Pneumatic radial tires

Abstract: A pneumatic radial tire having an improved resistance to uneven wear is provided by obtaining a crown profile of a tread portion, in which at least a radial half section of the crown profile with respect to an equatorial plane of the tire is divided into center and middle and shoulder regions and comprised of a composite arc formed by smoothly connecting at least three arcs having different radii of curvature to each other and each of distances (L 1 - L 3 ) and radii of curvature (R 1 - R 3 ) of these regions satisfies a particular relation: L 1 =(0.3 - 0.6) x W, R 1 = (8 - 20) x W, L 2 = (0.6 - 0.9) x W, R 2 = (1.5 - 3.5) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.5 - 1.0) x W.

Description: This invention relates to pneumatic radial tires for use in relatively small-size automobiles such as passenger cars, vans, pickups, trucks and the like, and more particularly to a pneumatic radial tire having an improved resistance to uneven wear of a tread rubber in a tread portion. When an automobile, especially the above type of relatively small-size automobile, is run on a road surface having many curves and slopes at a relatively high frequency, tires mounted onto the automobile tend to undergo such uneven wear that the wearing of the tread rubber located at both side regions of the tread portion (called shoulder region) becomes greater and faster than the wearing of the tread rubber located at a central region thereof. As the uneven wear progresses, the tire is rendered into a used tire while a great amount of unused rubber is left in the central region of the tread portion, which obstructs an effective utilization of the tire. For this end, it is attempted to round the crown profile of the tread portion in section as far as possible (i.e. the crown profile is defined by arcs having a small radius of curvature), which is said to be effective and widely adopted as a countermeasure for improving this type of uneven wear. However, it has been confirmed that this countermeasure is certainly effective for the control of the uneven wear under the condition of running on a curved road at a higher frequency, but undesirably promotes the uneven wear of the shoulder region during straight running. This results from the fact that the difference of ground contact length between the central region of the crown profile of the tread portion and the shoulder region is large during straight running. A tire according to the preamble of claim 1 is known from e.g. EP-A- 0 402 303. Furthermore, this document shows the axial width of the first radius to be 40% to 60% of the tread width and the radii are roughly given as R1= 10 x W, R2 = 2 x W and R3 = 0,45 x W. It is, therefore, an object of the invention to provide pneumatic radial tires having a long service life capable of contributing to the effective utilization of the tire by preventing premature wearing of the tread rubber at the shoulder region to uniformly wear the tread rubber along the crown profile in widthwise direction whether the main running is curved running or straight running and irrespective of the mixing ratio of both running forms. According to the invention, there is provided a pneumatic radial tire comprising a pair of bead portions, a pair of sidewall portions and a tread portion toroidally extending between the sidewall portions, a crown of the tread portion at a section of a plane including a rotational axis of a tire-wheel assembly when the tire is mounted onto a standard rim and inflated under a standard internal pressure is comprised of a composite arc smoothly connecting at least three arcs having different radii of curvature to each other; and when the crown is divided from the equatorial plane of the tire toward an end of the crown into a center region, a middle region and a shoulder region in said order and a distance ranging from the equatorial plane to an outer end of each of these regions is L and a radius of curvature of an arc in each of these regions is R, distance L 1 and radius of curvature R 1 of the center region, distance L 2 and radius of curvature R 2 of the middle region and distance L 3 and radius of curvature R 3 of the shoulder region satisfy the following relations with respect to a distance W ranging from an outermost end of the crown to the equatorial plane at a contact zone when the said assembly is pushed onto a flat plate under a standard load and after the load is released the improvement being: L 1 = (0.3 - 0.6) x W, R 1 = (8 - 20) x W, L 2 = (0.6 - 0.9) x W, R 2 = (1.5 - 3.5) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.5 - 1.0) x W. