  We reduce Yang-Mills equations for $SO^+(p,q)$, $Spin^+(p,q)$ and $SU(n)$
bundles, with constant and isotropic metrics, by developing the concept of
$SO^+(p,q)$-equivariance. This allows us to model the electroweak interaction
and $SO^+(p,q)$ bundles with a non-linear second order differential equation as
well as the weak and strong interaction with a non-linear wave equation.
