Suspension Tech: How to Calculate Your Spring Rate on Coil and Leaf Springs
Ever wonder what spring rate your OEM JK springs were, but didn’t have JK-Forum handy, or didn’t know what the true spring rate was on those aftermarket leaf springs you just bought off of Craigslist? First, shame on you for not making JK-Forum your homepage as you should. Second, this article will show you what you need to know and the calculations to help you.
First up is the coil spring and you’ll need to have a dial-caliper and ruler handy. First, get your spring’s outer diameter with your ruler. Using your dial caliper, get the spring’s wire diameter at one of the coils. Finally, get the active coil count. Active coils are the coils that have open ends. If you look at the coil in the picture, you’ll see it has 4.5 active coils despite having 6.5 coils.
Now, you’ll probably need a calculator unless you’re really good with math in your head.
Here is the equation: (modulus of spring steel x wire diameter⁴)/(8 x active coil count x mean coil diameter³)
The modulus of spring steel is a constant and is 11,250,000 pounds/inch². Eight is also a constant for the formula. Mean coil diameter is the outer diameter of the coil minus the wire diameter. If we have a spring that is 0.500-inches in wire diameter with a 3.5-inch OD, and has 6.5 active coils, we would have a spring rate of 500.8 pounds/inch.
Now, for you leaf spring guys. First, count the active leaves, but do not count the overload leaf. It is the one at the bottom or top that isn’t tied to the pack. Next, take your dial caliper and get the thickness of the main leaf, which is the uppermost leaf. Take your ruler and get the width of the main spring. Finally, get a tape measure and get the length of the main spring from the center of the eyelet to the center of the other eyelet.
The formula you’ll use is: ((leaf width x number of leaves)/12)*(((1,000*main leaf thickness)/ eyelet-to-eyelet length)³). Here, 12 and 1,000 are constants for this formula. For this example, if we have a main leaf with the width of 1.5 inches, 22 inches long, and 0.25 inches thick with four leaves, we’re looking at a spring rate of 734 pounds/inch.