[fe1rx]

There are a few different approaches to making this calculation and generally there are some hidden assumptions that can trip you up. The simplest approach is based on a measurement or calculation of static wheel deflection (x). The ride natural frequency is equal to 188 / SQRT(x) in cpm or 3.133 / SQRT(x) in Hz.

This assumes that your spring is linear rate, your motion ratio does not change with displacement, you are not riding on your bump stop, and spring preload is zero. Since at very least the last item is not true, some calculating is required to use this method. We can calculate the wheel static deflection assuming there was sufficient droop travel that there was zero preload, but it needs a lot of data not readily at hand (except that I have measured it):

FRONT:

1) 60 N/mm (342 lb/in) springs
2) Motion Ratio MR = 0.960
3) Wheel Rate WR due to spring = 342 x MR^2 = 315 lb/in
4) Wheel Rate due to bushings = 7 lb/in
5) Total front wheel rate = 322 lb/in
4) Total Corner Weight = 902 lbs
5) Unsprung Corner Weight = 115 lbs
6) Sprung Corner Weight = 902 - 115 = 787 lbs
7) Static wheel deflection= 787 lbs / 322 lb/in = 2.44 in
8) Natural Frequency = 3.133 / SQRT(2.44) = 2.00 Hz

This overestimates the ride natural frequency of the chassis, because it omits the tire deflection, which is about 0.41 inches assuming a tire rate of 2200 lb/in. Increasing static deflection to 2.85 in reduces the front natural frequency to 1.85 Hz.

REAR:

1) 140 N/mm (799 lb/in) springs
2) Motion Ratio MR = 0.563
3) Wheel Rate WR due to spring = 799 x MR^2 = 257 lb/in
4) Wheel Rate due to bushings = 23 lb/in
5) Total rear wheel rate = 280 lb/in
4) Total Corner Weight = 823 lbs
5) Unsprung Corner Weight = 120 lbs
6) Sprung Corner Weight = 823 - 120 = 703 lbs
7) Static wheel deflection= 703 lbs / 280 lb/in = 2.51 in
8) Natural Frequency = 3.133 / SQRT(2.51) = 1.98 Hz

This overestimates the ride natural frequency of the chassis, because it omits the tire deflection, which is about 0.37 inches assuming a tire rate of 2200 lb/in. Increasing static deflection to 2.88 in reduces the rear natural frequency to 1.84 Hz.

Hence my conclusion that 60 N/mm front and 140 N/mm rear are reasonable spring rates for our cars, if the primary objective is track performance (and if the vehicle has reasonable suspension travel and does not ride on its bump stops, etc.)