A flexible disk for connecting a transmission shaft to rotate with a mechanical part, the flexible disk being suitable for forming rotary motion about an axis of rotation and including in succession going from its center towards its periphery an empty central zone, a circular base, a transition zone having an outer face and an inner face, and then a peripheral collar. The transition zone presents a minimum thickness at the edge of the peripheral collar, the minimum thickness being determined with the help of the following thickness relationship in which “e” represents the thickness of the transition zone at a first distance “r” from the axis of rotation, “Q” is a constant that depends on the misalignment, “C” is the torque applied to the transmission shaft, “r” is the first distance, and “R” is the radius of the empty central zone: e = 1 2 ⁢ r 2 ⁡ ( Q C - 2 ⁢ r r 4 - R 4 )