GB2170153A - Pneumatic radial tire tread - Google Patents

Abstract

A pneumatic radial tire for heavy-duty road vehicles comprises a tread (1) which is divided into a pair of laterally outermost shoulder ribs (3) and at least one center rib (4) disposed between the laterally outermost shoulder ribs (3) by at least two main wide grooves (2) extending in a substantially circumferential direction of the tire, wherein at least one of the shoulder ribs (3) is divided into at least two small ribs (6, 7) by at least one auxiliary narrow groove (5) extending in the substantially circumferential direction and having a width which is not more than 3 percent of the width of the tread (1) and a depth which is not less than 30 percent of the depth of the main groove (2) located adjacent the shoulder rib (3), the auxiliary narrow groove (5) being disposed such that the distance, measured in a lateral direction of the tire, from a lateral edge (10) of the tread (1) to the auxiliary groove (5) is less than 50 percent of the width (A) of the shoulder rib (3). <IMAGE>

Description

SPECIFICATION Pneumatic radial tire The present invention relates in general to a pneumatic radial tire adapted to be used on heavy-duty road vehicles such, for example, as a truck, bus and the like and in particular to an improvement in configuration of the tread portion of the pneumatic radial tire especially suitable for running at high speed on a smooth road.

In accordance with an important aspect of the present invention, there is provided a pneumatic radial tire for heavy-duty road vehicles comprising a tread which is divided into a pair of laterally outermost shoulder ribs and at least one center rib disposed between the laterally outermost shoulder ribs by at least two main wide grooves extending in a substantially circumferential direction of the tire, wherein at least one of the shoulder ribs is divided into at least two small ribs by at least one auxiliary narrow groove extending in the substantially circumferential direction and having a width which is not more than 3 percent of a width of the tread and a depth which is not less than 30 percent of a depth of the main groove located adjacent the shoulder rib, the auxiliary narrow groove being disposed such that a distance, measured in a lateral direction of the tire, from a lateral edge of the tread to the auxiliary groove is less than 50 percent of a width of the shoulder rib.

In accordance with another important aspect of the present invention, there is provided a pneumatic radial tire for heavy-duty road vehicles comprising a tread which is divided into a pair of laterally outermost shoulder ribs and at least one center rib disposed between the laterally outermost shoulder ribs by at least two main wide grooves extending in a substantially circumferential direction of the tire, wherein at least one of the shoulder ribs is divided into at least two small ribs by at least one auxiliary narrow groove extending in the substantially circumferential direction and having a width which is not more than 3 percent of a width of the tread and a depth which is not less than 30 percent of a depth of the main groove located adjacent the shoulder rib, the small rib being formed at its laterally outer end portion opposing the auxiliary groove with a plurality of sipes so that rigidity of the laterally outer end portion is decreased.

In accordance with still another important aspect of the present invention, there is provided a pneumatic radial tire for heavy duty road vehicles comprising a tread which is divided into a pair of laterally outermost shoulder ribs and at least one center rib disposed between the laterally outermost shoulder ribs by at least two main wide grooves extending in a substantially circumferential direction of the tire, wherein at least one of the shoulder ribs is divided into at least two small ribs by at least one auxiliary narrow groove extending in the substantially circumferential direction and having a width which is not more than 3 percent of a width of the tread and a depth which is not less than 30 percent of a depth of the main groove located adjacent the shoulder rib, each of the small ribs being formed at its laterally outer end portion with a plurality of sipes so that rigidity of the laterally outer end portion is decreased, quantities of the sipes formed in the small ribs being gradually decreased in a lateral inward direction of the tire.

Description of the prior art Known tread configurations or patterns for pneumatic radial tires used for heavy-duty road vehicles suitable for running at high speed on smooth road are generally of the complete rib type, the rib-lug type and the rib block type. Such treads are provided with two to five or more wide grooves extending in the substantially circumferential direction of the tire and dividing the tread into a plurality of circumferentially extending land portions (which include land portions divided in the circumferential direction by transversely extending grooves and which will be hereinafter referred to as a rib).

Difficulties have been encountered in that various uneven wears occur in the circumferential ribs of the tread, particularly in the shoulder ribs located on the laterally outermost end portions of the tread. Among these uneven wears, there are an edge-drop wear in which only the laterally outermost end portion of the shoulder rib wears substantially linearly in the circumferential direction of the tire, an undulant wear in which the edge-drop wear grows and extends enevely on the circumference of the tread in the lateral inward direction of the tire and the shoulder rib undulates when viewed from the direction of the axis of rotation of tire, a shoulder-dropwear in which the undulant wear grows further to a wear of the whole of the shoulder rib, and a polygonal wear in which the undulate wear extends across the main groove and further to the center rib and the contour of the tire is no longer a true circle but a polygon. In a certain case, these wears are combined with one another by various working conditions of the tire and wear on the tread becomes intricate. However, essentially, wear on the tread occurs first in the outermost end portion of the shoulder rib and grows and extends enevely on the circumference of the tread in the lateral inward direction of the tire. If such uneven wear on occurs and are distributed unevely on the tread, appearance of the tire is impaired considerably and the tire is caused to vibrate undesirably, so that performance of the tire is remarkably deteriorated, thereby decreasing remarkably life of the tire itself.

