The unloaded length of a higher rate spring must be shorter than the
unloaded length of the stock spring, otherwise the car would sit higher
off the ground when you put the higher-rate springs in.
Suppose that at stock ride height, the loaded length of the spring is
7", and it's supporting 1300 pounds. Therefore, the stock spring will
have been compressed by 5", and the spring rate is 260 pounds per inch.
The 10-1/2" spring would have to be compressed by only 3.5" to achieve
the same ride height, and it's still supporting the same load. Therefore
it would have to have a spring rate of 371 pounds per inch.
Note that the front springs operate at about a 2:1 leverage ratio due to
where the spring perch is on the A arm.
Best regards,
Theo
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