PNEUMATIC RADIAL TIRE

Abstract: A pneumatic radial tire capable of effectively controlling the occurrence of wandering phenomenon when being used in vehicles such as passenger car, small-size truck, truck, bus and the like is provided, in which a second tread zone contacting with a mountain side of a slant road surface is arranged so as to project toward a side of a first tread zone contacting with a flat road surface and camber thrust of the radial tire is increased by the second tread zone during the running on ruts and the like under loading.

Description: This invention relates to pneumatic radial tires, and in particular to a pneumatic radial tire effectively controlling a complicated motion of the tire exceeding a driver's forecast or an occurrence of so-called wandering phenomenon to largely improve straight running stability when the tire is run on a slant face of an uneven road surface such as a rut or the like. Radial tires have excellent wear resistance and steering stability because cords in the carcass ply are arranged so as to extend substantially in a direction perpendicular to an equatorial plane of the tire. Therefore, the radial tires are frequently used as compared with bias tires in not only passenger cars but also vehicles such as small-size trucks, trucks, and buses as a result of recent advances in the high-speed performance of vehicles. However, the high-speed running of vehicles has developed in accordance with the improvement and expansion of road systems and the like. For this end, it is strongly demanded to sufficiently control the wandering phenomenon liable to be caused in the radial tire rather than the bias tire to thereby enhance the straight running stability and further improve the safety. Viewing the occurrence of wandering phenomenon in the radial tire, as shown in Fig. 1 of the accompanying drawings, when the tire T is rotated on a slant face S such as a rut or the like, each of load W, reaction force F R from road surface and camber thrust F C is applied to the tire T and hence lateral force F Y as a resultant force of horizontal components of these forces acts thereto. The tire of radial structure has high treading rigidity as compared with the tire of bias structure, and also the rigidity of the tread portion is higher than that of the side portion due to the structure of the radial tire, so that the camber thrust F C is made smaller than that of the tire of bias structure and hence lateral force F Y directing to a lower side of the slant face S becomes relatively large by a quantity corresponding to the reduced quantity of the camber thrust F C . As a result, the tire T strongly tends to slip downward on the slant face and it is difficult to ride over the rut and hence the wandering phenomenon is caused. As shown at radial section of the tire in Fig. 2, when the tire is rotated on the slant face S, camber thrust F C is caused based on the fact that the tread portion T r tends to strongly contact with the ground at an upper side or a mountain side of the slant face S and to rise upward at a valley side of the slant face S, and particularly falling-down deformation of a region Bu in the vicinity of the buttress of the tire side portion or so-called bulging deformation bside toward the mountain side is caused at the mountain side due to the strong ground contact of the tread portion T r and such a bulging deformation bside brings about bending deformation of the tread portion T r near to a ground contact end or so-called pushing-out deformation bsho, and such a pushing-out deformation bsho in a zone near to the ground contact end further gives shearing deformation S S as shown by dotted lines to the tread rubber in a portion near to the ground contact end, and such a shearing deformation creates a lateral force F CS directing to the upper side of the slant face S. As regards the camber thrust F C , in the bias tire having a small difference of rigidity among constitutional parts of the tire and a relatively soft structure, sufficiently large bulging deformation bside and pushing-out deformation bsho can be ensured and hence the camber thrust F C created becomes large, while since the rigidity of the tread portion in the radial tire is considerably higher than the rigidity of the side portion, a tendency of creating escape deformation in a direction opposite to the above as exaggeratedly shown by a phantom line becomes strong in the tire side portion and hence the shearing deformation S S of the tread rubber becomes naturally small and it is necessary to reduce the camber thrust F C . In order to increase the camber thrust in the pneumatic radial tire, therefore, it is effective that the bulging deformation bside of the region Bu is made large and the transmission efficiency of the bulging deformation bside is enhanced to make large the pushing-out deformation bsho in the zone near to the ground contact end to thereby increase the shearing deformation S S of the tread rubber at the ground contact end portion, or it is effective that the ground contact area of the tire to the slant face S is made large to increase a total amount of lateral force F CS directing to the upper side of the slant face S. Attention is also drawn to the disclosure of US-A- 4281702, from which a tire having properties of resistance to shoulder wear is known. It