Suspension Question Double Check

[rollercam]

I wanted to have some of the suspension experts on here chime in on the following conversation exchange. Below is the tail end of a conversation I was having on another forum. The car is a 2002 Z28 and the conversation is in reference to the front springs. I am thinking of replacing my static drop springs with a coil over set-up for some adjustability. The standard front drop spring for this car is usually around 1.25 inches drop with a spring rate of between 500 - 600 lbs/in. Company ABC says that the 14" 400 lb 2.5" coil over spring they offer on their coil over is in practice nearly equivalent for the reasons that are explained below. Is his answer accurate given the parameters used in the explanation?

Question from me:

Please forgive my ignorance, but I thought that these (2.5 inch inside diameter coil over springs) were linear rate springs and regardless of extension or compression the rate is the same. Could you explain preload to me and how the 14" inch 400 lb spring could be similar to an aftermarket 550 lb conventional lowering spring (mine are XYZ 1.25" lowering springs front and rear). I am not sure of the uncompressed spring height for the XYZ springs.

The answer I received:

Yes, they are linear rates. However, you need to calculate the amount of force the spring is actually pushing at ride height; then look at the rate as the suspension compresses. . .this determines handling and ride quality.

So let's do this for easy math (these are not real numbers, just something for illustration as we don't have actual free length numbers):
Ride height at the front spring is 11" (regardless of spring).

ABC coil over spring is 14" free height with a 400# rate.
ABC coil over spring is preloaded 3" at ride height (installed spring length is 11")

Conventional Dropped spring is 13" free height with a 550# rate.
Conventional Dropped spring is preloaded 2" at ride height (installed spring length is 11")

ABC coil over spring at ride height will be "pushing" 1200# force. (14-11=3; 3*400=1200)
Conventional Dropped spring at ride height will be "pushing" 1100# force. (13-11=2; 2*550=1100)

Now, compress the suspension 1" (go over a bump, roll through a turn, whatever). . .
Spring height is compressed 1", so it's now 10".
ABC coil over spring is now at 1600# force. (14-10=4; 4*400=1600)
Conventional Dropped spring is now at 1650# force. (13-10=3; 3*550=1650)

Now, hit a big bump and compress the suspension an additional 1". . .
Spring height is compressed 2" overall, so it's now 9".
ABC coil over spring is now at 2000# force. (14-9=5; 5*400=2000)
Conventional Dropped spring is now at 2200# force. (13-9=4; 4*550=2200)

As you can see, a longer lower rate spring will have more initial force at ride height but will gain less rate than the shorter higher rate spring.
What this means is the longer lower rate spring will initially resist more, but once it's moving not add as much force. So you have a great handling vehicle that doesn't jar your fillings out when you hit a bump.

[bret]

Their explanation is technically accurate. I don't have direct experience with that particular car so I can't comment of the logistical accuracy of the specific spring rates.