Pneumatic radial tires

Abstract: A pneumatic radial tire having good steering stability, resistance to hydroplaning and improved noise reduction comprises a carcass (2), a belt (4) and a tread portion (8) provided with a plurality of tread grooves and having an aspect ratio of not more than 50%, in which a ratio of ground contacting width (PW) of tire tread to rim width (RW) under defined measuring conditions is less than 0.85 and a negative ratio is not more than 25%.

Description: This invention relates to a pneumatic radial tire having an aspect ratio of not more than 50% for use on vehicles running on general-purpose roads, circuit courses and the like, and more particularly to a high-performance tire balancedly satisfying requirements of steering stability and resistance to hydroplaning and reducing noise. A conventional tire corresponding to the preamble of claim 1 is shown in figure 4. For example, a pneumatic radial tire used on a sport-type vehicle and having an aspect ratio of not more than 50% or so-called high-performance tire attaches much importance to steering stability and resistance to hydroplaning among performances required in usual passenger car tires, but does not treat other performances such as noise reduction as a problem. Recently, there has been a tendency to increase noise regulations throughout the world, so that it is demanded to reduce the noise of high-performance tires likewise the usual passenger car tires. In order to obtain sufficient steering stability in a high-performance tire, it is known to increase the ground contacting width of the tread as compared with the usual passenger car tires. Specifically, the tread ground contacting width of the high-performance tire is generally within a range of 0.85-1.00, particularly 0.9-1.0 as a ratio to a rim width. Although the resistance to hydroplaning is improved by the adoption of a directional pattern based on water flowing analysis, a negative ratio of tread grooves arranged in the tread (a ratio of total area of tread grooves formed in the tread portion to the area of the tread portion) is generally about 33-40%, which is larger than the negative ratio (about 30%) of the usual tire, from a viewpoint of compensating for the degradation of the resistance to hydroplaning resulting from the size of the ground contacting width. In addition, a groove width of a tread groove arranged in the tread portion of a high-performance tire is generally within a range of 8 mm to 10 mm or more, which is larger than a groove width (5-8 mm) of a main groove and a lateral groove in the usual tire, when the main groove extends along the circumference of the tire or obliquely at a relatively small inclination angle with respect to the circumference, or the lateral groove extends along a widthwise direction of the tire or obliquely at a relatively small inclination angle with respect to the widthwise direction, for ensuring a large groove area in respect of a small groove number so as not to decrease the block rigidity of the tread. In general, there is a relationship between the tread ground contacting width and the negative ratio considering the steering stability and the resistance to hydroplaning as mentioned below. That is, it is useful to increase the tread ground contacting width for further improving the steering stability. In this case, however, the discharging ability of water intruded in a ground contact region of the tire (particularly central region) toward the side of the tire lowers (which results from the fact that a distance from the central region to an end of the tread ground contacting width becomes long) to bring about degradation of the resistance to hydroplaning. Therefore, it is required to increase the negative ratio in order to compensate for such a degradation. On the other hand, it is useful to increase the negative ratio for further improving the resistance to hydroplaning. In the latter case, however, the substantial ground contact area of the tread decreases to degrade the steering stability. Therefore, it is required to increase the tread ground contacting width in order to compensate for such a degradation. If it is intended to further improve either the steering stability or the resistance to hydroplaning by the conventional technique of adjusting the tread ground contacting width and the negative ratio as mentioned above, therefore it is required to take means for preventing the degradation of the other performance, and both the tread ground contacting width and the negative ratio should be increased. For this purpose, the tire size should be made large. When both the tread ground contacting width and the negative ratio are increased, the total groove area of the tread grooves inclusive of the above main grooves and lateral grooves located in the ground contact region of the tread increases, and also one length of a front edge portion in the ground contact region of the tire increases. This results in the occurrence of a large noise because sound generated by contacting the tire with the ground in the ground contact region of the tire is resonantly enlarged in a tubular region defined between the tire and the road surface by the main groove to largely create so-called columnar resonance sound and also impact sound of a corner part of the lateral groove is increased. It is, therefore, an object of the invention to provide pneumatic radial tires simultaneously establishing steering stability and resistance to hydroplaning by using a technique entirely different from the conventional technique of increasing both the tread ground contacting width and the negative ratio and reducing the noise level. According to the invention, there is provided a pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead portions, a belt superimposed about a crown portion of the carcass and a tread portion disposed on the belt and provided with a plurality of tread grooves and having an aspect ratio of not more than 50%, characterized in that when the