In order to prevent the uneven wears, for example, the shoulder-drop wear and the polygonal wear, it has been proposed to raise contact-pressure of the shoulder rib by increasing radius of curvature of the crown of the tire or by providing relatively wider ribs. However, these methods do not sufficiently overcome the uneven wears, and have an adverse effect on the edge-drop wear and the undulate wear. By reason that the uneven wears occur particularly in the laterally outer half portions of the treads of the front tires for the heavy-duty road vehicles, the front and rear tires were rotated to alleviate the uneven wears. The rotation of the tire is not a drastic solution and recently in many cases tires are used without the rotation, so that various research and development are being paid extensively in the tire manufacturing industry to overcome the uneven wears.For the purpose of distributing the wear on the tread evenly and at the same time improving road-adherence of the tire, a pneumatic radial tire comprising a pair of relatively wider grooves and a plurality of narrower grooves is disclosed in United State Patent No. 3,411,559 (Re. 30,527). It has, however, been found that the uneven wears on the tread are not overcome satisfactorily by simply forming the narrower grooves on the opposite sides of the wider grooves.

It is, accordingly, an important object of the present invention to provide an improved pneumatic radial tire for heavy-duty road vehicles which can overcome the uneven wears and thereby enhancing remarkably life of the tire itself.

Local wear occurs in the laterally outer end portion of the shoulder rib due to transverse force produced during turning of the vehicle and drag, due to the difference in radii of the tread portion and the tread edge portion, produced during straight drive of the vehicle and then extends from the outer end portion of the shoulder rib in the circumferential, lateral inward and depth directions, so that the local wear grows to various uneven wears.

Accordingly, in accordance with an important aspect of the present invention, the shoulder rib of the tread is divided by auxiliary narrow grooves located in the predetermined position into small ribs of predetermined width so that the ground-contact pressure acting on the shoulder rib end when the transverse force is produced is decreased properly and distributed evenly over the shoulder rib. Thus, the local wear is prohibited from occurring in the shoulder rib and at the same time prevented effectively from growing and extending in the lateral inward direction of the tread.

In accordance with another important aspect of the present invention, a plurality of sipes are formed in the outer end portion of the small rib arranged inwardly of the auxiliary groove. Since the rigidity of the outer end portion of the small rib is decreased effectively by the sipes which serve to alleviate stress, the local wear once interrupted by the auxiliary groove is more effectively prevented from extending further in the lateral inward direction of the tire.

In accordance with still another important aspect of the present invention, a plurality of sipes are also formed in the outer end portion of the outermost small rib. As a result, since the rigidity of the outer end portion of the outermost small rib is also decreased, occurrence of the uneven wear in the outmost small rib is prevented more effectively. Since the transverse force which is causative of the uneven wear is larger in the outer portion of the tread, total projected area of the sipes on a radial plane which includes the axis of rotation of the tire is gradually decreased from the outermost small rib to the innermost small rib in this order. Accordingly, occurrence of the uneven wear is prevented more effectively.It is noted that if the sipes are provided more than necessary, such sipes of large quantity are apt to induce rib crack at the sipes and decrease resistance to wear.

The various features and advantages of a pneumatic radial tire for heavy-duty road vehicles according to the present invention will be more fully understood from a consideration of the following detailed description in conjunction with the accompanying drawings in which like reference numerals designate corresponding or similar members throughout the figures of the drawings and in which:: Figure 1 is a plan view of one embodiment of a pneumatic radial tire for heavy-duty road vehicles constructed in accordance with the present invention; Figure 2 is a transverse sectional view of a tread shown in Figure 1 taken substantially along line C-C in Figure 1; Figure 3 is a plan view of another embodiment constructed in accordance with the present invention, a plurality of parallel sipes being formed in the center portion of the tread; Figure 4 is a plan view showing another embodiment constructed in accordance with the present invention, two auxiliary narrow-zigzag grooves being formed in the shoulder ribs of the tread; Figures a similar view to Figure 4, except that three main wide-zigzag grooves are formed in the center portion of the thread;; Figure 6 is a plan view showing another embodiment constructed in accordance with the present invention, a plurality of sipes being formed in the lateral outer end portion of a small rib; Figure 7 is a transverse sectional view of a tread shown in Figure 6 taken substantially along line A-A in Figure 6; Figure 8 is a plan view showing another embodiment constructed in accordance with the present invention, a plurality of sipes extending in zigzag fashion; Figure 9 is a transverse sectional view of a tread shown in Figure 8 taken substantially along line B-B in Figure 8; Figure 10 is a plan view of another embodiment constructed in accordance with the present invention, a plurality of sipes being formed in each of small ribs; and Figure ii is a transverse sectional view of a tread shown in Figure 10 taken substantially along line A-A in Figure 10.

