"A compression spring cannot be closed and ground so consistently that its ends will always be square (in parallel planes at right angles to its axis).  In addition, the helix angles adjacent to the end coils will not have uniform configuration and closing tension, and these springs cannot be coiled so accurately as to permit all coils to close out simultaneously under load.  As a result of these end coil effects, the spring rate tends to lag over the initial 20 percent of the deflection range, often being considerably less than calculated.  As the ends seat during the first stage of deflection, the spring rate rises to the calculated value.  In contrast, the spring rate for the final 20 percent of the deflection range tends to increase as coils progressively close out.

"The spring rate over the central 60 percent of the deflection range is essentially linear.  If possible, critical loads and rates should be specified within this range.  Which can be increased to about 80 percent of total deflection by special production techniques.  However, these techniques add substantially to manufacturing cost and are usually unwarranted."