Solid axle two wheel bump vs single wheel bump ride frequency

[funcars]

I've calculated a solid rear axle ride frequency both ways I've seen it done - one from an old Steve Smith book calculates ride frequency based on single wheel bump which includes the motion ratio from the track width and spring mounting points. The other method from Milliken treats ride frequency more like a two wheel bump situation and for the leaf spring example uses the leaf rate directly. Both of these approaches give significantly different ride frequencies and wheel rates. Both approach roll rate calculations basically the same. With an independent suspension there would not be a difference between the two wheel bump and single wheel bump ride frequencies.

I'm in the process of changing front and rear spring rates along with installing a different rear suspension and want to get my ride frequency starting points close. I tend to lean more towards the Milliken method which results in a higher frequency and then calculate the roll rates separately. Is this the most realistic approach?



Thanks