Extension Spring Dimensions

Notes refer to diagram at bottom of page.

Per Spring Manufacturers Institute Handbook of Spring Design 

d          wire diameter

I.D.      Inside Diameter

O.D.    Outside Diameter

D.        Mean coil diameter = (O.D. + I.D.)/2 or O.D. - d

Lf            Free Length = The distance between the inner surfaces of the ends.  It is equal to the spring body length plus ends, where spring body length is given by Lbody = d(N+1).  N = Number of Coils.