Abstract. We consider skew product dynamical systems f : Θ × R → Θ × R, f (θ, y) = (T θ, f θ (y)) with a (generalized) baker transformation T at the base and uniformly bounded increasing C 3 fibre maps f θ with negative Schwarzian derivative. Under a partial hyperbolicity assumption that ensures the existence of strong stable fibres for f we prove that the presence of these fibres restricts considerably the possible structures of invariant measuresboth topologically and measure theoretically, and that this finally allows to provide a "thermodynamic formula" for the Hausdorff dimension of set of those base points over which the dynamics are synchronized, i.e. over which the global attractor consists of just one point.