  Chesnavich's model Hamiltonian for the reaction CH$_4^+ \rightarrow$ CH$_3^+$
is known to exhibit a range of interesting dynamical phenomena including
roaming. The model system consists of two parts: a rigid, symmetric top
representing the CH$_3^+$ ion and a free H atom. We study roaming in this model
with focus on the evolution of geometrical features of the invariant manifolds
in phase space that govern roaming under variations of the mass of the free
atom m and a parameter a that couples radial and angular motion. In addition,
we establish an upper bound on the prominence of roaming in Chesnavich's model.
The bound highlights the intricacy of roaming as a type of dynamics on the
verge between isomerisation and nonreactivity as it relies on generous access
to the potential wells to allow reactions as well as a prominent area of high
potential that aids sufficient transfer of energy between the degrees of
freedom to prevent isomerisation.
