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I'm not sure that you've actually identified the OEM spring rates by using KW springs to deduce the information.
I've obtained the spring rates directly from Eibach of their lowering springs for the Z4MC and they are approximately 15% stiffer than OEM and are as follows:
Front: Progressive spring, initial rate is 23N/mm (131 lbs/in) and the end rate is 34N/mm (194 lbs/in)
Rear: Progressive spring, initial rate is 70N/mm (399 lbs/in) with the end rate of 140N/mm. (799 lbs/in)
Most aftermarket adjustable suspension kits for the Z4MC have front spring rates in the region of 400 lbs/in and rear spring rates in the vicinity of 550 lbs/in, so the OEM front to rear sporing rate ratio is completely different with the OEM set up.
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