Studless tire including rib provided with first and second sipes

A studless tire comprising a tread provided with a plurality of blocks and at least one rib provided with first sipes and second sipes each extending thereacross,

• said at least one rib crossed only by said first sipes and second sipes,
• the first sipes and second sipes alternately arranged in the longitudinal direction of the rib at circumferential pitches Pc in a range of from 0.028 to 0.04 times a ground contacting width of the tread,
• each of the first sipes and second sipes comprising a deep central part, a shallow part on one side thereof and a mid-depth part on the other side thereof,
• the deep central part having a depth in a range of from 0.70 to 0.80 times a depth of a main longitudinal groove defining the rib, the mid-depth part having a depth being in a range of from 0.40 to 0.60 times the depth of the main longitudinal groove, and the shallow part having a depth in a range of from 0.04 to 0.35 times the depth of the main longitudinal groove,
• the shallow part and mid-depth part of each first sipe being positionally reverse to those of the second sipes so that, on each side of the rib, each shallow part alternating with an adjacent mid-depth part.