They are always considered a non-wear item. I beg to differ - metal that is constantly bent will eventually fatigue and they can wear out and sag after time and many miles. Shocks and springs are designed to work together, so unless you ensure your shocks/dampers are in proper working order, you're putting the springs under additional load from uncontrolled extension & compression. I've replaced factory coils on a few cars that were badly stressed. The difference between the stock and OEM replacement coils was very evident.

In your case, you're probably fine. Even if you've been bouncing around on bad shocks for awhile, they should be good for at least 150-200k.

 

- metal that is constantly bent will eventually fatigue and they can wear out and sag after time and many miles.

This is not true. Some metals, steels in particular, have a fatigue limit, meaning there is some load level below which failure will not occur no matter how many loading cycles it is subjected to. Creep, or sag, is also something that can be designed out based on material choice. I believe with most modern metals creep is not an issue. Plastics, however, are very prone to creep. That being said your springs should last the life of the vehicle.

 

Educate me then - what causes coils to wear out, bow, and sag then? Fact of the matter is that spring steel can become fatigued over time. Coil springs are designed for a certain travel distance - extensive over-compression of a coil spring can cause fatigue and creep.

 

Fact of the matter is that spring steel can become fatigued over time. Coil springs are designed for a certain travel distance - extensive over-compression of a coil spring can cause fatigue and creep.

Fatigue is not something that is guaranteed to occur, nor is creep. It is something that can be designed for through proper material selection and material usage. Things like corrosion and the inherent variability in material properties have an affect the fatigue life of materials. There are likely other reasons that would affect the fatigue characteristics of coil springs, but these are the two that stick out the most in my mind.

Extension of a coil spring will not produce a stress magnitude that is significantly different than that from compression. They will be opposite in sign, but the limits of the material will not be different in compression vs tension. You can pick up any elementary mechanics of materials book and see that this is the case.

 

OK ... so how does one determine if the issue is new springs or shocks/struts is needed? I have SVT springs that seems to have sagged. SVT Shocks/Struts seem to be ok and the push down test didn't get a springy feeling.

So what is the magical method to definitely make sure? Or just replace as needed based on the type of road you use?