A 'rear axle frequency' google search showed these posts in eng-tips.com.
Brian,
"damir130 6Sepp10
Hi everyone, congratulations on the high level of the discussions that have been going on here for the past few years. I've been reading and learning as much as I can, but there is one problem...
Are the coil spring perches directly on top of the axle, or are they on the arms? If they are on top of the axle, for jounce and rebound, just use the direct 1:1 motion. For body roll, use the ratio of the distance separating the spring perches to the distance separating the tire contact patches...