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Sway Bar Math What is wrong with my brain?

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I haven't focused on this yet, but for some reason I am having trouble figuring out the math I need to do here...

Currently running a Whiteline rear bar which has a rate between 266, 309, 357, 411 lbs/in.

I will be installing the Cortex bar which is much smaller with a range of 60-100lb/in.

A chassis mount bar does not work on my car (rubs the tires) and the Whiteline is a kludge to work with the Cortex T/A and Watts putting it really low to the ground in the middle. Hence the change.

So the question: How much additional spring rate do I need to add to the rear to be in the same ballpark of total roll rate that I currently have? For some reason, I can't put my head around what to measure to figure the rates here.

Thanks

DaveW
 
201
303
Exp. Type
Time Attack
Exp. Level
10-20 Years
75024
I'm not 100% sure on straight axles, but I think you need 3 measurements.
Roll center to center of tire patch = AA (to be used later)
(Roll center to swaybar mount/link X bar rate )/ AA = roll rate for bar
(Roll center to spring perch X spring rate ) / AA= roll rate for spring
 

Dave_W

Cones - not just for ice cream
1,016
1,326
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
You'll need to measure the distance between the bar arms, and also the distance between the springs. You know the current bar rates, and the current spring rates. Use the formulas in the link below (or anythinig like it) to calculate current total roll stiffness. Then calculate the new bar's roll stiffness and subtract that from the current total to find what roll stiffness you want from the new springs, then swap the formula around to solve for spring rate from the desired roll stiffness portion for the springs.
 
Thanks guys, this helps a ton. it has been 12+ years since I did all this math (and really put it in suspension analyzer when I designed my mod car) and the process was escaping me.

If the car wasn't currently *so* good, and my season didn't start with a national event, I wouldn't worry much about matching and just throw some springs at it and tune it from there.

Thanks,

DaveW
 

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