As stated, we have a highly intelligent crowd for peer-review. Thanks for signing up to make a post, Josh.
I pulled most of the info from a guide found here:
LINK to PDF
First point. Here's the traditional formula for a spring-mass oscillator:
F = sqrt(spring constant / mass)
It will output in rad/s. Radians are wondeful for Bode plots, but not good for normal people... so 1/2/pi = 0.159 is the conversion factor for Hz.
Second point may be very valid. It's a fine handwaving argument, but it may not be correct. I picked that up from a random forum post on the miata forums. Let's find out:
Tire dynamics should minimally affect suspension dynamics.
A decade of frequency separation is considered sufficient.
F_tire > 10 * F_susp
sqrt(tire rate / unsprung mass) > 10 * sqrt(spring rate / sprung mass)
If both values are more than one,
tire rate / unsprung mass > 100 * spring rate / sprung mass
tire rate / spring rate > 100 * unsprung mass / sprung mass
for our example:
6500 / 131 > 100 * 83 / 618
49.6 > 13.43 --> sufficiently separated
I guess I should amend the above statement. Thanks for pointing it out!
Continuing this thought:
If we calculate the max spring rate that can be used without being affected by tire dynamics (at stock pressures):
max front wheel rate = 484 lbs/in
max front spring rate = 526 lbs/in
max rear wheel rate = 422 lbs/in
max rear spring rate is = 548 lbs/in
At autox pressures, max spring rates would be 809 (front) and 843 (rear). In metric, that's 14.2k and 14.8k. Interesting, not that anyone would want to run them that stiff anyways.