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a schematic right-half radial section view of an assembly of a pneumatic radial tire according to the invention and a rim; Fig. 2 is a graph showing a distribution of ground contacting length at a ground contact area in an embodiment according to the invention; Fig. 3 is a graph showing a distribution of ground contact pressure at a ground contact area in an embodiment according to the invention; Fig. 4 is a schematical view illustrating an abrasion pattern of a conventional tire; and Fig. 5 is a graph showing a distribution of ground contact pressure in the conventional tire. In Fig. 1 is schematically shown a right-half radial section of a tire-rim assembly when a pneumatic radial tire 1 is mounted onto a standard rim 10 and inflated under a standard internal pressure. In this case, a left-half radial section is not necessarily the same as the right-half radial section, or both sections may be slightly different. Moreover, the rim is shown only by an outer profile line. Numeral 10F is a rim flange. The standard rim and standard internal pressure are based on a permitted rim and a load capacity-air pressure table corresponding to a size of a radial ply tire defined in JATMA YEAR BOOK 1992 (standard of The Japan Auto Tire Manufacturers Association). The pneumatic radial tire 1 shown in Fig. 1 comprises a pair of bead portions 2, a pair of sidewall portions 3 and a tread portion 4 toroidally extending between the sidewall portions 3. Moreover, the tire 1 is provided with a radial carcass 5 extending between a pair of bead cores 2c embedded in the bead portions 2 and reinforcing the bead portions 2, the sidewall portions 3 and the tread portion 4, a belt 6 reinforcing the tread portion 4, and a so-called layer 7 of nylon cords arranged substantially along the circumference of the tread portion 4 for reinforcing each end region of the belt 6. On the other hand, grooves formed in a tread rubber 4t are omitted. A crown 4c of the tread portion 4 (nomenclature of a tread or a treading surface at tire section) is comprised of a composite arc smoothly connecting at least three arcs having different radii of curvature to each other. In the illustrated embodiment, the composite arc is comprised of three arcs having radii of curvature R 1 , R 2 and R 3 . The wording "smooth connection" used herein means that the arcs are connected so as to become adjacent arced-curves in contact with each other or share a tangent on a shared point between the two adjacent arced-curves. The crown 4c is divided from an equatorial plane E of the tire toward an end of the crown 4c into a center region, a middle region and a shoulder region in this order. Among distances L of outer ends of these regions measured from the equatorial plane E, the center region corresponds to a region covered by a distance L 1 , and the middle region corresponds to a region defined between the distance L 1 and a distance L 2 , and the shoulder region corresponds to a region defined between the distance L 2 and a distance L 3 . All of these regions extend over the full circumference of the tread portion. In the full profile of the crown 4c, the center region has a radius of curvature R 1 , the middle region has a radius of curvature R 2 , and the shoulder region has a radius of curvature R 3 , respectively. The distances L 1 , L 2 and L 3 are determined in relation to an outermost end P of the crown 4c as mentioned below. That is, the outermost end P used herein means a crown end located at an outermost side of the tire 1 at a contact zone when an assembly of the tire 1 and the rim 10 inflated under the standard internal pressure is pushed onto a flat plate under the standard load and after such a load is released as shown in Fig. 1. As regards a distance W ranging from the outermost end P to the equatorial plane E, the distance L 1 is within a range of 0.3W - 0.6W, and the distance L 2 is within a range of 0.6W - 0.9W, and the distance L 3 is within a range of 0.9W - 1.0W. In this case, the outermost end P is an outermost end of the crown located at a side defining the distances L 1 , L 2 , L 3 with respect to the equatorial plane E. Moreover, the above contact zone is a contact region of the tread at such a state that the rotational axis of the assembly is parallel to a surface of the flat plate under the standard load. Furthermore, the radii of curvature R 1 , R 2 , R 3 are determined in relation to the distance W between the outermost end P of the crown 4c and the equatorial plane E of the tire likewise the above distances. That is, the radius of curvature R 1 is within a range of 8W - 20W, and the radius of curvature R 2 is within a range of 1.5W - 3.5W, and the radius of curvature R 3 is within a range of 0.5W - 1.0W, respectively. In order to form a composite arc by