Referring initially to Figure 1 of the drawings which is a plan view of a pneumatic radial tire for heavy-duty road vehicles embodying the subject matter of the present invention, a tire tread of the pneumatic radial tire for heavy-duty road vehicles is indicated generally by reference numeral 1. The tread 1 is provided with at least two (here three) main wide-zigzag grooves 2 extending in the substantially circumferential direction of the tire. The tread 1 is divided by the main grooves 2 into four zones constituted by a pair of shoulder ribs 3 located on the laterally outermost edge portions of the tread 1 and a pair of center ribs 4 located on the center portion of the tread 1. Each main groove 2 is constructed not so as to be closed in the ground-contact region of the tread 1 during cruising of the vehicle upon load.It is preferable that a groove width of the main groove 2 indicated by W2 be 4 to 8 percent of a width T of the tread 1 which is measured in the lateral direction of the tire which is substantially parallel to the axis of rotation of the tire. The shoulder ribs 3 are substantially free of transversal grooves which extend in the lateral direction of the tire and which interrupt their circumferential continuity.

Each shoulder rib 3 is divided into a plurality of small ribs 6 and 7 by at least one auxiliary narrow-zigzag groove 5 (here one) extending in the substantially circumferential direction of the tread 1. It is noted that the auxiliary groove 5 must be disposed such that a distance B, measured in the lateral direction of the tire, from a lateral edge 10 of the tread 1 to a mid-circumferential line of the auxiliary zigzag groove 5 indicated by M is less than 50 per cent of a width A of the shoulder rib 3, that is to say, the ratio B1A is less than 0.5. The mid-circumferential line M of the auxiliary zigzag groove 5 is located midway between a laterally outermost end 5a and a laterally innermost end 5b of the zigzag grove 5.The distance B from the lateral edge 10 to the auxiliary groove 5 is preferably not less than 20 perecent of the width A of the shoulder rib 3, more preferably 25 to 40 percent. In the case that the distance B is not less than 50 percent of the width A, since the ground-contact pressure at the lateral edge 10 of the tread 1 cannot be decreased sufficiently, the effect of the presence of the auxiliary groove 5 cannot be otained.In the case that the distance b is extremely small as compared with the width A of the shoulder rib 3 so that the auxiliary groove 5 is located near the lateral edge 10 of the tread 1, the ground-contact pressure at the lateral edge 10 of the shoulder rib 3 becomes small sufficiently so that occurrence of local wear in the small rib 6 is alleviated, but the ground-contact pressure at a laterally outermost end 11 of the small rib 7 is increased so that local wear tends to occur in the small rib 7.

The auxiliary groove 5 has a groove width W5 which is not more than 3 percent of the width T of the tread 1 so that the ground-contact pressure does not vary rapidly across the shoulder rib 3. It is preferable that the width W5 of the auxiliary groove 5 be between approximately 0.3 and 2 percent of the width T of the tread 1 so that the auxiliary groove 5 is closed in the ground-contact region of the tread 1 during cruising of the vehicle upon load. In order to prevent uneven wears on the tread 1, it is desired that a depth dS (Figure 2) of the auxiliary groove 5 be not less than 30 per cent of a depth d2 (Figure 2) of the main groove 2, preferably within 100 percent. It is most preferable that the widths of the small ribs 6 and 7 be each smaller than that of the center rib 4 from the standpoint of balance of the ground-contact pressure.It is noted that if the lateral end of a rib is shaped into zigzag, the width of the rib is measured by measuring the mid-circumferential line located midway between a laterally outermost end and a laterally innermost end on the zigzag line as the lateral end of the zigzag rib.

Referring to Figure 2, the auxiliary grooves 5 do not always extend substantially vertically from the outer surface of the tread 1. The auxiliary grooves 5 may extend at angle of a with respect to a normal line n substantially vertical to the outer surface of the tread 1. It is most preferable, as shown in Figure 2, that the auxiliary grooves 5 extend from the outer surface of the tread 1 laterally outwardly at an angle of not more than 20 degrees with respect to the normal line n.