is, therefore, an object of the invention to provide pneumatic radial tires sufficiently controlling the occurrence of wandering phenomenon on the slant face of a rut or the like to largely improve the straight running stability by increasing the camber thrust of the radial tire without degrading the properties inherent to the radial tire. The pneumatic radial tire according to the invention comprises a pair of bead portions, a pair of sidewall portions, a tread portion toroidally extending between both sidewall portions, a radial carcass reinforcing these portions and a belt reinforcing the tread portion at the outer circumferential side of the radial carcass. In a first motor vehicle pneumatic radial tire of the invention, the tread portion is comprised of a first tread zone contacting with a flat road surface and a second tread zone projecting toward the side of the first tread zone and contacting with a mountain side of a slant road surface, and the first tread zone is a maximum width region of a ground contact portion of the tire on a flat road surface under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of the truck and bus tires at an inflation state under a normal air pressure in accordance with a maximum loading capacity according to JATMA standard, and the second tread zone is a region extending outward from the first tread zone in the widthwise direction of a ground contact portion on the flat road surface when a camber angle of 10° is applied to the tire under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of the truck and bus tires at an inflation state under a normal air pressure in accordance with a maximum loading capacity according to JATMA standard. In a preferable embodiment, the tire further comprises a reinforcing portion projecting outward from the second tread zone in the widthwise direction . In a more preferable embodiment, an outer profile line of the reinforcing portion at radial section of the tire is located inward from a phantom extending line of an outer profile line of the second tread zone in the radial direction of the tire. The reinforcing portion may be comprised of plural ribs arranged at given intervals in the circumferential direction of the tire, if necessary. It is preferable that the tread width is within a range of 80-105% of a tire section width at an inflation state under a normal internal pressure; and it is preferable that the maximum width of the belt is within a range of 60-100% of the tire section width at the inflation state under a normal internal pressure. In the first pneumatic radial tire, the tread portion is largely projected by a portion corresponding to the second tread zone in the widthwise direction of the tire as compared with the conventional radial tire, so that the ground contact area of the tire during running on the slant face as shown in Fig. 2 is made larger as compared with the conventional tire, whereby the total amount of lateral force F CS directing to the upper side of the slant face S can be increased. Since the second tread zone projecting in the widthwise direction of the tire has a high rigidity as a part of the tread portion T r , the escape deformation from the portion near to the ground contact end of the tread portion T r to the tire side portion as exaggeratedly shown by the phantom line in Fig. 2 is effectively prevented to increase the bulging deformation bside. In addition, it is possible to transmit the pushing-out deformation bsho based on the bulging deformation bside to a wide range of the second tread zone contacting with the slant face S, whereby the shearing deformation S S of the tread portion T r and hence camber thrust F C are effectively increased. On the other hand, in a radial tire having no second tread zone, the position of the ground contact end can not be moved to the upper side of the slant face during the contact with the slant face of the rut or the like and hence the ground contact pressure at the ground contact end portion and the vicinity thereof largely rises and also the rigidity ranging from the ground contact end portion to the tire side portion is low, and escape deformation occurs as shown in Fig. 2 Therefore, the tire having the second tread zone is a tire of radial structure and can effectively enhance lateral force F CS and hence camber thrust F C . Even in the conventional radial tire, if there is a shoulder shape such as round shoulder, taper shoulder or the like, when a camber angle of 10° is applied to the tire, a side region of the tread portion may contact with the ground at a width of not more than about 5 mm. However, such a ground contact region is not a ground contacting portion of the tread having a high rigidity, so that it is impossible to bring about the effect aimed at by the invention. In the case of arranging the reinforcing portion so as to push outward from the second tread zone in the widthwise direction, such a reinforcing portion ensures the bulging deformation bside of a portion near to the buttress and can effectively increase the bulging deformation bside, and as a result the pushing-out deformation bsho of a portion near to the ground contact end and the shearing deformation S S of tread rubber are increased. Moreover, when the outer profile line of this reinforcing portion in meridional section of the tire is located inside a