Pneumatic tires

Abstract: A pneumatic tire comprises a carcass 1 of at least one carcass ply, a belt 2 comprised of at least three belt layers, and a tread 3 provided with plural circumferential grooves 3a and island portions 3b defined by these circumferential grooves 3a, in which the tread 3 is comprised of at least two rubber layers, one of which layers being a foamed rubber layer 4 not exposed at a side of ground contact surface of the tire. In this case, the foamed rubber layer 4 is extended up to a buttress portion of the tire, whereby noises produced during the running of the tire are effectively reduced without the degradation of wear resistance.

Description: This invention relates to a pneumatic tire, and more particularly to a pneumatic tire which can effectively reduce noise produced during the running of the tire without degrading wear resistance. As a means for reducing noise in a pneumatic tire produced during the running of the tire, it has hitherto been attempted to develop a tread pattern by giving attention to the portion of the tire directly contacting with the ground. Lately, however, it has been confirmed that resonance sound resulting from vibration of a sidewall portion becomes a large factor as a noise of the tire in addition to sound generated in the ground contact portion. Attention is drawn to the disclosures of FR-A-1533215 and EP-A-0370724. FR-A-1533215 discloses that a web composed of foamed rubber is embedded in recesses formed in a tire tread portion inward from buttress portions in the widthwise direction. The web is not continuous in the circumferential direction but is arranged at a certain interval in the circumferential direction (see Fig. 4), so that the usual tread rubber is existent between the webs. EP-A-0370724 discloses a pneumatic tire which comprises 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 comprises a base rubber made of a foamed rubber, and a cap rubber made of a non-foam rubber, the cap rubber being arranged radially outside the base rubber. It is an aim of the present invention to particularly reduce the resonance sound in the sidewall portion of the tire without degrading wear resistance of the tire. According to the invention, there is provided a pneumatic tire comprising a carcass of at least one carcass ply extending between a pair of bead cores, a belt superimposed around a crown portion of the carcass outward in a radial direction of the tire and comprised of at least three layers, and a tread arranged on the belt outward in the radial direction and provided with plural circumferential grooves and island portions defined by these circumferential grooves, said tread being comprised of at least two rubber layers, wherein one of said rubber layers is a foamed rubber layer not exposed at a side of ground contact surface of the tread, characterized in that said foamed rubber layer is extended up to a buttress portion of the tire so as to envelop an end portion of the belt. In preferred embodiments of the invention, the foamed rubber layer is arranged in a zone located at a position corresponding to not more than 50% of a groove depth of the circumferential groove toward the belt, particularly in a zone ranging from the shoulder portion of the tire to the buttress portion thereof, and also the foamed rubber preferably has an expansion ratio of 5-50%. The invention will be further described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 is a diagrammatic left-hand sectional view of a first embodiment of a pneumatic tire according to the invention; Fig. 2 is a schematic view illustrating a state of generating noise at a sidewall portion of the tire; Figs. 3 to 5 are diagrammatic left-hand sectional views of other embodiments of pneumatic tires according to the invention, respectively; and Fig. 6 is a graph showing a relation between sound pressure and frequency. Fig. 1 shows a left-half section of the main part of a first embodiment of a pneumatic tire having a tread width TW, according to the invention. Numeral 1 is a carcass of at least one carcass ply extending between a pair of bead cores (not shown), numeral 2 a belt arranged on the carcass 1 outward in the radial direction of the tire and comprised of four belt layers in the illustrated embodiment, and numeral 3 a tread arranged on the belt 2 outward in the radial direction and provided with plural circumferential grooves 3a and island portions 3b defined by these circumferential grooves 3a. Further, numeral 4 is a foamed rubber layer. In the illustrated embodiment, the foamed rubber layer 4 is arranged in a zone corresponding to not more than 50% of a groove depth h in the circumferential groove 3a of the tread 3 so as to