Bottom bracket assembly of a bicycle, comprising: - a shaft having an axis (X-X) extending along a longitudinal direction; - a pair of bearings (31, 41) adapted to support said shaft in rotation with respect to a housing box of the bottom bracket assembly provided in a bicycle frame; - a left crank arm associated with a first end portion of said shaft through first coupling means, wherein said first end portion comprises a grooved outer surface and said left crank arm comprises a through hole which is at least partially grooved for the sliding coupling with said grooved outer surface; - a right crank arm associated with a second end portion of said shaft; - a through-cavity extending along said longitudinal direction and entirely passing through said left crank arm, said shaft and said right crank arm; - adjustment means for adjusting the axial position of said left crank arm with respect to said pair of bearings (31, 41); wherein: - said first coupling means are arranged in a radially outer position with respect to said shaft so as not to create obstructions in the through-cavity; - said first end portion of said shaft comprises, in an axially outermost position with respect to said grooved outer surface, an external threading; - said bottom bracket assembly further comprising an internally threaded ring nut screwed onto said external threading and in axial abutment onto said left crank arm; characterised in that: - said through-cavity is without any obstructions and is defined by a surface lacking any sharp or sudden variation in diameter; - said adjustment means are axially arranged between said left crank arm and the respective bearing of said pair of bearings (31, 41); - said ring nut is positioned at an axially outermost position of said shaft. - a shaft having an axis (X-X) extending along a longitudinal direction; - a pair of bearings (31, 41) adapted to support said shaft in rotation with respect to a housing box of the bottom bracket assembly provided in a bicycle frame; - a left crank arm associated with a first end portion of said shaft through first coupling means, wherein said first end portion comprises a grooved outer surface and said left crank arm comprises a through hole which is at least partially grooved for the sliding coupling with said grooved outer surface; - a right crank arm associated with a second end portion of said shaft; - a through-cavity extending along said longitudinal direction and entirely passing through said left crank arm, said shaft and said right crank arm; - adjustment means for adjusting the axial position of said left crank arm with respect to said pair of bearings (31, 41); - said first coupling means are arranged in a radially outer position with respect to said shaft so as not to create obstructions in the through-cavity; - said first end portion of said shaft comprises, in an axially outermost position with respect to said grooved outer surface, an external threading; - said bottom bracket assembly further comprising an internally threaded ring nut screwed onto said external threading and in axial abutment onto said left crank arm; - said through-cavity is without any obstructions and is defined by a surface lacking any sharp or sudden variation in diameter; - said adjustment means are axially arranged between said left crank arm and the respective bearing of said pair of bearings (31, 41); - said ring nut is positioned at an axially outermost position of said shaft.