The question is to have a uniform drop of e.g. 2.5cm and keep the car level you might need to reduce the spring length by a different amount front and back since the spring will compress when the car sits on it.

How will you be able to make sure the car is level with the new springs?

I like to over-complicate things :biggrin:

You said you like to over-complicate things so here's the theory...

To work out what the length of a spring needs to be to give a specified ride height for a given spring rate, we need to have a reference point - a spring rate and free length for a spring that gives us a known ride height.

Thanks to Peaty on Scoobymods, we can use Pink STi springs for a Forester XT as our referrence. I'll use the rears in this example but the fronts are the same principle.

These have a free length of 13.75in and a spring rate of 167lb/in (fronts are 11.75in and 195lb/in).

Now that we know this, we'll work out the compressed length of the spring.

The equation for this is:

Deformed Length = Free Length - (Load / Spring Rate)

Since the Earth's gravity exerts a constant force on it's surface, we don't need to know the weight of the car in order to work out our ride height. All we need as a referrence is the force required to compress the spring a set distance, i.e. the spring rate.

In the case of Peaty's STi Pink springs, this is is 167lb/in. Thus, our refernce points are 167lb and 12.75" (13.75" - 1").

Now that we have our load and compressed length reference points, we can re-arrange the equation above to calculate the free length of the spring required to maintain the same ride height for our new spring rate. Thus equation is now:

Free Length = Deformed Length + (Load / Spring Rate)

If we wanted to increase our spring rate to say 180lb/in (as per King Springs) then to maintain the same ride height as the STi springs, the free length of the new spring would be:

(167 / 180) + 12.75 = 13.68" or 0.07" shorter than the STi one.

Now that we know the free length of our new spring, we just need to add or subtract the amount of lift/drop that we want.

Q.E.D.

(I might be wrong of course, but this is what came out when I scratched my head a bit ;o)