Clones in 3-connected frame matroids

We determine the structure of clonal classes of 3-connected frame matroids in terms of the structure of biased graphs. Robbins has conjectured that a 3-connected non-uniform matroid with a clonal class of size q−1q−1 is not GF(q)GF(q)-representable. We confirm the conjecture for the class of frame matroids.