...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.
True and for your example of hitting an obstacle square, I'd say you are correct. But I think we would all agree getting into the gnats behind of calculations of roll rates is way more complex than what any of us here want or need to get in to. I do use the motion ratio wheel rates for my calculations simply because of the transient nature of handling is loading the springs differently on each turn. Even then its a simple ball park figure that may be adjusted up or down based on how the car feels to me, what the tire temps read, and what the stopwatch says.
I get that. The car is pretty neutral right now with some slight understeer. I'm fine with that. I am looking to build a similar car within a year or so and was thinking of having it a step or two softer. Again, it is easy enough to do with torsion bars and sway bars due to the diameters/lever arms but....
I used Direct Connection's "neutral handling" methodology to find my Swinger's needed rear leaf spring rate and its a decent starting point. You can do the same using your
current car as a starting point since its "
pretty neutral" handling is what you want to emulate only "
softer".
1) Calculate your current cars Front roll couple. That value will represent 75%-80% the cars total roll stiffness. Thus the 20%-25% remaining will be the range of your CURRENT car's rear roll couple. I'm guessing if you back calculate the leaf spring rate, your MP XHD may be in the range of 110-140 lb/in
2) Now substitute in "softer" TB redo the calcs and see what the rear leafs you'd need. Just error on the side of too soft because a rear bar can bridge the gap and bump up the rear roll couple.
Agree this info provides a good starting point but would remind other readers it is not a set in stone set of values. The neutral handling line was developed from sanctioned competition cars with specific limitations on tire size, overall weight, and biased weight. Street cars can have a much greater variety of variables and a wider range of these variables that can impact where this neutral point occurs.
I'd also add, because of the limited number of torsion bars and sway bars, anyone doing this work should dial in what they want for front rates and then build the rear to compliment it. It much cheaper and easier to modify leaf spring packs for rate than it is to get a custom t-bar size made. T-bars are also limited by the socket size on most of cars to be no more than 1.24, max, unless you are energetic enough to convert ABE bodies to C body sockets, ala the original SCCA TA series race cars.
Why no mention of sway bars?
Instead of running a really stiff spring rate on the street, why not run a softer spring rate and stiff sway bars?
This is possible to a degree. With a street car you still need to have adequate spring rates to prevent bottoming out on features you encounter with regular street driving, which will still necessitate a step or three up from stock. But, this is still part of a larger calculation of roll couple, mentioned higher up, that needs to be maintained. You need heavy enough springs rates to support the car, then use the sway bars to tune its turning characteristics.
The popular soft spring/ big bar set up in modern nascar is actually softer than most stock rates and are used on a smooth surface to allow wind resistance to actually push the car down closer to the ground. Set ups of this type are not practical for the street and require way more shock control than most of us would be willing to pay for.
Is the rate for the hollow front bar supposed to equal that of the solid one?
I think I read that somewhere...
Again, a more complex question in simple clothes with a wide range of "it depends" in the middle. In simple terms, two identical diameter sways bars, one solid, one hollow, will find the solid bar producing more rate. For a hollow bar to produce equal rate requires its overall diameter go up and/or its wall thickness to increase. For example, the popular 1.125" solid bar replaced by a 1.375" diameter, .25" wall tubular bar of identical configuration will have nearly identical rates. But in this case, the hollow bar may weigh 8-10 pounds less. Its increased diameter can also create installation problems for some k frame configurations that require some modifications.
Bar configuration and mounting has a profound impact on its applied rate. The diameter is only one of several variables that impact how much force it applies to a wheel. Looking at calculations I've done, I've found that a solid .75" factory frame hung bar applies almost as much wheel rate as many aftermarket solid .875" axle hung bars because of the variety of factors that may include not only diameter, but also lever arm length, axle mounting position, frame mounting position, resistance width, lever arm mounting angle, and link length.
