A supporting disc for a supporting disc bearing of open-end spinning rotors has a disc-like base body, on whose outer circumference a tire-like covering is applied. On a front side of the supporting disc facing the operator's side of the spinning aggregate, a cylinder-shaped permanent magnet for non-contact speed monitoring is arranged at a distance from the axis of the supporting disc. For the purpose of the correction of mass imbalances, the permanent magnet is inserted into a through bore hole, which is filled out by the permanent magnet only up to a gradation of diameters which serves as a stopping surface. As the permanent magnet is arranged in the transition area between the base body and the tire-like covering, the part of the through bore hole not filled out by the permanent magnet can be filled out by the covering.