The bead breaking unit for tyre changing machines comprises an arm having a first end jointed to the base of a tyre changing machine on a substantially vertical axis and a second end associated to a bead breaking tool.The arm is suitable for oscillating between a home position, wherein the bead breaking tool is placed away from the base, and a working position, wherein the bead breaking tool is placed in contact with a wheel that has to undergo bead breaking resting against at least one side pad protruding at least in part from the base.Moreover, the bead breaking unit comprises an adjustment device for adjusting the slant of the resting surface of the pad suitable for defining at least two working configurations.