smoothly connecting these arcs having the radii of curvature R 1 , R 2 and R 3 to each other, a center of the radius of curvature R 1 is placed on the equatorial plane E of the tire, and a center of the radius of curvature R 2 is placed on a line of the radius of curvature R 1 passing through a point of the crown 4c at the distance L 1 , and a center of the radius of curvature R 3 is placed on a line of the radius of curvature R 2 passing through a point of the crown 4c at the distance L 2 . The inventors have fully observed the uneven wear of tread rubber created in the shoulder region of the conventional tire when the frequency of the cornering operation in an automobile is high and a lateral force input from a road surface into the treading surface of the tread portion in the cornering is large or the side force or cornering force generated in the tire is large, and arrived at first and second conclusions as mentioned below. It is known that when a force is applied to the surface of the tread rubber contacting with the road surface and slippage is created between the tread rubber surface and the road surface, a stripe pattern is formed on the tread rubber surface in a direction substantially perpendicular to the force applying direction. This pattern is called an abrasion pattern. As a result of detailed observation on the abrasion pattern in the uneven worn tire, it has been confirmed that this pattern is generated over the whole of the treading surface in its widthwise direction such that the direction of the linear stripe extends substantially along the circumference of the treading surface as shown in Fig. 4. Particularly, it is important that an interval (pitch) between linear stripes in the center region of the treading surface is narrow (small), while the pitch in the shoulder region is considerably larger than that in the center region. Moreover, Fig. 4 shows a profile of the crown (treading surface) together with schematic abrasion patterns in frames indicated by arrows corresponding to the center region and shoulder region of the crown. The size of the pitch in the linear stripe pattern is proportional to the wearing rate of the rubber. That is, a small pitch means that the wearing rate is slower, while a large pitch means that the wearing rate is faster. In the abrasion pattern of Fig. 4, therefore, it is a matter of course that the worn amount of the shoulder region is considerably larger than that of the center region. This results because a relation of (worn amount of tread rubber) α (ground contact pressure) x (friction coefficient) x (lateral force) is established in all regions of the treading surface in the widthwise direction of the tread portion. Finally, this is said to be the difference of ground contact pressure between the regions because the lateral force applied to the treading surface of the tread portion and the friction coefficient at the ground contact region are equal over the full width of the crown. In fact, the distribution of ground contact pressure in the same new tire as the conventional tire used to the observation of uneven wear is measured to obtain results as shown in Fig. 5. In Fig. 5, an ordinate (E') is coincident with the equatorial plane E of the tire. As seen from Fig. 5, the ground contact pressure in the shoulder region indicates a large peak and the vicinity of this peak position corresponds to the abrasion pattern in the frame shown in the left side of Fig. 4. That is, when lateral force is applied to the treading surface of the tread portion at a high frequency, uneven wear produced in the shoulder region is mainly influenced by the ground contact pressure. Therefore, a first conclusion is that it is necessary to make the distribution of ground contact pressure in the widthwise direction of the crown equal as far as possible for preventing the occurrence of this uneven wear. As an effective countermeasure for preventing uneven wear of the tread rubber due to frequent cornering, it has hitherto been adopted to round the crown shape. When the uneven wear due to premature wearing of the tread rubber in the shoulder region of the tire provided with such a rounded crown is examined in detail during straight running of the automobile, it has been confirmed that a portion having a shorter ground contacting length in the treading surface of the tread portion or the shoulder region always indicates a braking behaviour to the road surface during the running of the tire. That is, it has been confirmed from the observation of the abrasion pattern that so-called braking force based on the above behaviour is applied as an external force to the shoulder region substantially along the running (rotating) direction of the tire. When the