XRAY - Model racing cars

Does anyone know how the "C" corresponds with the spring rate? Is that the spring constant? How do I calculate the spring rate? I thought a rate should mean force/distance like grams/mm or lbs/in or N/m. What are the spring rates for the xray springs?

35 8182 XRAY XB808 FRONT SPRING SET C = 0.65 - WHITE (2)
35 8183 XRAY XB808 FRONT SPRING SET C = 0.70 - GREY (2)
35 8185 XRAY XB808 FRONT SPRING SET C = 0.80 - GREY-BLUE (2)
35 8186 XRAY XB808 FRONT SPRING SET C = 0.86 - BLUE (2)
35 8187 XRAY XB808 FRONT SPRING SET C = 0.92 - VIOLET (2)
35 8188 XRAY XB808 FRONT SPRING SET C = 0.98 - PURPLE (2)
35 8282 XRAY XB808 REAR SPRING SET C = 0.47 - WHITE (2)
35 8283 XRAY XB808 REAR SPRING SET C = 0.50 - GREY (2)
35 8285 XRAY XB808 REAR SPRING SET C = 0.57 - GREY-BLUE (2)
35 8286 XRAY XB808 REAR SPRING SET C = 0.61 - BLUE (2)
35 8287 XRAY XB808 REAR SPRING SET C = 0.65 - VIOLET (2)
35 8288 XRAY XB808 REAR SPRING SET C = 0.70 - PURPLE (2)

Good one & dont really know. As a guess is C the coil or metal diameter under the coating. Other than colour the only way to see any difference is when placing two different springs together there is a difference in coil pitch or spacing.

According to this thread on RC Tech you can multiply the C value by 5.71 to produce the lbs/in rating.
http://www.rctech.net/forum/electric-ro ... nd-wt.html
Also this thread, while about the on-road springs, demonstrates the relationship between lbs/in rated springs and C rated springs.
viewtopic.php?t=6746

As it says in the link above, the units of C are N/mm, so the higher the value the greater the force in Newtons required to deflect the spring by 1mm.