  A class of dynamical shift conditions is shown to lead to a strongly
hyperbolic evolution system, both in the Z4 and in the BSSN Numerical
Relativity formalisms. This class generalizes the harmonic shift condition,
where light speed is the only non-trivial characteristic speed, and it is
contained into the multi-parameter family of minimal distortion shift
conditions recently proposed by Lindblom and Scheel. The relationship with the
analogous 'dynamical freezing' shift conditions used in black hole simulations
discussed.
