...and these are simple free rate calculations. A spring's installed roll rate can also vary based on mounting layouts and shackle angles. For your Charger, with its parallel mounting, it is a good starting point as the splayed layouts are only found in Late B, E, F, J, and M bodies.
To figure out the wheel rate of leaf springs, you need the center to center distance of the axle housing mounting pad points and center to center of the tread width. Divide pad width by tread width, multiply by 100, that's your wheel rate.
Your rear anti-sway bar also contributes to wheel rate as well.
We have had this discussion before, but imo, in say hitting railroad tracks head on, the pad distance is not factor, spring rate is wheel rate, as both springs move the exact same amount as the wheel moves.. Where I disagree with the pad dimension factor, whatever one loses in rate with the pad not being directly over the center tire patch on a one wheel bump, one regains by the other leaf now being closer and it then makes up equally for what little was lost with the spring pad being mounted closer to the center. I am not addressing roll rates here, they are harder to calculate with an OEM mounted leaf, which has designed in anti roll properties already.