phantom extension line of the outer profile of the second tread zone, the functions inherent to the reinforcing portion can sufficiently be developed without the increase of extra weight while ensuring a sufficient ground contact width to the slant face S. In this tire, when the tread width is within a range of 80-105% of a tire section width, the danger of breaking the tread end can sufficiently be removed while sufficiently securing the increase of camber thrust F C . Further, when the maximum width of the belt is within a range of 60-100% of the tire section width, the above bulging deformation bside is effectively transferred to the vicinity of the ground contact end and also the occurrence of so-called belt end separation can effectively be prevented. A second motor vehicle pneumatic radial tire according to the invention comprises a tread portion comprised of a first tread zone contacting with a flat road surface and a second tread zone projecting toward the side of the. first tread zone and contacting with a mountain side of a slant road surface, in which a half width (TW 1 ) of the first tread zone is a maximum ground contact width in a ground contact portion of the tire on the flat road surface from an equatorial plane of the tire under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of the truck and bus tires at an inflation state under a normal air pressure in accordance with the maximum loading capacity, and a width (TW 2 ) of the second tread zone satisfies the following condition: TW 2 /TW 1 >4.0 x 10 -2 when this width is a ground contact width extending outward from the half-width (TW 1 ) of the first tread zone among the maximum ground contact width in the ground contact portion of the tire on the flat road surface from the equatorial plane of the tire when a camber angle of 10° is applied to the tire under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of the truck and bus tires at an inflation state under a normal air pressure in accordance with a maximum loading capacity, and a distance (b) from a side edge of the first tread zone to an outer profile line of the tire at a position corresponding to 0.95 times a height (H) of the carcass from a bead core at a state of inflating under an air pressure corresponding to 10% of a normal air pressure satisfies the following condition with respect to the width (TW 2 ) of the second tread zone: b/TW 2 >1.2. Moreover, the term "carcass height" used in the specification and claims means a height from the bead core to an innermost carcass ply when there are plural carcass plies. In such a tire, it is preferable that a total tire thickness of a middle part in the tire side portion locating within a range of 0.5-0.8 times the carcass height (H) from the bead core at the meridional section of the tire in a normal direction of the radial carcass is thinner than a total tire thickness of the other part in the tire side portion; and that a maximum width position of the radial carcass is located within a range of 0.6-0.8 times the carcass height (H) from the bead core. Furthermore, it is preferable that the total tire thicknesses of the tire side portion in the normal direction of the radial tire at positions of 0.85 times and 0.4 times the carcass height (H) from the bead core at the meridional section of the tire are thicknesses exceeding 1.65 times the similar total tire thickness at the maximum width position of the carcass. Also, it is preferable that the width of the tread at a state of inflating under an air pressure corresponding to 10% of the normal air pressure is within a range of 80-95% of the tire section width; and that the maximum width of the belt at a state of inflating under an air pressure corresponding to 10% of the normal air pressure is within a range of 60-90% of the tire section width. In such a pneumatic radial tire, when the interrelation of the half-width of the first tread zone (TW 1 ) and the width of the second tread zone (TW 2 ) are TW 2 /TW 1 >4.0 x 10 -2 , the effective increase of camber thrust F C can be particularly ensured while sufficiently maintaining the ground contact width of the second tread zone when the tire is rotated on the slant face such as a rut or the like, while when the interrelation of the distance (b) from the side edge of the first tread zone to the outer profile of the tire and the width of the second tread zone (TW 2 ) is b/TW 2 >1.2, the escape deformation of the buttress and the second tread zone as shown by a phantom line in Fig. 2 is restrained, while the pushing-out deformation bsho accompanied with the bulging deformation bside of the second tread zone is increased to enhance the rigidity of the second tread zone and hence a large shearing force to the road surface is generated in the contact of the second tread zone with the ground, whereby the effective increase of camber thrust F c can be realized. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a schematic view illustrating a state of generating wandering phenomenon; Fig. 2 is a meridional section view illustrating a state of generating camber thrust; Fig. 3 is a meridional section view of a first pneumatic radial tire according to the invention; Fig. 4 is a schematic section view illustrating a state of generating camber thrust in the tire of Fig. 3; Fig. 5 is a schematic view illustrating another embodiment of a first pneumatic radial tire; Fig. 6 is a meridional section view of a comparative tire having no second tread zone; Fig. 7 is a meridional section view of a half portion of a second pneumatic radial tire; and Fig. 8 is a section view of a second pneumatic radial tire similar to Fig. 7. In Fig. 3 showing a meridional section of a first pneumatic radial tire, 1, 2 are a pair of bead portions and a pair of sidewall portions, respectively, and 3 a tread portion toroidally extending between the sidewall portions. Moreover, the word "tire side portion" or "side portion" used in the specification and claims includes the bead portion 1 and the sidewall portion 2, respectively. Further, a radial carcass 4 continuously extends from one bead portion 1 to the other bead portion 1. In this case, the radial carcass 4 is comprised of two plies wound at their side portions around a bead core 5 embedded in each bead portion 1 from the inside toward the outside and one so-called down ply, and reinforces the above portions 1, 2, 3. Also, a belt 6 is arranged on an outer periphery side of a crown portion of the radial carcass 4 to reinforce the tread portion 3. The illustrated belt 6 is comprised of three belt layers, among which a middle layer has a widest width. In this tire, the tread portion 3 is comprised of a first tread zone 7 contacting with a flat road surface during the running of the tire under loading and a second tread zone 8 projecting toward the side of the first tread zone 7 and contacting with a mountain side of a slant road surface. In this case, the first tread zone 7 is a maximum width region of a ground contact portion of the tire on a flat road surface under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of the truck and bus tires at an inflation state under a normal air pressure in accordance with a maximum loading capacity according to JATMA standard. Now, the loading condition used herein is determined by considering the magnification and frequency of load actually applied to the tire, and is based on the fact that a maximum load is frequently loaded in trucks and buses and a load corresponding to about 70% of the maximum load is loaded in vehicles other than the above. Furthermore, the second tread zone is a region extending outward from the first tread zone in the widthwise direction of a ground contact portion on the flat road surface when a camber angle of 10° is applied to the tire under the respective loading condition in truck and bus tires and tires other than the above at the inflation state under the above normal air pressure. The reason why the camber angle of 10° is given to each of the tires is based on the knowledge that when examining the tire behavior during the running on ruts in detail, the deformation state on the slant face of the rut is substantially the same as that in the application of camber angle of 10° to the tire. When the tire having such a structure is run on a slant face S under loading as shown in Fig. 4, the ground contact area can be increased by a region of ground contact width of the second tread zone 8 owing to the presence thereof as compared with the conventional radial tire, whereby lateral force F CS directing toward the upper side of the slant face S and hence camber thrust F C can directly be increased. Further, the second tread zone 8 of high rigidity not only prevents the escape deformation of a portion near to the buttress but also positively contributes to the increase of the pushing-out deformation of a portion near to the ground contact end as previously mentioned, from which point the effective increase of camber thrust F C can also be realized. In this case, it is preferable that the radius of curvature of the second tread zone 8 is not less than 40 mm at a section shown in Fig. 3 for ensuring sufficient ground contacting property of this zone. In Fig. 5, a reinforcing portion 9 projecting outward from the second tread zone 8 in the widthwise direction is arranged in the aforementioned tire, whereby the strength and rigidity in the second tread zone 8 and buttress portion are increased. The outer profile line of the reinforcing portion 9, preferably at the meridional section of the tire, is located inward from a phantom extending line of the outer profile line of the second tread zone 8 in the radial direction of the tire, whereby the contact of the reinforcing portion 9 with the ground is prevented during the running of the tire on the slant face S under loading. Furthermore, the projection quantity of the reinforcing portion 9 toward the side of the tread portion can be determined by considering durability to heat generation, weight, cost and the like required for satisfying the running conditions, and easiness of mounting a tire chain on a snow road. The projection quantity is gradually decreased inward in the radial direction of the tire and disappears at an upper end of the sidewall portion 2. Such a reinforcing portion 9 brings about the increase of the bulging deformation bside under the improved reinforcing action and can effectively contribute to the further increase of the camber thrust F C . Moreover, when the reinforcing portion 9 is annular and continuously arranged in the circumferential direction of the tire, or when it is arranged at slight intervals through slits having a narrow width or