tire is mounted onto a rim and vertically placed on a flat plate under an internal pressure of 2.5 bar at a static state and a load corresponding to 70% of the maximum load capacity determined by load index in accordance with the above internal pressure is applied thereto, the ratio of ground contacting width (PW) of tire tread to rim width (RW) is less than 0.85 and the negative ratio of the total area of the tread grooves to the total tread area is not more than 25%. The term "tread groove" used herein includes fine grooves and sipes in addition to grooves having a relatively wide groove width such as main grooves and lateral grooves arranged in the tread portion. Furthermore, the term "rim" used herein preferably means a measuring rim defined by ETRTO (The European Tyre and Rim Technical Organisation - Standards Manual 1990). In a preferable embodiment of the invention, the tread groove is a main groove extending at an inclination angle within a range of 0-45° with respect to the circumference of the tire and/or a lateral groove extending at an inclination angle within a range of 0-44°, preferably 5-30°, with respect to the widthwise direction of the tire. In other preferable embodiments of the invention, the groove width of a portion corresponding to not less than 80% of all main grooves arranged in the tread portion is not more than 5 mm; and the groove width of a portion corresponding to not less than 90% of all lateral grooves arranged in the tread portion is not more than 6 mm; and the groove width of a portion corresponding to not less than 70% of all tread grooves including the main grooves and lateral grooves is within a range of 0.5-3.0 mm; and the maximum width (CW) of the carcass line of the carcass is smaller than the rim width (RW) and the distance (CH) between the position of the rim flange and the position of the maximum width of the carcass line shifted in the widthwise direction of the tire and projected onto the equatorial plane of the tire at section of the tire is not more than 1/3 of the distance (TH) between the position of the rim flange and the position of the sectional height of the tire projected onto the equatorial plane. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a diagrammatically left-half section view of an embodiment of a pneumatic radial tire (or high-performance tire) according to the invention; Fig. 2 is a schematically developed view of a tread pattern in a tire used in Examples 3 and 4; Fig. 3 is a diagrammatically left-half section view of another embodiment of a pneumatic radial tire according to the invention; and Fig. 4 is a diagrammatically left-half section view of a conventional high-performance tire. In Fig. 1 is sectionally shown a typical embodiment of a pneumatic radial tire according to the invention and Fig. 2 shows a tread pattern of this tire, in which numeral 1 is a pneumatic radial tire, numeral 2 a radial carcass, numeral 3 a crown portion of the radial carcass, numeral 4 a belt, numeral 5 a main groove, numeral 6 the widthwise direction of the tire, numeral 7 a lateral groove, and numeral 8 a tread portion. The tire 1 is a pneumatic radial tire having an aspect ratio of not more than 50% and comprises the radial carcass 2 toroidally extending between a pair of bead portions and the tread portion 8 arranged outward on the crown portion 3 of the carcass and provided with the belt 4, main grooves 5 and sub-grooves 7. In such a tire according to the invention, the ratio of tread ground contacting width to rim width and one negative ratio are made small as compared with those of the conventional high-performance tire. That is, the ratio of tread ground contacting width (PW) of the tire to rim width (RW) under the aforementioned conditions defined by ETRTO is less than 0.85, preferably within a range of 0.70-0.82, and the negative ratio is not more than 25%, preferably 15-22%. By adopting such a construction, the reduction of noise can be attained while balancedly satisfying both the steering stability and the resistance to hydroplaning. That is, the discharge ability of water intruded in the ground contact region of the tire (particularly central region) toward the side of the tire can be enhanced by making the tread ground contacting width small, whereby the resistance to hydroplaning can be improved. In this case, a substantial decreasing quantity of tread ground contact area accompanied with the decrease of the tread ground contacting width can be supplemented by setting the negative ratio to a small value, whereby the steering stability becomes equal to or more than that of the conventional high-performance tire. In addition, since the tread ground contacting width is small, the front edge length of the ground contact region of the tire is short and it is possible to decrease a sound generating source to reduce impact sound by a corner part of the lateral groove, and hence it is possible to reduce the noise level. The main groove 5 may be a straight groove extending along the circumference of the tire (Fig. 2) or a zigzag circumferential groove or a main slant groove extending obliquely at an inclination angle of 0-45°, preferably 5-30°, with respect to the circumference of the tire so as to form a directional pattern. In case of the main slant groove, there may be a straight inclination or a curved inclination or a combination thereof and also the inclination angle may be constant or gradually be increased, according to need. The number of the main grooves 5 is not particularly restricted, but when the main groove 5 is a straight groove as shown in Fig. 2, the number of main grooves is preferably 2-5. Furthermore, a portion corresponding to not less than 80% of all main grooves arranged in the tread portion preferably has a groove width of not more than 5 mm from a viewpoint of noise reduction. When columnar resonance sound generated at the main groove 5 by contacting the tire with the road surface is examined in relation to the groove width of the main groove 