Spring Rate Help With Math!

Jan 16, 2023
Figuring out a starting spring rate for your project can be difficult, even with corner weights of your vehicle it may be difficult to figure out what spring set up you should run, shock angle and position can all lead to a complicated trigonometry problem that will take half the day.

With a few simple measurements and formulas you can have great success, figuring out the corner force that your coil-over sees is not overly complicated but you will need a few items
1. tape measure
2. a jack that can lift the suspension into a full droop scenario
3. calculator or brush up on your long division
The first thing we need to do is determine the active spring rate that we have. For single rate that number is the rate of your spring as advertised by the manufacturer, for dual rate we use this formula to determine active rate, (upper rate x lower rate) / the sum of the upper and lower = active rate.
10" 100lb/in upper
12" 200lb/in lower
(100x200)/300= 66.66666lbs/in

Secondly we need to know the free length of the springs, we can take this information either before installing them or from the manufacturers listings. Our example springs have a total combined length of (10" upper + 12" lower)=22". We will also need to know the thickness of our slider/spring divider in our example we will use .5" our total free length comes to (22" of spring+.5 slider)=22.5"

Ok now what? what do we do with this information?
We need to grab our tape measure and jack to figure out a few things, we need to know what our corner force is and we need to see how much droop and preload we have on our current set up. we can get all of this information with 2 measurements and simple math.

Step one
Measure the total length of your springs at ride height from top of top spring to bottom of bottom spring in our example we have 16.5"

Step two
Jack up the chassis of the vehicle and allow the suspension to fully droop out (leave your limit straps installed) our measurement comes out to 20.5"

Step three
Time for some math
To figure out our droop we take our ride height and subtract it by our full droop measurement 20.5-16.5=4. We have 4" of droop.

To figure out our preload we take our full droop and subtract if from our free length 22.5-20.5=2. We have 2" of preload.

To get our corner force number we take the preload plus the droop and multiply that by our active rate, (4+2)x66.6666666= (6)x66.6666666=399.99999

On our example vehicle we are using 399.99lbs/in to attain our ride height, Corner Force = 399.99lbs/in

What can we do with this number?
Now that we know how much energy or spring force it takes to hold up our chassis we can use this number to make changes without guessing what preload we need or combined rate.

"Generally" we like to set up vehicles with 2"-4" of preload this all depends on application and intended use and is greatly dictated buy the available springs and travel within the spring.

If you find that you have little to no, or even negative(loose) spring preload this formula will still help you get your springs corrected!

lets work on a messed up coil over set up on Dales Ranger
Dale has a beamed ranger that has less than stellar ride quality. He says his springs are loose and the truck seems overly rough. We had Dale take his measurements here is what he had

upper spring 14x400/lbin
lower spring 16x450/lbin
slider thickness .5"
= Total spring length 30.5" and 211.77lbin active rate.
Dale measured his ride height measurement at 27.5", when he measured his droop measurement the springs were very loose so we had him measure from the mating faces on his coil-over he got 32.5" that is -2" of preload, no good!
Lets finish our formula out
(Preload+Droop)x Active Rate = corner force (-2+5)x211.77= 3x211.77= 635.31 corner force

Dale likes his ride height and we agree that 5" of droop seems right for his application but the -2" of preload is not correct at all.

We know the corner force of his truck and we like our droop we just need to decide what preload we want, for Dale we want 2.5" of preload lets figure out what spring rate his truck wants.

lets take his droop add our desired preload and we can divide that by the corner force and it will give us our new desired combined spring rate.
635.31 corner force /(5" droop+ 2.5" of preload)/ = 635.31/7.5= 84.708.

Our new combined spring rate for 2.5" of preload and 5" of droop would be 84.708lbs/in ! Much different from the 211lb/in we had before.

If we use our formula from earlier we can play around with options of springs that get us to the 84.708 or we can call it 85lb/in to make things easy, a 150lb/in over 200lb/in looks almost perfect at (150x200)/350= 85.714. close enough for what we need!

I hope this little story helps out on getting your toys sprung correctly.

Dale soon after cut his truck in half to start a back half project that he never finished, dont be like dale, run some preload and have fun out there your truck shouldnt be kicking your A**


Apr 9, 2022
heres a worksheet i created for spring rate math


*was view only originally, anyone should be able to edit it now
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