Warning: take Fat Cat Motorsports spreadsheets with a grain of salt
#1
Warning: take Fat Cat Motorsports spreadsheets with a grain of salt
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.
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.
#9
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In fairness, that seems pretty irrelevant to the main point of the original post - which was to claim a possible mathematical error in an online tool a lot of people have used (or cited) for initial car setup.
#10
I remember having a phone conversation with Keith @ FM that mentioned the spreadsheet was not he most accurate due to something with sway bar calculations as well.
I guess I have always used the spreadsheet as a relative comparison either with a change in my setup or in comparing my setup to a known setup of another "well handling Miata".
Don't know if that's worthwhile but I guess I thought the error would be in both calculations and delta would be the same/close enough for me.
I guess I have always used the spreadsheet as a relative comparison either with a change in my setup or in comparing my setup to a known setup of another "well handling Miata".
Don't know if that's worthwhile but I guess I thought the error would be in both calculations and delta would be the same/close enough for me.
#11
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all i know is, everyone says try to achieve 60% FRC based on that chart, when I did, it was the worst handling miata ever. Opposed to my current setup where I'm at like 45% and it's amazing.
#14
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When it comes to suspension setup, absolute numbers are as meaningless to me as the tessellation rate for some specific video card or the GDP of Bolivia. But if I can plug in some specific baseline numbers from a known setup (such as my own car) and then play around with the tool to get answers like "X will exhibit more understeer relative to Y" then there's some utility in that. A lot of it is common-sense to people who live and breathe suspension, but somewhat foreign and unintuitive to those of us who don't.
#17
The real proof in the pudding is going to the track and testing your changes there.
Last edited by Lincoln Logs; 02-22-2013 at 03:39 PM.
#18
Shaikh is a darn good engineer and honest business owner. He's also human. Is there a mistake in his spreadsheet? Don't know, haven't looked into it. I did E-mail Shaikh to look at this thread so he can research it. He's a small shop and is insanely busy (especially with the autocross season rapidly approaching). If there is a mistake, I'm 100% confident it will be corrected -- although expecting that to happen overnight is silly.
Shaikh delivers a quality product and is a good guy. I'm one of his customers. The implication we're getting from you is that he is either dishonest or incompetent or both. We know better.
Shaikh delivers a quality product and is a good guy. I'm one of his customers. The implication we're getting from you is that he is either dishonest or incompetent or both. We know better.
Last edited by hornetball; 02-22-2013 at 10:30 PM.
#19
Shaikh is a darn good engineer and honest business owner. He's also human. Is there a mistake in his spreadsheet? Don't know, haven't looked into it. I did E-mail Shaikh to look at this thread so he can research it. He's a small shop and is insanely busy (especially with the autocross season rapidly approaching). If there is a mistake, I'm 100% confident it will be corrected -- although expecting that to happen overnight is silly.
Shaikh delivers a quality product and is a good guy. I'm one of his customers. The implication we're getting from how you chose to post this is that he is either dishonest or incompetent or both. We know better.
Shaikh delivers a quality product and is a good guy. I'm one of his customers. The implication we're getting from how you chose to post this is that he is either dishonest or incompetent or both. We know better.
#20
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Like others are saying, I saw the spreadsheet as a starting point to get things in order with my suspension, and I have no complaints. I bought FCMs because I liked them after I got a couple test rides in other cars. I got a ride in a Koni + GC setup and didn't like it. I also got a ride in a car with FM + AGX & FM + tokico and didn't like either. Little secret: I'm no engineer so the numbers are irrelevant to me. I used the spreadsheet to come up with something that looked fair, I tried it, changed spring rates, tried it again, changed the sway bars, tried it again... until I got it where I liked it. Do I turn faster lap times or pull more G's on the skidpad? Iono, I'm just an enthusiast. Is my car acceptably comfortable on the street and predictable at track days? Yes, and that's all I wanted out of my car.
Again: Since when do any of us blindly listen to vendorspeak? The last time I did that it turned into a $3,200 nightmare. I tend to think I learned something from the experience.