Pneumatic radial tire for heavy loads

A pneumatic radial tire for heavy loads, having steel belt layers provided on the inner side of a tread, and a tread pattern having a plurality of divisional ribs separated by a plurality of main grooves extending in the circumferential direction of said tire, characterized in that at least the axially inner vertical surfaces of left and right outermost main grooves out of said main grooves are formed so as to extend zigzag in the circumferential direction of said tire, straight narrow grooves which extend continuously in the circumferential direction of said tire being provided at the shoulder side edge portions of ribs axially inward and adjacent to said outermost main grooves, the width W.sub.4 of said narrow grooves being set to a level in the range of 0.5-1 mm, the axes of said narrow grooves which extend in the direction of the depth thereof being inclined in the downwardly inward direction with respect to a normal of said tread surface, said narrow grooves forming laterally separate rib bodies and narrow ribs having a height which is smaller than that of said rib bodies and an axially outer edge which extends in a zigzag line in the circumferential direction of the tire, a ratio of a maximum width W.sub.2 of said narrow ribs to a total width W.sub.1 of said ribs on the axially inner side of said outermost main grooves being set to a level satisfying the relation W.sub.2 /W.sub.1 .ltoreq.0.3, an angle .alpha..sub.1 between the side surface which faces the relative outermost main groove of a maximum width portion of each narrow rib and a normal of said tread surface being set smaller than an angle .alpha..sub.2 between the side surface which faces the relative outermost main groove of a minimum width portion of each narrow rib and said normal whereby a circumferential tangential line of which a zigzag side surface on each said outermost main groove meets a bottom surface thereof extends linearly.