envelop the end portion of the belt 2. As a result of vibration analysis during the running of the tire, it has been found that among noises produced from the tire, a pattern noise is most dominant, but sound resulting from resonance of the sidewall portion becomes a large factor. In order to reduce such noises, it is advantageous to use a foamed rubber having fine bubbles therein for damping vibration from the road surface. As a tire using such rubber, there is known a studless tire which is concerned with running performances on snow and ice roads in winter season. However, when the foamed rubber used in the studless tire is merely applied to a tread in tires running on general-purpose road surfaces, the wear resistance is considerably poor as compared with the usual tire, so that it is difficult to put the tire having a tread of the foamed rubber for running on usual roads into practical use. According to the invention, vibrations in the sidewall portion based on a noise generation mechanism as shown in Fig. 2 are damped by the foamed rubber layer 4 arranged in a zone ranging from a base portion of the tread to shoulder and buttress portions, whereby resonance sound in the sidewall portion can effectively be mitigated without degrading the wear resistance. In the invention, the reason why the foamed rubber layer 4 is preferably arranged in the zone corresponding to not more than 50% of the groove depth h in the circumferential groove 3a of the tread 3 is due to the fact that when the position of the foamed rubber layer 4 exceeds 50% of the groove depth h, the foamed rubber layer 4 is exposed before a middle worn stage of the tread rubber to undesirably degrade the wear resistance from the middle worn stage. Therefore, the position of the foamed rubber layer 4 is preferably restricted to the zone corresponding to not more than 50% of the groove depth h of the circumferential groove 3a. Moreover, the foamed rubber layer 4 is preferably arranged in a zone from the shoulder portion to the buttress portion, i.e. it is arranged in the shoulder island portion defined by the circumferential groove 3a so as to envelop the end portion of the belt 2 from the base portion of the tread. It is more effective that the end portion of the foamed rubber layer 4 is located inward from the end portion of the belt in the widthwise direction of the tire. Furthermore, the expansion ratio of the foamed rubber is preferably restricted to a range of 5-50%. When the expansion ratio is less than 5%, the effect of damping vibration may not be obtained, while when it exceeds 50%, the rigidity becomes too small and hence the dynamic strain during the running under a load becomes large to undesirably degrade the wear resistance. Moreover, the expansion ratio is represented by the following equation: V (%) = (ρ 0 /ρ 1 - 1) x 100 wherein V is an expansion ratio, ρ 0 (g/cm 3 ) is a density of solid phase portion of the foamed rubber and ρ 1 (g/cm 3 ) is a density of the foamed rubber. In the invention, the foamed rubber layer 4 may be arranged over the full length of the tread 3 in the widthwise direction thereof as shown in Fig. 3. However, the central region of the tread 3 in the widthwise direction undergoes large dynamic strain during the running under a load and has a small effect of damping the transmission of vibration from the tread 3 to the sidewall portion, so that it is most preferable to arrange the foamed rubber layer 4 in a zone ranging from the shoulder portion of the tread to the buttress portion thereof as shown in Fig. 1. When the foamed rubber layer 4 is arranged over the full length of the tread 3 in the widthwise direction, it is preferable that the thickness of the foamed rubber layer 4 is made thin in the central region of the tread 3 as shown in Fig. 4. Moreover, the foamed rubber layer 4 may be exposed from the outer surface of the tire at the buttress portion as shown in Fig. 5. In this case, the foamed rubber may be degraded by ultraviolet rays or ozone. Therefore, the same rubber as used in the sidewall portion is preferably arranged on the buttress portion so as not to expose the foamed rubber from the surface of the tire in order to prevent the degradation by ultraviolet rays or ozone as shown in Fig. 1. The following examples are given in illustration of the invention and are not intended as limitations thereof. There were provided four test tires having a tire size of 11R22.5 14PR and the same tread pattern, in which the foamed rubber layer 4 having an expansion ratio of 18% was arranged as shown in Figs. 1, 3, 4 and 5, respectively. These tires were mounted on a nominal rim and and then noise and wear tests were made by running under a nominal internal