PNEUMATIC RADIAL TIRE

Abstract: A pneumatic radial tire incorporated, in a region lying between a bead core and at least this portion of the outer surface of the bead portion which is in contact with a rim on which the tire is mounted, a rubber sheet is incorporated, which has a dynamic modulus at 20° C, E′(20° C), of at least 6.0 MPa and a ratio of the dynamic modulus E′(20° C) to a dynamic modulus at 60° C, E′(60° C), namely E′ (20° C)/E′ (60° C), of at least 1.2.

Description: The present invention relates to a pneumatic radial tire, and more particularly, a pneumatic radial tire improved in or relating to the road noise generation without sacrifice to the resistance to the bead detachment from rim. Road noise mean noise to become generated inside a a running car as a result of tires of the car contacting a roughened pavement surface. That is to say, the road noise generation is such a phenomenon in which when tires are caused to vibrate by delicate disturbances which they receive from a pavement surface, the vibration of the tires excites various portions of the car body to vibrate through suspension mechanisms of the car, whereby noise become generated inside the running car. Thus, in order to suppress the noise generation, it may be well devised to let the tires alleviate and/or absorb disturbances which they receive from the pavement surface so as to suppress to minimum the degree of transmission of vibration from tires to the car body. Conventionally, as means for suppressing the transmission of vibration, it has been proposed to make use of a rubber having a large energy loss, namely a rubber having a large loss tangent, tan δ, or to increase the volume of rubber, in the tread portion of the tire. However, according to such proposed means, the rolling resistance of the tire becomes disadvantageously increased and the fuel consumption economy is greatly sacrificed. Also, it has been proposed to dispose a soft rubber in a region below a bead core in the bead portion of the tire. According to this proposed measure, however, the resistance to the bead detachment from rim of the tire tends to be deteriorated and the tire tends to be unable to perform a stable running in a condition thereof having a lower inflation pressure than the prescribed normal value. A primary object of the present invention is to provide a pneumatic radial tire, particularly for passenger cars, with which the generation of road noise is suppressed without the risk of lowering the resistance to the bead detachment from rim. Another object of the invention is to provide a pneumatic radial tire with which the generation of road noise is suppressed without increasing the rolling resistance of the tire. The pneumatic radial tire for attaining the above objects according to the present invention is characterized in that in a region lying between a bead core in a bead portion and at least an outer surface portion of the bead portion in contact with a rim on which the tire is mounted, there is a rubber sheet incorporated, which has a dynamic modulus of elasticity at 20° C, E′(20° C), of at least 6.0 MPa and a ratio of this dynamic modulus E′(20° C) to a dynamic modulus of elasticity at 60° C, E′(60° C), namely E′(20° C)/E′(60° C), of at least 1.2. More preferably, the above ratio E′(20° C)/E′(60° C) of the rubber sheet is within a range of 1.2 to 1.6, and in addition, the rubber sheet has a loss tangent, tan δ, of at least 0.2. By incorporating the above rubber sheet in a region between an outer surface portion of the bead portion in contact with the rim and the bead core as above, it is possible to obtain a desirable resistance to the bead detachment from the rim and, at the same time, suppress the generation of the road noise. Also, it is possible to suppress the road noise generation without increasing the rolling resistance of the tire. In or for the present invention, the dynamic modulus of elasticity of the rubber, E′, means the dynamic storage modulus determinable on a viscoelasticity spectrometer (commercially obtainable for example at Iwamoto Seisakusho K.K., Japan), at respective prescribed temperatures and under the load condition in which a test sample of rubber is applied with an initial strain of 10 % and then a dynamic strain of a frequency of 20 Hz and an amplitude of ± 2 % (or 8 to 12 % of the original length of the test sample). For purposes of the description of the invention, the modulus values found at 20° C are shown by E′(20° C), while those at 60° C are shown by E′(60° C). Fig. 1 is a meridian sectional view, showing an embodiment of pneumatic radial tires according to the present invention; Fig. 2 is an enlarged partial sectional view, showing the bead portion of the radial tire of Fig. 1; and Fig. 3 is a schematic plan view, diagramatically showing the J-letter shaped running test course used in the carrying out of evaluations of test tires with respect to their resistance to the bead detachment from the rim. In or for the present invention, radial tires have a sectional structure for example as shown in Fig. 1 of the accompanying drawings. The illustrated typical example of radial tires is essentially composed of a pair of a left bead portion 10 and a right bead portion 10 each having a bead core 5 embedded therein, a pair of a left side-wall portion 2 and a right side-wall portion 2 continuous to the left and the right bead portions 10 and 10 respectively, and a tread portion 