  We develop rigorous, analytic techniques to study the behaviour of biased
random walks on combs. This enables us to calculate exactly the spectral
dimension of random comb ensembles for any bias scenario in the teeth or spine.
Two specific examples of random comb ensembles are discussed; the random comb
with nonzero probability of an infinitely long tooth at each vertex on the
spine and the random comb with a power law distribution of tooth lengths. We
also analyze transport properties along the spine for these probability
measures.