pressure of 7.00 kgf/cm 2 and a nominal load of 2500 kgf. The measured results are shown in Table 1 together with the measured results of a comparative tire having the same structure as in the test tire except that the tread was made from usual rubber without using the foamed rubber. Each of these test tires comprised a radial carcass of single steel cord ply extending between a pair of bead cores and a belt of four steel cord belt layers, and had a rib tread pattern provided with four circumferential grooves and five continuously extending circumferential island portions defined by these grooves. The noise test was conducted by an indoor noise test (drum having an outer diameter of 3 m) in which a microphone was placed at a position separated from the tire by 50 cm to measure sound. A sound pressure near to 500 Hz being a resonance sound of the sidewall portion was determined from the measured value in accordance with a relation between frequency and sound pressure, whereby noise was represented by an index on the basis that the comparative tire was 0. The smaller the index value, the smaller the noise. The wear test was conducted by actually mounting the tire on a vehicle and running over a distance of 50,000 km to measure a worn amount. The worn amount was represented by an index on the basis that the comparative tire was 100. The larger the index value, the better the wear resistance. Acceptable Example (Fig.1) Acceptable Example (Fig.3) Acceptable Example (Fig.4) Acceptable Example (Fig.5) Comparative tire Noise - 0.8 - 1.1 - 1.0 - 0.8 0 Wear 100 94 96 100 100 As seen from Table 1, all the tires according to the invention generate relatively low noise and do not undergo considerable degradation of wear resistance. In Fig. 6 is shown a relation between sound pressure and frequency in the comparative tire and the tire having the structure of Fig. 1 according to the invention. As seen from Fig. 6, the sound pressure near to 500 Hz as a resonance sound of the sidewall portion in the tire according to the invention is mitigated by about 1 dB as compared with that of the comparative tire. Although the invention is not limited to the above embodiments, various modifications may be made. For example, lug pattern, rib-lug pattern or block pattern may be used as a tread pattern, or organic fiber cord ply may be used as a carcass ply and the same organic fiber cord ply may be used as a belt layer. As mentioned above, according to the invention, noises generated during the running of the tire, particularly noise resulting from the resonance of the sidewall portion, can be reduced without degrading the wear resistance.

Claims: A pneumatic tire comprising a carcass (1) of at least one carcass ply extending between a pair of bead cores, a belt (2) superimposed around a crown portion of the carcass outward in a radial direction of the tire and comprised of at least three layers, and a tread (3) arranged on the belt (2) outward in the radial direction and provided with plural circumferential grooves (3a) and island portions (3b) defined by these circumferential grooves (3a), said tread (3) being comprised of at least two rubber layers, wherein one of said rubber layers is a foamed rubber layer (4) not exposed at a side of ground contact surface of the tread, characterized in that said foamed rubber layer (4) is extended up to a buttress portion of the tire so as to envelop an end portion of the belt (2). A pneumatic tire as claimed in claim 1, characterized in that said foamed rubber layer (4) is arranged in a zone located at a position corresponding to not more than 50% of a groove depth of the circumferential groove (3a) toward the belt. A pneumatic tire as claimed in claim 1 or 2, characterized in that said foamed rubber has an expansion ratio of 5-50%. A pneumatic tire as claimed in any of claims 1 to 3, characterized in that said foamed rubber layer (4) is arranged only in a zone ranging from the shoulder portion of the tire to the buttress portion thereof. A pneumatic tire as claimed in any of claims 1 to 3, characterized in that said foamed rubber layer (4) is arranged over a full width of the tread. A pneumatic tire as claimed in claim 5, characterized in that a thickness of said foamed rubber layer (4) is made thin in a central portion of the tread (3) in the widthwise direction. A pneumatic tire as claimed in any of claims 1 to 6, characterized in that said foamed rubber layer (4) is not exposed in the buttress portion from the outer surface of the tire.

Inventors: NAOI TOSHIKATSU, TOKUNAGA SEIICHIRO, NAOI, TOSHIKATSU, TOKUNAGA, SEIICHIRO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1992
Main classification: B60C9
IPC codes: B60C9, B60C11