Pneumatic radial tire with four main grooves

A pneumatic radial tire having

• a tread being provided with four main groves extending continuously in the circumferential direction of the tire being arranged symmetrically with respect to the tire equator so as to divide the tread into five ribs, wherein said five ribs include two shoulder ribs, two middle ribs and a central rib,
• said four main grooves consisting of a pair of axially inner main grooves (G2) being disposed one on each side of the tire equator and a pair of axially outer main grooves (G1) being disposed one between each tread edge and each inner main groove,
• each shoulder rib (RS) between the outer main groove and the tread edge being provided with shoulder lateral grooves (g1L, g1R) extending from the outer main groove to the tread edge so as to be divided into blocks,
• each middle rib (RM) between the inner main groove and the outer main groove being provided with middle lateral grooves (g2L, g2R) extending from the inner main groove to the outer main groove so as to be divided into blocks,
• the central rib (RC) between the two inner main grooves being provided with central lateral grooves (g3L, g3R) extending from the inner main grooves towards the tire equator and terminated in the central rib so as to provide continuity in the circumferential direction of the tire, each of the central lateral grooves (g3L, g3R) being gradually decreased in depth from the axially outer end to the inner end thereof so as to provide the maximum depth at the axially outer end and the minimum depth at the axially inner end, and the difference between the maximum depth and the minimum depth is 0.4 to 0.6 times the depth (DH2) of the inner main groove (G2),
• all the lateral grooves on each side of the tire equator being inclined in the same direction but in the opposite direction to the lateral grooves on the other side of the tire equator,
• each of the lateral grooves (g1L, g1R, g2L, g2R, g3L and g3R) being curved so that the inclination of the groove center line thereof with respect to the tire equator gradually increases from the axially inner end to the outer end thereof,
• the angle .theta.1 of the center line of the shoulder lateral groove (g1L, g1R) at the axially inner end being in the range of 65 to 80 degrees with respect to the tire equator (CO),
• the angle .theta.2 of the center line of the middle lateral groove (g2L, g2R) at the axially inner end being in the range of 45 to 60 degrees with respect to the tire equator (CO),
• the angle .theta.3 of the center line of the central lateral groove (g3L, g3R) at the axially inner end being in the range of 40 to 55 degrees with respect to the tire equator (CO),
• the central lateral grooves (g3L) on one side of the tire equator being circumferentially shifted from the central lateral grooves (g2R) on the other side of the tire equator,
• between both sides of each inner main groove (G2), the middle lateral grooves (g2L, g2R) being shifted from the central lateral grooves (g3L, g3R) towards a circumferential direction (D),
• between both sides of each outer main groove (G1), the shoulder lateral grooves (g1L, g1R) being shifted from the middle lateral grooves (g2L, g2R) towards the circumferential direction (D),
• the circumferential shifts (LS1) between the shoulder lateral grooves (g1L, g1R) and the middle lateral grooves (g2L, g2R) being 0.2 to 0.3 times the mean circumferential pitch length PM of the lateral grooves (g1L, g1R, g2L, g2R, g3L and g3R),
• the circumferential shifts (LS2) between the middle lateral grooves (g2L, g2R) and the central lateral grooves (g3L, g3R) being 0.3 to 0.5 times the mean circumferential pitch length PM,
• the circumferential shifts (LS3) between the central lateral grooves (g3L) on one side of the tire equator and the central lateral grooves (g3R) on the other side of the tire equator being 0.25 to 0.5 times the mean circumferential pitch length PM, wherein
• said circumferential shifts between the shoulder lateral grooves and the middle lateral grooves and between the middle lateral grooves and the central lateral grooves (LS1 and LS2) are defined as the circumferential distance between a lateral groove end opened at the axially inner groove wall of each main groove and a nearest lateral groove end opened at the axially outer groove wall of the same main groove, measured from the center line to the center line of those lateral grooves,
• said circumferential shifts between the central lateral grooves on one side of the tire equator and the central lateral grooves on the other side of the tire equator (LS3) are defined as the circumferential distance between the terminal end of each central lateral groove (g3L) on one side and the terminal end of the nearest central lateral groove (g3R) on the other side measured from the groove center line to the groove center line,
• said mean pitch length PM is defined as the whole circumferential length (L) of the tire measured around the tire equator (CO) divided by the number (NP) of the pitches of the lateral grooves,
• in a state that the tire is mounted on its regular rim and inflated to its regular inner pressure and loaded with its regular tire load,
• the quotient (WG/W) of the total (WG) of the axial widths (WG1, WG2) of the four main grooves (G1 and G2) divided by the ground contact width (W) measured between the axial edges (SE) of the ground contacting area (S) being in the range of 0.25 to 0.32,
• on each side of the tire equator, the ground contact width (W) and the total (LG1+LG2) of the axial length (LG1) measured from the ground contacting area edge (SE) to the axially inner edge of the shoulder rib (RS) and the axial length (LG2) measured from the ground contacting area edge (SE) to axially inner edge of the middle rim (RM) satisfying the following relation:
• 0. 44.ltoreq.(LG1+LG2)/W.ltoreq.0.50, and
• the angle .theta.2 is larger than the angle .theta.3.