So I have a coilover spring rate question... basically, I would like to get a pair the lower coils to verify that everything is going to fit/clear on my front end before I weld up the coilover mounts.
I have been trying to figure out a baseline that others have used for a similar vehicle... but that seems like the numbers can vary so much, between similar vehicles.
Can anyone provide some additional information on figuring out spring rates for coilovers?
Your combined spring rate with two coils before you hit the dual rate stop is much softer than either of the coils alone. Say you have a 100 pound over a 200 pound coil in your dual rate system. If you load it with 200 pounds of sprung weight, both coils see the full 200 pounds, so the 100 pound coil will compress 2 inches, and your 200 pound coil will compress 1 inch. 200 pounds of weight will compress your shock in this system 3 inches. 200/3 = 66.67. So a 100 over 200 will give you a total of 66.67 pounds per inch spring rate, until you hit the dual rate stop. Once you hit the dual rate stop, you take the top spring out of the equation, and your spring rate jumps to whatever the bottom spring is. In this case, 200 pounds. That will definitely stiffen things up a lot before you crash into the bump stops.
So to take any guess at what springs you need to buy, you need to know how much weight is sitting on the springs first. If the shocks are sitting vertically, you can just measure the sprung weight and use that figure. A MUCH better way to do it is to get any coil of known rate, put it on and measure how much it compresses. Then you'll KNOW how much weight was on it.
Once you have a guess of how much weight is on each shock, the next thing you need to determine is how much you want the shock compressed at ride height. I like to set mine somewhere in the middle of their travel at ride height, but some people want more compression or more extension for whatever reason.
In order to avoid the use of tender springs (I prefer to) you want the springs to be compressed just a little at full extension of the shock, so they don't go slack and fall out of the cups.
So if you know the weight, and you know how much you want the shock to compress at ride height, you just divide the weight by the number of inches you want the shock to compress at ride height to get your desired spring rate.
For example, lets say you have 14" travel coilovers, you want them to sit in the middle of their travel at ride height, and you've determined that your front coilovers are each holding 500 pounds of unsprung weight, and you don't want to use tender springs. You want your 500 pounds of weight to compress your springs 7 inches. 500/7=71.43. Since you want to avoid tender springs, you want to a combined spring rate a little under the calculated amount, so you can add a little extra tension to the springs to bring them up to ride height.
The formula for finding combined spring rate is:
CR = (R1 * R2) / (R1 + R2)
CR = Combined spring rate
R1 = Spring rate of top spring
R2 = Spring rate of bottom spring (and total spring rate after the stop)
So if we go for about 70 pounds spring rate, we might end up with 100 over 180 for a spring rate of 70. Or 80 over 200, or 140 over 140. Any two springs that add up to 280 will give you the same combined spring rate. So at this point I would go to Summit Racing and see what springs are available for cheap. Often they'll have one spring rate for 25 bucks, and another spring rate ten or twenty pounds different for 80. You should be able to find a combination that works for you for a reasonable price.
With all that said... You are not going to get it right the first time! You will end up replacing at least one spring on each corner. So make your best guesses and go through all the calculations above, but plan on making a change to get everything dialed in right. Buy inexpensive springs, because you will end up with at least 4 of them sitting in a drawer in your garage!