Registered: 1st May 01
Location: Hurstbourne Tarrant
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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!
1. Wire diameter.
This affects rate since greater diameter wire is stronger than lesser diameter wire. So, when wire diameter is increased, spring rate increases
2. 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
3. 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. As the number of active coils increases, the spring rate decreases.
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 500#/inch will compress 1" when a 500# weight is placed onto the spring. If another 500 pound weight is put onto the spring the spring will compress another inch. At this point the load on the spring has increased to 1000 pounds. The rate of the spring, however, remains constant at 500#/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 springs' 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.