During straight drive of the vehicle, since the ground-contact pressure of the tread center portion between the outermost main grooves 2 becomes larger than that of the tread shoulder portions disposed outwardly of the outermost main grooves 2, the tread center portion wears away more rapidly than the tread shoulder portions. In order to alleviate such center wear concurrently and enhance life of wear, as shown in Figure 3 which shows a tire having four main wide grooves, each center rib 4 may be formed with a plurality of parallel sipes 13 each of which is connected at its both ends with the main grooves 2. In this case, a space P of adjacent sipes 13 measured in the circumferential direction of the tire is desired to be 5 to 15 percent of the tread width T, more preferably 6 to 10 percent.It is preferable that each sipe 13 be arranged at angle X of between 0 and t 45 degrees with respect to the rotational axis 0 of the tire and the depth of each sipe 13 be 30 to 100 percent of the depth of the main groove 2. The sipes are intended to mean narrow incision which is substantially closed in the ground-contact region of the tread during cruising of the vehicle upon load, and preferably the width u of the sipe 13 is between 0.3 and 1.5 mm. In order to alleviate the center wear concurrently, the above-noted arrangement in which the center rib 4 is provided with a plurality of such sipes 13 is applicable to the embodiment as shown in Figure 1 and all of embodiments which will be hereinafter described in this specification.

While it has been illustrated and described that each shoulder rib 3 is provided with the single auxiliary narrow groove 5, the shoulder rib 3 may be provided with a plurality of the auxiliary narrow grooves each of which is constructed and arranged as stated above. This arrangement will be more effective when the number of the main zigzag groove 2 is small and the width A of the shoulder rib 3 is large. In this case, the width of each of the small ribs provided in the shoulder rib is also smaller than that of each of the center ribs.

According to the present invention, Figures 4 and 5 show a shoulder rib 3 provided with two auxiliary narrow ribs 5.

As described above, since the uneven wear on the shoulder rib occurs in the laterally outer half portion of the tread more than the laterally inner half portion of the tread when the tire is employed in the vehicle, the auxiliary groove 5 may be provided in only one of the shoulder ribs. In this case, the one shoulder rib provided with the auxiliary groove 5 is arranged to be located in the outside of the vehicle. It is noted that the arrangement in which the auxiliary groove 5 is provided in only one of the shoulder ribs is applicable to all of embodiments which will be hereinafter described.

Figure 6 shows another embodiment constructed in accordance with the present invention in which a plurality of sipes 9 are formed in a laterally outer end portion 15 of the small rib 7so that the rigidity of the outer end portion 15 of the small rib 7 is decreased sufficiently, for the purpose of preventing local wear ocurred in the vicinity of the lateral edge 10 of the tread 1 from extending in the lateral inward direction of the tire to the small rib 7. The sipes 9 extend in the substantially lateral direction of the tire from the outermost end 17 of the small rib 7 toward the center portion of the tread 1 so that rigidity of the laterally outer end portion 15 of the small rib 7 is decreased as compared with the small rib 6.It is noted that the wear on the small rib 6 disposed outwardly of the small rib 7 does not have a serious effect on appearance and performance of the tire. In this embodiment, although the position of the auxiliary groove 5 is not restricted as in the previous embodiment as shown in Figure 1, it is most preferable that the distance between the auxiliary groove 5 (the outermost auxiliary groove 5 if a plurality of the auxiliary groove 5 are provided) and the lateral edge 10 of the tread 1, which is substantially equal to a width W6 of the small rib 6, be not less than 20 but less than 50 percent of the width W3 of the shoulder rib 3.

In the embodiment shown in Figure 6, the sipes 9 are spaced apart a predetermined space S from one another in the circumferential direction of the tire. In order that the rigidity of the outer end portion 15 of the small rib 7 is decreased such that the effect of the present invention is obtained, at least a plurality of the sipes 9 must be formed in one pitch of the zigzag of the auxiliary zigzag groove 5. The predetermined space of adjacent sipes 9 in the circumferential direction of the tire is preferably 10 to 200 percent of the depth d2 (Figure 7) of the main groove 2, more preferably 20 to 60 percent.It is preferable that a length of the sipe 9 indicated by L be 10 to 30 percent of a width W7 of the small rib 7 formed with the sipes 9 and that a depth of the sipe 9 indicated by dg in Figure 7 be 20 to 100 percent of the depth dS of the auxiliary groove 5.

Referring to Figures 8 and 9, there is shown another embodiment constructed in accordance with the present invention. A plurality of sipes indicated by reference numeral 16 are provided in the small rib 7 and extend in zigzag fashion along the auxiliary groove 5 in the substantially circumferential direction of the tire.

