  We present experimental results for spherical particles rising and settling
in a still fluid. Imposing a well-controlled center of mass offset enables us
to vary the rotational dynamics selectively by introducing an intrinsic
rotational timescale to the problem. Results are highly sensitive even to small
degrees of offset, rendering this a practically relevant parameter by itself.
We further find that for a certain ratio of the rotational to a vortex shedding
timescale (capturing a Froude-type similarity) a resonance phenomenon sets in.
Even though this is a rotational effect in origin, it also strongly affects
translational oscillation frequency and amplitude, and most importantly the
drag coefficient. This observation equally applies to both heavy and light
spheres, albeit with slightly different characteristics for which we offer an
explanation. Our findings highlight the need to consider rotational parameters
when trying to understand and classify path properties of rising and settling
spheres.
