They are magnetic Neodymium treads covered with Rubber . In the center they are threaded for fixing. The coating with Rubber gives the tread apart from the magnetic and mechanical adhesion so that the tread to suction, not to slip and not to injure the surface on which it steps ! Ideal for holding large objects on sensitive metal surfaces ! Such as e.g. lighting fixtures or advertising on car roofs (sirens, signs etc. ). Many popular also are the rubber treads with plastic cable holder.