If you want to do it once.
Weigh the car at race weight. Get the weight on the rear wheels alone.
Remove the current springs.
Measure the free arch,
Measure the how far each compress when a heavy weight is placed on them.
>The way I do this is by standing on them after weighing myself, or whoever is the victim.
> be care full when getting off the spring, it can throw you!
Now you can calculate the spring rate of each one, and you've measured the free arch.
Depending on how beat up the spring is, the rate and free arch may be less than new. Especially the right side.
But it tells you exactly where the springs are at. So you can then select new ones and know what they will do.
One spec that can't be easily measured or figured is the spring ratio.
Look at the number of leaves, where the second leaf end is positioned under the eye, and the thickness of the main leaf and the 2nd leaf.
This gives a pretty good idea of what you have, and what to ask about when shopping.