braking force exceeds the effect of controlling the wear at a lower ground contact pressure, the worn amount of the tread rubber represented by the above proportional relation is considerably larger in the shoulder region than in the center region, which results in the occurrence of uneven wear in the tire provided with the rounded crown during straight running of the tire. This type of the uneven wear is a wearing form called self-excited wearing as compared with the forcible wearing associated with cornering. Therefore, the second conclusion is that it is necessary to make the ground contacting length along the rotating direction of the ground contact area of the tread portion over the full width of the crown equal as far as possible or to make the distribution of the ground contacting length the same as far as possible for preventing the uneven wear produced during straight running of the tire. From the above first and second conclusions, it has been found that it is necessary to simultaneously establish the equalization of the ground contact pressure distribution and the equalization of the ground contacting length distribution as far as possible, or to make both the distribution states the same together as far as possible in order to prevent the uneven wear produced during cornering and straight running. In order to equalize these distributions, it is useful to standardize a certain specified element at a ground contacting state of the treading surface in the tread portion 4. According to the invention, the distance W from the outermost end P of the crown 4c after the complete removal of the load to the equatorial plane E of the tire is adopted as the above specified element instead of a maximum ground contact half-width under the standard loading because it has been confirmed that there is caused a difference between the maximum ground contact half-width and the distance W after the removal of the load. That is, it is actually effective to adopt the position of the outermost end P in the assembly of the tire 1 and the rim 10. Furthermore, it is reasonable to adopt the tire form near to the actual running state as far as possible for realizing the first and second conclusions. In this connection, according to the invention, the crown 4c of the tread portion 4 in the assembly of the tire 1 and the standard rim 10 inflated under the standard internal pressure is comprised of a composite arc formed by smoothly connecting at least three arcs (three arcs in the embodiment of Fig. 1) having different radii of curvature (R 1 , R 2 , R 3 in the embodiment of Fig. 1) to each other, whereby the equalizations of the ground contact pressure and the ground contacting length can easily be realized in conformity with the actual state. When the crown 4c having radii of curvature R 1 , R 2 , R 3 is divided into the center region, middle region and shoulder region (only in one-side zone from the equatorial plane E) to measure a distance L measured from the equatorial plane E to an outer end of each of these regions, the simultaneous establishment between the equalization of ground contact pressure distribution and equalization of ground contacting length distribution is obtained or both the distributions may be arranged close to the same state when the distance L 1 and radius of curvature R 1 of the center region, the distance L 2 and radius of curvature R 2 of the middle region, and the distance L 3 and radius of curvature R 3 of the shoulder region satisfy the following relations with respect to the distance W ranging from the outermost end P of the crown 4c to the equatorial plane E: L 1 = (0.3 - 0.6) x W, R 1 = (8 - 20) x W, L 2 = (0.6 - 0.9) x W, R 2 = (1.5 - 3.5) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.5 - 1.0) x W. This is explained with reference to Figs. 2 and 3 showing results measured on an upper half portion of ground contacted shape and ground contact pressure distribution from the center (equatorial plane E) to the shoulder in an example according to the invention (shown by a solid line), a conventional example la creating uneven wear through the action of lateral force at a high frequency (shown by dotted lines) and a conventional example 2a creating uneven wear during straight running (shown by dot-dash lines). The tire to be measured is a pneumatic radial tire for passenger cars having a tire size of 175/80R14. Moreover, the ordinate (E') in Figs. 2 and 3 is coincident with the equatorial plane E of the tire, while only an outer profile of the crown at ground contact state is shown in Fig. 2 for measuring the ground contacting length. As to the ground contacting length at the ground contacted state shown in Fig. 2, the conventional example la shows a