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Coil Spring Rates

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Spring Rate Calculation

The actual rate of a coil spring under compression can be calculated from the following measurements:

Measurements

WD - Wire Diameter

The outside diameter of the wire used in the spring

NC - Number of Coils

The number of free coils in the length of the spring

CD - Coil Diameter

The mean diameter of the coils within the spring

Calculation

SR - Spring Rate

= TM x WD⁴ / 8 x NC x CD³
(Where TM is torsional modulus of spring material)

The form below allows the spring rate to be calculated for a coil spring manufactured in steel (torsional modulus = 11,250,000).

Wire Diameter	inches
Coil Diameter	inches
Number of Coils	(free)
	lb/ft

This figure relates to the spring rate of the coil itself, based on the number of active coils being free and not coil-bound. In many cases, the effective spring rate is quite different to that shown in this calculation due to the suspension geometry in use.

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Note that measurements used here are in Imperial terms (inches, lb/ft), not metric measures.
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