In Figure 8, the sipes 16 are discontinuous but may be continuous. It is preferable that the total quantity (length times depth) of the sipes 16 as shown in Figures 8 and 9 be substantially equal to the total quantity of the sipes 9 as shown in Figures 6 and 7 and that a space, indicated by Wg, between the auxiliary groove 5 and the sipes 16 be 10 to 25 percent of the width W of the small rib 7 formed with the sipes 16.

The sipes 9 and 16 in the embodiments shown in Figures 6 through 9, are also constructed so as to be closed in the ground-contact region of the tread during cruising of the vehicle upon load as hereinbefore described about the sipes 9 in Figure 3. Awidth of each of sipes 9 and 16 indicated by t is smaller than the width W5 of the auxiliary groove 5 and preferably 0.3 to 1.5 mm. Each of the constructions of the embodiments shown in Figures 6 through 9 is substantially identical to that of the embodiment shown in Figures 1 and 2, exceptforthe sipes 9 and 16.

Referring to Figures 10 and 11,there is shown another embodiment forming the subject matter of the present invention. In this embodiment, the shoulder rib 3 located on the lateral edge portion of the tread 1 is divided by two auxiliary narrow-zigzag groove 5 extending in the circumferential direction of the tire into a small rib 6 located on the laterally outermost end portion of the tread 1, a small rib 7b disposed laterally inwardly of the small rib 6, and a small rib 7c disposed laterally inwardly of the small rib 7b. As in the case of the first embodiment as shown in Figure 1, the shoulder ribs 3 are substantially free of transversal grooves which extend in the lateral direction of the tire and which interrupt their circumferential continuity.

In order to minimize local wear on a laterally outer end portion 20 of the small rib 6 and at the same time prevent the local wear from extending in the lateral inward direction ofthetread 1 from the laterally outer end portion 20 of the small rib 6, the small rib 6 is formed with a plurality of sipes 9 each of which extends in the substantially lateral direction of the tire from the outermost end 10 thereof for the purpose of decreasing sufficiently the rigidity of the laterally outer end 20.Similarly, in order to minimize local wear on a laterally outer end portion 21 of the small rib 7b disposed laterally inwardly of the small rib 6 and at the same time preventthe local wearfrom extending in the lateral inward direction of the tread 1 from the outer end portion 21 of the small rib 7b, the small rib 7b is formed with a plurality of sipes 9' each of which extends in the substantially lateral direction of the tire from the outermost end 1 7b of the small rib 7b for the purpose of decreasing the rigidity of the outer end portion 21. The sipes 9' formed in the small rib 7b are smaller in number and number than the sipes 9 formed in the small rib 6 disposed laterally outwardly of the small rib 7b.Similarly, in order to minimize local wear on a laterally outer end portion 22 of the small rib 7c disposed laterally inwardly of the small rib 7b and at the same time prevent the local wear from extending in the lateral inward direction of the tread 1 from the outer end portion 22 of the small rib 7c, the small rib 7c is formed with a plurality of sipes 9" each of which extends in the substantially lateral direction of the tire from the outermost end 1 7c of the small rib 7c for the purpose of decreasing the rigidity of the outer end portion 22.

The sipes 9" formed in the small rib 7c are smaller in number and number than the sipes 9' formed in the small rib 7b. Thus, the quantities of the sipes 9, 9' and 9" are gradually decreased from the outermost small rib 6 to the innermost rib 7c in this order.

The sipes 9 of the small rib 6 are equidistantly spaced apart from one another in the circumferential direction of the tire and extend substantially vertically from the outer surface of the tread 1 toward the axis of rotation of the tire. Likewise, the sipes 9' and 9" of the small ribs 7b and 7c, respectively, are equidistantly spaced apart from one another in the circumferential direction of the tire and extend substantially vertically from the outer surface of the tread 1 toward the axis of rotation of the tire. In order that the rigidity of the outer end portion 20 of the small rib 6 is decreased such that the effect of the present invention is obtained, at least one sipe 9 must be formed in one pitch of the zigzag of the auxiliary zigzag groove 5.Likewise, in order that the rigidities of the outer end portions 21 and 22 of the small ribs 7b and 7c are decreased such that the effect of the present invention is obtained, at least one sipe 9' and one sipe 9" must be formed in one pitch of the zigzag of the auxiliary zigzag groove 5.

