First of all, I don’t usually contribute to forums, but today I’m making an exception because I believe everyone should be aware of the obvious mistake present in FCM’s spreadsheets. I have been using those spreadsheets myself to calculate FRC% until I discovered the error in the roll stiffness calculation. I did email FCM to inform them that the spring’s contribution in roll stiffness is twice the actual value, but I did not receive any answer and the spreadsheets are still the same.

Since I don’t expect anybody to simply believe my words, I will demonstrate the error with references:

So let’s start with FCM’s result when only considering the front springs for the roll stiffness (roll bars diameter at 0):

(The spreadsheet can be found there:

FCM_MSDS_1_6NA.xls)

Now let’s calculate the roll rates:

Using the equations Milliken book (p.589 and 596)

Or the equivalent equation from OptimumG technical papers (

http://www.optimumg.com/docs/Springs...Tech_Tip_2.pdf)

With the FCM’s default values:

IRf = .72

tf = 55.5 in -> 4.625 ft (front track)

Kspringf = 700 lb/in

KrideF = 700 lb/in * (.72)^2 = 362.88 lb/in

KrollF = (12in/1ft * 362.88 lb/in * (4.625)^2) /2 = 46 573 lb-ft/rad

46 573 lb-ft/rad *pi/180 = 812.86 lb-ft/deg

Then if we compare this value with the one from Fat Cat Motorsports the mistake is obvious.

Calculated value: 812.86 lb-ft/deg

FCM value: 1625.8 lb-ft/deg ->1625.8 lb-ft/deg /2 = 812.9 lb-ft/deg

Finally, those spreadsheets can still be used, but would require a bit more work. The solution would be to use the spreadsheet to calculate the roll stiffness contribution of each individual component and dividing by 2 those for the springs and then add them to calculate the FRC% manually. Otherwise the spreadsheet will give you false FRC% since it considers the springs for twice their actual spring rates in terms of roll.