You need to know the sprung corner weight (CW). I'll use our current coil over (C/O) project as the example since it is fresh.

We bought springs in 200 lb/in x 14" long. We installed them on the shocks with a spacer to hold the rig at our design ride height (DRH) After they were installed, we checked several things to ensure we are getting a reasonably accurate amount of compression on the springs. Shake the rig by rocking side to side with the front tires chocked to help remove any suspension bind and try to get the rig to settle to what it is designed at. Trans in neutral so the rear tires can roll to relieve bind, cargo that is going to be carried in the rig where it goes. Or basically "weigh" the rig how it will be driven most of the time.

That gave us 4.125" of compression in the front, 3.0" of compression in the rear. The math is basic, compression x rate = sprung weight.

Front = 825

Rear = 600

The sprung weight is used with the compression of the shock length from fully extended to get your target spring rate. In our case we are using 14" travel C/Os so here is how that breaks down.

Front DRH is 6" of shaft showing so 8" of shaft is in the body. How far the shaft moved into the body is how far the spring stack compressed which is why we use that number.

Our tuners like 1" of preload (PL) in the front and 2" in the rear. A discussion with your tuner should be had to get what they like. Preload is the amount you compress the spring stack from fully extended when installed on the shock.

That puts us at 9" or 8" + 1" of PL = Total.

The math is dividing the CW by the amount the shaft moved into the shock + PL at DRH.

825 divided by 9 = 91.6 lbs Target Rate.

Once you have the TR, then the guessing game starts. Our tuner doesn't want the upper and lower spring to be more than 100 lbs apart in rate. It is easy to see what springs are out there by going to some place like Poly Performance and Kartek or even Eibach to check the offerings.

The math for 2 springs on a C/O to get the combined rate is product over the sum. Or, multiply the two rates and then divide that by them added together.

After that, then you start with a few selections to see which way it needs to go to get you the rate you are after.

Target Rate = 91.6

200/250 or 50,000 divided by 450 =111.1 which is higher than our TR. We'll start dropping them down with numbers based on what we are able to get in a spring.

175/250 or 43750 divided by 425 = 102.9 so still a bit high.

150/250 or 37,500 divided by 400 = 93.75 which is very close to our target spring rate. It needs to be understood that the gas charge in the reservoir will add some small amount of lift to the spring stack so we shoot for a target rate slightly under what the math gives us for the original target rate.

150/225 or 33,750 divided by 375 = 90 which would work for us but I'd like just a bit less plus I couldn't readily find a 225 lbs/in x 16" lower spring.

150/200 or 30,000 divided by 350 = 85.7 so that's very close to what I think we can use and in fact what we did use.

The above is just to show that you can move the numbers in and out of the formula based on commonly available springs to understand the process easier which is why I called it "guessing" because you are plugging in numbers to see how that moves the rates around to get to what you are after.

Of note, a 14" C/O typically runs a 16" lower spring and a 14" upper. 12" runs a 14" lower and a 12" upper. Our experience is with Fox and King shocks and this works on those. I don't know if others are the same, they should be but you should verify.

To get your preload correct, once the spring stack is installed on the shock, hold it vertical and screw the lower nut down until it just touches the top spring, then screw the lock nut down on that until it touches. Then hold the lock nut and start screwing the main nut down to compress the stack, measure between the two until you reach your preload then screw the lock nut down and lock it in.

The above is pretty accurate. When we got the stacks on the shocks, suspension settled a bit, the rear came in at 7.25" of shaft showing at ride height with a target of 7", the front came in at 6.125 with a target of 6.0. We'll start driving it and get the shocks loosened up, get everything moving and used to moving to settle in and then we'll dial it back in to hit our target numbers. I showed how to arrive at a target rate for the front, the process is the same for the rear.

Disclaimer- I need to go back over this more intently to check for errors, if you see any, point them out.