Factory Front and Rear Spring Rates???

[Cobra #3170]

I hope this isn't too long and boring but maybe it will help. There are many other factors that effect handling like front and rear camber, weight distribution, tire and wheel size, geometry, toe, roll axis, CG ect. but this should help with spring selection.

The important factor is not the spring rate, it is the front and rear wheel rates that determine how any car behaves. The motion ratio of the suspension (the amount the spring moves vs. the wheel movement) squared times the spring rate gives you the wheel rate. If your car has a motion ratio of say .7 and a spring rate of 600 pounds your wheel rate will be 294 lb/". Anti roll bar rates are typically under 30% of the wheel rate to keep from lifting the inside wheel in a corner (at least in a narrow car like a Cobra). You can balance almost any spring rates front to rear so the car is neutral, the important factors are ride quality and grip on rough surfaces. The shock also needs to be matched to the spring rate and unsprung weight to achieve the best overall performance.
The anti roll bars and front to rear ride height ratio can be adjusted to achieve oversteer, understeer, or neutral handling if the front to rear rates are close to optimum for the particular car. My front springs are 200 lb/" higher than my rears but the wheel rate is higher in the rear than the front because of the higher rear motion ratio. A crude check can be made by placing the car on 4 wheel scales and then jacking the front and rear up with two floor jacks placed at the chassis center line simultaneously, one in front and one in back. You jack the front up .5", jack the rear up .5" and then take scale readings. The change in readings is what you want, if it goes from 650 pounds to 600 pounds in .5 inches of movement your wheel rate would be 100 lbs/ in
Repeat this process until you get 3-4 scale readings (1-2") and you can determine front and rear wheel rates. If you have a calculator you can then determine your motion ratio by dividing the wheel rate by the actual spring rate (printed on the coils of Eibachs and etched on the ends of Hypercoils). Take the square root of that number and that will give you the approximate motion ratio for the car at that location. Once you know your wheel rates it can give you a basis for making spring changes and keeping your car neutral when you make a change. All you need to do is select springs that maintain the existing front to rear wheel rate ratio. This is assuming you like the way your car handles. If you don't like it, upping the front wheel rate will increase under steer, upping the rear wheel rate will increase over steer, the reverse is also true. Shock rebound should also be increased with increased spring rates. All This is a very general and may not apply in extreme cases of wheel rates, but it should help guide your selection. There are a number of programs that will calculate anti roll bar rates, just keep in mind that they should be under 30% of wheel rate and should maintain the front to rear balance once you have an optimum number.