02-22-2013, 08:38 PM
Join Date: Dec 2005
Total Cats: 9
Originally Posted by SrDevelopment
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.
Thank you for pointing this out! Could have used an independent double-check on the FRC/roll stiffness calcs when I first did this years ago. I don't recall seeing an email from you (did a search for Frank) but definitely see my error reading through your post and my copy of RCVD. It also explains why I (and others) felt more effect from sway bars (at either end) than calculated. Now it'll be truly more quantitative
which has been my goal all along.
I'm in the process of correcting my internal FRC spreadsheet then will update our site but it could take a week, building some tight-deadline WRX single-adjustables right now. I've been planning to release new versions for a long time and now I can incorporate the corrections plus some added features such as bump stop contribution to FRC, more accurate motion ratios, user adjustable sway bar parameters and a few more metrics. Spreadsheets for other vehicles will go up and be more accurate as a result of your feedback. I'll post again when all the changes are implemented and 'live.'
Then you'll be able to take the spreadsheet with a nice dash of Himalayan sea salt
Last edited by Shaikh_A; 02-22-2013 at 08:52 PM.