Because I'm sure there are people of many different levels of technical knowledge here, I'm going to point out a couple of things that people sometimes understandably misunderstand.
About 99% of the "catalog springs" (or "catalogue springs" for furrners) listed for race car coil-over shocks are linear rate.
What does that mean?
A 200 lb spring will compress 1" if you put a 200 lb load on it. At that point it is still a 200 lb spring, i.e. it still has a 200 lb per inch rate.
A 200 lb spring will compress 2" if you put a 400 lb load on it. At that point it is still a 200 lb spring, i.e. it still has a 200 lb per inch rate. It has not become a "400 lb" spring.
A 200 lb spring will compress 3" if you put a 600 lb load on it. At that point it is still a 200 lb spring, i.e. it still has a 200 lb per inch rate. It has not become a "600 lb" spring.
It will be a 200 lb spring right up to the point it's not a spring... its coil bind or solid height.
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Misunderstanding #1: A 200 lb will not compress until there is a 200 lb load on it. Wrong. If you put a 100 lb load on it, it will compress a half inch.
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Misunderstanding #2: If you stack a 170 lb spring on top of a 200 lb spring, the 200 lb spring will not compress at all until the 170 lb spring is coil bound. Wrong. The 200 lb spring is supporting the 170 lb spring, so it has load on it. Since it has load on it, it is therefore compressed by some amount (see Misunderstanding #1 above).
In fact, since they are in series, they both are seeing the SAME load.
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Misunderstanding #3: Stacking two springs of different rates will create a progressive rate spring. Wrong. It creates a linear rate spring softer than either of the two springs involved. Consider #1 and #2 above, plus the fact that, all else being equal (coil diameter, coil spacing, and wire size), a longer spring is a softer spring... to understand this.
So what's the new linear rate?
The following is verbatim from Hyperco, right up the road from me... whose "Hypercoils have been utilized by every Indy 500 winner since 1965"
Combined Spring Rate
This is used when two springs are stacked on top of each other.
Combined Spring Rate =
(Spring Rate 'A' x Spring rate 'B')
÷ (Spring Rate 'A' + Spring Rate 'B')
For example if the rate for spring 'A' is 200 and the rate for spring 'B' is 500, the combined rate is calculated as follows:
Combined Spring Rate =
(200x500) ÷ (200+500) = 143Plugging Cheapracer's 200 and 170 lb springs into this yields a linear spring rate of 92 lbs per inch...
UNTIL one of the springs is coil bound.
How do you know when/where that happens to which? It's not as hard as you might imagine when you keep in mind that both springs at all times have the SAME load applied to them. If there is 100 lb load, the 200 lb spring will be compressed (100/200) a half inch and the 170 lb spring will be compressed (100/170) 0.588" ... the shock travel is 1.088". Cross check that against a 92 lb spring... (100/92) = 1.087" (slight error, 'cause the 92 was a rounded figure). The free height and solid height are usually known from the catalog... subtract solid from free and you know available deflection, so you can easily figure out what load it takes to bind each spring (I'm too tired to decide if it will always be the softer first, I think spring dimensions come into that too).
Here's the rub. Cheapracer implied, I think, that there would be a 30 lb jump in spring rate (or none?) when the 170 lb spring goes solid. Wrong (please don't piss on me!). It jumps from 92 lbs to 200 lbs suddenly... not gradually. That's probably not good.