Rotors are grouped into two categories: rigid and flexible. Rigid rotors can be balanced at lower speeds, as long as there is sufficient centrifugal force to detect the unbalance. Flexible rotors, on the other hand, deflect outward from the rotational axis, and the center of rotation moves away from the rotational axis as the speed increases. Unlike rigid rotors, flexible rotors must be balanced in stages, starting with lower speeds and slowly working up to operating speed.