Coil spring “settling”

So I'm well familiar with the concept that new coil springs take a while to "take a set" or settle to their final height. 
 

I'm about to pop some different springs in the front of my truck and they're typically a bit annoying to install until they've been used a while and settled down some. So I figured I'd do a little science experiment to try and set the springs prior to installing them the first time. 
 

So, is it time that they're compressed that settles a spring, or is it cycles that settles a spring ? Should I let them sit under load for a week or would I be better served to squeeze and release them repeatedly ? 
 

This is all a strictly academic discussion. I figured I'd put each one under a load as shown in the picture and let it sit for a few days. 
 

I think any apparent short-term settling has more to do with changes in hardware around the spring than to the spring itself.  Springs can sag over time but it typically takes many years or even multiple decades.

I don't know whether time under load or cycling causes it, it could be one or the other or some of both. Some quick searching turns up theories involving fatigue from cycling, flattening from overload conditions, material creep from time under load, or even a loss of tension due to age alone.

Usually time will settle them, as well as driving.  The springs on the front of my S10 (still working on so not driveable yet) have settled down a good bit just in a few weeks.  

I've always been bothered by this idea because I'm the sort of dingbat who wants stuff to behave like the physics books, and springs shouldn't change shape without enough force for plastic deformation...

But if they do, there must be an explanation. Apart from it being surrounding hardware and other compliance, could it be stress relief from the forming process? What do we know about stress relief?

Could it be that use pushes the parts of the spring which have the most residual stress into plastic deformation such that they are more relaxed back at neutral afterward? I would think it would only take a deep compression cycle to make this happen. On the vehicle, this would be more like a big load, a big bump at speed, or something else that cycled it as far as it would go in normal use as opposed to time.

I don't know the answer, just trying to reconcile theory and practice.

I'm interested in the results! But, also, that compressed spring on the right looks like a time bomb set to launch your arbor plate at you at the most inopportune moment.

I hope you wore safety squints while compressing that spring on that press! Holy smokes!

If you feel like living dangerously, one trip to max load is the "right" way to do it. I'm not a materials expert but I wouldn't think that low load cycling or holding would do much of anything. The point is to introduce a stress level at least close to peak working load.

I don't believe they settle. More likely it's bushings taking a new set if they haven't been reset properly. Good metallurgy should not change. 

If it permanently changes shape, it has yielded. Springs are normally "set" at the factory to give a uniform pre-stress.  If that step isn't done, then the setting will happen unpredictably in the field.

I recommend giving unevenly set springs the official US Army wilderness survival test: put them in a paper bag, carry them out to the woods, and welcome them back if they make their way home. 

In reply to Keith Tanner :

In RX-7land, it's accepted that springs settle, by a lot.  When the cars were 20-25 years old, people were complaining often that their lowering springs made their car sit a lot higher.  Yes, because your car was 2" below its intended ride height due to settling, so your 1" lowering springs make it sit higher.

There's no loaded bushings in those cars aside from the front strut tops.  It's all spring aging.

Now, with old American iron where the springs sat somewhat loosely in cups in the frame and in the control arms, a lot of the settling very much was the springs wiggling around until they found their forever home.  That takes only a couple hundred miles or so.  After ten-fifteen years, the springs will sag.

It's never been adequately explained to me if this aging is due to loss of spring rate due to micro-cracking, or due to loss of load due to plastic deformation.  I've seen both arguments.

confuZion3 said:

I hope you wore safety squints while compressing that spring on that press! Holy smokes!

No kidding! I backed away from my monitor by instinct.

In reply to Pete. (l33t FS) :

I was referring to new springs, not ones that are 30 years old and gradually losing their temper. Your experience is why we don't quote a lowering amount, we quote a final ride height.

No bushings? I find that hard to believe. What do the control arms pivot on? Probably rubber bushings, and those do have a significant spring rate. If they are torqued with the wheels in droop, they will be trying to lift the car back to that same position and will accomplish a certain amount of lift on the car. When I changed out my rubber bushings for spherical bearings on the track Miata, I had to make a significant change to my shock valving due to the drop in effective spring rate. 

The first question we ask when someone calls and complains that their car is "sitting high" on our springs is if they reset the bushings. The answer is almost always no*. The NA/NB show this characteristic to some extent, the NC and ND even more so. 

* except for that one guy who, after a bunch of troubleshooting, asked "does it matter if the engine isn't in the car?"

According to every materials science textbook I read (so, one) springs wear out via fatiguimg, not just through loading

I would bet that most people see settling with new springs because they are also adding/replacing suspension components and seeing the newly torqued bolts and the fresh bushings settle into their new positions.  AKA nothing to do with the spring side of things

In reply to Mr_Asa :

What's meant by "wear out" in that context? I'd have leaned toward thinking of eventual fatigue failure in the form of cracking/breaking, not changing shape. But I am not a materials scientist.

In reply to Jesse Ransom :

Eventually they'd lose spring rate, then depending on how they are made they'd crack.

Leaf springs, having no support would crack, coils would just settle against each other, or potentially buckle.

In reply to Keith Tanner :

Ah, I see what you mean.  I had been thinking of setups (like Miatas) where the spring loads against the control arms, and shock bushings, so bushing creep will also make the car sit lower.

To directly answer your question, unless you really get crazy with how far off you are from ride height, it doesn't seem to matter that much.  At least, cars that have never been apart before will sit lower and lower as they age.  You'd think that the bushings would actually help the car sit up where it's supposed to be 

I also suspect that Mazda used the same springs for a 2200lb SA and a 2600lb fully loaded FB, and the lowering springs were engineered in 1979...

In reply to Pete. (l33t FS) :

The spring rate of the bushings won't stop the car from settling if the springs are sagging. But as the bushings themselves fail, they may actually stop helping hold the car up and thus contribute to the settling. 

Their contribution all depends on the suspension design. Like I said, the multi-link cars have more influence from the bushings. SA/FBs may not get a lot of lift from bushings, but I'll bet it's not zero.

Do the RX-7s have different markings on their springs? Miatas do, there were very definitely changes over the years. The 1996 cars sit about 2" higher than the 1995s, and I'm pretty sure automatic cars got distinct springs.  

In reply to Keith Tanner :

The Books say that right hand drive cars had left and right side springs.

As far as I could ever see, North American cars all had the same springs on both sides, and they were all the same springs.  This, and an unfortunately ill-adjusted body welding rig at the factory for the Series 3s, means that North American cars, especially the -84-85 cars, sit a lot lower on the left.  In the front, the left side has the battery, the steering box, the starter, the master cylinders, the A/C compressor, the power steering pump if equipped (and its monstrously heavy bracket) AND the engine and radiator are mounted in the chassis cocked over to the left. So the left front has a lot more corner weight than the right.

In the back, you have the driver and the fuel tank on the left, and they welded the car together crooked starting for the MY84 update.

It's one of those things that you can't unsee once you know that it's there.

IIRC the only suspension differences are that in '81 the rear swaybar went to 15mm from 18mm, and the GSL-SE got stiffer dampers to counter the gummy as-installed tires (.9g of cornering in late 83!)  I am sure I could find the Fiche to find spring part numbers...

In the MG world, having the driver side lower than the passenger is called the "bachelor lean". That was probably metallurgy :)