When I choose spring rates I do (two) calculations.
First what is the weight of the car at that axle. Divide that number by 2. this gives you F (force or weight)
Since F=kx next is to determine the amount fo droop you want or the shock has. This amount of droop gives you the minimum preload on the spring at ride height. If you have 3" of droop you dont want the spring to become unloaded. take F/x=K. K is your MAXIMUM spring rate.
Now this is where personal preference comes in. I design around a 1G bump. How much travel do you have in your shocks from ride to full compression, this is your x for the next equation. Take F times 1G, or whatever number you want to design around. F/x=K this is you MINIMUM spring rate. Now you have a range.
The only way to adjust this is to adjust your shock length and your shock length at ride height.
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