Total of projected areas 9a (Figure 11) of the sipes 9 of the outermost small rib 6 on a radial plane which includes the axis of rotation of the tire is preferably 100 to 800 mm2 per unit length (1 cm) in the circumferential direction of the tire, more preferably 200 to 600 mm. The ratio between the total of the projected areas 9a of the sipes 9 of the small rib 6 per unit length and total of projected areas 9b (Figure 11) of the sipes 9' of the small rib 7b per unit length is preferably 0.2 to 0.8, more preferably 0.3 to 0.7. The ratio between the total of the projected areas 9b of the sipes 9' of the small rib 7b per unit length and total of projected areas 9c (Figure 11) of the sipes 9" of the small rib 7c per unit length is preferably 0.2 to 0.8, more preferably 0.3 to 0.7.Thus, the quantities of the sipes 9, 9' and", that is, the totals of the projected areas 9a, 9b and 9c of the sipes 9, 9' and 9" are gradually decreased from the outermost small rib 6 to the innermost small rib 7c in this order. Such reduction is achieved as follows.

The lengths of the sipes measured in the lateral direction of the tire may be gradually decreased from the outermost small rib 6 to the innermost small rib 7c in this order. In this case, the length La of the sipe 9 of the outermost small rib 6 is preferably not less than 2 percent of the tread width T, more preferably not less than 3 percent. The length Lb of the sipe 9' of the small rib 7b disposed inwardly of the outermost small rib 6 is preferably not less than 10 percent of the width Wb of the small rib 7b. The length Lc of the sipe 9" of the innermost small rib 7c is preferably not less than 10 percent of the width Wc of the small rib 7c.

The depths of the sipes measured in the radial direction of the tire may be gradually decreased from the outermost small rib 6 to the innermost small rib 7c in this order. In this case, the depth ha (Figure 11) of the sipe 9 of the outermost small rib 6 is preferably 30 to 120 percent of the depth d2 of the main groove 2 located adjacent the shoulder rib 3, more preferably 50 to 100 percent. The depth hb of the sipe 9' of the small rib 7b disposed inwardly of the outermost small rib 6 is preferably 20 to 120 percent of the depth dS of the auxiliary groove 5 located laterally outwardly of the small rib 7b. The depth hc of the sipe 9" of the innermost small rib 7c is preferably 20 to 120 percent of the depth d5 of the auxiliary groove 5 located laterally outwardly of the small rib 7c.

The pitches of the sipes, that is, the spaces between two circumferentially adjacent sipes, measured in the circumferential direction of the tire may be gradually increased from the outermost small rib 6 to the innermost small rib 7c in this order. In this case, the pitch Sa (Figure, 10) of the sipe 9 of the outermost small rib 6 is preferably 2 to 7 percent of the tread width T, more preferably 3 to 6 percent.

It is noted that at least two of the length L, the depth h and the pitch S of the sipe may be varied at the same time so that the totals of the projected areas 9a, 9b and 9c of the sipes 9, 9' and 9" are gradually decreased from the outermost small rib 6 to the innermost small rib 7c in this order.

In the case that the length L, the depth h and the pitch S of the sipe are varied in the circumferential direction of the tire (in this embodiment, the length of the sipes 9'), the length L, the depth h and the pitch S are averaged in the circumferential direction of the tire, respectively. Each of the sipes 9, 9' 9" is constructed so as to be closed in the ground-contact region of the tread during cruising of the vehicle upon load. The width of each of the sipes 9, 9' and 9" is narrower than that of the auxiliary groove 5. It is preferable that a width of the sipe indicated by t be 0.3 to 1.5 mm. This width t of the sipe may be varied. From the standpoint of maneuverability, it is desired that the tread end is constructed to be round, as shown in Figure 11, such that the radius of curvature of the tread end is not more than 20 percent of the tread width T.In this case, the tread width T is a width between intersection points 12 atwhich the extension lines extending upwardly from the outer surface of the shoulder portion intersect the extension lines extending laterally from the outer surface of the tread 1. Also, in Figure 10, the swing width g of the main groove 2 between the laterally innermost end and the laterally outermost end on the zigzag line thereof is preferable substantially equal to or more than the swing width f of the auxiliary groove 5 between the laterally innermost end and the laterally outermost end on the zigzag line thereof.In this embodiment, although the position of the auxiliary groove 5 is not restricted as in the previous embodiment as shown in Figure 1, it is most preferable that the distance between the auxiliary groove 5 (the outermost auxiliary groove 5 if a plurality of the auxiliary groove 5 are provided) and the lateral edge 10 of the tread 1, which is substantially equal to the width W6 of the small rib 6, be not less than 20 but less than 50 percent of the width W3 of the shoulder rib 3. Further, it is noted that the subject matter of the present invention can be attained by either one of the shoulder ribs 3 constructed and arranged as shown in Figure 10.

The effect of the present invention is tested as follows.

All of the tires tested are pneumatic radial tires for trucks and buses having a tire size of 12 R 22.5 16 PR.

