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):
https://www.miataturbo.net/members/s...fcm-1-3112.jpg
(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)
https://www.miataturbo.net/members/s...n-eq1-3113.jpg
https://www.miataturbo.net/members/s...n-eq2-3114.jpg

Or the equivalent equation from OptimumG technical papers (http://www.optimumg.com/docs/Springs...Tech_Tip_2.pdf)
https://www.miataturbo.net/members/s...mg-eq-3115.jpg

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.

I always had anyways.

The optimum-G papers used to have an error in their damping calculation as well (damping curve was correct at the wheel, not for an actual damper in its mounting position)...Not the best of references if you want to go beyond FRC's.

Is OP from Stewart Development? :confused:

We take most vendors claims with a grain of salt around here.

His post is coming from Montreal. So I highly doubt it is Stewart.

Well I believe Milliken is a good enough reference don't you think so?

You can do whatever you want of this information.

I've got FCMs on my car, no complaints here.

I will say that I would have purchased the Xidas from 949 had they existed at the time I was buying suspension.

You, yes you. Post more often.

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.

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.

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.

Do you even track, bro?

(I know you lift.)

Fuck the track.

^ This.

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.

I more or less stopped using that sheet when I noticed that it doesn't consider lever arm length on sways. It assumes the same length as OEM, but adjustable bars are adjustable.

I decided that I could come up with something better, but it hasn't happened yet. :D

You can always just copy his sheet in excel. All the math is pretty much out there for you to do it. It'll just take a half hourish to make the sheet the first time.

Most recently as I have developed my suspension set up I've used it as a reference as well. I almost think of it like a dyno; you want to go to the same one every time if possible so you can more reliably measure the change from the last time.

The real proof in the pudding is going to the track and testing your changes there.

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.

As a party with no vested interest in this - no relation to FCM and have never tried their products, although I have read some of Shaikh's informative articles at TTAC - that's not what I inferred about the OP at all. He simply said that the spreadsheet should be taken with a grain of salt due to a math error (which he claims to have emailed FCM about, first), no more or less. Whether his claim is true, I have no idea, but OP sure seems to have some basis, relevant citations included.

My phone seems to have cut off the part where I was trying to add "...and my car hasn't exploded yet or burst into flames, inaccurate calculation or not." Not sure if that makes it more or less relevant actually, I know better than to post from my phone anyway :giggle:

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.