Spring Rates - Suspension 102

Spring Rate, What is it?

Spring rate is expressed as the amount of force required to compress a spring a set distance. The measurement is expressed in metric (kg/mm) or in standard (lbs/in).

What does it mean when they say 8K or 500lb Spring Rate?

When someone says “8K” they are shortening the metric measurement of the spring rate; in this case 8 kg/mm. What this means is that if you apply 8 kilograms of weight to the top of the spring it will compress 1mm. If you apply another 8 kilograms (16kg total) of weight it will compress another 1mm.

The other spring is being measured in pounds per inch; in this case 500 lbs / inch. If you apply 500 lbs of weight to the top of the spring it will compress 1 inch. Now if you apply another 500 lbs of weight it will compress another 1 inch so (1000 lbs will = 2″ of compression).

How can I Convert Spring Rates?

for a metric spring rate (kg/mm) to a standard spring rate (lbs / in) use the formula

XX kg/mm x 56 = ( Spring rate in lbs/in )

Example  8kg/mm x 56= 448lb Spring


For a standard spring rate (lbs / in) to metric spring rate (kg/mm) use the formula

XXX lbs/in ÷ 56 = ( spring rate in KG/mm

Example  500lb/in ÷ 56  = 8.9K Spring Rate

So for a 8 kg/mm spring you would have a 448lb spring, and a 500lb spring is equal to a 8.9 kg/mm spring.

What is the Difference Between a Linear Spring and a Progressive Spring?

Linear Springs

Linear springs have the same spring rate through out their travel range. The spring force increases linearly until it’s limit. This makes for a very predictable and steady handling experience at the cost of some ride harshness.

Progressive Springs

Progressive springs can come in many types. They can be multiple rate progressive springs, or they can be a fairly standard progressive profiles. The profile of the spring from top to bottom starts out narrow and gains width as it reaches the base of the spring. This has the effect of steadily increasing the spring rate for every inch of travel. This makes for a car that seems timid at low speeds but very rigid during strong cornering loads. While this seems to be an ideal situation for a street car the actual result is a car that seems to be somewhat unpredictable, especially during those last few inches of compression length.

Higher isn’t Always Better

There is a point where you can have too high of a spring rate. When you have too much spring rate the car will be harsh riding and will begin to understeer / oversteer sooner. It might feel very responsive and feel like it “handles” better, but ultimately you’ve made the car less capable. Bear that in mind before you run out and slap some high rate springs on your car.