progressive rate springs have a very light initial spring weight. this allows for a very comfortable street ride and handles the sharp little wash board roads by allowing the spring to move more freely. then deeper in the spring the spring rate rises so when you hit the big bumps, dips, jumps etc you wont bottom out. You will spend most of the time enjoying the soft ride but you will still have the firm spring when you need it.
Then you have to factor in shock valving and get it to work in unison with the springs for the best overall ride. It needs to be light enough to handle the small stuff but still be firm to resist blowing through the valving on the hard stuff. its a LONG process of trial/error and fine tuning to perfectly valve a shock for your spring, vehicle wight, terrain and intended use.
too many times people will buy generic lift company spring and slap on an off the shelf valved fox/king shock and think they have done wonders to their suspension. A little bit better quality progressive rate spring and a properly valved shock will be a HUGE difference.
Kind of like a stock cummins motor vs. a fully built smarty tuned, larger turbo, injectors, upgraded fuel delevery, etc. Yeah they are both the same motor but the one that is tuned will outperform the off the shelf one by a large margin.
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2007.5 6.7l XRT/MHP Suspended by Carli Bilstein 2.65'' and some Carli extras
Ford Ranger Prerunner 408ci 520HP, full chromoly, equal length i-beams/3 link...KINGs off...BILSTEINs under development