Bottom bracket for bicycles comprising: - a central pin having a longitudinal axis extending along a predetermined direction and comprising a first axial end portion and a second axial end portion; - a first adapter radially outer with respect to, and crossed by, said central pin and configured to be associated with a first end of a box for housing the bottom bracket in a frame of a bicycle; - a first rolling bearing radially arranged between said central pin and said first adapter; - a sealing gasket arranged in axially outer position with respect to said first rolling bearing and radially outer with respect to said central pin to insulate said first rolling bearing in a fluid-tight manner from the outside, said sealing gasket being directly exposed to the outside and being elastically deformable between an undeformed condition and a deformed condition; wherein said sealing gasket comprises a first axially outer end provided with a first lip and said first lip being in constant fluid-tight relationship with said central pin; and characterised in that the sealing gasket comprises a second lip axially more inner with respect to the first lip and when the sealing gasket passes from the undeformed condition to the deformed condition, said second lip moves from a more outer radial position, in which the second lip is radially outside of the first lip and in which the second lip does not exert a fluid seal on said central pin, to a more inner radial position, in which the second lip is in fluid-tight relationship with said central pin. - a central pin having a longitudinal axis extending along a predetermined direction and comprising a first axial end portion and a second axial end portion; - a first adapter radially outer with respect to, and crossed by, said central pin and configured to be associated with a first end of a box for housing the bottom bracket in a frame of a bicycle; - a first rolling bearing radially arranged between said central pin and said first adapter; - a sealing gasket arranged in axially outer position with respect to said first rolling bearing and radially outer with respect to said central pin to insulate said first rolling bearing in a fluid-tight manner from the outside, said sealing gasket being directly exposed to the outside and being elastically deformable between an undeformed condition and a deformed condition; wherein said sealing gasket comprises a first axially outer end provided with a first lip and said first lip being in constant fluid-tight relationship with said central pin; and characterised in that the sealing gasket comprises a second lip axially more inner with respect to the first lip and when the sealing gasket passes from the undeformed condition to the deformed condition, said second lip moves from a more outer radial position, in which the second lip is radially outside of the first lip and in which the second lip does not exert a fluid seal on said central pin, to a more inner radial position, in which the second lip is in fluid-tight relationship with said central pin. wherein said sealing gasket comprises a first axially outer end provided with a first lip and said first lip being in constant fluid-tight relationship with said central pin; and characterised in that the sealing gasket comprises a second lip axially more inner with respect to the first lip and when the sealing gasket passes from the undeformed condition to the deformed condition, said second lip moves from a more outer radial position, in which the second lip is radially outside of the first lip and in which the second lip does not exert a fluid seal on said central pin, to a more inner radial position, in which the second lip is in fluid-tight relationship with said central pin.