For a given natural frequency (determined by spring rate and suspension type), If the car is left to it's own devices after a bump, critical damping is the force setting that allows the car to come to rest the quickest.

It is not the same as the ideal force curve.

However, there is an emprical indirect relationship.

After testing (not by me), there is a racer (Dennis Grant, Far North Racing) that has found that 65% of critical from 0 - 3 in/sec kneeing digressively to 30% of critical from 3 - max in/sec approximates the ideal force curve very well.

Since I don't have the money to plot suspension histograms, I usually take that advice when getting my shocks valved.

2.2 Hz translates to around 500 lb/in? It was a bit stiff, now that I recall, I don't have my calculations anymore.

EDIT: I remember, 2.5 and 2.6 Hz was somewhere like 500/525......way too stiff.

 

Well it all depends on vehicle weight, a race car, fully lightened will need lighter springs for a given frequency than a full weight street car. 2.2 will be somewhere between maybe 300-350, and 2.5 will be around 400-500. This is based on a number of assumptions, your numbers may be different.

For NA track cars on DOT race tires, something like a 400/275 split is good with any number of different sway bar combinations, depending on the driver (everything from large front, no rear to large front and large rear). That's about the rates that I'm using (7/5 in kg/mm with an RB front and stock rear bar on 225/50/15 Hankook Z211 R's). For stickier, and wider tires (real slicks), stiffer springs are likely to be needed.