Patent Description:
Conventionally, as known from <CIT>, <CIT> and <CIT>, plies with a plurality of cords are used as a component of pneumatic tyres. In particular, <CIT> discloses a pneumatic tyre as specified in the preamble of claim <NUM>.

It is also known that the above cords affect various performances of pneumatic tyres. For example, <CIT> has proposed a pneumatic tyre capable of reducing rolling resistance by setting the intermediate elongation of carcass cords.

When tyres travel, plies with cords having multiple filaments may undergo repeated compression deformation, causing the multiple filaments to break little by little, and then reducing the rigidity of plies. In particular, when driving at high speeds, tyres are often deformed, there has been a problem that the steering stability tends to decrease due to the decrease in rigidity of plies.

The present invention has been made in view of the above circumstances and has a major object to provide a pneumatic tyre capable of sustaining excellent steering stability at high speeds.

According to the present invention, the above object is achieved by a pneumatic tyre as specified in claim <NUM>.

Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. <FIG> illustrates a meridian cross-sectional view of a pneumatic tyre (hereinafter, simply referred to as "tyre") <NUM> in accordance with an embodiment of the present invention. In <FIG>, the tyre <NUM> is in a normal state. As illustrated in <FIG>, the tyre <NUM> according to the present embodiment is a pneumatic tyre for passenger car. However, the present invention may be applied to heavy-duty tyres or motorcycle tyres.

As used herein, when the tyre is based on a standard, the "normal state" is such that the tyre <NUM> is mounted onto a standard wheel rim with a standard pressure but loaded with no tyre load. If a tyre is not based on the standards, the normal state is a standard state of use according to the purpose of use of the tyre and means a state of no tyre load. As used herein, unless otherwise noted, dimensions of portions of the tyre are values measured under the normal state.

As used herein, the "standard pressure" is a standard pressure officially approved for each tyre by standards organizations on which the tyre is based, wherein the standard pressure is the "maximum air pressure" in JATMA, the maximum pressure given in the "Tire Load Limits at Various Cold Inflation Pressures" table in TRA, and the "Inflation Pressure" in ETRTO, for example.

As illustrated in <FIG>, the tyre <NUM> according to the present embodiment includes a carcass <NUM>. The carcass <NUM>, for example, is composed of a single carcass ply 6A. The carcass ply 6A includes a plurality of carcass cords covered with a topping rubber. The carcass cords, for example, are oriented at an angle of from <NUM> to <NUM> degrees with respect to the tyre circumferential direction. For the carcass cords, organic fiber cords such as nylon, polyester or rayon may preferably be used, for example.

The carcass ply 6A includes a main portion 6a and a pair of turn-up portions 6b. The main portion 6a extends, through a pair of sidewall portions <NUM> and a tread portion <NUM>, between axially spaced bead portions <NUM>. The turn-up portions 6b are connected to the main portion 6a and turned up around bead cores <NUM> from axially inside to outside of the tyre to extend outwardly in the tyre radial direction.

In the present embodiment, the tread portion <NUM> includes a tread reinforcing layer <NUM>. The tread reinforcing layer <NUM>, for example, includes a belt layer <NUM>. The belt layer <NUM>, for example, includes two belt plies 8A and 8B. The belt plies 8A and 8B, for example, include a plurality of belt cords inclined at an angle with respect to the tyre circumferential direction and a topping rubber covering the belt cords. Preferably, the belt cords are oriented at an angle of from <NUM> to <NUM> degrees with respect to the tyre circumferential direction.

The tread reinforcing layer <NUM>, for example, further includes a band layer <NUM>. The band layer <NUM>, for example, is composed of a single band ply 9A. The band ply 9A includes one or more band cords that are oriented at an angle equal to or less than <NUM> degrees with respect to the tyre circumferential direction. In some preferred embodiments, the band ply 9Ais configured as a so-called jointless band ply in which a single band cord is spirally wound in the tyre circumferential direction.

<FIG> illustrates an enlarged perspective view of a ply <NUM> included in the tyre in accordance with the present embodiment. As illustrated in <FIG>, the tyre <NUM> according to the present embodiment includes the ply <NUM> which includes a plurality of cords <NUM> covered with a topping rubber <NUM>. The ply <NUM> may apply to at least one of the carcass ply 6A, the belt plies 8A, 8B and the band ply 9A described above.

