I will consider the geodesic flow of a rank 1 Riemannian surface which is expansive but not Anosov. I will sketch the proof that in this context (as in many comparable contexts) the Hausdorff dimension of the set of vectors with only zero Lyapunov exponents is large. The proof is based on work by Bowen-Walters about symbolic codings of any (general) expansive flows and the construction of bridging measures.