uniform distribution over a full range from the center to the shoulder, while the conventional example 2a shows such a distribution that the ground contacting length is maximum at the equatorial plane E and gradually decreases up to a region of about 0.6W and rapidly decreases toward the shoulder end. On the contrary, the example according to the invention shows such a distribution that the ground contacting length is constant in the center region and slightly increases in the middle region and rapidly decreases in the shoulder region. Moreover, the ground contact area is substantially the same in all of these tires. The ground contact pressure distribution corresponding to the ground contacted state of Fig. 2 is shown in Fig. 3, whereby the ground contact pressure at the shoulder region is described below. As seen from Fig. 3 showing results measured on many experiments for the ground contact pressure distribution, the pneumatic radial tire for passenger cars is obliged to have a tendency that the ground contact pressure of the treading surface of the tread portion 4 becomes higher in the shoulder region of the crown 4c. The region having such a high ground contact pressure is largely worn through the action of large lateral force as compared with the other regions to produce uneven wear. Particularly, it has been confirmed that the conventional example la rapidly raises the ground contact pressure when the distance L measured from the equatorial plane E is between 0.6W and 0.9W and the region having the high ground contact pressure is existent from this distance up to about 1W. For this end, it is necessary to make the radius of curvature R 3 in the shoulder region of the crown as small as possible to reduce the ground contact pressure. When the radius of curvature R 3 is too small, the distribution of the ground contacting length is close to that of the conventional example 2a shown in Fig. 2. In order to avoid such a ground contacting length distribution, the minimum value of the radius of curvature R 3 is restricted to 0.5W, whereby the resistance to uneven wear during straight running is advantageously maintained. On the other hand, the great rise of the ground contact pressure is suppressed by restricting the maximum value of the radius of curvature R3 to 1W. Moreover, the distance L 3 of the shoulder region is restricted to a range of 0.9W - 1W because it may be required to change the performances on wearing in accordance with commercial demand. As to the middle region, the ground contact pressure starts to rise at a position that the distance measured from the equatorial plane E is about 0.6W as shown in Fig. 3, so that it has been confirmed that the middle region is a region showing a distribution of a high ground contact pressure next to the shoulder region. Therefore, the distance L 2 is set to a range of 0.6W - 0.9W, while the middle region is defined between the distance L 1 and the distance L 2 . In this case, it is desirable to reduce the ground contact pressure of the shoulder region by setting the radius of curvature R 2 in the middle region to a large value as far as possible. However, when the radius of curvature R 2 is too large, even if the radius of curvature R 3 in the shoulder region is made as small as possible, the highest ground contact pressure in the shoulder region can not be controlled as compared with the conventional example 1a and hence uneven wear is caused by the input of large lateral force. In order to avoid this problem, the maximum value of the radius of curvature R 2 is restricted to 3.5W. As a result, ground contact pressure higher than that of the conventional example 1a is obtained in a zone A enclosed by phantom line on the curve of the example in Fig. 3, so that the ground contact pressure of the shoulder region is considerably lower than that of the conventional example 1a. On the other hand, when the radius of curvature R 2 is less than 1.5W, the ground contacting length of the shoulder region is too short and the resistance to uneven wear in the straight running is degraded. In the center region, it is desirable that the value of the radius of curvature R 1 is made as large as possible for reducing the ground contact pressure of the shoulder region. However, if the radius of curvature R 1 is too large, there is formed a so-called butterfly-type ground contact profile in which a portion having a short ground contacting length is formed in the center region and the ground contacting length of the shoulder region becomes longer, whereby the resistance to uneven wear in straight running is degraded. In order to avoid such an inconvenience, therefore, the upper limit of the radius of curvature R 1 is restricted to 20W. On the