What are the spring rates of the different JCW springs?
I am mainly interested in the rate of the JCW springs installed on the GP.

 

Not listed anywhere...protected info

 

Just a guess, but couldn't an alingment shop put the spring on some sort of rig and check its rate?

 

A shop that sets up race car suspension is likely to have a spring rate checking tool, but a standard alignment shop is not.

There is a formula for calculating spring rate based on wire diameter, coil spring diameter and number of full coils in the spring. It works well for straight springs but not for the barrel-shaped front Mini springs.

What I found interesting was that the front JCW springs for my Mini were exactly the same wire diameter (to the .001") as the stock springs, and the largest diameter and end diameters of the JCW springs were also the same as stock (2005). Coil count: same also. The only difference was that the free-length (uncompressed) for the JCW was slightly shorter than stock. This is an argument for the JCW front springs being aproximately the same spring rate as stock. (The rear JCW springs used a few thousandths larger wire diameter than stock, so I presume they are slightly stiffer.)

Note: A few thousandths difference in diameter can be significant, because the spring rate increases as a "cubed" (4th power) function of the increase in diameter.

 

My front JCW springs also are the same wire diameter as stock but I measured spring diameter a little less than a couple millimeters smaller. When I had some fitment issues with the fixed camber plates the stock springs had worse bind than the JCWs.