 0.65, in which b and W are the width of the base rubber 8 and the ground contact width of the tread when a normal internal pressure is applied to the tire 1 fitted to a normal rim under application of a normal load, respectively. The reason is that if the ratio b/W is less than 0.15, anti-vibration riding comfortability is lower than that of the conventional tires as mentioned later. On the other hand, if the ratio is more than 0.65, the cornering stability is lowered as compared with that of the conventional tires. In addition, it is preferable that the expansion ratio (Vs) of the base rubber 8 falls in the range from 10 to 50%. The reason is that if the expansion ratio is less than 10%, the anti-vibration riding comfortability is lower than that of the conventional tires as mentioned later. On the other hand, if the expansion ratio is more than 50%, the cornering stability is lower than that of the conventional tires, and the complete wear life is rapidly lowered. Usually, the expansion ratio (Vs) is represented by the following equation. Vs = {(ρ₀ - ρ g )/(ρ₁ - ρ g ) - 1} × 100(%) (1), wherein: ρ₁ is the density (g/cm³) of the foamed rubber; ρ₀ is the density (g/cm³) of a solid part of the foamed rubber; and ρ g is the density (g/cm³) of a gas contained in the foam of the foamed rubber. Because the value of ρ g is negligibly small as compared with that of ρ₁, the above-carried equation "(1)" can be replaced with the following equation. Vs = (ρ₀/ρ₁ - 1) × 100(%) (2) Further, it is preferable that the gauge ratio t/T falls in the range from 0.05 to 0.50, in which t and T are the maximum gauge of the base rubber 8 and that of the tread rubber 5, respectively. The reason is that if the gauge ratio t/T is less than 0.05, the anti-vibration riding comfortability is lower than that of the conventional tires as mentioned later. On the other hand, if the gauge ratio is more than 0.50, the base rubber 8, which has a greater wearing rate, extends from the surface of the tire at the wear final stage to rapidly lower the complete wear life, and to lower the cornering stability compared with the conventional tires. If a deep main groove 17 is formed in the surface portion 16 of the tread of the tire 1, the base rubber 8 may appear at the groove bottom of the main groove 17. The base rubber 8 made of the foamed rubber is weaker than the non-foam rubber, and is easily broken. Therefore, the base rubber must not project from the groove bottom of the main groove 17. In the above embodiment, those portions of the base rubber which overlap with the main grooves 17 are deformed toward the radially inner side of the tire. When the above tire 1 is fitted to a vehicle and the vehicle is run under rotation of the tire 1, compression forces act upon the tread rubber 5 within the ground contact area owing to the load, and vibrations are inputted to the tire due to the uneven ground surface. Since foamed rubber of which the compression modulus is reduced to about 1/2 of the non-foam rubber is used for the base rubber 8 of the tire 1 in this embodiment, vibrations inputted are absorbed and attenuated to a large extent by the base rubber 8 capable of effectively reducing its volume under application of the compression stresses, so that the anti-vibration riding comfortability of the vehicle can be greatly improved. On the other hand, since the cap rubber 9 arranged radially outside the base rubber 8 is made of the non-foam rubber having a small wearing rate, wear resistance will not drop. Next, a first test example will be explained. Test Example 1 On conducting this test, Comparative Tire 1 in which a tread rubber was entirely made of a non-foam rubber, and Test Tires 1 through 7 to which the present invention was applied, were prepared. Various dimensions of each of the tires are shown in Table 1, and their tire size was 185/70SR14. Next, each of the tires was fitted to a vehicle, and its anti-vibration riding comfortability was evaluated by driver's feeling. Results are shown in Table 1 by index. As is clear from Table 1, the anti-vibration riding comfortability was improved for the Test Tires. As shown in Figs. 2 and 3, as the expansion ratio (Vs) increases or as the gauge ratio t/T increases, the anti-vibration riding comfortability is improved. Further, the complete wear life was measured by running each of the tires in this test. The complete wear life means a running distance at which at least one portion of the remaining main groove became 1.0 mm or less in depth. The main groove means the deepest groove in the tread, and is generally a wide groove extending continuously in a circumferential direction. Results are shown in Figs. 4 and 5. From the results, it is seen that as the expansion ratio (Vs) increases or as the gauge ratio t/T increases, the complete wear life lowers. If the expansion ratio (Vs) exceeds 50% or if the gauge ratio t/T exceeds 0.5, the complete wear life rapidly drops. Thus, it is preferable that the expansion ratio (Vs) is not more than 50% and that the gauge ratio t/T is not more than 0.50. Next, a second test example will be explained below. Test Example 2 On conducting the second test example, Comparative Tires 2 and 3 in which a tread rubber was entirely made of a non-foam rubber, and Test tires 8 through 14 to which the present invention was applied, were prepared. The test tire 10 is outside the scope of the present invention because the ratio b/W is greater than 0.65. Comparative Tire 2 is a tire in which contradictory properties, that is, the anti-vibration riding comfortability and the cornering stability, were both satisfied by selecting an appropriate kind of a non-foam rubber. Thus, Comparative tire 2 had a practically sufficient anti-vibration riding comfortability and cornering stability. The tire size of each of the tires