<FIG> illustrates an enlarged cross-sectional view of one of the cords <NUM>. As illustrated in <FIG>, each cord <NUM> includes a plurality of filaments <NUM> twisted together. The plurality of filaments each has an outer diameter d. In some preferred embodiments, each cord <NUM> according to the present embodiment includes a plurality of preliminary twisted yarns <NUM> in which a plurality of filaments was twisted together in advance. The preliminary twisted yarns <NUM> are twisted together. In the present embodiment, each cord <NUM> is composed of two preliminary twisted yarns <NUM>. Further, in each cord <NUM> according to the present embodiment, since the two preliminary twisted yarns <NUM> are twisted together, portions 13a of outer surfaces of the preliminary twisted yarns <NUM> (corresponding to contact surfaces of the two preliminary twisted yarns <NUM>) are flattened. As a result, the cross-sectional shape of each cord <NUM> is a substantially oval shape in which the cross-sectional width is small at the center thereof.

Each of the cords <NUM> has a ratio D/d of an average cord diameter D to the outer diameter d of each filament being equal to or more than <NUM>. In such a cord <NUM>, since the thin filaments <NUM> are tightly twisted together, even during high-speed running where a large tensile stress repeatedly acts on the cords <NUM>, the deformation generated per filament <NUM> tends to be small and the stress that the cord <NUM> can exert can be increased. For this reason, the breakage of the filament <NUM> inside the cord <NUM> can be suppressed, and excellent steering stability can be maintained at high-speed running. Note that the cords <NUM> having the ratio D/d of <NUM> or more can be produced by appropriately combining known production methods.

The average code diameter D is calculated by the simple average of a major axis D1 and a minor axis D2 in the cross section of each cord <NUM>. The major axis D1 means the maximum diameter of each cord <NUM>. The minor axis D2 means the largest diameter of each cord <NUM> in the direction orthogonal to the major axis D1. Each cord <NUM> according to the present embodiment has a constant cross-sectional shape and extends in the length direction of the cord <NUM>. Alternatively, the cross-sectional shape and cross-sectional area of the cord <NUM> may change in the length direction of the cord <NUM>. In this case, it is preferable that the average cord diameter D be measured at the position where the cross-sectional area of the cord <NUM> is minimum. This is because the substantial tensile strength of a cord <NUM> depends on the configuration of the cord <NUM> at the position where the cross-sectional area of the cord <NUM> is minimum.

In general, in the ply <NUM> having the above-mentioned cords <NUM>, one or more of the plurality of filaments <NUM> constituting the cord <NUM> are gradually broken due to repeated compression deformation when the tyre travels. As a result, the rigidity of the ply <NUM> may decrease. In particular, when driving at high speeds, the tyre is deformed frequently, so the steering stability tends to decrease due to the decrease in rigidity of the ply <NUM>.

Although various studies have been conducted on cords of the plies, much attention has not been paid to the relationship between cords and a rubber member constituting the tyre. The inventors have focused on the relationship between the cords <NUM> of the ply <NUM> constituting the pneumatic tyre and a rubber member of the tyre, which had not received much attention in the past. Then, the inventors have made completed the present invention by analyzed these in detail.

As illustrated in <FIG>, the tyre <NUM> includes a pair of sidewall rubbers <NUM>. The sidewall rubbers <NUM>, for example, are disposed outwardly in the tyre axial direction of the carcass <NUM> in the pair of sidewall portions <NUM> to form outer surfaces of the sidewall portions <NUM>. In the present invention, the ratio D/d and a loss tangent tanδ of at least one sidewall rubber <NUM> satisfy the following formula (<NUM>); <MAT>.

Here, the loss tangent tanδ of the at least one sidewall rubber <NUM> is a value measured using a dynamic viscoelasticity measuring device (Iplexer series) manufactured by GABO under the following conditions in accordance with the provisions of JIS-K6394. The test sample at the time of measurement is a rubber piece taken from the sidewall rubber <NUM>, having a length of <NUM> in the tyre circumferential direction, a width of <NUM> in the tyre radial direction, and a thickness of <NUM>.

In the present invention, by satisfying the above formula (<NUM>), the heat generation of the sidewall rubber <NUM> can be reduced, and the decrease in rigidity of the sidewall rubber <NUM> can be suppressed. On the other hand, since the ratio D/d is sufficiently large for the heat generation of the sidewall rubber <NUM>, breakage of some filaments <NUM> (shown in <FIG> and the same applies hereinafter) in the ply <NUM> (shown in <FIG> and the same applies hereinafter) can be suppressed, and the decrease in rigidity of the ply <NUM> can effectively be suppressed. In addition, since the decrease in rigidity of the sidewall portions <NUM> and the ply <NUM> can be suppressed, cornering force can be generated with good response when a steering angle is applied to the tyre <NUM>. Due to such an action, the tyre <NUM> according to the present invention can continuously exhibit excellent steering stability at high-speed running.

