F-O-A Suspension Calculator How To

FOA Coilovers will typically use a Dual Rate Spring setup. We use a Main Spring and a Tender Spring.  The Main Spring is the bottom spring that covers the shaft of the shock. The Main Spring should have the same or higher spring rate and a longer length than the Tender Spring. The rate of the Main Spring will be the Secondary Rate.

The Tender Spring is the top spring that covers the body of the shock. The Tender Spring should have the same or lower spring rate and will most likely be shorter than the Main Spring.

A Dual Rate spring configuration will have a Primary Rate that is a combination of the Main Spring and Tender Spring. When the Tender Spring coil binds or hits the stopper, the Secondary rate will take effect.

To figure spring rate, we must know the vehicles WHEEL TRAVEL, SPRUNG WEIGHT, and DROOP PERCENTAGE. We need to know the suspension configuration as well. (A-Arm, I-Beam, Four Link rear, etc.) See Figures 1-3 below.

p-figure1p-figure2 p-figure3






You will need to measure the distance from the suspension pivot, to the lower shock mount, and the distance from the suspension pivot to the end of the arm, or tire centerline. If your shock will be mounted at an angle, you will need to measure that angle as well. For our calculator, 0 degrees is straight up and down. The further away from vertical, the higher the angle number..10 degrees, 15 degrees, etc.

The SPRUNG WEIGHT is the weight that each shock is supporting (i.e., the corner weight less unsprung weight). Unsprung weight is the tire and wheel, hub, upright, brakes, and about half the weight of the shock and A-Arm or I-Beam. For the rear suspension of a truck, the unsprung weight is the tires and wheels, the entire rearend and half the weight of the shock and trailing arm.

The DROOP PERCENTAGE is the desired ride height as a percentage of total wheel travel. (i.e if you have 10″ of wheel travel, and you want the vehicle to sit right in the middle of the wheel travel at ride height, that would be 50% droop. If you wanted it to sit lower..say 7″ into the wheel travel, that would be 70% droop.) Most desert cars get around 40% droop, with rockcrawlers getting up to 70% droop.

Below is the information needed to properly calculate Spring Rates.

________ inches 1. Wheel Travel
________ pounds 2. Sprung weight of each applicable vehicle corner
________ inches 3. Pivot to lower shock mount distance (D1, D3, or D5)
________ inches 4. Pivot to end of suspension arm or tire centerline (D2, D4, or D6)
________ percent 5. Desired droop travel
________ degrees 6. Shock angle from vertical (in degrees)


Enter the above info into the F-O-A Suspension Calculator to get your recommended spring rates.


Source [CRAWLpedia] Off-Road Encyclopedia www.CRAWLpedia.com.