the like in the circumferential direction, the functions inherent thereto can effectively be developed. On the other hand, in order to effectively control the increase of tire weight while maintaining the reinforcing effect through the reinforcing portion 9, as shown in Fig. 5, it is preferable that the reinforcing portion 9 is constructed with a plurality of ribs 10 arranged at relatively large intervals in the circumferential direction of the tire. In the first pneumatic radial tire, it is further preferable that the tread width TW is within a range of 80-105% of the tire section width TS under the inflation of the above normal air pressure, and that the maximum width of the belt BW is within a range of 60-100% of the tire section width TS. In other words, when the tread width TW is less than 80% of the tire section width TS, there is a danger that the camber thrust F C can not sufficiently be increased, while when said ratio exceeds 105%, the increase of camber thrust F C reaches a limit and the danger of breaking the tread end becomes higher. Further, when the belt maximum width BW is less than 60% of the tire section width TS, there is a danger of damaging the effective transmission of the bulging deformation bside of a portion in the vicinity of the buttress to a portion near to the ground contact end, while when said ratio exceeds 100%, the increase of camber thrust F C reaches a limit and also it is apt to cause separation at the belt end portion. A comparison test relating to the effect of controlling the wandering phenomenon in the pneumatic radial tire of the above structure will be described below. Test Tire Various radial tires for small-size trucks having the structure shown in Fig. 3 and a tire size of 195/85R16 114/112L LT are manufactured by varying the tread width and belt maximum width under dimensions shown in Table 1 as invention tires 1-3. In these tires, the tire section width TS is 191 mm, and the outer profile line of the tread portion is comprised of an arc having a radius of curvature of 300 mm over a region outward from the equatorial plane of the tire to 37.6 mm in the widthwise direction of the tire, an arc having a radius of curvature of 100 mm over a region outward from the equatorial plane of the tire to 37.6-80.0 mm in the widthwise direction of the tire, and an arc having a radius of curvature of 50 mm over an outermost region in the widthwise direction of the tire. In this case, the first tread zone 7 is existent in a region outward from the equatorial plane of the tire to each side of 62.5 mm in the widthwise direction of the tire, and the second tread zone 8 is existent outward from the first tread zone 7 in the widthwise direction. Further, a reinforcing portion 9 as shown in Fig. 5 is provided in tires having the same structure as mentioned above as invention tires 4 and 5. In this case, each of the ribs 10 constituting the reinforcing portion 9 taperingly projects outward from a side edge of the second tread zone 8 to 10 mm in the widthwise direction of the tire, and the projection amount gradually decreases inward in the radial direction of the tire and disappears at the upper end portion of the sidewall portion 2. The rib 10 has a width of 20 mm in the circumferential direction of the tire. These ribs 10 are arranged at an interval of 16 mm in the circumferential direction in a total number of 64 on the circumference. As a comparative tire, there is prepared a tire having the structure shown in Fig. 6. This comparative tire differs from the tire shown in Fig. 3 only in that the second tread zone 8 is not existent. Test Method Each of these tires is inflated under a normal air pressure of 6.0 kgf/cm 2 and mounted onto a small-size truck of 2 ton capacity (rear wheel is double-wheel type). The small-size truck is run on a paved road including ruts at a state of loading under a maximum authorized payload by a test driver, during which straight running stability is evaluated by feel. The results are also shown in Table 1 by an index evaluation that the comparative tire is 100 (the larger the index value, the better the property). As seen from this Table, the tires according to the invention effectively prevent the wandering phenomenon and can largely improve the straight running stability. Tire Tread width TW (mm) Belt maximum width BW (mm) Tire section width TS (mm) Reinforcing portion Straight running stability Remarks Comparative tire 125 (0.66) 125 (0.66) 189 none 100 Fig. 6 Invention tire 1 180 (0.95) 125 (0.66) 189 none 124 Fig. 3 Invention tire 2 180 (0.95) 162 (0.86) 189 none 137 Fig. 3 Invention tire 3 192 (1.02) 162 (0.86) 189 none 142 Fig. 3 Invention tire 4 180 (0.95) 162 (0.86) 189 presence 139 Fig. 5 Invention tire 5 192 (1.02) 162 (0.86) 189 presence 145 Fig. 5 Note) Numerical value in parenthesis of tread width and belt maximum width is a ratio to tire section width. The second pneumatic radial tire according to the invention will now be described with reference to Fig. 7. In this case, a solid line indicates the invention tire, and broken lines indicate the conventional tire. Moreover, explanations are omitted with respect to the same structural portions as described in the first pneumatic radial tire. In this tire, an interrelation between a half-width TW 1 of the first tread zone 7 and a width TW 2 of the second tread