There has been a lot of talk about front and rear shocks and some good information. Not much has been discussed about front and rear spring selection (coil overs).

Generally, from what I remember, the spring rate is whatever is needed to just hold the car up. Is this still true?

My truck is 58/42% at 3300 lbs which yields 1,914/1,386 weight.

What would be the front and rear spring rates that would be optimal?

I more complicated than that. It depends on angles and attaching points

It does matter but not as much as you think. If the shocks are behind the axle the rears will need to be either 110### or 130. Track use only 110#, street may require 130# for compression control to keep from bottoming out. Mine weighs 1520# on the rear. I measured the unsprung weight. Dana, old school American 200S, M/T 10.5wx31, Will-wood. Then 1/2 of the Koni aluminum coil overs and ladder bars. That weight was 330#. My springs are compressed 3.5" at ride height with driver. So 130x3.5"=455#/side. 455x2=910. 1520-330 (unsprung)=1190. That being said I'm surprised there is a 280# (1190/910) difference with the shocks only 4" behind the axle. Obviously the calculation doesn't match reality

Doug

what he said....^^^ after reading a couple different formulas, I called my chassis guy, borrowed a set of springs, and then purchased another set to get that "happy" spot.

I am running 150 in rear and 500 in front. I was thinking my rears were a little stiff (thinking 130) but I was mainly concerned with my fronts and think they are too stiff.

Can you get about 2" compression on your spring sitting? I have 350 in mine at around 1750 on the nose

Without knowing what shocks, suspension, shock location and installed length at ride height, street, strip or both, ET, spring free length, spring compressed length at ride height now etc. anything would be a guess. Race only, shocks behind the housing, ladder bars, typical backhalf deal? Probably a step too tight on both ends.

sixpack, I will have to check as I don't remember.

CM, front are single QA1 coil over, rear ladder coil over, dbl adj Afco, NOT back halfed.

After some research, I am going to try 130 lbs rear, 450 front.

Slightly off topic, but not really. Is there an easy way to determine what springs I already have in my car?

Most any spring will have a tag or the number etched on the
flat of the spring on the end with the pounds per inch... if
you dont want to do the formula.. set the bottom spring adjuster
to 1" up from the bottom... is the shock at the design length
with the car at race weight... if you have to move the adjuster
more than 1" then you need a different springs... if its a A-body
then a 130 will be close but might need a change one way or another

the spring rate is whatever is needed to just hold the car up

2 things are missing:

1. there's more than one way to get the car to the same ride height (assuming you know what you want already):
A. short stiff spring - stops transferring weight when it tops out
B. long weak spring, the extra length is pre-load - transfers weight up to its full length

2. spring stiffness must be enough to control the weight of the unsprung components (axle etc.). An 8-1/4" and a Dana need different rear springs regardless of the rest of the tuneup.

Not really. In my case, the coil overs are a given length. So because there is less energy loss per inch in the weaker spring , it would top out quicker with the weak spring than the stiff one.

I have never heard of controlling the unsprung components. How would you know when your spring values are too low to handle the unsprung weight. We all know what would happen if you had too low a spring pressure for the sprung weight. And when I lightened my car, I had to change springs several times to get it down from it's 4x4 stance.

I agree with comment #1
Comment #2 is dead wrong. The spring doesn't control the housing. That would be the job of the shock.
Doug

Comment #2 is dead wrong

LOL

I think you mean "I don't understand what you said".

Let's save time in future.

I won't correct the mistakes, and you can just say "You're wrong" anyway.

The spring has to be the correct rate to hold the car up at the desired ride height when compressed to the desired length. The spring rate and the resulting compressed length is a tuning decision. Having the spring in coil bind before the shock is fully compressed OR having a spring that holds the car up with zero spring compression are the only wrong choices. The choices between those 2 extremes are determined by how you want or need the spring to influence suspension movement. If a spring "tops out" or reaches free length at any time during a run, something is wrong.

A lighter spring has to be compressed more which stores more energy than a heavy spring which is only slightly compressed. This energy can be used to help get the housing out in a car that needs help getting the tire applied at the hit. Higher horsepower applications don't need that influence and will use a heavier spring with less energy stored at ride height. Think about the 6 cylinder torsion bars helping get the front end separated on cars that need the pitch rotation. Same effect.

While the weight of the unsprung components may influence a spring choice (total weight-unsprung weight=sprung weight), the spring doesn't control what the axle does. It influences it, but the shock is what controls it. Suspension type and geometry, spring rates and shock adjustments all work together to determine what happens as the car moves down the track.

My small block Duster has 10" X 300lb front springs. They work with both of my engines as they are similar in weight. I have a set of 10" X 350lb springs doing nothing if anyone is interested.

I'll say it! I don't understand what you mean.

Ok let me ask then. What does spring rate have to do with unsprung weight in the scheme of things? The shock controls the rate at which the sprung and unsprung weight move towards or away from one another. Am I missing something?
Doug