the inventors have made various studies with respect to the relation between the width w of railway wear and the ratio of amplitude w 1 to tread width b when the pitch p of zigzags of the groove is set to 11 mm in the rib - type radial tire having a size of 10 . 00 r 20 , 14 pr and four grooves . as a result , it has been found that when the ratio w 1 / b is within a range of 0 . 5 to 2 . 0 %, preferably less than 1 . 7 %, the railway wear decreases considerably as seen from the graph of fig3 . when the ratio w 1 / b exceeds 2 . 0 %, the railway wear is apt to be caused , while when the ratio w 1 / b is less than 0 . 5 % or the edge of the zigzag groove becomes straight , the railway wear is also caused . the term &# 34 ; tread width b &# 34 ; used herein means a ground contact width of the tire as shown in fig1 . further , the degree of railway wear is generally expressed by the stepwise height h formed in the convex part 3 of the circumferential rib 2 facing to the zigzag groove 1 as shown in fig2 . however , since the stepwise height h and the width w are correlative with each other , the larger the stepwise height h , the larger the width w . therefore , the inventors have evaluated the railway wear by the width w in fig3 . in fig4 is shown the relation between the ratio of pitch p to tread width b and the width w of railway wear when the amplitude w 1 is constant . from the result of fig4 it has been found that the ratio p / b should be within a range of 2 . 0 to 7 . 0 %, preferably 2 . 5 to 6 . 0 % in order to diminish the railway wear . when the ratio p / b is less than 2 . 0 %, there are many problems in the production and effect of the zigzag groove , while when the ratio p / b exceeds 7 . 0 %, the railway wear itself increases considerably . in the above mentioned rib - type radial tires , the number of the zigzag groove is usually at least two , preferably four . moreover , a width w 2 of the groove should be within a range of 4 . 5 to 7 . 5 %, preferably 5 . 0 to 7 . 0 % of the tread width b considering the draining performance and the like during the travelling on wet road . particularly , in case of the heavy duty pneumatic radial tires for truck and the like , the width of the zigzag groove cannot unreasonably be enlarged because it is necessary to ensure a wide area for the rib in order to improve the wear life of tire . the width w 2 of the zigzag groove is a distance between the two sides or extention lines thereof defining the groove 1 measured in a direction perpendicular to the side of the groove as shown in fig1 . the invention will be described with reference to the following examples . in fig5 is shown an embodiment of the tread pattern according to the invention wherein zigzag grooves 1 extending in a circumferential direction of tread are arranged at substantially equal intervals in a widthwise direction of tire . in the embodiment of fig6 showing the enlarged groove portion of fig5 the tire has the following relations , i . e . w 2 / b = 6 . 9 %, w 1 / b = 1 . 1 % and p / b = 5 . 6 %, provided that the tread width b is set to 180 mm . carcass : one ply of metal cords ( cord angle is 90 ° with respect to the circumferential direction of tire .) belt : three layers of metal cords ( cord angle of each layer is 15 ° with respect to the circumferential direction of tire .) then , the railway wear was actually tested with respect to the above tire and the tire of the prior art ( the zigzag groove having a width w 2 of 9 . 6 mm , an amplitude w 1 of 5 . 0 mm and a pitch p of 25 mm is used in accordance with the tread pattern shown in fig1 i . e . w 2 / b = 5 . 3 %, w 1 / b = 2 . 8 %, p / b = 13 . 9 %) to obtain the following results . mounting position for tire : front wheel ( the tires were changed in its right and left positions every 5 , 000 km .) the degree of railway wear is measured to obtain a result expressed by h and w defined in fig2 as shown in the following table . ______________________________________ h ( stepwise height ) w ( width ) ______________________________________tire of the prior art 1 . 2 mm 7 . 0 mmtire of the invention 0 . 4 mm 0 . 8 mm______________________________________ as seen from the above data , the tire of the invention considerably improves the railway wear when compared with the tire of the prior art and contributes to improve durability and tire performances in the rib - type heavy duty pneumatic radial tire by the provision of zigzag grooves extending circumferentially of tire in the shape apart from the conventional rib - type tread pattern . in fig7 is shown a cross section of a practically preferred zigzag groove 1 wherein the side wall of the groove is substantially 90 ° with respect to a tread outer surface over a height from the tread outer surface h 1 corresponding to at least not more than 30 % of a depth h of the groove . such a cross section of the zigzag groove 1 further diminishes the railway wear . as shown in fig8 sipes 6 may be provided in an edge 5 of a circumferential rib 2 defined between the zigzag grooves 1 extending in a circumferential direction of the tread , particularly convex parts 3 , said sipes being parallel to an axial direction of tire and opened toward the groove 1 , whereby the wet performance of tire can be further improved . in the sipe 6 , the opened width is preferably 0 . 3 to 1 . 0 mm , the depth is preferably not less than 50 % of the depth of the groove , and the length in the axial direction of tire is preferably within a range of 75 to 150 % based on the amplitude w 1 of the groove 1 . moreover , the sipes 6 are usually located at intervals corresponding to the zigzag pitch p in the circumferential direction of tire or may be arranged in concave parts 4 of the circumferential rib 2 or at a middle position between the convex part 3 and the concave part 4 . in any case , when the length of the sipe 6 is too long , the wet performance is improved , but an eccentric wear known as heal - and - toe wear is caused in the tread portion around the sipe 6 , so that the use of the very long sipe 6 is not favorable . as mentioned above , according to the invention , eccentric wear , particularly railway wears in the heavy duty pneumatic radial tires can considerably be diminished .