zone 8, that is the interrelation between the half-width (TW 1 ) of the first tread zone as a maximum ground contact width in a ground contact portion of the tire on a flat road surface from an equatorial plane of the tire under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in one case of tires having a size smaller than that of the truck and bus tires at an inflation state under a normal air pressure in accordance with the maximum loading capacity and the width (TW 2 ) of the second tread zone as a ground contact width extending outward from the half-width (TW 1 ) of the first tread zone among the maximum ground contact width in the ground contact portion of the tire on the flat road surface from the equatorial plane of the tire when a camber angle of 10° is applied to the tire under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of the truck and bus tires at the inflation state under the above normal air pressure, is TW 2 /TW 1 >4.0 x 10 -2 . In addition, an interrelation between the width TW 2 of the second tread zone and a distance (b) between points P, Q when an intersect between a phantom line segment drawn from the bead core 5 to a height position corresponding to 0.95 times a carcass height H, which is a height H from the bead core 5 to an innermost carcass ply at an inflation state under an air pressure corresponding to 10% of the above normal air pressure, with a normal line drawn from a side edge of the first tread zone 7 to the phantom line segment is P and an intersect of the phantom line segment with the outer profile line of the tire is Q, is b/TW 2 >1.2. The illustrated tire satisfying the above conditions can develop functions similar to those of the above first pneumatic radial tire under each action of the first tread zone 7 and the second tread zone 8 constituting the tread portion 3. When the width TW 2 of the second tread zone 8 is not more than 4.0 x 10 -2 the half-width TW 1 of the first tread zone 7, the ground contact width of the second tread zone can not sufficiently be ensured during the running of the tire on the slant face under loading and hence the required increase of camber thrust F c can not be obtained. Observing the deformation behaviou of the second tread zone 8 during the running of the tire on the slant face, it is usually effective to increase the rigidity of a portion in the vicinity of the point P in order to create a large pushing-out deformation bsho based on the bulging deformation bside, so that the buttress portion can directly be reinforced and also the second tread zone 8 can indirectly and effectively be reinforced by malting sufficiently large the rubber thickness of a portion outward from the point P in the axial direction of the tire to render the ratio of rubber thickness b to width TW 2 of the second tread zone 8 to be more than 1.2, and consequently the pushing-out deformation bsho of the second tread zone 8 is effectively increased and also a large shearing force to road surface is created in the ground contact portion of the second tread zone 8, whereby camber thrust F C can largely be increased. In other words, when the ratio is less than 1.2, the reinforcing effect is low and it is difficult to create the large pushing-out deformation bsho resulting from the bulging deformation bside. This is particularly effective when a reinforcing portion 11 further projecting from a tread end or a side edge of the second tread zone 8 is defined in the outer profile line of the tire passing the point Q as shown, in which the reinforcing portion 11 itself functions likewise the reinforcing portion 9 described on the first pneumatic radial tire and can effectively contribute to the increase of camber thrust F C . In such a tire, it is preferable that a total tire thickness of a middle part in the tire side portion locating within a range of 0.5-0.8 times the carcass height H from the bead core 5 at the meridional section of the tire in a normal direction of the radial carcass is thinner than a total tire thickness of the other part in the tire side portion, and that a maximum width position of the radial carcass is located within a range of 0.6-0.8 times the carcass height H. According to the former case, when the thickness of the middle part in the tire side portion is made smaller than the thickness of each of the buttress portion and the bead portion, not only can the increase of the pushing-out deformation bsho be ensured while maintaining the buttress portion at a high rigidity, but also the rigidity of the middle part is made small to concentrate the bulging deformation bside of a portion near to the buttress and the bulging deformation bside can also be increased, and they can effectively contribute to the increase of camber thrust F C . This is particularly remarkable when each total tire thickness at positions corresponding to 0.85 times and 0.4 times the carcass height H from the bead core 5 exceeds 1.65 times the total tire thickness at the position of carcass maximum width. In other words, when said ratio is not more than 1.65 times, it is difficult to sufficiently increase the bulging deformation bside. Moreover, the reason why the positions corresponding to 0.85 times and 0.4 times the carcass height H are specified is based on the knowledge that in order to conduct the concentration of deformation into the