Front Sway Bar wheel rate Opt 2 = sway bar rate * (Motion Ratio^2)
Leaf spring wheel rate = (listed rate) * (Motion Ratio^2) * 2


These two formulas are both supposed to determine the wheel rate.
They both have spring rate times motion ratio squared.
Yet one of the formulas has a x2, and the other doesn't.
Why aren't these formulas the same if they're doing the same thing?

It seems like the *2 at the end of the Leaf Spring Wheel Rate Formula shouldn't be there.

Using 70 B body as an example, distance between leaf spring perches is 44 inches, divided by track width of 59.2 inches = .7432432 motion ratio.
that motion ratio squared is .5524104
multiplied by a 160 pound spring rate = 88.38566
then when you multiply that by 2, you get 176.77132
It seems odd that the wheel rate would be higher than the spring rate.

Couple things going on here.

First quote; No, you would not double the sway bar rate calculations because the sway bar works over its whole length. The sway bar interacts between both wheels so its motion ratio calculated rate will link the two wheels. Its not isolated to a single wheel.

Second quote; Yes, they are both calculating wheel rate, but they are two different components, so the rate is achieved in different calculations. In this case, the spring must be doubled, because the original formula was to determine total roll couple, not isolated spring rate. Since you clipped it out of context of the rest of the formulas, the *2 appears to double the spring rate for a single spring. Rather it is doubling the spring rate in the overall picture of the whole rear roll rate of two springs and a single anti-roll bar. So, you have 176.77# for the two leaf spring rates. You now need motion ratio adjusted sway bar rates to know the total rear roll couple rate.

Thanks, that makes perfect sense.
I wasn't sure, and thought I'd ask.

Your comments here and in other threads have been extremely helpful towards my understanding of this stuff.
I really appreciate it, THANK YOU!

I'm pretty sure that I understand what you're saying.


Just to be clear though (which may also help anyone else reading along), Here is the Rear Roll Couple formula as it was presented above.



It makes perfect sense. There are two leaf springs, so you multiply the spring rate by 2.
I'm not using a rear sway bar on the car (yet), so I don't have to worry about that part of the formula.
I do need to calculate a leaf spring wheel rate though, so I look for the formula, and it's presented as...



I recognize that the middle portion "(leaf spring width / track width)" is the motion ratio of the leaf springs.

Using 70 B body as an example, distance between leaf spring perches is 44 inches, divided by track width of 59.2 inches = .7432432 motion ratio.
that motion ratio squared is .5524104
multiplied by a 160 pound spring rate = 88.38566

I believe you're saying that the "Leaf Spring Wheel Rate" formula should Not include the "*2" on the end.

Also, that the leaf spring wheel rate does get multiplied by 2 in the "Rear Roll Couple" formula.

And that multiplying the leaf spring wheel rate by 2 in Both formulas (as it's shown) would result in an incorrect answer.

You've got it!