The tread pattern of the tires is constituted by three main grooves and four ribs, and the internal construction of the tread is substantially identical to that of a known TBR tire. The kinds of the tested tires are shown in detail in Table 1. The prior art tire A is of the type having no auxiliary grooves and no sipes. The prior art tires B, C and D, and tires E and F of Figure 1 according to the present invention are of the type having an auxiliary groove and no sipes. The tire G of Figure 6 according to the present invention is of the type having an auxiliary groove and sipes. The tire H according to the present invention is substantially similar to the tire G, except for two auxiliary grooves. The tire I of Figure 10 according to the present invention is of the type having two auxiliary grooves and different sipes of different quantity.

TABLE 1 Kind of tire A B C D E F G H I Tread width T 200 200 200 200 200 200 200 200 200 Main groove 2 width2 12 12 12 12 12 12 12 12 12 depth d2 15 15 15 15 15 15 15 15 15 Auxiliary groove 5 width --- 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 depth do --- 12 12 12 12 12 12 12 12 angle a --- 8" 8" 8" 8t 8" 0" 0" 0" Width W3 of 45 45 45 45 45 45 45 45 45 shoulder rib 3 Positionbof --- 31.5 27 22.5 18 13.5 18 15 15 outermost auxiliary groove 5 Width W6 of outer --- 30.5 26 21.5 17 12.5 17 14 14 small rib 6 Width W7 orWb of --- 12.5 17 21.5 26 30.5 26 13 13 inner small rib 7 or 7b Width W of inner --- --- --- --- --- --- --- 14 14 Sipe9,9',9" length La --- --- --- --- --- --- --- --- 8 length L or Lb --- --- --- --- --- --- 8 8 5 length lie --- --- --- --- --- --- --- 8 3 depth ha --- --- --- --- --- --- --- --- 12 depthhorhb --- --- --- --- --- --- 12 12 12 depth hue --- --- --- --- --- --- --- 12 12 pitch Sa --- --- --- --- --- --- --- --- 6 pitch S or Sb --- --- --- --- --- --- 6 6 12 pitch Sc --- --- --- --- --- --- --- 6 24 width t --- --- --- --- --- --- 0.5 0.5 0.5 (Unit: mm) These nine kinds of the tires are tested under condition of smooth road of 100 percent, average velocity of 80 km)hr, and normal load in accordance with ETRTO standard. The resistances to wear of the shoulder ribs 3 are measured at a travelling distance of 50,000 km, and the uneven wears on the shoulder ribs 3 are evaluated by visual inspection. The results are shown in Table 2.

It is noted that the resistances to wear of the shoulder ribs 3 are obtained by dividing the travelling distance by the maximum reduction in the groove depth of the outer main groove 2 located adjacent the shoulder rib 3, at the outer groove wall of the groove walls of the outer main groove 2. In Table 2, the resistance to wear of the prior art tire A is indicated as 100 by an index number, and the more the number is large, the more the resistance to wear is good. As noted above, the local wear occurred in the tread end extends in the lateral inward direction of the tire and grows into the uneven wear. Accordingly, the uneven wear can be evaluated properly by the size of the wear reached to the laterally innermost end of the shoulder rib3.

Kind of Resistance to State of uneven wear on the shoulder tire wear rib 3 A 100 Shoulder-drop wear on the shoulder rib 3 and polygonal wear on the center rib 4 were observed.

B 101 A large number of undulate wears on the shoulder rib 3 as well as edge-drop wear were observed.

C 104 Ditto.

D 109 Almost ditto.

E 114 The outermost small rib 6 worn slightly as compared with the inner rib 7, but the undulate wear was almost unobserved.

F 115 Ditto.

G 115 The outermost small rib 6 worn slightly, but the undulate wear was further mitigated as compared with tires E and F.

H 117 Ditto.

I 125 The uneven wearwas almost unobserved.

From the foregoing results, it will be seen that an improved pneumatic radia-l tire for heavy-duty road vehicles which can overcome the uneven wears and thereby enhancing remarkably life of the tire itself is afforded by the present design. While certain representative embodiments and details have been shown for the purpose of illustrating the subject matter of the present invention, it will be apparent to those skilled in this art that various changes and modification may be made in the embodiments without departing from the spirit or scope of the invention.

Claims (20)

1. A pneumatic radial tire comprising a tread which is divided into a pair of laterally outermost shoulder ribs and at least one center rib disposed between said laterally outermost shoulder ribs by at least two main wide grooves extending in a substantially circumferential direction of said tire, wherein at least one of said shoulder ribs is divided into at least two small ribs by at least one auxiliary narrow groove extending in said substantially circumferential direction and having a width which is not more than 3 percent of a width of said tread and a depth which is not less than 30 percent of a depth of said main groove located adjacent said shoulder rib, the auxiliary narrow groove being disposed such that a distance, measured in a lateral direction of said tire, from a lateral edge of said tread to said auxiliary groove is less than 50 percent of a width of said shoulder rib.

