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Coil Spring Rate Calculator

 

 

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Definitions:

Spring Rate is half the difference between the loads 1 inch above and 1 inch below a specified position. Another definition would be: The amount of force it takes to compress the spring 1-inch and is expressed in ld/in.  The lower the rate, the softer the spring.  If the front of your truck is sagging you need more spring load, not more spring rate.

Spring Load is the amount of weight the spring is designed to carry at a certain height. This is also called the Design Load or Load Rate.  Think weight carrying capacity.

Load Rate is not to be confused with Spring Rate.  Load Rate is the amount of weight a spring is designed to carry at a certain height.

Let's say a spring has:

  • A rate of 200 lb/in

  • Designed for a 3-Inch deflection

  • When deflected 3-inches the spring is supporting 600 lbs

  • Therefore, the spring has a Load Rate of 600 lbs.  Not a Spring Rate of 600 lbs.

Unsprung Weight is the weight of the tires, wheels, knuckles, hubs, axles, and half the weight of the springs, shocks, control arms, and/or links.

Sprung Weight is the weight of the body, chassis, drivetrain, tools, parts and the other half of the total weight of the springs, shocks, control arms and/or links.

Wheel Rate is the spring rate actually measured at the wheel (or tire).  The wheel rate is usually lower than the true spring rate due to factors such as spring position and control arm or axle leverage that can effectively lessen the spring rate at the wheel versus the actual spring rate at the spring.  If you move the spring closer to the tire (and the spring travels parallel to the wheel), the wheel rate and spring rate will become almost the same.

Variable Rate Springs have a soft initial spring rate and to absorb the subtle irregularities of the road/trail progressing to a firmer rate to handle large bumps.  These springs increase in rate as they are compressed.

Effect On Rate:

In the formula below you will see Wire Diameter, Coil Diameter and Number Of Coils.  Here is how they effect rate:

 

Dimension Increased Effect On Rate
Wire Diameter Higher
Coil Mean Diameter Lower
Number Of Coils Lower

 

Coil Spring Rate Formula:

11,250,000 - Torsional Modules For Steel (Constant)

CSPWD - Coil Spring Wire Diameter (How thick is the wire?)

8 - Constant

NOAC - Number Of Active Coils (Coils that are free to move.  Not coils seated.  It may be that 1/2 of the top and bottom coil is seated causing a spring with 8 coils to have the distance of 7 free.)  

CMD - Coil Mean Diameter (The diameter from center to center of the coil. Measure the diameter of the coil from wire center to wire center. If you know the coil is .5 inches thick and the outside diameter of the coil spring is 3 inches, then the Coil Mean Diameter would be 2.5 inches)

(11,250,000 x (CSWD x 4))

(8 x NOAC x (CMD x 3))

All Measurements in inches - Enter as decimal
For Inch Fractions To Decimal Chart, Click HERE

 

Coil Spring Wire Diameter Coil Spring Mean Diameter Number of
Active Coils
= Coil Spring Rate (lb/in)

 
 

 

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