Hereinafter, a more detailed configuration of the present embodiment will be described. Note that each configuration described below shows a specific aspect of the present embodiment. Thus, the present invention can exert the above-mentioned effects even if the tyre does not include the configuration described below. Further, if any one of the configurations described below is applied independently to the tyre of the present invention having the above-mentioned characteristics, the performance improvement according to each additional configuration can be expected. Furthermore, when some of the configurations described below are applied in combination, it is expected that the performance of the additional configurations will be improved.

In the present invention, the ply <NUM> can be applied to the carcass ply 6A, the belt plies 8A and 8B, and/or the band ply 9A. In this embodiment, the ply <NUM> described above is applied to the carcass ply 6A and the band ply 9A. By setting the ply <NUM> which satisfies the above relationship as the carcass ply 6A, it is considered that the reaction force of the ply <NUM> can be increased while suppressing the decrease in rigidity of the side portions of the tyre. In addition, by setting the ply <NUM> which satisfy the above relationship as the band ply 9A, a large reaction force can be generated by the ply <NUM> in the tread portion, and the reaction force can be transmitted with good response in the side portions whose rigidity is maintained high. Hence, it is considered that the tyre according to the present embodiment can continuously exhibit excellent steering stability at high-speed running.

The present invention is not limited to the above-mentioned embodiment, and the above-mentioned ply <NUM> may be applied to belt plies 8A and 8B. Further, in another embodiment according to the present invention, the ply <NUM> described above may be applied to a reinforcing ply (not illustrated) for reinforcing the tread portion <NUM>, the sidewall portions <NUM> and/or the bead portions <NUM>. Such a reinforcing ply can help to further improve the steering stability at high-speed running.

In some more preferred embodiments, the ratio D/d and the tanδ of at least one of the sidewall rubbers satisfy the following formula (<NUM>); <MAT>.

This can further enhance the above effects.

The loss tangent of the at least one sidewall rubber <NUM> is preferably equal to or more than <NUM>, more preferably equal to or more than <NUM>, but preferably equal to or less than <NUM>, more preferably equal to or less than <NUM>. Such a sidewall rubber <NUM> can be prepared by appropriately adjusting and combining known materials.

The above-mentioned sidewall rubber <NUM> can be obtained by appropriately combining known materials. For the rubber components of the sidewall rubber <NUM>, isoprene rubber, butadiene rubber (BR), styrene butadiene rubber (SBR), acrylonitrile butadiene rubber (NBR), chloroprene rubber (CR), butyl rubber (IIR), styrene-isoprene-butadiene copolymer rubber (SIBR) and the like can be employed. These may be used alone or in combination of two or more.

Further, fillers such as carbon black and silica, plasticizers such as oil and resin, processing aids such as fatty acids, vulcanization agents such as sulfur, vulcanization accelerators and the like can be appropriately added to the above-mentioned rubber components. The loss tangent tanδ of the sidewall rubber <NUM> according to the present embodiment can be adjusted by changing the glass transition point of the rubber component and the type and amount of the filler, the plasticizer, the vulcanizing agent, and the vulcanization accelerator as appropriate. In particular, by relatively reducing the amount of filler and plasticizer and relatively increasing the amount of vulcanizing agent and vulcanization accelerator as compared with the conventional rubber material, the loss tangent tanδ of the sidewall rubber <NUM> can be set in the above range.

As illustrated in <FIG>, the cords <NUM> included in the ply <NUM> are organic fiber cords. The filaments <NUM> of the cords <NUM> according to the present embodiment include polyester fibers. The cords <NUM> like this can help to reduce the cost of manufacturing tyres. However, the material applied to the cords <NUM> is not limited to such an embodiment, and various materials such as nylon, rayon or aramid can be applied.

The number of cords <NUM> contained in a <NUM> width of the ply <NUM>, for example, is in a range of from <NUM> to <NUM>, preferably <NUM> to <NUM>. Also, the fineness of each cord <NUM> is preferably equal to or more than <NUM> dtex, more preferably equal to or more than <NUM> dtex, still further preferably equal to or more than <NUM> dtex, but preferably equal to or less than <NUM> dtex, more preferably equal to or less than <NUM> dtex, still further preferably equal to or less than <NUM> dtex. In the present embodiment, each cord <NUM> is composed of two preliminary twisted yarns <NUM>. The fineness of each preliminary twisted yarn <NUM> is preferably equal to or more than <NUM> dtex, more preferably equal to or more than <NUM> dtex, but preferably equal to or less than <NUM> dtex, more preferably equal to or less than <NUM> dtex.

