  We study a quite general family of dynamical $r$-matrices for an auxiliary
loop algebra ${\cal L}({su(2)})$ related to restricted flows for equations of
the KdV type. This underlying $r$-matrix structure allows to reconstruct Lax
representations and to find variables of separation for a wide set of the
integrable natural Hamiltonian systems. As an example, we discuss the
Henon-Heiles system and a quartic system of two degrees of freedom in detail.
