When the original springs are no longer able to maintain proper ride height or are not capable of handling vehicle load requirements, new springs are in order.
All springs sag with age. The constant load they bear leads to creep within the molecular structure of the metal. Over time, the spring weakens and begins to lose height (coil spring) or arch (leaf spring).
As the spring settles, ride height decreases and causes undesirable changes in wheel alignment. That is why alignment technicians are always supposed to measure ride height before they realign the wheels. More than an inch of sag usually means time for new springs.
Spring sag also reduces suspension ability to support its normal load. This, combined with decreased ride height, means less clearance between suspension stops and chassis. The result can be bottoming on rough roads or when carrying extra weight.
Spring sag can also have a negative effect on safe handling and braking. A wheel's ability to maintain traction during acceleration, braking and cornering depends to a large extent on the load it carries.
A weak spring does not carry its fair share of the load, so the wheel with the weakest spring is most likely to break traction and spin or skid during hard acceleration, cornering or braking. Weak springs also allow more body roll, which puts added strain on the shocks, struts and other suspension components.
Weak springs need to be replaced and when they are, both springs on the same axle should be replaced (both fronts or both rears). This maintains the same side-to-side ride height.
Trying to "shim up" a weak spring with inserts or a spacer is not the professional way to address the underlying problem. Shims and spacers can be used to restore ride height, but they cannot duplicate the original ride.
Such products usually make a spring more rigid (stiffer spring rate) and decrease the amount of suspension travel the spring can handle because of less space between the coils. Nor can shims or spacers provide many of the benefits that are available with various types of specialty replacements like variable rate springs (those that become increasingly stiff as the load is increased).