Choosing the right spring to give your steed that look and performance you want.

Wheel Rate

Wheel Rate is the amount of force the wheel is exerting on the road. As you can see the wheel rate is a function of Spring Rate and the Square of the Motion Ratio. On the mustangs Modified McPherson Strut design the motion ratio is 0.5. When converting to coil overs and making it a true McPherson strut the motion ratio is closer to 0.9. Wheel Rate is expressed in lbs/in. Meaning if the wheel rate were 400lb/in it would take 400lbs to compress 1 inch.

Motion ratio is important because it determines how effective your springs are and how well your suspension is designed. By running a few numbers you can quickly see how different spring rates affect the Modified McPherson and true McPherson design. With coilovers you can run a much less stiff spring and achieve much higher wheel rates.

WR = Wheel Rate
SR = Spring Rate

Spring Rates

Spring Rate is the rate of which a spring will compress and usually denoted in lbs/in (pounds per inch). That is to say it takes x amount of force to compress the spring 1 inch. Say you apply 100lbs of pressure to the spring and it compresses 1 inch, then the spring rate would be 100lb/in. A linear spring will continue in this fashion, you apply 200lbs and it should compress 2 inches. If it doesn't then you have a progressive rate spring. A progressive spring rate is where the force required to compress the spring changes as the spring is compressed. Below you can tell which springs are progressive and which are linear. Progressive rate springs will be denoted by having a range such as 500-560lb/in. A fixed rate spring will only have one number, such as 650lb/in.

K = Spring Constant (Spring Rate)
W = Spring Wire diameter (in)
G = 12,000,000 (constant for steel)
N = Active coil count + .5
D = Diameter of the spring as measured from the center of the wire

Stock Spring Rates

Aftermarket Spring Rates

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