Re:
You need to figure out your sprung corner weights, then you can figure out what spring rates you need. Having known spring rates allows you to do this easily.
First: Measure the springs fully extended with no pre load by running the top adjuster down to where it barely touches the spring (a 16" and 18" spring with the dual rate slider should be around 34.5").
Second: Drop it down so it's sitting under it's own weight and measure the springs again (example: mine compressed from 34.5 to 24 9/16")
Third: Do some math
(Tender Spring* Main Spring)/(tender spring+main spring), so for your front.... (150*200)/(150+200)=85.715 inch pounds
Calculate sprung corner weights by multiplying the number from above by the amount the springs compressed. Since I don't know this number for your buggy I'll use mine for example (34.5-24 9/16)*85.715=851.792#'s Do this for each corner and you'll know your exact sprung weights.
I also don't know how much shaft you want to show front and rear, but here's the formula once you decide that which will allow you to back into your spring rate's.
Shaft showing= Shock travel-((corner weight/initial spring rate)-Preload distance)
Also need to mention that I don't know ****, I subscribe to Wayne's tuning logic and base all my stuff of off of that!
