Patent Application
20140299247
The present invention relates to a pneumatic radial tire for a passenger vehicle.
Bias tires having relatively narrow cross sectional widths were predominantly used in vehicles up to around 1960 because vehicles in those days were relatively lightweight, had relatively low cruising speed required thereof and thus did not put so much stress on the tires. However, radial tires having wide and flat structures are predominant these days because good driving stability in high speed running, as well as good wear resistance, is required of tires as highway networks are developed and vehicle speed increases (e.g. PTL 1).
However, increasing widths of tires decreases free space in a vehicle and deteriorates comfortablility therein. This is becoming a big issue because electric vehicles which have been developed for use in recent years, in particular, must have sufficient space for accommodating driving units such as a motor for controlling torque of rotating tires around drive shafts and in this regard ensuring sufficient space in vicinities of tires thereof is increasingly important.
Further, there has been increasingly a demand for a better fuel efficiency in recent years as people are more concerned about environmental issues. It has been conventionally known that increasing diameter and width of a tire is effective in terms of decreasing rolling resistance value (RR value) of the tire for better fuel efficiency thereof. Increasing diameter and width of a tire, however, also increases weight of the tire and air resistance of a vehicle, thereby resulting in an increase in resistance experienced by the vehicle and too much load on the tire.
In this connection, decreasing diameter and width of a tire to reduce weight thereof then results in deterioration of controllability of the tire.
PTL 1: JP-A 07-040706
It is in general difficult to achieve good fuel efficiency, good comfortablility in a vehicle (free space in a vehicle) and other good performances of a tire simultaneously in a compatible manner, as described above. A technique of comprehensively improving these performances in a compatible manner has therefore been demanded.
The present invention aims at solving the problems described above and an object thereof is to provide a pneumatic radial tire for a passenger vehicle where durability and wear resistance of the tire have been improved with ensuring high fuel efficiency and wide free space in a vehicle.
The inventors of the present invention keenly studied to solve the problems described above.
As a result, the inventors first discovered that reducing a tire width and increasing a tire diameter or, more specifically, controlling a cross sectional width SW and an outer diameter OD of a radial tire under an appropriate SW-OD relationship is very effective in terms of ensuring good fuel efficiency and wide free space of a vehicle using the radial tire.
Further, the inventors newly discovered that it is effective to optimize configuration of a crown portion of a radial tire having small width and large diameter in terms of improving durability and wear resistance of the tire.
The present invention has been contrived based on the aforementioned discoveries and main structural features are as follows.
(1) A pneumatic radial tire for a passenger vehicle, having a carcass constituted of plies as radially-disposed carcass cords and provided in a toroidal shape across a pair of bead portions, a belt constituted of at least one belt layer, and at least one belt reinforcing layer as a rubber-coated cord layer extending in the tire circumferential direction, the belt and the belt reinforcing layer being provided on the outer side in the tire radial direction of the carcass, characterized in that:
provided that SW and OD represent cross sectional width and outer diameter of the tire, respectively, SW/OD≦0.26 when SW<165 (mm) and
SW and OD satisfy a formula shown below when SW≧165 (mm)
OD≧2.135×SW+282.3; and
provided, in a cross section in the width direction of the tire, that: m1 represents an imaginary line passing through a point P on a tread surface at the tire equatorial plane and extending in parallel to the tire width direction; m2 represents an imaginary line passing through a ground contact end E and extending in parallel to the tire width direction; LCR represents a distance in the tire radial direction between line m1 and line m2; and TW represents a tread width of the tire, a ratio LCR/TW≦0.045.
In the present invention, the aforementioned “ground contact end E” represents in a cross section in the tire width direction of a tire each of respective end points in the tire width direction of a ground contact surface of the tire in contact with a flat plate when the tire is placed upright with respect to the flat plate in a standard state where the tire is assembled with a rim and inflated at the maximum air pressure under the maximum load respectively prescribed for each vehicle on which the tire is to be mounted.
The aforementioned “tread width TW” and “distance LCR” of a tire are to be measured, respectively, in a state where the tire is assembled with a rim, inflated at the maximum air pressure prescribed for each vehicle on which the tire is to be mounted, and has no load exerted thereon. The “tread width TW” specifically represents a distance in the tire width direction between the ground contact ends E, E of the tire in this (no load) state.
The “distance LCR” specifically represents a distance in the tire radial direction between line m1 and line m2 of the tire in this state.
