coilovers advice

[NoHachi]

First off, you would do yourself a big favour by reading up on this subject and how everything interacts. It will make it a lot easier for you to figure out what suits you and how to get to that.

Springs, if you chose 4kg/mm fronts, look for something around 3kg/mm rears.

Spring length, visualise this: when the car is in the air (jump or similar), the shock is fully extended. This position is the first datapoint. If your spring free length is shorter then this length then the spring can rattle and unseat. Your secondary spring on the front is a very weak spring that is normally fully compressed, but in this situation opens and keeps everything in place. Its springrate is matched to the weight of the wheel, not the car. So that it can control tire movement when there is no weight resting on it. This is a good thing.

Now, imagine a very weak and a very stiff spring. Put a weight of 250kg = 300kg on both of them (weight of quarter of the car). The stiffer spring will move A LOT less. Since both of them need to support the same weight, the stiffer spring will be shorter then the weak one ( which NEEDs to be longer).

You can calculate the length of the spring you need by using the calc on page 47 here:
http://performance-suspension.eibach.com...-17_US.pdf