2. A pneumatic radial tire as claimed in claim 1, in which said distance from said lateral edge of said tread to said auxiliary groove is not less than 20 percent of said width of said shoulder rib.

3. A pneumatic radial tire as claimed in claim 1, in which a width of each of said small ribs is smal!erthan that of said center rib.

4. A pneumatic radial tire as claimed in claim 1, in which said auxiliary groove extends from an outer surface of said tread radially inwardly and laterally outwardly at an angle of not more than 20 degrees with respect to a normal line substantially vertical to said outer surface of said tread.

5. A pneumatic radial tire as claimed in claim 1, in which said center rib is formed with a plurality of parallel sipes each of which is connected at its both ends with said main grooves.

6. A pneumatic radial tire comprising a tread which is divided into a pair of laterally outermost shoulder ribs and at least one center rib disposed between said laterally outermost shoulder ribs by at least two main wide grooves extending in a substantially circumferential direction of said tire, wherein at least one of said shoulder ribs is divided into at least two small ribs by at least one auxiliary narrow groove extending in said substantially circumferential direction and having a width which is not more than 3 percent of a width of said tread and a depth which is not less than 30 percent of a depth of said main groove located adjacent said shoulder rib, said small rib being formed at its laterally outer end portion opposing said auxiliary groove with a plurality of sipes so that rigidity of said laterally outer end portion is decreased.

7. A pneumatic radial tire as claimed in claim 6, in which a distance, measured in a lateral direction of said tire, from a lateral edge of said tread to a laterally outermost auxiliary narrow groove of said auxiliary narrow grooves is not less than 20 percent and less than 50 percent of a width of said shoulder rib.

8. A pneumatic radial tire as claimed in claim 6, in which said sipes extend in a substantially lateral direction of said tire and are spaced apart a predetermined space from one another in said circumferential direction of said tire.

9. A pneumatic radial tire as claimed in claim 8, in which said predetermined space between said sipes is 10 to 200 percent of a depth of said main groove.

10. A pneumatic radial tire as claimed in claim 6, in which said sipes extend discontinuously in zigzag fashion along said auxiliary groove in said substantially circumferential direction of said tire.

11. A pneumatic radial tire as claimed in claim 6, in which said sipes extend continuously in zigzag fashion along said auxiliary groove in said substantially circumferential direction of said tire.

12. A pneumatic radial tire as claimed in claim 6, in which said center rib is formed with a plurality of parallel sipes each of which is connected at its both ends with said main grooves.

13. A pneumatic radial tire comprising a tread which is divided into a pair of laterally outermost shoulder ribs and at least one center rib disposed between said laterally outermost shoulder ribs by at least two main wide grooves extending in a substantially circumferential direction of said tire, wherein at least one of said shoulder ribs is divided into at least two small ribs by at least one auxiliary narrow groove extending in said substantially circumferential direction and having a width which is not more than 3 percent of a width of said tread and a depth which is not less than 30 percent of a depth of said main groove located adjacent said shoulder rib, each of said small ribs being formed at its laterally outer end portion with a plurality of sipes so that rigidity of said laterally outer end portion is decreased, quantities of said sipes formed in said small ribs being gradually decreased in a lateral inward direction of said tire.

14. A pneumatic radial tire as claimed in claim 13, in which a distance, measured in a lateral direction of said tire, from a lateral edge of said tread to a laterally outermost auxiliary narrow groove of said auxiliary narrow grooves is not less than 20 percent and less than 50 percent of a width of said shoulder rib.

15. A pneumatic radial tire as claimed in claim 13, in which between adjacent small ribs is 0.2 to 0.8 a ratio between totals of projected areas of said sipes disposed within a unit length in said circumferential direction of said tire, on a radial plane which includes an axis of rotation of said tire.

16. A pneumatic radial tire as claimed in claim 13, in which lengths of said sipes formed in said small ribs, measured in a lateral direction of said tire, are gradually decreased in said lateral inward direction of said tire.

17. A pneumatic radial tire as claimed in claim 13, in which depths of said sipes formed in said small ribs, measured in a radial direction of said tire, are gradually decreased in said lateral inward direction of said tire.

18. A pneumatic radial tire as claimed in claim 13, in which pitches of said sipes formed in said small ribs, measured in said circumferential direction of said tire, are gradually increased in said lateral inward direction of said tire.

19. A pneumatic radial tire as claimed in claim 13, in which said center rib is formed with a plurality of parallel sipes each of which is connected at its both ends with said main grooves.

20. A pneumatic radial tire, substantially as herein described with reference to, and as shown in, any of the Figures of the accompanying drawings.