“The maximum load prescribed for each passenger vehicle” represents in the present invention the largest load value among respective four load values exerted on four tires of the passenger vehicle when the prescribed upper limit number of occupants ride in the passenger vehicle.
(2) A pneumatic radial tire for a passenger vehicle, having a carcass constituted of plies as radially-disposed carcass cords and provided in a toroidal shape across a pair of bead portions, a belt constituted of at least one belt layer, and at least one belt reinforcing layer as a rubber-coated cord layer extending in the tire circumferential direction, the belt and the belt reinforcing layer being provided on the outer side in the tire radial direction of the carcass, characterized in that:
provided that SW and OD represent cross sectional width and outer diameter of the tire, respectively, SW and OD satisfy a formula shown below
OD≧−0.0187×SW2+9.15×SW−380; and
provided, in a cross section in the tire width direction of the tire, that: m1 represents an imaginary line passing through a point P on a tread surface at the tire equatorial plane and extending in parallel to the tire width direction; m2 represents an imaginary line passing through a ground contact end E and extending in parallel to the tire width direction; LCR represents a distance in the tire radial direction between line m1 and line m2; and TW represents a tread width of the tire, a ratio LCR/TW≦0.045.
According to the present invention, it is possible to provide a pneumatic radial tire for a passenger vehicle having excellent durability and wear resistance with ensuring high fuel efficiency of and wide free space in a vehicle.
How a pneumatic radial tire for a passenger vehicle of the present invention (which tire will be referred to simply as a “tire” hereinafter) has been realized will be described below.
First, the inventors of the present invention paid attention to a fact that a tire cross sectional width SW (see
A tire cross sectional width SW of a radial tire smaller than that of the conventional radial tire also causes a good effect of reducing an air resistance value (Cd value) of the vehicle because an area of the tire viewed from the front thereof decreases.
However, there is a demerit in this case in that a rolling resistance value (RR value) of the tire increases due to an increase in magnitude of deformation of a ground contact portion of a tread when the internal air pressure of the tire remains the same.
The inventors of the present invention, in view of the aforementioned situation, discovered that the problem can be solved by utilizing the characteristics inherent to a radial tire. Specifically, the inventors of the present invention realized that, in the case of a radial tire having a smaller magnitude of tread deformation than a bias tire, it is possible to make the radial tire be less affected by a rough road surface and thus reduce a rolling resistance value (RR value) thereof when the internal air pressure remains the same by increasing the outer diameter OD (see
In short, reducing width and increasing outer diameter of a tire effectively ensure a wide space in a vehicle, respectively, although they are in a trade-off relationship in terms of a rolling resistance value (RR value). Reducing tire width also successfully decreases an air resistance value (Cd value) of a vehicle.
In view of this, the inventors of the present invention keenly studied optimizing a relationship between a tire cross sectional width and an outer diameter of a tire such that an air resistance value (Cd value) and a rolling resistance value (RR value) of a vehicle improve as compared with the conventional radial tire.
Specifically, the inventors of the present invention, paying their attention to a relationship between a tire cross sectional width SW and an outer diameter OD of a tire, carried out a test including mounting test tires of various tire sizes (some of them were non-standard products) on a vehicle and measuring an air resistance value (Cd value) and a rolling resistance value (RR value) for each type or size of the test tires. A condition satisfied by SW and OD when both of an air resistance value and a rolling resistance value were superior to those of the conventional radial tire was empirically deduced based on the measurement results.
The experiment results from which the optimum relationship between SW and OD was obtained will be described in detail hereinafter.
A plurality of pneumatic radial tires, each having a pair of bead portions 1 and a carcass 2 radially disposed to extend in a toroidal shape across the pair of bead portions 1 as shown in
The tire exemplarily shown in
The belt reinforcing layer 5 can be formed by using cords made or organic fibers such as nylon, Kevlar® or the like. The belt reinforcing layer 5 exemplarily shown in
A plurality of main grooves 6 each extending in the tire circumferential direction are formed in the tread 4 (one main groove in each half portion of the tire exemplarily shown in
In the tire as exemplarily shown in
Further, a ratio LCR/TW of a distance LCR with respect to a tread width TW, as defined above, is 0.05 in the tire as shown in
A number of test tires having various cross sectional widths and outer diameters were prepared based on the tire structures described above.