middle part located within a range of 0.5-0.8 times the carcass height H, it is advantageous to sufficiently thin a portion of this range, particularly the position of carcass maximum width with respect to portions at the positions of 0.85H and 0.4H outside the above range. In the latter case, when the position of the carcass maximum width is specified within a range of 0.6H-0.8H, it is possible to increase the bulging deformation bside during the running of the tire on the slant face under loading by making small the radius of curvature of the carcass ranging from the tread end portion to the buttress region and also it is possible to increase the pushing-out deformation bsho by approaching the position of creating the bulging deformation bside toward the buttress region. In addition, carcass tension upon inflation under the air pressure is made small within this range, so that further increase of the bulging deformation at this range can be ensured. In such a tire, it is further preferable that the tread width at the inflation under an air pressure corresponding to 10% of the normal air pressure is within a range of 80-95% of the tire section width, and that the belt maximum width is within a range of 60-90% of the tire section width. This can realize the direct increase of camber thrust F C and the increase of bulging deformation bside likewise the first pneumatic radial tire. When the tread width exceeds 95% of the tire section width, the danger of lowering the durability to heat generation due to the increase of rubber thickness at the end portion of the belt becomes higher, while when the belt maximum width exceeds 90% of the tire section width, the danger of lowering the belt durability and the durability to heat generation accompanied with the increase of strain in the end portion of the belt becomes higher. Moreover, in order to sufficiently contact the second tread zone 8 with the ground in this tire, it is preferable that the radius of curvature of the outer profile line at the meridional section of the tire is not less than 30 mm. Also, the reinforcing portion 11 may be comprised of an annular projection continuously extending in the circumferential direction of the tire, or plural ribs likewise the first pneumatic radial tire. Although the practical embodiment of the second pneumatic radial tire is described with respect to a tire shown in Fig. 7 having a tire size of TBR 11R 22.5 14 PR, the above features are similarly applied to a tire shown in Fig. 8 having a tire size of TBR 215/70 R17.5. In this case, a solid line indicates the invention tire, and broken lines indicate the conventional tire. The invention tire shown in Fig. 7 has TW 1 x 2 = 185.0 mm, TW 2 = 9.6 mm, b = 19.7 mm, b/TW 2 = 2.05, TW 2 /TW 1 = 10.83 x 10 -2 , while the conventional tire shown in the same figure has TW 1 x 2 = 185.0 mm, TW 2 = 0.0 mm, b = 15.5 mm. Further, the invention tire shown in Fig. 8 has TW 1 x 2 = 167.0 mm, TW 2 = 8.0 mm, b = 15.4 mm, b/TW 2 = 1.93, TW 2 /TW 1 = 9.58 x 10 -2 , while the conventional tire shown in the same figure has TW 1 x 2 = 167.0 mm, TW 2 = 0.0 mm, b = 12.1 mm. A comparison test between the invention tire and the conventional tire shown in Fig. 7 will be described below with respect to the performance of controlling the wandering phenomenon . Test Tire There are provided invention tires 6-9 and a conventional tire having a tire size of 11R22.5 14PR and dimensions shown in Table 2. In the invention tires, the outer profile line of the tread portion is comprised of an arc having a radius of curvature of 580 mm outward from the equatorial plane of the tire to 62.8 mm in the widthwise direction of the tire, an arc having a radius of curvature of 200 mm outward from the equatorial plane of the tire to 62.8-91.5 mm in the widthwise direction of the tire, and an arc having a radius of curvature of 200 mm outward therefrom in the widthwise direction of the tire. In this case, the first tread zone 7 is existent in a region outward from the equatorial plane of the tire to each side of 92.5 mm in the widthwise direction of the tire, while the second tread zone 8 is existent outward from the first tread zone 7 in the widthwise direction. Moreover, the conventional tire has no second tread zone because the tread portion is comprised of only the first tread zone. Test Method Each of these tires is mounted onto a rim of 7.50 x 22.5, inflated under a normal air pressure of 7.0 kgf/cm 2 and then mounted onto a truck of 11.5t capacity (wheel arrangement 2-D·4). The truck is run on a paved road including ruts at a state of loading under a maximum authorized payload by a test driver, during which straight running stability is evaluated by feel. The results are also shown in Table 2 by an index evaluation that the conventional tire is 100 (the larger the index value, the better the property). As seen from this table, the tires according to the invention can largely improve the straight running stability. As seen from the above, the pneumatic radial tires according to the invention can effectively control the occurrence of wandering phenomenon when applied to vehicles such as passenger cars, small-size trucks, trucks, buses and the like and particularly can realize excellent straight running stability even on road surfaces including ruts during high-speed running of the vehicle.