used in this Test Example was 185/70SR14, and the ground contact width of the tread was 121 mm. Various dimensions of the tires are shown in Table 2. Next, each of the tires was fitted to the passenger car vehicle, and the vehicle was run on a test course. The anti-vibration riding comfortability and the cornering stability were evaluated by the driver's feeling. Results are shown in Table 2 and Figs. 6 to 11. As is seen from Test Example 2, and Figs. 6, 7 and 8, as the ratio b/W increases, or as the gauge ratio t/T increases, or as the expansion ratio (Vs) increases, the anti-vibration riding comfortability is improved. However, if the ratio b/W is less than 0.15, or if the gauge ratio t/T is less than 0.15, or if the expansion ratio (Vs) is less than 10%, the anti-vibration-riding comfortability is lower than that of the above Comparative Tire 2. The ratio b/W is not less than 0.15, and it is preferable that the gauge ratio t/T is not less than 0.05, and that the expansion ratio (Vs) is not less than 10%. On the other hand, as is clear from Table 2, and Figs. 9, 10 and 11, as the ratio b/W increases, or as the gauge t/T increases, or as the expansion ratio (Vs) increases, the cornering stability drops. If the ratio b/W exceeds 0.65, or if the gauge ratio t/T exceeds 0.50, or if the expansion ratio (Vs) exceeds 50%, the cornering stability becomes poorer than that of the above Comparative Tire 2. The ratio b/W is not more than 0.65, and it is preferable that the gauge ratio t/T is not more than 0.50, and that the expansion ratio (Vs) is not more than 50%. In Table 2, the 10% compression modulus means a value obtained by measuring compression stresses occurring when static compression forces were applied to a flat rubber plate having lengthwise and breadthwise lengths being both 40 mm and a thickness of 10 mm from upper and lower sides, and dividing compression stress by compression strain, the compression stress being a value when the thickness of the flat rubber plate is reduced by 10%. Fig. 12 is a sectional view of a second embodiment of the present invention. In this embodiment, the widthwise center of a base rubber 21 is spaced from the equatorial plane 10 of the tire such that the base rubber 21 is entirely arranged on one side of the equatorial plane 10. Wear resistance and anti-vibration riding comfortability can both be attained in the pneumatic tire in which the fitting side is preliminarily determined. Otherwise, the construction and function are the same as in the first embodiment. Fig. 13 is a sectional view of a third embodiment of the present invention. In this embodiment, narrow base rubbers 22 and 23 are provided on opposite sides of the tire equatorial plane 10, respectively, and a cap rubber 9 is arranged above and between the base rubbers 22 and 23. The total value of widths c and d of the base rubbers 22 and 23, respectively, is equal to the width b of the base rubber 8 in the first embodiment. An applied load in the ground contact area becomes near uniform, so that both the prevention of the occurrence of uneven wearing and improvement of the anti-vibration riding comfortability can be attained. Otherwise, the construction and function are the same as in the first embodiment. In the above-mentioned embodiments, most of the radially outer side surface of the base rubber is flat. However, according to the present invention, the radially outer surface of the base rubber may be corrugated. Thus, a rapid increase in the exposed area of the base rubber at the final wearing stage can be prevented. As explained above, according to the present invention, the anti-vibration riding comfort-ability can be improved to a large extent without adversely affecting the wear resistance.

Claims: A pneumatic tire (1) comprising a toroidal carcass (3), a belt (4) composed of at least two rubberized cord layers (6) arranged radially outside the carcass, and a tread (5) surrounding the periphery of the belt, the tread rubber comprising a base rubber (8) made of a foamed rubber and a cap rubber (9) made of a non-foam rubber, said cap rubber being arranged radially outside the base rubber, characterized in that the width of the base rubber is less than that of the belt, and in that b/W is in a range from 0.15 to 0.65, in which b and W are respectively the width of the base rubber and the ground contact width of the tread when a normal internal pressure is applied to the tire. A pneumatic tire as claimed in Claim 1, characterized in that the expansion ratio of the base rubber is in a range from 10 to 50%. A pneumatic tire as claimed in Claim 1 or 2, characterized in that a widthwise center of the base rubber (8) is substantially in conformity with the equatorial plane (10) of the tire, and the cap rubber (9) is also arranged on opposite sides of the base rubber. A pneumatic tire as claimed in claim 1 or 2, characterized in that the widthwise center of the base rubber (21) is spaced from the equatorial plane (10) of the tire such that the base rubber is entirely arranged on one side of the equatorial plane. A pneumatic tire as claimed in claim 1 or 2, characterized in that base rubbers (22,23) are provided on opposite sides of the equatorial plane (10) of the tire, and a cap rubber (9) is arranged above and between the base rubbers (22,23), in that the total value of widths (c,d) of the base rubbers (22,23) is less than the width of the belt, and in that (c+d)/W is in a range from 0.15 to 0.65. A pneumatic tire as claimed in any of Claims 1 to 5, characterized in that t/T is in a range from 0.05 to 0.50, in which t and T are maximum gauges of the base rubber and the tread rubber, respectively.

Inventors: KOBAYASHI KAZUOMI, KOBAYASHI, KAZUOMI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1989
Main classification: B60C9
IPC codes: B60C9