Question about spring rate versus spring age

[BrakeL8r]

Hi everyone -- hope everyone had a good Thanksgiving.

Does anyone know how a spring's rate is affected (if at all) by its age/use? I'm moving some newish aftermarket shocks from a '91 Miata over to a '97 Miata. The springs on both cars are original equipment. Someone had made a comment earlier when we put the new shocks on the '91 that the old springs would have the advantage of giving a slight lowering affect, since springs naturally sag as they get age. Does this imply that their spring rate decreases?

Thanks in advance,

Michael
ST4 Miata #176

[Dave_G]

Spring rate is determined by four things:
* the diameter of the spring wire
* the diameter of the spring coils
* the number of coils
* the modulus of rigidity of the spring material

The only one of these that might conceivably change over time is the modulus of rigidity, though I've never heard of this happening. Most people I've heard weigh in on this claim that coil springs do NOT sag over time.

The later NA Miatas ('96 and '97), however, did have taller springs than the '90-95 models. Your '97 will have a naturally taller stock ride height from the factory than your '91.

[edgetoo]

All springs sage with time. Depending on the material and design, some
much more so than others. (they also sometimes brake due to flaws,
fatigue, and/or poor design.) As springs sag, the spring rate changes.
(Lower) Measuring and logging spring rate, and free (unloaded) length is
useful in tracking changes. (If your class requires components to be as
delivered stock, you can use these parameters to select the most
advantageous stock parts in the parts bin.) Ones' local prep shop will use
this data in setting ride height and cross weighting. This together
with "track" alignment can improve grip and better handling.

Ed Givler

[kfoote]

Spring rate is determined by four things:
* the diameter of the spring wire
* the diameter of the spring coils
* the number of coils
* the modulus of rigidity of the spring material

The only one of these that might conceivably change over time is the modulus of rigidity, though I've never heard of this happening. Most people I've heard weigh in on this claim that coil springs do NOT sag over time.

The later NA Miatas ('96 and '97), however, did have taller springs than the '90-95 models. Your '97 will have a naturally taller stock ride height from the factory than your '91.

*Free length is also a factor

In most cars with soft springs (like anything you would find in a street car) the modulous of rigidity can decrease over time due to fatigue, corrosion, or other ecternal forces. This will effectively decrease the spring rate, however, simultaneously, the free length will also decrease (what is commonly referred to as "sag"), thus making the spring stiffer, and this. The net result is unless there is a severe loss of the modulous of rigidity, though the car may end up lower, the effective spring rate will remain about the same. The effective spring rate increase due to sag usually outpowers the decrease in modulous of rigidity for a given change in ride height in today's street cars very quickly due to the progressive rate designs of virtually all current street cars, so the net result of spring sagging is usally a spring rate increase, not a decrease.

That having been said, the differences in spring rate that we are talking about here are generally far too small to be noticeable.