Claims: A motor vehicle pneumatic radial tire comprising a pair of bead portions (1), a pair of sidewall portions (2), a tread portion (3) toroidally extending between both sidewall portions, a radial carcass (4) reinforcing said portions, and a belt (6) reinforcing the tread portion at the outer circumferential side of the radial carcass, in which the tread portion (3) is comprised of a first tread zone (7) contacting with a flat road surface and a second tread zone (8) projecting toward the side of the first tread zone and contacting with a mountain side of a slant road surface, and the first tread zone (7) is the maximum width region of the ground contact portion of the tire on a flat road surface under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of truck and bus tires at an inflation state under a normal air pressure in accordance with a maximum loading capacity according to JATMA standard, and the second tread zone (8) is the region extending outward from the first tread zone in the widthwise direction of the gound contact portion on the flat road surface when a camber angle of 10° is applied to the tire under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of truck and bus tires at an inflation state under a normal air pressure in accordance with a maximum loading capacity according to JATMA standard. A tire as claimed in claim 1, characterized by further comprising a reinforcing portion (9) projecting outward from the second tread zone (8) in the widthwise direction. A tire as claimed in claim 2, characterized in that an outer profile line of the reinforcing portion (9) at radial section of the tire is located inward from the phantom extending line of the outer profile line of the second tread zone (8) in the radial direction of the tire. A tire as claimed in claim 2 or 3, characterized in that the reinforcing portion (9) is comprised of plural ribs (10) arranged at given intervals in the circumferential direction of the tire. A tire as claimed in any of claims 1 to 4, characterized in that the tread width (TW) is within a range of 80-105% of a tire section width (TS) at an inflation state under a normal internal pressure. A tire as claimed in any of claims 1 to 5, characterized in that the maximum width of the belt (BW) is within a range of 60-100% of the tire section width (TS) at the inflation state under a normal internal pressure. A motor vehicle pneumatic radial tire comprising a pair of bead portions (1), a pair of sidewall portions (2), a tread portion (3) toroidally extending between both sidewall portions, a radial carcass (4) reinforcing said portions, and a belt (6) reinforcing the tread portion at the outer circumferential side of the radial carcass, in which the tread portion (3) is comprised of a first tread zone (7) contacting with a flat road surface and a second tread zone (8) projecting toward the side of the first tread zone and contacting with a mountain side of a slant road surface, and a half-width (TW 1 ) of the first tread zone (7) which is the maximum ground contact width of the ground contact portion of the tire on a flat road surface from an equatorial plane of the tire under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of truck and bus tires at an inflation state under a normal air pressure in accordance with the maximum loading capacity, and the width (TW 2 ) satisfies the following condition: TW 2 /TW 1 >4.0 x 10 -2 when said width of the second tread zone (8) is the ground contact width extending outward from the half-width (TW 1 ) of the first tread zone (7) among the maximum ground contact width in the ground contact portion of the tire on a flat road surface from the equatorial plane of the tire when a camber angle of 10° is applied to the tire under a normal loading in the case of truck and bus tires and under a loading corresponding to 70% of normal loading in the case of tires having a size smaller than that of truck and bus tires at an inflation state under a normal air pressure in accordance with a maximum loading capacity, and the distance (b) from a side edge of the first tread zone (7) to an outer profile line of the tire at a position corresponding to 0.95 times the height (H) of the carcass from a bead core (5) at a state of inflating under an air pressure corresponding to 10% of a normal air pressure satisfies the following condition with respect to the width (TW 2 ) of the second tread zone (8): b/TW 2 >1.2. A tire as claimed in claim 7, characterized in that the total tire thickness of the middle part in the tire side portion locating within a range of 0.5-0.8 times the carcass height (H) from the bead core (5) at the meridional section of the tire in a normal direction of the radial carcass is thinner than the total tire thickness of the other part in the tire side portion. A tire as claimed in claim 7 or 8, characterized in that the maximum width position of the radial carcass is located within a range of 0.6-0.8 times the carcass height (H) from the bead core. A tire as claimed in any of claims 7 to 9, characterized in that the total tire thicknesses of the tire side portion in the normal direction of the radial tire at positions of 0.85 times and 0.4 times the carcass height (H) from the bead core (5) at the meridional section of the tire are thicknesses exceeding 1.65 times the similar total tire thickness at the maximum width position of the carcass. A tire as claimed in any of claims 7 to 10, characterized in that the width of the tread (TW) at a state of inflating under an air pressure corresponding to 10% of the normal air pressure is within a range of 80-95% of the tire section width (TS). A tire as claimed in any of claims 7 to 11, characterized in that the maximum width of the belt (BW) at a state of inflating under an air pressure corresponding to 10% of the normal air pressure is within a range of 60-90% of the tire section width (TS).

Inventors: MATSUMOTO HIROYUKI, MATSUMOTO, HIROYUKI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1996
Main classification: B60C11
IPC codes: B60C13, B60C3, B60C11