Winter type tire tread

A tread for a directional winter type tire comprising an elastomeric substance extending circumferentially about the axis of rotation of the tire, said tread having:

• a pair of shoulders which flank the lateral edges of a ground engageable surface;
• an uneven number of, but at least three, circumferentially extending axially spaced straight grooves;
• a plurality of zig-zag shaped grooves disposed circumferentially about the tire in a repetitive manner and extending across the tread from the equatorial plane EP towards the lateral edges of the tire, the zig-zag shaped grooves situated on one side of the equatorial plane being substantially symmetrical to those situated on the opposite side of the equatorial plane, each zig-zag shaped groove having a first leg, originating in a circumferentially extending center groove, and every second leg substantially coincident with the respective circumferentially extending groove, apart from the shouldermost legs, the centerlines of the different legs forming with the equatorial plane angles having a magnitude below or equal to 80.degree., apart from the centerlines of the shouldermost legs which have magnitudes ranging between 90.degree. and 100.degree., said circumferentially extending and zig-zag shaped grooves defining block elements, arranged in circumferentially extending axially innermost center rows and axially outermost shoulder rows;
• a plurality of zig-zag sipes includes in each block element each sipe zig-zag having a straight line first, a straight line second and a middle portion connecting them, the first portion entering the footprint of the tire before the middle and the second portion, the second portion leaving the footprint of the tire after the first and middle portion;
• and
• the block elements in the center rows have substantially the shape of a parallelogram, each parallelogram having two acute and two obtuse angles joined respectively by a large and a small diagonal, the large diagonal of the parallelograms forming with the equatorial plane an angle comprised between 30.degree. and 60.degree. on one side of the equatorial plane and an angle comprised between -30.degree. and -60.degree. on the opposite side of the equatorial plane.