I found some good options to replace the rear weak coils that come with my completely dead Nivomats (that will also be replaced with standard shocks)but I am not sure how strong they should be:

In this site:
http://www.stockcarproducts.com/springs4.htm
they sell 3 options:
they are all 5"x13" coils and the 92-960 needs 10" of compressed coils to get a normal look so I need the coils to compress 3 inches when loading the empty car, and this car weighs 2300 pounds in total, I guess the front coils will carry like 70% of this weigh but I am not sure.
I don't know how to weigh each corner of the car, does each tire hold the same amount of pounds (1/4th of the whole car) or the front ones suffer more due to the engine and tranny?

Well, the coils rate options are:
125 lb per inch,
275 lb/in and
300 lb/in

I would like to have a normal ride for 4 persons (wife and 2 sons)and I was thinking of getting the 125 lb/in rate... which will need 3"x125=375 pounds each coil to loose the 3 inches I need...that means the car needs to have 750 pounds of weigh in the rear side which is like the 32% of the entire car.

Can anybody please have any ideas or experience on this topic?
I don't know what's the rate of the existing coils that come with the Nivomats.

Everyone is right, this is a special system. These are a selve leveling shocks the Nivomats. Stick with the original Volvo nivomats made by Sachs of Germany.I just changed them both on my 94 960 for 900.CDN. and the spings from volvo for 300CDN. I did the whole package because I figured for the money I won't have to go back a second time. Your springs don't need change pending upon how much abuse they've taken. By the way the prices were parts only less taxes and labour. My belief is do it right the first time you'll thak yourself later.

Your symptom is absolutely on-target for failed Nivomats. Unlike the other 99.xx% of the cars on the road, this system DOES use the shock absorbers to support some of the car's mass. Unless you intend on converting to a non-Nivomat system, your springs should stay.

Your balance figure is way off. 70% in front is about right for front-wheel drive but the RWD Volvo is so well balanced because it is about 51/49% ratio.

Hi

I'd be surprised if the coils need replacing. It's amazing what new shocks do. Soggy shocks make it seem as though the springs aren't working properly. But in my past experience, new shocks do the job.
--
Alan Clarke, Australia

www.ipd.com sells a complete replacement setup for the Nivomats. They sell regular shock and spring setups which eliminate the nivomat shock all together.

The springs you have will be just fine.