First, there was prepared as Reference tire 1 a tire having tire size: 195/65R15, which tire size is used in vehicles of the most common types and thus suitable for comparison of tire performances. There was also prepared as Reference tire 2 a tire having tire size: 225/45R17, which is what is called an “inch-up” version of Reference tire 1.
Further, there were also prepared other test tires (test tires 1 to 52 and conventional test tires 1 to 9) of various tire sizes. These test tires were each assembled with a rim, inflated at internal pressure of 220 kPa and subjected to the tests described below.
Table 1 shows relevant characteristics of the respective test tires.
With regard to tire sizes, a variety of tire sizes including the conventional sizes prescribed in JATMA (The Japan Automobile Tyre Manufacturers Association, Inc.) in Japan, TRA (THE TIRE and THE RIM ASSOCIATION INC.) in the United States, ETRTO (European Tyre and Rim Technical Organisation) in Europe and the like and those beyond these conventional sizes were widely studied.
<Rolling Resistance (RR Value)>
Rolling resistance was measured by: assembling each of the test tires described above with a rim to obtain a tire-rim assembly inflated at internal pressure as shown in Tables 2-1 and 2-2; exerting on the tire-rim assembly the maximum load prescribed for a vehicle on which the tire is mounted; and running the tire at drum rotation speed of 100 km/hour to measure a rolling resistance thereof.
The evaluation results are shown as index values relative to “100” of Reference tire 1.
The smaller index value represents the smaller rolling resistance.
<Air Resistance (Cd) Value of Vehicle>
Air resistance was determined by: assembling each of the test tires described above with a rim to obtain a tire-rim assembly inflated at internal pressure as shown in Tables 2-1 and 2-2; mounting the tire-rim assembly on a vehicle of 1500 cc displacement; and blasting air on the tire at speed corresponding to 100 km/hour and measuring an air pressure value experienced by the tire by a balance installed on the floor under the tire.
The results were converted to index values relative to “100” of Reference tire 1 for evaluation. The smaller index value represents the smaller air resistance.
The evaluation results are shown in Tables 2-1, 2-2 and
It has been revealed from the test results shown in Tables 2-1 and 2-2,
SW/OD≦0.26 when SW<165 (mm); and
OD≧2.135×SW+282.3 when SW≧165 (mm)
It has been revealed from the test results shown in Tables 2-1 and 2-2,
OD≧−0.0187×SW2+9.15×SW−380
Further, the inventors of the present invention discovered that test tires 1 to 7 and 17, each satisfying SW/OD≦0.24, more reliably obtain the aforementioned good effect than other test tires, as shown in Tables 2-1, 2-2 and
Next, the following tests were carried out for each of test tires 1 to 18 in order to evaluate fuel efficiency and comfortability (degree of free space) of a vehicle on which the tire was mounted.
<In-Use Fuel Economy>
A test was carried out based on the JOC 8 test cycle prescribed by Ministry of Land, infrastructure, Transport and Tourism (MUT) of Japan. The evaluation results are shown as index values relative to “100” of Reference tire 1. The larger index value represents the better fuel efficiency.
<Comfortability>
Each of the test tires was mounted on a vehicle having 1.7 m width and the resulting width of the rear trunk was measured. The evaluation results are shown as index values relative to “100” of Reference tire 1. The larger index value represents the better comfortability.
The test results thus obtained are shown in Table 3 below.
It is understood from Table 3 that some of the test tires satisfying neither relationship formulae (1) nor relationship formula (2) (see
The inventors of the present invention revealed from the findings described above that it is possible to reduce both air resistance value and rolling resistance value of a pneumatic radial tire in a state where the tire is mounted on a vehicle and also enhance fuel efficiency and comfortability of the vehicle by setting cross sectional width SW and outer diameter OD of the tire to satisfy the aforementioned relationship formulae (1) and/or relationship formula (2).
In this connection, the inventors of the present invention noticed that the tire satisfying the aforementioned relationship formulae (1) and/or relationship formula (2) experiences a problem inherent to a tire having narrow-width and large-diameter that: an input of force (pressure) from a road surface increases due to a narrowed width or a decrease in a ground contact area of the tire, thereby increasing a rate of deflection of a tread in the tire radial direction to increase a magnitude of strain between the belt and the bet reinforcing layer when the tire is rotated, which facilitates occurrence of separation between the belt and the belt reinforcing layer to deteriorate durability of the tire and also makes shearing force generated in rubber between the belt land the belt reinforcing layer uneven in the tire circumferential and width directions, i.e. makes distribution of ground contact pressure uneven, to deteriorate wear resistance properties of the tire. The inventors made a keen studied to solve this problem and discovered a tire structure which can solve the problem.
