Spring Rate Calculator

The steps below will help you to find an approximate spring rate for your coil-over application. Enter your suspension measurements in the white boxes below.
You should consider the desired spring/shock travel carefully. The amount of travel (compression) the softer the suspension will calculate.
This calculator will only give approximations depending on the accuracy of your information. Use at your own discretion.

SELECT FRONT OR REAR


Street Stock Front End

Street Stock
(9 inch Ford axle)

Step 1 – Determine Sprung Weight

Corner Weight: (1/2 the front or rear weight)

Use an accurate racecar scale that will weigh each corner of your car. Make sure the vehicle is loaded with driver, ballast, and fuel as you will race it.

The weight balance between the front and rear of a vehicle can range widely from 50%/50% (Front/Rear) to 60%/40% (Front/Rear).
The basic Street/Pure Stock GM Metric clip with weight jacks front and rear,
employing a 9 inch Ford rear axle with disk brakes.
Recommended numbers: 48% (front), 52% (rear), 53% (cross)

Unsprung Weight: (1/2 the front or rear weight)

“Unsprung weight” is vehicle weight not supported by the springs. Including: Tire/wheel assembly; brake rotors, calipers and components; spindle; weight applied to the lower control arm.
Rear Unsprung weight includes: Rear axle housing with third member, axles, all brake components and shock absorbers.
Front Unsprung corner weight is anywhere from 85 (light components) to 120 (stock components)
For the Street Stock input enter 95 pounds.
Rear Unsprung corner weight is usually around 135 (light components to 175 (stock components)

Sprung Weight:

Sprung weight = Corner weight – Unsprung weight. It is the weight supported by the spring and is the only weight used in calculating spring rates.

Step 2 – Calculating Motion Ratio

The “motion ratio” is the mechanical advantage (leverage) the wheel/tire has over spring compression .  

Enter Dimension A:


Dimension A – Measure the distance from the control arm pivot point on the subframe (centerline of the bushing) to the point on the control arm directly under the center of the spring or the “coil-over assembly.”

Enter Dimension B: (see above illustration)


Dimension B – Measure the distance from the lower control arm pivot point on the frame
to the centerline of the ball joint (or the center of the wheel).

Motion Ratio:

Motion Ratio = Dimension A / Dimension B
It should be noted that the pivot point to the ball joint equals arm length
The measurement, pivot point to the center of the tire equals wheel length
The real Motion Ratio = Arm Length divided by Wheel Length.

Step 3 – Coil Spring Angle

Spring Angle: (see illustration above)


Using a protractor or similar measuring device, measure the angle of the centerline of the spring or “coil-over assembly” from the horizontal of the control arm (or axle if calculating the rear). In most cases, this will be somewhere between 75 degrees and 90 degrees. Most conventional springs and many strut suspensions are very close to 90 degrees, and 90 degrees can be used for the angle. This measurement helps determine the “force angle” and resultant spring force applied to the control arm.

Step 4 – Select your shock stroke and spring length

Measure the shock main shaft. 50% travel for a nine inch shock = 4.5 inches.
You will want to select the shock stroke that will fit your suspension design.

Step 5 – Spring Rate

Below are approximate spring rates for your coil-over application. This calculator will only give approximations depending on the accuracy of your information. Use at your own discretion.  

Cruiser / Daily Driver

Recommended spring rate to optimize ride quality. Is best used for crusing, day to day use, and long distance driving.

Performance Driving

Recommended spring rate for performance driving in events like an autocross or simple road course.

Racing

Recommended spring rate for full-on racing with sticky tires.

Spring Rate:

Preload:

Part Number:

 

Spring Rate:

Preload:

Part Number:

 

Preload:

Part Number:

 
Note: Weight Jacks

Preload is the turn-number you will add to tighten the lower coil mount on the spring. This will compress the spring.  The desired amount of stroke depends on the shock length and spring height.
When the car is resting at ride height, adjust the jacking-bolt to the desired height. Adding more preload will raise the corner height, it will also change the cross-percentage (wedge).

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