Quote from colcob :

You're right, its much clearer. Still wrong though.
0.5*pi is not the same as 1/(2*pi) . Thats just algebra

Bugger. Silly brackets.

So...

Frequency = 0.5 / (Pi * (Mass/Stiffness) ^0.5)


And (in the wrong equation) force would not become mg, since it included downforce. But of course it shouldn't be in the equation at all.

Okay, so to sum up... downforce does not affect suspension frequencies. So 0.6 of my calc is wrong too, but only because of asking for weight+downforce (ignoring the issue that 2Hz to 1 tonne as a linear ratio is a bad assumption).