I had a lengthy argument about the longevity of springs. The issue was whether they wear out.
Assume that the coil spring is 20 years old and visually appears fine - no cracks. Also, no noticeable problem.
Any metallurgical discussion would be welcomed.
Thanks in advance.

 

A few years ago I changed 25 year old springs on a Jaguar XJ6... the new springs raised the ride height up by almost an inch. Based on that, and nothing more scientific, I'd say that springs relax to a certain extent over time.

But, I'd also hazard a guess that spring decline is a logarithmic function - it relaxes most early on in its life, perhaps the first few years, but not so much between 20 and 30 years old.

 

If I remember right, springs never lose rate, but will start to collapse. But, there are lots of race teams that consider them maintence items.

 

I would think that a soft oem spring driven HARD will degrade faster and moreso than a hard aftermarket spring driven occasionally on track.

 

Bed springs wear out.

 

That was really helpful. I'll have to remember that.....

 

Think about it. Both are spring steel. Both are made pretty much the same way (rolls of wire bent in a machine). Both hold up a pretty much constant weight over time. It's actually a pretty good comparison and one the guy who runs H&R gave us one time in a presentation here.

Springs definitely can go bad. Look at about any Ford Taurus on the road. They are all dragging the back ends around. It's a known problem. I have seen OEM springs, and those from both H&R and Eibach lose spring rate over time. It's not nearly as pevelant in the good aftermarket parts as the cheaper parts, though.
I'm not sure if all this helps you here, but yes, they can go bad.

 

I understand they can go "bad" as evidenced by a difference in ride height. But if the ride height has not changed, can they still go bad. In other words, are you saying that spring rate will change without a detectable difference in ride height.

 

Yes. Ride height is observed in an unloaded state, the spring is being asked to do the least amount of work possible. Load up a car in the corner and whatever wear the spring has experienced now becomes more pronounced. What appears to working in the garage may not be working as intended on track.

 

Springs do wear out and need replacement. As for when, that is up to you. Think about a wind up spring on an overhead garage door. When it is new the garage door's weight is offset by the spring. You can raise the door to the half way point and it will stay there or to the 3/4 point and the door will stay there. Now one year later most of the weight of the door will be lifted but the garage door will not stay half open. You must wind the spring a few turns to gain the old balance.

 

I spoke with my boss (who is a professor of metallurgy) and she told me that the issue with springs, especially in heat environments, is that eventually the durometer (hardness) of the metal begins to degrade over time. She couldn't describe the rate of decay as a function without more information, but she did say that performance would probably degrade first, followed by static compression/height.

 

John:
Do you still have the original 964 Cup springs in your car? I would think that Dan Jacobs, Spencer ***, or one of the other Porsche shops up there have a coil spring tester, or access to one. Pull your shocks, remove the springs and have them tested.

I have been told over the years that coil springs do lose some of there rate, but nut sure what the percentage is.

 

Don't even start thinking about your valve springs...that will keep you up at night...

 

I threw out a poorly framed question. What I really wanted to know was about a qualilty car spring such as an OEM Porsche spring or an older
Cup spring such as mine (they have not been replaced). Last year, I was told that springs do not, as a practical matter, wear out. They wil break first. Theoretically, I guess, the spring rate can change. And yes, if you have a Taurus or the like, it can get droopy. The posts here seem to confirm that. But I know from studying metallurgy for over two years that not all metal is the same. (Unfortunately spring rate was not what I focused on.) Its properties are highly dependent on its method of manufacture. And I have no idea how the metal in car springs (mine specifically) is manufactured.

The short answer, as you point out, is to have the springs tested. But I was looking for a scientific explanation of what occurs over time to a qualitiy car spring that does not appear to have undergone any noticeable change.

 

All metals have an endurance limit. Typically you would design for a life of 10^7 cycles. There is no "infinite design life."