As illustrated in <FIG>, to further improve the above effects, the ratio D/d of the cords <NUM> is preferably equal to or more than <NUM>, more preferably equal to or more than <NUM>. On the other hand, when the ratio D/d is excessively large, manufacturing cost of the cords <NUM> tends to increase. Thus, the ratio D/d is preferably equal to or less than <NUM>, more preferably equal to or less than <NUM>.

As illustrated in <FIG>, the average cord diameter D of the cords <NUM>, for example, is preferably in a range of from <NUM> to <NUM>, more preferably <NUM> to <NUM>. The outer diameter d of the filaments <NUM>, for example, is preferably in a range of from <NUM> to <NUM>, more preferably <NUM> to <NUM>. However, the cords <NUM> and the filaments <NUM> are not limited to such dimensions.

Preferably, the cords <NUM> has a small heat shrinkage rate. The heat shrinkage rate of the cords <NUM> is preferably equal to or less than <NUM>%, more preferably equal to or less than <NUM>%, still further preferably equal to or less than <NUM>%. Such a cord <NUM> does not shrink excessively even at high-speed running, and can further improve steering stability of the tyre.

As used herein, the "heat shrinkage rate" means the "dry heat shrinkage rate after heating" of the cord after heating at a temperature of <NUM> degrees C for five minutes under no load, which is measured in accordance with JIS-L1017, Section <NUM> (b) "Dry heat shrinkage rate after heating (method B)".

Preferably, the cords <NUM> has an intermediate elongation equal to or less than <NUM>%, more preferably equal to or less than <NUM>%, still further preferably equal to or less than <NUM>%. Such a cord <NUM> can help to sustain excellent steering stability of the tyre. As used herein, the "intermediate elongation" means the elongation (%) when a constant load specified by the standard is applied in accordance with the chemical fiber tyre cord test method of JIS L1017.

The composition of the cords <NUM> above applies to the composition of the cords taken from new and unused tyre <NUM>.

While the particularly preferable embodiments of the tyre in accordance with the present invention have been described in detail, the present invention is not limited to the illustrated embodiments, but can be modified within the scope of the invention as defined by the claims.

Pneumatic tyres of size <NUM>/60R16 that satisfy the present invention were prepared based on the specifications in Tables <NUM>-<NUM>. For comparison, pneumatic tyres of reference examples <NUM> to <NUM> were also prepared. The tyres of reference examples <NUM> to <NUM> had substantially the same configuration as the tyres of Examples except for the specifications shown in Tables <NUM> to <NUM>. For each test tyre, steering stability at high speeds (when the tyre is new and when the tyre is used late) was tested. The common specifications and test methods for test tyres are as follows.

Steering stability when driving at high speeds in the above test vehicle was evaluated by the driver's sensuality. In Tables, "steering stability of new tyre" is an evaluation of the steering stability of a new test tyre that has been run-in. In Tables, "Steering stability of used tyre" is an evaluation of the steering stability of the test tyre after traveling <NUM> on a general road. The test results are indicated in Tables using a score with the steering stability of Ref. <NUM> as <NUM>. The larger the value, the better the steering stability at high speeds.

Tables <NUM> to <NUM> show the test results.

The sidewall rubber compounds A to C shown in Tables <NUM> to <NUM> are as shown in Table <NUM> below.

Note that the total score of steering stability when the tyre is new and when the tyre is used late may be used as an index for comprehensive evaluation of steering stability during high-speed running.

Claim 1:
A pneumatic tyre (<NUM>) comprising:
a ply (<NUM>) having a plurality of cords (<NUM>) each having an average cord diameter D; and
a sidewall rubber (<NUM>) having a loss tangent tanδ, as measured in accordance with the provisions of JIS-K-<NUM> and under the conditions specified in the description,
wherein
the plurality of cords (<NUM>) each comprises a plurality of filaments (<NUM>) twisted together, the plurality of filaments (<NUM>) each having an outer diameter d,
characterized in that
the plurality of cords (<NUM>) each has a ratio D/d of the average cord diameter D to the outer diameter d being equal to or more than <NUM>, and
the ratio D/d and the loss tangent tanδ of the sidewall rubber (<NUM>) satisfy the following formula (<NUM>); <MAT>