A tread ( 1 ) comprising a plurality of grooves of longitudinal and transverse general orientation ( 2 b , 2 c , 31, 32 ), the grooves of longitudinal orientation comprising main grooves ( 2 b , 2 c)of depth G and secondary grooves ( 31, 32 ) of depth G' less than the depth G of the main grooves, the central part (C) of the tread, defined axially by two grooves of longitudinal orientation, and ofawidth of between 0.25 and 0.5 times the width W of the tread, being provided with at least one main groove ( 2 c) of longitudinal orientation and of average depth G. The depth G' of the secondary gro oves ( 31, 32 ) is between 0.4 and 0.6 times the depth G of the main grooves ( 2 b , 2 c). The transverse grooves ( 5 c) of the ribs of the central part (C) comprise over at most half of their length a connecting bridge ( 6 c), the surface radially to the outside of which is at a distance Hc from the running surface of between 0.4 and 0.8 times the depth G of the main grooves. The transverse grooves ( 5 b) of the ribs of each edge (B) comprise over at most half of their length a connecting bridge ( 6 b) which prevents closure of said transverse grooves. The surface radially to the outside isata distance Hs from the running surface which is at most equal to 0.3 times the depth G of the main grooves.