Pneumatic radial tire for motorcycle

A pneumatic radial tire for a motorcycle comprising;

• a belt containing cords extended substantially in a circumferential direction of a tread, a directional tread pattern defined by main slant grooves each having an acute-side inclination angle of not less than 45° with respect to the circumferential direction of the tread and extending from a tread center or a vicinity thereof up to a tread shoulder region, and a slant sub-groove extending between the slant main grooves and having an acute-side inclination angle with respect to the circumferential direction of the tread smaller than that of the slant main groove, wherein a groove width of the slant main groove is within a range of 3-15% of a peripheral length between both ends of the tread when the tire is mounted onto a standard rim and inflated with an air pressure corresponding to 80% of a maximum air pressure with no loading;
• said slant sub-groove having a main portion not opening into the slant main groove;
• said slant sub-groove having an acute-side inclination angle of 0-30° with respect to the circumferential direction of the tread, a groove width narrower than that of the slant main groove, a length in the circumferential direction of the tread corresponding to 30-80% of an arranging pitch of the slant main grooves, and a depth corresponding to 50-100% of a depth of the slant main groove.