other hand, when the radius of curvature R 1 is less than 8W, even if the radii of curvature R 2 , R 3 have the aforementioned upper limits, the ground contacting lengths of the middle and shoulder regions are largely shorter than that of the center region and hence the resistance to uneven wear in the straight running is degraded. As mentioned above, the uneven wear produced in the shoulder region can advantageously be improved according to the invention. The aforementioned crown profile is adapted to the improvement of the resistance to uneven wear during running under a service condition having a proper mixing ratio of curved running and straight running. However, there are existent a biased service condition mainly composed of curved running having a relatively large curvature and a biased service condition mainly composed of straight running in accordance with road conditions. Under the above biased service conditions, it is effective to further improve the resistance to uneven wear by adopting the crown profile so as to satisfy the relations of the distances L 1 -L 3 and radii of curvature R 1 -R 3 with respect to the distance W as mentioned below, respectively. That is, it is preferable that the following relations of the distances L 1 -L 3 and radii of curvature R 1 -R 3 with respect to the distance W are satisfied under the biased service condition mainly composed of curved running: L 1 = (0.5 - 0.6) x W, R 1 = (8 - 12) x W, L 2 = (0.8 - 0.9) x W, R 2 = (1.5 - 2.5) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.5 - 0.75) x W. On the other hand, it is preferable that the following relations of the distances L 1 -L 3 and radii of curvature R 1 -R 3 with respect to the distance W are satisfied under the biased service condition mainly composed of straight running: L 1 = (0.3 - 0.5) x W, R 1 = (12 - 20) x W, L 2 = (0.6 - 0.8) x W, R 2 = (2.5 - 3.5) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.75 - 1.0) x W. Moreover, the resistance to uneven wear is considerably improved when the following relations of the distances L 1 -L 3 and radii of curvature R 1 -R 3 with respect to the distance W are satisfied under the service condition having a proper mixing ratio of curved running and straight running: L 1 = (0.4 - 0.6) x W, R 1 = (8 - 15) x W, L 2 = (0.75 - 0.85) x W, R 2 = (2.0 - 3.0) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.5 - 0.8) x W. The following examples are given in illustration of the invention and are not intended as limitations thereof. A pneumatic radial tire for passenger cars to be tested having a tire size of 175/80R14 is the tire 1 having the structure as shown in Fig. 1 in which the radial carcass 5 is a single rubberized ply containing polyester cords, and the belt 6 is comprised of two steel cord cross layers, and the layer 7 is a rubberized layer containing nylon cords of 1260 d/2. The tire 1 is mounted onto a standard rim 10 of 5J x 14 and inflated under a standard internal pressure of 190 kPa to form a tire-rim assembly. When a load of 495 kg is applied to this assembly and then released, the distance W between the equatorial plane E and the outermost end P on the treading surface of the tread portion 4 is 60 mm. There are provided 21 test tires, in which 13 tires are examples according to the invention and 2 tires are conventional examples and 6 tires are comparative examples. The distances L 1 , L 2 , L 3 from the equatorial plane E and the radii of curvature R 1 , R 2 R 3 of the crown 4c in these tires with respect to the distance W are shown in Table 1. In this case, for instance, numerical value of 0.5 means 0.5W and so forth. Then, each of these test tires is mounted onto an automobile and actually run under the following two running conditions to conduct uneven wearing tests. Running condition (1): The automobile is mainly run on general-purpose streets and slopes having many curves frequently subjected to input of lateral force (side force or cornering force) Running condition (2): The automobile is mainly run on an expressway for straight running. After the running over a given distance, the worn amount per unit width of 5 mm is measured in the widthwise direction of the crown 4c to determine a ratio of worn amount in the shoulder region to worn amount in the center region. As the value of the worn amount ratio becomes near to 1.0, the better the resistance to uneven wear. The measured values of the worn amount ratio are shown in the columns "Running condition" of Table 1. In order to check the effect of the invention in a pneumatic radial tire for passenger cars having a smaller aspect ratio, there are provided three test tires having a tire size of 205/70R14 as Example 14 and Comparative Examples 7 and 8. Each of