I'm reading about Ehrenfest's paradox where one considers a rotating disk. If one let r' be the radius of the disc in an inertial frame and r be the radius of the disc when it is at rest. Then the periphery must be Lorentz' contracted such that ##2\pi r' < 2\pi r##, but since the radial line is perpendicular to the direction of motion of the disk it is not Lorent'z contracted so that ##r'=r##. Thus the paradox. The supposed kinematical solution is to consider the disk being accelerated up to a given angular velocity. By an analysis involving the relativity of simultaneity it is found that it is inconsistent to require that the disk be accelerated up to the angular velocity and additionally require it being done while keeping the periphery 'Born rigid'. I.e. accelerated in such a way that the rest length of each line element along the periphery remains constant. This is claimed to resolve the paradox and my question is why it does that. More specifically; why does one need to assume a 'Born rigid' acceleration of the disk in order to accept the paradox?