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Too many variables. The final ride height and rate of a coil spring are affected by the wire diameter, the coil diameter, the number of turns (coils) in the spring, and the free length.
And that's assuming it's not wound as a progressive-rate spring.
It is far from being as simple as saying "If the free length is 21" it'll give you 3" of lift."
ok so correct me if I am wrong........
Any one company that offers a 3" lift for a XJ has more than one option for a spring to arrive at a 3" lifted height?
I didn't ask for a complete manufactured coil spring rate / dimensional chart. I had hoped that there might be that someone somewhere has compliled a listing of spring dia / lengths by lift kit manufacturer for each companys lift options!
Here's one I've accumulated over the years from various peoples postings. Take it with grain of salt....
Spring Coil Rates:
Tera 4" TJ coils give 4.5" of lift on an XJ and have a 200 lb/in rate , 20.25" tall unloaded
Tera 3" TJ coils give 3.5" of lift on an XJ and have a 170 lb/in rate
Rusty's 4.5" coils give 5" of lift and have a 150 lb/in rate, 22" tall unloaded
RE's 4.5" ZJ coils give 6"-6.25" of lift on an XJ and have a 200 lb/in rate, 22" tall unloaded
Rustys 3" coils give 3" and have 150 lb/in rate.
Rancho 3" coils, at least the old coils I had give about 2 1/4" of lift on an XJ and have a very high 235 lb/in rate.
OEM is supposedly 147 lb/in rate.
OME coils give 2.25" - 2.5" of lift and have a 148 lb/in rate
I didn't ask for a complete manufactured coil spring rate / dimensional chart. I had hoped that there might be that someone somewhere has compliled a listing of spring dia / lengths by lift kit manufacturer for each companys lift options!
To answer the first part of your second post, yes, any spring manufacturer has several options for how to arrive at a certain ride height. If they use thicker wire, they get a stiffer spring. If they then use more windings (more coils) they then get a flexier spring again. What they do is a function of balancing ride height against spring rate.
I don't know if any manufacturer other than the Jeep factory offers multiple spring rates that all sit at the same height. In other words, does Rusty (or RE, or Teraflex, or OME, or ...) offer one 3" lift spring with a 180# rate and another 3" lift spring with a 220# rate? Dunno. My guess is probably not. My sense is that each of the aftermarket vendors has an idea of how stiff they think a spring "should" be, and they have their springs made accordingly.
Trailmaster springs are very stiff. I've heard that Rusty's are comparatively soft. OME are supposed to ride more or less like stock (which means the rate is similar to stock).