For Drag Racing or Road Racing? If it's for Drag Racing do you want the technical answer or the short answer?
Short answer is "ask around and find out what others are using"...
Technical answer is long and involves calculations but is basically done around the total amount of travel of the control arms (from bump to bump) and the available spring lengths/rates and the amount of shock extension available.
Here’s a simple one for coilovers..
Lets say the weight of the front of the car is 1500lbs according to our scales.
We want the lightest/longest spring possible (as long as it doesn’t coil bind) and we need the length of the spring at coil bind or the diameter of the spring “wire” so we can calculate the coil bind. The spring MFR should have the coil bind length.
Lets say that we want to try a 14” long / 80lb spring. We want to calculate how far the spring is going to compress with the 80lb spring. Since the car weighs 750lbs per side:
Compressed: 750 / 80 = 9.375”
So the length of the spring while installed in the car:
14 - 9.375 = 4.625”
Now you need to find out if they will coil bind when they are compressed to 4.625”. They probably will..
If it was me I would work backwards by measuring the compressed/extended strut lengths and "desired" compressed spring then calculate what spring I need then look thru the catalogs and hopefully find something that works for me...
For example: I would turn the spring spanner (spring mount) down as far as it would go and this would be the desired spring length while at ride height..
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