5, it has been confirmed that when the groove width is more than 5 mm but not more than 15 mm, it tends to increase the columnar resonance sound. Moreover, a groove width of more than 15 mm is preferable from a viewpoint of noise reduction, but it is very difficult to attain a negative ratio of not more than 25%. The lateral groove may be a lug groove extending along the widthwise direction of the tire or a slant lateral groove extending obliquely at an inclination angle of 0-44°, preferably 5-30°, with respect to the widthwise direction of the tire as shown in Fig. 2. In the latter case, the arranging shape and inclination angle of the lateral groove may be selected according to need. The number of the lateral grooves 7 is not particularly restricted. Furthermore, a portion corresponding to not less than 90% of all lateral grooves arranged in the tread portion preferably has a groove width of not more than 6 mm from a viewpoint of noise reduction because when the groove width exceeds 6 mm, the impact sound by the corner part of the sub-groove becomes larger. In addition, a portion corresponding to not less than 70% of all tread grooves arranged in the tread portion inclusive of the main grooves and the lateral grooves preferably has a groove width of 0.5-3.0 mm as a natural result through the aforementioned action to the noise reduction. When the maximum width (CW) of the carcass line of the carcass is smaller than the rim width (RW) and the distance (CH) between the position of the rim flange and the position of the maximum width of the carcass line shifted in the widthwise direction of the tire and projected onto an equatorial plane 9 of the tire at section of the tire is not more than 1/3 of the distance (TH) between the position of the rim flange and the position of the sectional height of the tire projected onto the equatorial plane 9, a carcass length ranging from the rim flange to the end of the ground contacting width (periphery length) can be made shorter than that of the conventional high-performance tire (Fig. 3), whereby the rigidities of the side portion in all directions are enhanced to increase all rigidities in a loading direction, widthwise direction and circumferential direction of the tire and hence the steering stability can be further improved. Moreover, the increase of the rigidity in the loading direction of the tire can improve vibration damping property but also develop an effect of reducing rolling in the change of running lane. Although only one embodiment of the invention has been described, various modifications may be conducted within the scope of the claims. The following examples are given in illustration of the invention and are not intended as limitations thereof. Example 1 There is prepared a tire having a tire size of 255/40ZR17 as shown in Fig. 3, which comprises a radial carcass 2 of a single carcass ply toroidally extending between a pair of bead portions, a belt superimposed about a crown portion 3 of the carcass and comprised of two rubberized cord layers and a tread portion 8 provided with two main grooves 5 extending in the circumferential direction of the tire and plural lateral grooves 7 extending at an inclination angle of 30° with respect to the widthwise direction of the tire, in which the ratio (PW/RW) of tread ground contacting width to rim width under the aforementioned conditions defined by ETRTO is 0.82 and the negative ratio is 22%. Further, all main grooves arranged in the tread portion have a groove width of 9 mm, while all lateral grooves arranged in the tread portion have a groove width of 7.0 mm. Moreover, the ratio of maximum width of carcass line to rim width is 1.07, and the distance (CH) between position of the rim flange and position of the maximum width of carcass line is 50% of the distance (TH) between position of the rim flange and position of tire sectional height. The other tire structure is the same as the conventional tire. Example 2 The tire of this example has substantially the same structure as in Example 1 except that the number of the main grooves is four, and a portion corresponding to 90% of all main grooves arranged in the tread has a groove width of 4.5 mm and the remaining portion has a groove width of 6 mm. Example 3 The tire of this example has substantially the same structure as in Example 2 except that a portion corresponding to 95% of all lateral grooves arranged in the tread portion has a groove width of 5 mm and the remaining portion has a groove width of 6-10 mm. Example 4 The tire of this example having a radial section as shown in Fig. 1 has substantially the same structure as in Example 3 except that the ratio of maximum width of carcass line to rim width is 0.98, and the distance between position of the rim flange and position of the maximum width of carcass line is 31% of the distance between position of the rim flange and position of tire sectional height. Comparative Example 1 The tire of this comparative example has substantially the same structure as in Example 2 except that the ratio (PW/RW) of tread ground contacting width to rim width is 0.95 and the groove width of the main groove is 5 mm (constant). Comparative Example 2 The tire of this comparative example has substantially the same structure as in Example 2 except that the negative ratio is 35% and the groove width of the main groove is 11 mm (constant). Conventional Example The tire of the conventional example is the conventional high-performance tire shown in Fig. 4 having substantially the same structure as shown in Fig. 1, in which the ratio (PW/RW) of tread ground contacting width to rim width is 0.95 and the negative ratio is 35% and the distance (CH) between position of the rim flange and position of the maximum width of carcass line is 50% of the distance (TH) between position of the rim flange and position of tire sectional height. All of the above tires have a basic tread pattern as shown in Fig. 2. Test Method Tests for evaluating the steering stability, resistance