coil spring terminology ?

[Josh]

I am looking at the Moog site for the spec on their rear variable rate coil springs. They have a spring rate, installed height, spring rate and free height among other specs. Does anybody know just what installed height means in relation to the load? Do they mean at the installed height that the spring is at the load specified?
I am trying to lower the rear of my 1979 Impala. The stock spring for this is a variabe rate coil with a rate of 206# and an installed height of 11" and a load of 865# and a free height of 15.1". I had replaced these a couple years ago with a similar spring with a rate of 220#, installed height of 10" and a load of 850#, free height= 13.8". These are failry harsh and I need about 300# in my trunk to get the ride height to where I want it. There is plenty of travel in the back of this car. Any help is appreciated!

[exwestracer]

Figuring rate is tough with a variable spring; because the rate will be different for every inch of compression. Let's look at a linear spring for starters...

The spring installed height is the distance between the spring perches with the car at the desired ride height. You'll measure this with the chassis on jacks (or blocks) and NO springs in the rear. Let's say that at a desired 5" ride height, the distance between upper and lower spring plates is 10". That is your installed height. This is important to know, because there are many ways to get there... If the sprung weight over that axle is 1000lb, then each spring is seeing apprx. 500 lb. To get to our 10" spring installed height, we could use a 500lb/in spring with an 11" free length (compresses 1 inch), or a 250lb/in spring with a 12" inch free length (compresses 2"), OR a 125lb/in spring with a 14" free length (compresses 4").

You must have a very good idea what the SPRUNG weight on the rear axle is under normal driving conditions (all parts on the car, 1/2 tank of gas, etc.). A simple way to get close is to put the car on scales and record the rear weight. Then jack the car up and remove the springs. LOOSEN ALL of the control arm mounting bolts and disconnect the shocks. Lower the chassis back down to ride height and leave the weight on the jacks. The scale should now only be reading the weight of the rear axle assembly. Subtract that from the rear total to get the sprung weight. It WILL be off a few pounds, but should be close enough.

[SLO_Z28]

I wouldn't touch a variable rate spring, there are a number of issues that can arise from them. You're better off getting the proper rate conventional spring and a properly valved digressive dampener.

[exwestracer]

i wouldn't touch a variable rate spring, there are a number of issues that can arise from them. You're better off getting the proper rate conventional spring and a properly valved digressive dampener.

"like!"

[Josh]

O.K. I am checking out linear rate springs. If I know I had 865 pounds of load at an 11" installed height, then would in not make sense that finding a spring with the same load at 9" installed height should lower my ride height 2"? Moog has a couple that would get me close.

[exwestracer]

Yes, just make sure that the spring is stiff enough to stop compressing (during cornering) before you run out of travel. Also, remember that total axle sprung weight needs to be divided in half to figure load on each spring.