1 connecting the pair of side-wall portions 2 and 2 to each other. On the inner side of the tire, a carcass 4 is arranged, which comprises at least a single carcass layer and of which each end portion is turned up from the inner side to the outer side of the bead core 5 in a manner of enrobing a bead filler 6. In the tread portion 1, at least two belt layers 7 are disposed in a cord arrangement such that the belt cords of one layer cross those of the other layer. The further reference numeral 15 denotes a rim on which the radial tire is mounted. In the bead portion 10 shown in enlargement in Fig. 2, an inner lower corner portion is a bead toe 11, and an outer lower corner portion is a bead heel 12. The reference numeral 3 denotes a rim cushion rubber. In a region between this outer surface portion of the bead portion 10 which is in contact with the rim 15 and the bead core 5, a rubber sheet 13 comprising a hard rubber is incorporated. In a region outside of the bead core within the region of the rim cushion rubber 3, the rubber sheet 13 extends along a turned-up end portion of the carcass 4 up to a location appreciably above the upper end of a rim flange. In a region below the bead core 5, the rubber sheet 13 extends from the bead heel 12 to the bead toe 11 parallel to the plane of a bead seat, and after it is turned at the position of the bead toe 11, extends up to a location slightly above the upper edge of the bead core 5. On the outer side of the rubber sheet 13, further, a reinforcing layer 8 comprising a fiber fabric may be arranged as needs be. In or for the present invention, the hard rubber constituting the rubber sheet 13 is required to have a characteristic represented by a dynamic modulus at 20° C, E′(20° C), of at least 6.0 MPa and a ratio of this E′(20° C) to a dynamic modulus at 60° C, E′(60° C), namely E′(20° C)/E′(60° C), of at least 1.2. That is to say, the hard rubber for use should necessarily be such a one as having a characteristic of a large susceptivity to thermal influence such that at a low temperature, it can show a high dynamic modulus of elasticity, while its dynamic modulus of elasticity lowers as the temperature is relatively high. When the dynamic modulus at 20° C, E′(20° C), is smaller than 6.0 MPa, the resistance to the bead detachment tends to be lowered. If the rubber sheet has a ratio E′(20° C/E′(60° C) of at least 1.2, it is possible to meet that at the time the tire temperature is raised after an initial running of the tire, the rigidity in a region below the bead core can be lowered and thereby disturbances which the tire may receive from the pavement surface can be absorbed and alleviated, whereby the generation of road noise can be suppressed. However, if the above ratio E′(20° C)/E′(60° C) is excessively large, then the rigidity in the bead portion when the tire temperature is high tends to be so greatly lowered that the steering stability of the tire is adversely affected. Accordingly, preferably the above ratio E′(20° C)/E′(60° C) should not exceed the above upper limit of 1.6. It may be imagined that if the hardness of the rubber sheet 13 is lowered when the tire is at a high temperature, the rigidity in the region below the bead core in the bead portion may possibly become lowered as stated above and thereby a bead detachment from the rim may possibly take place. However, in connection with the bead detachment from the rim, in addition to structural factors such as the rigidity in the region below the bead core in the bead portion, the inflation pressure of the tire forms an important factor of it. Therefore, if the inflation pressure of the tire is sufficiently high, then the risk of the bead detachment can be little even if the rigidity in the region below the bead core is relatively low. That is to say, a bead detachment from the rim is likely only during an initial period of time immediately after the initiation of running of the tire when the tire inflation pressure remains to be relatively low, and after the air pressure inside the tire is increased after the tire temperature is raised due to tire's own generation of heat or due to heat which tire receives from brake members, the risk of the bead detachment becomes suppressed. Rather than this, on account of a lowering of the hardness of the rubber sheet, the vibration absorption of the tire becomes enhanced and the road noise generation becomes suppressed. In order to further enhance the effect of suppressing the road noise generation, preferably the sheet rubber should have a loss tangent, tan δ, at 60° C, of at least 0.2 according to the present invention. Rubber compositions satisfying the above required characteristics of the rubber sheet may be obtained by blending various additives such as carbon black, vulcanization accelerator and so forth in one or more of rubber components such as natural rubber (NR), acrilonitrile-butadiene rubber (NBR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), and isobutylene-isoprene rubber (IIR). It is possible to appropriately selectively combine together particular types of the rubber components and of the additives, and similarly selectively determine the blending ratios thereof. Now, the results/effects of the present invention will be described in greater detail with reference to Examples of the invention. Examples: There were prepared four different rubber compositions A, B, C and D having respective different blending