Now on with the information..... interesting stuff.
This is total compression force in lbs. This is the raw data.
This is incremental spring rate in lb/in. This is the "Spring Rate", and it is derived from the raw data above.
Front springs.... y-axis is lb/in.:
A few important observations on the front springs:
1) This front spring data shows an interesting difference between the Stock/HFP springs and the other "drop" springs in that the stock and HFP springs are more linear. This is attributable to the drop springs needing to lower the car; in other words, the first few inches of compression must be softer to lower the car. The Neuspeed Sport, Swift Sport, and Neuspeed Race are the most linear of the drop springs.
2) Prokit, Sportline, and Skunk2 all bind early..... essentially at coil bind before 7" of compression where all the others go beyond 7" of compression, but I don't know how big a concern that is.
3) Related to item #2 above, there is a very non-linear increase in spring rate of the Prokit, Sportline, Skunk2, and Progress springs deep into their compression. This could be designed into the springs, but the reality is that they will feel harsh under heavy compression (big bumps).
4) The KW V3 front spring is a higher rate, which I would expect from a coilover type setup, but it's not overly stiff and should be a very nice street coilover setup.
Rear springs....y-axis is lb/in.:
Each rear tire has 560 lbs on it, and that does not equate to spring compression force exactly because the spring is far inboard of the wheel (any wheel force is multiplied at the spring). The distance on the lower control arm from the inner pivot to the spring is 12 inches, and the distance from inner pivot to wheel/tire contact patch "vector" is 21 inches. This means that a wheel force must be multiplied by 21/12 to equal the force at the spring; so 560 lbs at the wheel is 980 lbs at the spring. 980 lbs is a between 4 and 5 inches on most of these springs (See raw data). The KW V3 is obviously higher, which makes sense because it is a coilover type setup, but it is interesting that the Neuspeed Sport springs "primary" coils are in the same range as the KW V3.
A few important observations on the rear springs:
1) As with the front springs, the Prokit, Sportline and Skunk2 all had binding issues and could not be measured to 6" of compression. And again the Stock and HFP springs are ultra linear (very nice!).
2) Stock springs and Swift springs are very similar in back. Prokits are very similar to HFP. Sporline and Progress are similar rear rates at static ride height. Skunk2 is higher than all but Neuspeed Sport and Race, which are in a class of their own. Neuspeed Sport has a more linear performance in the primary coils than the Neuspeed Race. The Neuspeed Race have the most drop of any spring; so the spring rate increase may be related to keeping the car from bottoming out as easily, which the Neuspeed Sport doesn't have to worry about so much.
Stock, HFP, Prokit, Swift:
Neuspeed Sport, Progress, Sportline, Skunk2, Neuspeed Race:
Edmunds.com has the 2006 Civic Si listed at 2877 lbs. Weight distribution is 61% Front and 39% rear..... so, that in pounds is 1755/1122, or 880 lbs per front tire and 560 lbs per rear tire.