Spring spec question - Rear Shocks

It's a tough tolerance for the vendors to hold even on new springs -- i.e., the tolerance is so large that auto manufactuers typically subsort them when new into matched pairs. Add in many years of age/use, and nothing unusal about what you've measured IMO.

 

I am no expert but here's my general understanding.

Spring rate is determined by the thickness of the wire when a coil is wound, so a 250 lb spring is thicker than a 200 lb spring.

Some springs may "settle" over their life, reducing the effective ride height. Literature, such as the specs for the 575, for example, identifies a 5mm drop in ride height as being within spec.

Spring rate needs to be measured over a range of movement. The first inch of movement can require less force than the second inch. So, if our spring is rated at 200lb, it may take 180lb to crush the spring by an inch, and 380 (180 + 200) to crush the spring by 2 inches and 580 to crush the spring by 3 inches. This will continue until coil bind.

Given this, two observations:

It is clear your stock springs have taken reduced set by the (0.55 inch/13mm) change in the free length. How has this changed the ride height? A 13 mm reduction in free length may be half this under static load and correcting for the motion ratio may put you close to tolerance for the stock spec.

Second, how did you measures the spring rate? First inch, second inch, third inch? Average over the range?

Last, there is some quite good material online describing spring rates and measurement of spring rates. One, and I'm sorry, I don't recall the reference, was written by a speed shop in Indianapolis which described the measurement quite well. I am sure you can google it and it will turn up in the first page of results.

Beyond that, I have no specific knowledge of the C4 to advise you. If you do decide to go replace the springs, I recently sourced HGTC equivalent springs for my 575. An off the shelf unit worked for the front, rears were custom wound. Prices very reasonable ($50/each F, rears $120/each I think).

Philip

 

True only if assuming that they are comprised of the same material and temper --- material composition and temper (both of which determine the stiffness) are critical performance elements in coil spring design. Also, coil packing (relative density) greatly influences overall spring rates.

To the OP ----

Unless your car has very low mileage, I would regard the changes you've measured (assuming accurate measurements) to be well within "normal wear / usage limits". Providing that you are not noticing any substantial degradation of the car's handling performance or any bottoming out / travel limit of the suspension, I would not bother to replace those springs.

 

David here what I was drawing upon:
From Suspension Spring, makers of Blue Coil Springs:
"Some thoughts on coil spring.... There has been a theory that spring rates change (pounds per inch of compression). A spring rate is derived from two things. First, the size of the material used, and second, the total number of inches of material used. If none of these things change, your spring rate will remain the same".

As I said, I am no expert, Phlip

 

Philip ---

I was merely elaborating a bit on what you had posted.

No offense or criticism intended --- your comments are true, relevant, and valuable information. In particular your points about making sure the measurements (of spring force v. deflection) are done correctly and over the correct range of displacement are most important.

Please allow me to add a finer point on the issue. Essentially, coil springs that are designed to be linear (some are designed to behave highly non-linear) do follow, in theory, Hooke's Law [ F = -kx ]. This means of course, that the more you deflect (compress, for instance) the spring, the more resistive force it will exert (i.e., the force required to compress it for the first inch will be less than the force required to compress it for the next inch, etc.). In other words, progressive displacement is accomplished only by progressively increasing the applied force.

Generally speaking, specs for measuring a coil spring's rate are set to apply over the range of [ initial position = free length (to) final position = some specific target displacement amount /// or alternatively, final rest position attained under a fixed static load applied ]. The idea being that this is not only easiest to measure, but this is also the range where a typical coil spring will behave in the most consistent and repeatable manner.

Although an important detail often overlooked is that Hooke's Law is strictly true only for an "ideal" spring, in which, the spring rate "k" is a true constant. However, in reality, no coil spring is perfectly ideal, and the k value is not actually an absolute constant --- it will vary somewhat over the entire range of the spring's deflection. Sometimes this variance is trivial, sometimes it can be quite significant, which really underscores the importance of measuring any spring's rate over a wide range of its deflection, and deriving from that, a sort of "average rate" in order to best characterize the spring --- this is what you were alluding to with your comments, I believe.

Back to the OP's original concern.....

Ferrari's made in the early days, say before the mid-80's were true works of art --- but not necessarily --- true works of science / engineering. In some respects, there were manufacturing inconsistencies and variances in many aspects of each car's overall build and its components quality / performance. Therefore, I have never put too much credence in certain "specs" --- especially with regard to some individual components. For something as variable and difficult to control (especially back then) as the consistency of a suspension coil spring.........well, it is easy to imagine there was likely a lot of variance from car to car even when they left the factory. So, it would be difficult to say with a high level of certainty to what degree your particular springs have sagged / fatigued --- if it all --- just by comparing them to Ferrari's general spec for all C4 rear springs .

For me, I would be more concerned about how closely the springs match each other now in free length and stiffness, rather than how much they may appear to deviate from the spec (i.e., guideline ).

 

This is the only parameter that the auto manufacturers use to match up pairs in assembly -- compressed length when at the nominal working load (because it affects the coachwork leveling -- if the actual spring rate at that point is slightly different for the two springs that's not a big concern). Since this is usually a fairly large deflection from the free state (and a big load), its not an easy thing to DIY measure.

 

David and Steve, thanks for the expansion. No harm, no foul. I did not mean to come across as defensive and appreciate the education -- we are all victims of what we've learned and it helps to understand both the first principles and the practical applications.

Philip