I'm looking at coil spring rate info and can't find on Google information about whether a spring's torsional modulus can change over time.
All the pages I found say that torsional modulus is a constant -- reflecting a physical property -- and a couple of websites note that while springs can sag, their rate does not change. (I realize that temperature can affect this property but it seems irrelevant in the context of car springs.)
So is it possible for the spring's metal -- um, springiness -- to change with age?
Thx..
It can and does, over normall useage the rate of change is so slow that it shouldn't be a problem.
The key here is to note the term "over normall useage". If the spring is being deformed to the point where it starts undergoing plastic deformation, then fatigue will quickly set in and the spring rates go to hell.
Torsional modulus is a constant. Spring rates are a constant. Spring free height/deflection angle (for torsion bars) can change due to constant high loads ("creep") or poor control of the metallurgy during manufacturing.
A good example of free height deflection (sag) is with 1960's GM barges with high weights and low spring rates, ("Good ride" in those days). Those cars frequently sagged the coil springs (over several years of usage) due to the weight and unconcern about the spring quality.
Plastic deformation of the spring material does not change the modulus or the rate. Plastic deformation changes the free height and puts the spring at risk for fracture or fatigue fracture.
Temperature does not affect modulus or rate at operating temps for a vehicle.
Rog
When I said spring rates will go to hell, I was refering to cracks forming due to fatigue. This would occur right before a catastrophic failure.
Plastic deformation can, and often times does, change the torsional modulus and the resulting spring rate. A good example of this that you can check yourself is a paperclip. Flex one without deforming it. Now bend it (deforming it), and reflex it. The springiness will feel softer and weaker. It will also re-deform more readily. Plastic deform it a few more times and it becomes extremely weak and floppy, with no springiness. Then it snaps.
Does that mean coil springs do the exact same thing? No. Coil springs do not have enough motion to get into plastic deformation. The range of motion and deflection is well within the non-plastic deformation range. Exception would be cheap and poorly heat treated aftermarket springs. Those can plastic deform under the weight of the car sitting in the driveway.
