A process for building a tyre comprising:
- forming a carcass structure comprising at least one carcass ply having opposite edges in engagement with respective annular anchoring structures;
- forming a belt structure comprising at least one first belt layer and at least one cushion insert enclosing an axially external edge of said at least one first belt layer;
- and
- forming a tread band at a position radially external to said belt structure, wherein forming said belt structure comprises at least:
- forming a first portion of said at least one cushion insert through laying of at least one continuous elongated element of elastomeric material into coils disposed close to each other on a substantially cylindrical laying surface of an auxiliary drum set to a starting diameter, the auxiliary drum is composed of consecutive circumferential sectors having respective circumferential shoulders on the laying surface with said shoulders mutually abutting when the auxiliary drum is set to a minimum diameter, wherein the circumferential sectors form the laying surface;
- after forming the first portion, radially expanding the auxiliary drum while maintaining the laying surface thereof at a substantially cylindrical shape, from the starting diameter to a first work diameter by radially moving said circumferential sectors, wherein radially moving said circumferential sectors enables diametrical sizes of the auxiliary drum to be varied;
- after said expanding, forming said at least one first belt layer at a radially external position to said first portion by application of belt strip-like elements disposed consecutively close to each other along a circumferential extension of the laying surface in such a manner that said first portion axially projects outward from the axially external edge of said at least one first belt layer;
- and
- forming a second portion of said cushion insert through laying of at least one continuous elongated element of elastomeric material into coils disposed close to each other in a radially external position to said axially external edge of said at least one first belt layer and to a stretch of the first portion projecting from the axially external edge in an axially external direction, so as to enclose said axially external edge between the first and second portions of the cushion insert,
- wherein the elastomeric material of at least one of said first portion and said second portion has a composition different from that of an elastomeric material of the tread band and a sidewall, respectively, of said tyre.