  We show the existence of fragile-to-strong transitions in kinetically
constrained systems by studying the equilibrium and out-of-equilibrium dynamics
of a generic constrained Ising spin chain which interpolates between the
symmetric and fully asymmetric cases. We find that for large but finite
asymmetry the model displays a crossover from fragile to strong glassy
behaviour at finite temperature, which is controlled by the asymmetry
parameter. The relaxation in the fragile region presents stretched exponential
behaviour, with a temperature dependent stretching exponent which is predicted.
Our results are confirmed by numerical simulations.