The tire structure for improving durability and wear resistance of a pneumatic radial tire for a passenger vehicle satisfying the aforementioned relationship formulae (1) and/or relationship formula (2), thus discovered, will be described hereinafter.
As shown in
In the tire exemplarily shown in
It is critically important that the ratio LCR/TW is equal to or less than 0.045 in the tire of the present invention, as described above.
Other structures and the like of the tire shown in
The “width W1 in the tire width direction of the belt reinforcing layer 8” represents a width in the tire width direction of the belt reinforcing layer having the narrowest tire-width-direction width in a case where there exist a plurality of belt reinforcing layers.
An effect of the present invention will be described hereinafter.
According to the present invention, a crown portion of the tire tread is made flat and a ground contact area thereof increases so that an input of force (pressure) from a road surface is mitigated and a rate of deflection of the tread in the tire radial direction decreases to improve durability and wear resistance of the tire by setting the ratio LCR/TW to be in the aforementioned range.
The ratio LCR/TW>0.045 fails to ensure a satisfactorily large ground contact area, thereby increasing a rate of deflection of the tread to deteriorate durability and wear resistance of the tire, as described above. The ratio LCR/TW≦0.04 is preferable in terms of more reliably obtaining good durability and wear resistance of the tire.
On the other hand, the ratio LCR/TW≧0.025 is preferable in terms of avoiding deterioration of ground contact properties and ensuring good performances in cornering power, wear resistance and fuel efficiency of the tire.
Young' modulus of cords of the belt reinforcing layer 8 is preferably 15,000 MPa in the present invention.
Young' modulus of cords of the belt reinforcing layer 8≧15,000 MPa enhances ring rigidity of the tire to suppress deformation thereof in the tire circumferential direction, thereby suppressing deformation thereof in the tire width direction, as well, due to incompressibility of rubber.
As a result, a relatively wide region in the tire circumferential direction of a ground contact surface is deformed by an input of force in the tire width direction from a road surface, whereby the ground contact surface takes on a configuration in which ground contact length in the tire circumferential direction does not change so much in the tire width direction.
Accordingly, partial wear resistance and cornering power of the tire improve in this case.
In this connection, too high Young's modulus of cords of the belt reinforcing layer 8 decreases ground contact length of the tread as a whole and deteriorates the maximum cornering force. Young's modulus of cords of the belt reinforcing layer 8 is therefore preferably 30,000 MPa.
Further, each belt layer 7a, 7b is a slant belt layer including belt cords extending to be inclined at an angle 45° with respect to the tire circumferential direction and the belt 7 is constituted of a plurality of the slant belt layers 7a, 7b such that belt cords of one slant belt layer intersect belt cords of the other slant belt layer alternately.
The belt layer preferably has the structure described above because the belt cords disposed to be inclined at a large angle with respect to the tire circumferential direction decrease out-of-plane bending rigidity in the tire circumferential direction of the tread, increases elongation in the tire circumferential direction of rubber when a ground contact surface of the tread is deformed and thus successfully suppresses a decrease in ground contact length of the tire, thereby well improving cornering power and partial wear resistance of the tire.
In this connection, the inclination angle of the belt cords of the belt layer with respect to the tire circumferential direction is preferably 75° in terms of avoiding deterioration of ground contact properties and ensuring good performances in cornering power, wear resistance and fuel efficiency of the tire.
In the tire of the present invention, a ratio W1/W2 of a width W1 in the tire width direction of the belt reinforcing layer 8 with respect to a width W2 in the tire width direction of the narrowest width belt layer 7a of the two belt layers 7a, 7b constituting the belt 7 preferably satisfies 0.8≦W1/W2≦1.05, as shown in
The ratio W1/W2 is preferably 1.05 because then a width of the belt reinforcing layer is made substantially equal to or slightly narrower than a width of the narrowest width belt layer, which not only decreases rigidity of the belt reinforcing layer to reduce strain generated due to difference in rigidity between the belt and the belt reinforcing layer and thus suppress deterioration of durability of the tire caused by the strain but also makes shearing force generated in rubber between the belt land the belt reinforcing layer uniform in the tire circumferential direction to further improve wear resistance properties of the tire. Further, the ratio W1/W2≦1.05 improves ground contact properties of the tire, thereby improving cornering power, cornering force and fuel efficiency thereof, as well.