to hydroplaning and noise level are made with respect to each of the above tires, as follows. (1) Steering stability The steering stability is evaluated on dry road surface and wet road surface, respectively. The steering stability on dry road surface is evaluated by a driver's overall judgement for feeling results of braking and traction properties, handling responsibility, road gripping property during steering and controllability exceeding slip limit when the test tire is actually run on a test course consisting of a high speed circuit including a long straight portion, a handling road having many curved portions and a flat road having a radius of about 100 m at speeds ranging from low speed to high speed. The steering stability on wet road surface is evaluated by a driver's overall judgement for the same feeling results as described above when water is sprinkled on a road surface of the test course at a water depth of about 0-5 mm. (2) Resistance to hydroplaning The resistance to hydroplaning is evaluated by an average value of three measured results at a running speed causing hydroplaning when the test tire is actually run on the test course having a water depth of 6 mm in a straight running direction. (3) Noise level The test tire is run at a speed of 60-80 km/h and an internal pressure of 3.0 bar under a load corresponding to the seating capacity and then the internal pressure is again adjusted to 3.0 bar and thereafter run at a speed of 55 km/h, during which noise level is measured by placing two microphones at an interval of 50 cm at a position separated from a center line of the running vehicle by 7.5 m and from the ground surface by a height of 1.2 m to evaluate the noise level. The evaluated results are shown in Table 1. In Table 1, the steering stability on each of dry and wet road surfaces is represented by an index value on the basis that the conventional example is 100, while the resistance to hydroplaning is represented by an increment of speed (km/h) based on the occurrence speed of hydroplaning in the conventional example. The larger the numerical value, the better the property. The noise level is represented by a lowering quantity (dB) based on the noise level in the conventional example, in which the smaller the numerical value, the better the noise reduction. Steering stability on dry road surface Steering stability on wet road surface Resistance to hydroplaning Noise level Example 1 100 100 +4 -1.2 Example 2 102 101 +4 -1.9 Example 3 101 100 +5 -2.1 Example 4 106 105 +5 -2.2 Comparative Example 1 103 90 -6 -0.7 Comparative Example 2 95 101 +6 -0.8 Conventional Example 100 100 standard standard As seen from Table 1, the tires of Examples 1-4 exhibit excellent steering stability, the resistance to hydroplaning and the noise level. According to the invention, it is possible to provide a pneumatic radial tire having a low aspect ratio or so-called high-performance tire in which the steering stability and the resistance to hydroplaning can simultaneously be established without increasing the tire size, and the noise level is reduced.

Claims: A pneumatic radial tire (1) comprising a radial carcass (2) toroidally extending between a pair of bead portions, a belt (4) superimposed about a crown portion (3) of the carcass and a tread portion (8) disposed on the belt and provided with a plurality of tread grooves (5, 7) and having an aspect ratio of not more than 50%, characterized in that when the tire is mounted onto a rim and vertically placed on a flat plate under an internal pressure of 2.5 bar at a static state and a load corresponding to 70% of the maximum load capacity determined by load index in accordance with the said internal pressure is applied thereto, the ratio of ground contacting width (PW) of tire tread to rim width (RW) is less than 0.85 and the negative ratio of the total area of the tread grooves to the total tread area is not more than 25%. A pneumatic radial tire as claimed in claim 1, characterized in that the tread grooves comprise main grooves (5) extending at an inclination angle within a range of 0-45° with respect to the circumference of the tire and/or lateral grooves (7) extending at an inclination angle within a range of 0-44° with respect to the widthwise direction (6) of the tire. A pneumatic radial tire as claimed in claim 1 or 2, characterized in that the groove width of a portion corresponding to not less than 80% of all main grooves (5) arranged in the tread portion (8) is not more than 5 mm. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that the groove width of a portion corresponding to not less than 90% of all lateral grooves (7) arranged in the tread portion (8) is not more than 6 mm. A pneumatic radial tire as claimed in any of claims 1 to 4, characterized in that the groove width of a portion corresponding to not less than 70% of all tread grooves including the main grooves (5) and lateral grooves (7) is within a range of 0.5-3.0 mm. A pneumatic radial tire as claimed in any of claims 1 to 5, characterized in that the maximum width (CW) of the carcass line of the carcass (2) is smaller than the rim width (RW) and the distance (CH) between the position of the rim flange and the position of the maximum width (CW) of the carcass line shifted in the widthwise direction of the tire and projected onto the equatorial plane (9) of the tire at section of the tire is not more than 1/3 of the distance (TH) between the position of the rim flange and the position of the sectional height of the tire projected onto the equatorial plane (9). A pneumatic radial tire as claimed in any of claims 1 to 6, characterized in that said ratio of ground contacting width (PW) of tire tread to rim width (RW) is 0.70-0.82. A pneumatic radial tire as claimed in any of claims 1 to 7, characterized in that said negative ratio is 15-22%.

Inventors: KANEKO TAKASHI, KANEKO, TAKASHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1997
Main classification: B60C11
IPC codes: B60C11, B60C3