What is Spring Rate?
How Springs Work Under Load
There are basically three different spring designs presently used in cars:
These springs are used in different situations and with differing results. Since the designs are so varied, it only follows that the dynamics of each design are also varied. You must remember, though, that the three factors listed below are the only factors that affect spring rate — wire diameter, mean diameter and number of active coils.
- Closed and ground on both ends. Coil-overs and rear conventional springs are this type.
- Closed on both ends, ground on one end. Conventional front springs are normally this type.
- Closed and ground on one end, open on one end. This is similar to a cut-off conventional spring.
WHAT IS SPRING RATE?
Spring rate refers to the amount of weight needed to compress a spring an inch (Example: 500# per inch) To understand and properly check a spring for rate you need to know the factors that determine the rate of the spring. Fortunately, there are only three things that affect spring rate, so there's not that much to remember!
- Wire diameter. This affects rate since greater diameter wire is stronger than lesser diameter wire. So, when wire diameter is increased, spring rate increases.
- Mean diameter of spring. Mean diameter is the overall outside diameter of the spring less one wire diameter. When mean diameter increases, the spring rate decreases.
- Active coils. Determination of the number of active coils varies according to spring design. Count the total coils minus two for springs with both ends closed (includes all AFCOILS). Count the total coils minus one for springs with one end closed and one end open. As the number of active coils increases, the spring rate decreases.
As a coil spring compresses, the inactive (dead) end coils gradually contact adjacent, active coils. The contact causes the active coils to deaden, which increases the rate of the spring. The rate creep that results usually stops after the first inch of spring travel and does not appear again until spring travel approaches coil bind. Generally speaking, this type of rate creep is of little consequence with springs softer than approximately 500#/inch. When you use springs stiffer than 500#/inch, rate creep becomes more pronounced.
Example: A racer replaced a 720# coil-over spring with a 750# Afcoil. The racer believed he had stiffened his right front spring; however, the chassis behaved as though he had gone to a softer spring. Upon rating both springs, he found that the 720# spring was rated for its first inch of travel (720#/in.) and produced a much higher (780#/in.) rate for its second inch where it actually operated on the race car. Because Afcoils are designed to give their nominal rate closest to their actual working range of travel (this particular 750 spring rated 735#/in. for its first inch of travel and 755#/in. for its second inch), this racer actually softened up his race car even though the spring rate markings indicated the opposite!
HOW SPRINGS WORK UNDER LOAD
If a spring’s rate is linear (most racing springs have linear rates), its rate is not affected by the load put onto the spring. For example, a linear rate spring rated at 200#/inch will compress 1" when a 200# weight is placed onto the spring. If another 200# weight is put onto the spring, the spring will compress another inch. At this point, the load on the spring has increased to 400#. The rate of the spring, however, remains constant at 200#/inch.
If the load put onto a spring increases the rate of the spring, the spring is said to have a progressive rate. Progressive rate springs are sometimes used on torque arms to absorb engine torque. Keep in mind that the load (or preload) put onto a progressive rate spring can greatly increase the rate of the spring.
Typically, progressive rate springs are made by varying the spacing between the spring's active coils. During compression, the close coils bottom out and deaden. This reduces the amount of active coils and spring rate increases as a result.
Springs that are designed to include coils of different diameter or are wound using a tapered wire will also produce a progressive rate.
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