On the other hand, the ratio W1/W2≧0.8 prevents a width of the belt reinforcing layer from being too narrow, ensures satisfactorily high ring rigidity and thus reliably attains good wear resistance and cornering power of the tire.
The width W2 is preferably 0.85 to 1.10 times as much as the tread width TW.
The tire of the present invention preferably has air volume ≧15,000 cm3 because a tire for a passenger vehicle must have an air volume ≧15,000 cm3 in order to reliably have the minimum loading capacity essentially required of a passenger car running on public roads.
Test tires of Examples 1 to 5 and test tires of Comparative Examples 1 to 4 were prepared and subjected to tests described below for evaluating various performances of the tires in order to confirm an effect of the present invention.
Specifically, each of the test tires of Examples 1 to 5 and Comparative Examples 1 to 4 has a pair of bead portions and a carcass provided to extend in a toroidal shape across the pair of bead portions, as shown in
Further, the test tire has a belt constituted of two belt layers, a belt reinforcing layer, and a tread provided on the outer side in the tire radial direction of the carcass in this order.
The two belt layers are slant belt layers provided such that belt cords of one layer intersect belt cords of the other layer alternately and that steel cords as the belt cords of each layer extend to be inclined at an inclination angle of ±60° with respect to the tire circumferential direction.
Cords of the belt reinforcing layer are made of nylon and extend substantially in the tire circumferential direction.
<Rolling Resistance (RR Value)>
Rolling resistance was measured by: inflating each of the test tires at air pressure as prescribed for each vehicle on which the tire was to be mounted; exerting on the tire 80% of the maximum load prescribed for the vehicle on which the tire was to be mounted; and running the tire at drum rotation speed of 100 km/hour to measure a rolling resistance thereof.
The evaluation results are shown as index values relative to “100” of Comparative Example 3. The smaller index value represents the smaller rolling resistance.
<Durability>
Durability was measured by: inflating each of the test tires at air pressure as prescribed for each vehicle on which the tire was to be mounted; exerting on the tire the maximum load prescribed for the vehicle on which the tire was to be mounted; subjecting the tire to a drum durability test at high speed, i.e. running the tire on a drum under the aforementioned conditions and increasing the speed from 120 km/hour stepwise by 10 km/hour for every 5 minutes until the tire had a trouble; recoding the speed at which the trouble occurred as a measurement value for evaluating durability of the tire. The evaluation results are shown as index values relative to “100” representing the speed at which a trouble occurred in the Comparative Example 3 tire. The larger index value represents the better durability.
<Cornering Power>
Cornering power of each test tire was measured by using a flat belt type tire testing apparatus for measuring the cornering characteristics thereof at the air pressure and under the maximum load as respectively prescribed for each vehicle on which the tire was to be mounted, at drum rotation speed of 100 km/hour.
The cornering power values thus measured were converted to index values relative to the cornering power value “100” of Comparative Example 3 tire for evaluation. The larger index value represents the larger and thus more preferable cornering power.
<Wear Resistance>
Wear resistance of each test tire was determined by: running the tire 30,000 km on a drum testing machine at 80 km/hour at the air pressure and under the maximum load as respectively prescribed for each vehicle on which the tire was to be mounted; and measuring a remaining groove depth after the running on the drum as a wear resistance value. The wear resistance values thus measured were converted to index values relative to the wear resistance value “100” of Comparative Example 3 tire for evaluation. The larger index value represents the better wear resistance.
The relevant characteristics and evaluation results of the respective test tires are shown in Table 4.
It is understood from Table 4 that the tires of Examples 1 to 5 unanimously exhibit lower rolling resistance values and better or higher durability, cornering power and wear resistance values than the tires of Comparative Examples 1 and 3.
Further, it is understood from comparison of Example 2 with Comparative Example 2 in Table 4 that the tire of Example 2 in which the relationship of SW and OD has been optimized exhibits lower rolling resistance value and better or higher durability, cornering power and wear resistance values than the tire of Comparative Example 2.
Yet further, it is understood from comparison of Example 2 with Comparative Example 4 in Table 4 that the tire of Example 2 in which the ratio LCR/TW been optimized exhibits lower rolling resistance value and better or higher durability, cornering power and wear resistance values than the tire of Comparative Example 4.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-241672 | Nov 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/007052 | 11/2/2012 | WO | 00 | 5/1/2014 |