these tires is mounted onto a standard rim of 51/2J x 14 and inflated under a standard internal pressure of 195 kPa to form a tire-rim assembly. When a load of 590 kg is applied to this assembly and then released, the distance W between the equatorial plane E and the outermost end P on the treading surface of the tread portion is 72 mm. The distances L 1 , L 2 , L 3 and the radii of curvature R 1 , R 2 , R 3 with respect to the distance W in these tires are shown in Table 2. After the test for uneven wear is carried out under the same running conditions (1) and (2) as described above, the resistance to uneven wear is evaluated in the same manner as described above to obtain results as shown in Table 2. Example Comparative Example 14 7 8 L 1 (×W) 0.5 0.6 0.5 R 2 (×W) 8.5 9.0 6.0 L 2 (×W) 0.83 0.95 0.9 R 2 (×W) 1.75 2.0 1.5 L 3 (×W) 1.0 1.0 1.0 R 3 (×W) 0.7 0.2 0.15 Running condition (1) 1.2 1.8 1.6 Running condition (2) 1.3 1.2 1.6 As seen from the evaluation results of Tables 1 and 2, the tires according to the invention exhibit a good wearing state different from the uneven wear because the worn amount of the shoulder region in the tread rubber is equal to that of the center region under both straight and corner running conditions and the value of the worn amount is not so large. On the contrary, the tires of the conventional and comparative examples exhibit a wearing state clearly comprising uneven wear because the worn amount of the shoulder region is considerably larger than that of the center region under either or both of the above running conditions. As mentioned above, uneven wear of the shoulder region in the conventional tires is generated under either or both of straight and corner running conditions. On the contrary, according to the invention, the occurrence of uneven wear of the shoulder region can effectively be prevented under both the above running conditions by optimizing the crown profile of the tread portion, whereby there can be provided pneumatic radial tires having improved resistance to uneven wear and a long service life.

Claims: A pneumatic radial tire (1) comprising a pair of bead portions (2), a pair of sidewall portions (3) and a tread portion (4) toroidally extending between the sidewall portions, a crown (4c) of the tread portion (4) at a section of a plane including a rotational axis of a tire-wheel assembly when the tire is mounted onto a standard rim (10) and inflated under a standard internal pressure is comprised of a composite arc smoothly connecting at least three arcs having different radii of curvature to each other; and when the crown (4c) is divided from the equatorial plane of the tire toward an end of the crown into a center region, a middle region and a shoulder region in said order and a distance ranging from the equatorial plane to an outer end of each of these regions is L and a radius of curvature of an arc in each of these regions is R, distance L 1 and radius of curvature R 1 of the center region, distance L 2 and radius of curvature R 2 of the middle region and distance L 3 and radius of curvature R 3 of the shoulder region satisfy the following relations with respect to a distance W ranging from an outermost end of the crown to the equatorial plane at a contact zone when the said assembly is pushed onto a flat plate under a standard load and after the load is released, characterized in that : L 1 = (0.3 - 0.6) x W, R 1 = (8 - 20) x W, L 2 = (0.6 - 0.9) x W, R 2 = (1.5 - 3.5) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.5 - 1.0) x W. A pneumatic radial tire as claimed in claim 1, characterized in that the distances L 1 - L 3 and radii of curvature R 1 - R 3 with respect to the distance W satisfy the following relations: L 1 = (0.5 - 0.6) x W, R 1 = (8 - 12) x W, L 2 = (0.8 - 0.9) x W, R 2 = (1.5 - 2.5) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.5 - 0.75) x W. A pneumatic radial tire as claimed in claim 1, characterized in that the distances L 1 - L 3 and radii of curvature R 1 - R 3 with respect to the distance W satisfy the following relations: L 1 = (0.3 - 0.5) x W, R 1 = (12 - 20) x W, L 2 = (0.6 - 0.8) x W, R 2 = (2.5 - 3.5) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.75 - 1.0) x W. A pneumatic radial tire as claimed in claim 1, characterized in that the distances L 1 - L 3 and radii of curvature R 1 - R 3 with respect to the distance W satisfy the following relations: L 1 = (0.4 - 0.6) x W, R 1 = (8 - 15) x W, L 2 = (0.75 - 0.85) x W, R 2 = (2.0 - 3.0) x W, L 3 = (0.9 - 1.0) x W, R 3 = (0.5 - 0.8) x W.

Inventors: KADOWAKI HIROSHI, OHSAWA YASUO, KADOWAKI, HIROSHI, OHSAWA, YASUO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1996
Main classification: B60C11
IPC codes: B60C11, B60C3