specifications as shown in the below Table 1, and using the rubber compositions A, B, C and D respectively for the rubber sheet in the bead portion of radial tires having a structure as shown in Fig. 1, there were built a tire of the invention I, a tire of the invention II, a comparative tire I and a comparative tire II. The tires had a same tire size of 195/65 VR 15, the following described belt layer and carcass layer, and a reinforcing layer comprising a plain-weave nylon fabric provided on an outer side of the rubber sheet. Belt Layer: 2 rubberized layers having steel cords of a 1 x 5 (0.25) twist structure, in a cord density of 40 cords/50 mm and in a bias cord arrangement at 24° to the circumferential direction of the tire Carcass Layer: a single layer having 1,000 D/2 polyester fiber cords, in a cord density of 55 cords/50 mm and at a cord angle of substantially 90° to the tire circumferential direction Respective test tires were mounted on a rim of a size of 6 1/2 JJ x 15, and subjected to J-letter shaped cornering tests for the evaluation of the resistance to the bead detachment and also for road noise feeling tests on test cars as below described. J-letter shaped cornering test : Respective test tires were mounted on a test car (passenger car) and subjected to a cornering test at a velocity of 35.5 km/hr on a test course as shown in Fig. 3, comprising a rectilinear course part 30 and a semi-circular course part 31 having a radius of 6 m and connected to the rectilinear course part 30. The tests were carried out in a manner such that the inflation pressure of the tire mounted on the front left wheel of the test car was step-wise reduced from 2.0 kg/cm² each time by 0.1 kg/cm², and inflation pressure values at which a rim touch or a bead detachment from the rim took place were determined. Tests were conduted 5 times for each of the varied inflation pressure values, and for the rating of the tires, reciprocal numbers of the inflation pressure values at the time of the generation of the rim touch or the bead detachment are shown by indices with the value found of the comparative tire I taken as 100. A larger value (index) means a higher and more desirable resistance to the bead detachment. Road noise feeling test on test car : Respective tires mounted on test cars (passenger cars) were subjected to a preliminary running at a velocity of 50 km/hr for 10 minutes to raise the temperature of the tire, and immediately thereafter, were run at a velocity of 50 to 60 km/hr on a road having large and small concave or convex irregularities on the surface such as gravel-paved roads, Belgian roads and crack-formed roads, to determine the loudness, the timbre and the degree of acoustic unpleasantness of noise generated inside the test cars. Determinations were made by five different drivers by feeling, and their evaluated values were averaged. A rating of tires was made by the following 5-point rating method taking the evaluation made of a reference tire as 0. - 2 : worst; - 1 : bad; 0 : reference; + 1 : good; and + 2 : best Results of the J-letter shaped cornering tests and the road noise feeling tests on test cars are shown in the below Table 2. Tire of Invention Comparative Tire I II I II Rubber Coposition of Table 1 B C A D J-letter Shaped Cornering Test 120 115 100 110 Road Noise Test on Test Cars + 1 + 0.5 0 - 0.5 From the above Table 2, it is seen that in comparison to the comparative tire I, each of the tires of the present invention I and II has a conspicuously improved characteristic with respect to each of the resistance to the bead detachment from the rim and the road noise generation. In contrast to this, in comparison to the comparative tire I, the comparative tire II shows poorer characteristic with respect to the road noise generation, even though its resistance to the bead detachment is improved.

Claims: A pneumatic radial tire comprising a pair of a left bead portion (10) and a right bead portion (10) each having a bead core (5), and at least a single carcass layer (4) having its end portions turned up from the inner side to the outer side of the tire in a manner of enrobing the bead cores, characterized in that in a region between the bead core (5) and at least an outer surface portion of the bead portion in contact with the rim (15) on which the tire is mounted, there is incorporated a rubber sheet (13) having a dynamic modulus of elasticity at 20° C, E′(20° C), of at least 6.0 MPa and a ratio of the dynamic modulus E′(20° C) to a dynamic modulus of elasticity at 60° C, E′(60° C), namely E′(20° C)/E′(60° C), of at least 1.2. A pneumatic radial tire as claimed in claim 1, wherein the ratio E′(20° C)/E′(60° C) is 1.2 to 1.6. A pneumatic radial tire as claimed in claim 1, wherein the rubber sheet has a loss tangent, tan δ, at 60° of at least 0.2. A pneumatic radial tire as claimed in claim 1, wherein the rubber sheet is incorporated in an inner layer of the bead portion. A pneumatic radial tire as claimed in claim 1, wherein the rubber sheet is disposed along an outer surface of the bead portion. A pneumatic radial tire as claimed in claim 1, wherein the rubber sheet extends up to a region inner of the bead core in the bead portion. A pneumatic radial tire as claimed in claim 1, which is for a passenger car.

Inventors: HANADA, RYOJI, MISAWA, MAKOTO
Applicants: THE YOKOHAMA RUBBER CO., LTD.
Application year: 1989
Main classification: B60C15
IPC codes: B60C15