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.

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 ;)

Shaikh is a boss, news at 11.

Can you add in data points for the length of sway bar while you are at it?

+1
Heh, I had to fake this by calculating the equivalent increased OD of the sway bar for a certain [shorter] arm length. Would be nice to just be able to enter it in the spreadsheet though.

Is adding swaybar arm length even possible given the number of swaybars available? Granted it probably could be done with a couple swaybars.

I'm pretty sure the spread sheet already do that, its just hidden in the back ground.

Yes, can (and will) make sway bar arm length adjustable along with other parameters. I've got a database on sway bar values which one can reference or change based on your own measurements. Naturally, up to the user to enter sane values, I imagine some funny results happening otherwise ('it goes to 11!').

I appreciate your support, guys.

Awesome, thanks! let us know when the new spreadsheet revision is ready!

+1.

Howdy MT crew. It'll be just shy of 4 years to the day when I made my last post on this thread and I've got two cool spreadsheets to share where I expect all the calculations to be accurate and useful. All fields in blue are inputs, boxed fields in white or not color are calculated outputs. I will create a specific page for all spreadsheets but for right now I've only got direct links to each one. The first sheet is fairly mobile friendly, the second may not be quite as much with the graph function present.

FRC / Setup Comparison sheets
FCM_Ride_Harmony_1_6NA_Mazda_Miata_Online
FCM_Ride_Harmony_1_8NA_Mazda_Miata_Online
FCM_Ride_Harmony_NB_Mazda_Miata_Online
FCM_Ride_Harmony_NC_Mazda_Miata_Online
FCM_Ride_Harmony_ND_Mazda_Miata_Online
FCM_Ride_Harmony_Fiat_Abarth_Online_new
FCM_Ride_Harmony_FD_Mazda_RX7_Online

Damping Evaluation sheets
FCM_Ride_Harmony_1_6NA_Mazda_Miata_Damping
FCM_Ride_Harmony_1_8NA_Mazda_Miata_Damping
FCM_Ride_Harmony_NB_Mazda_Miata_Damping
FCM_Ride_Harmony_NC_Mazda_Miata_Damping
FCM_Ride_Harmony_ND_Mazda_Miata_Damping
FCM_Ride_Harmony_Fiat_Abarth_Damping
FCM_Ride_Harmony_FD_Mazda_RX7_Damping

===

The first sheet is the revised FRC / Setup Comparison spreadsheet for all Miata generations - NA 1.6, NA 1.8, NB, NC, and now ND. I've also made sheets for the FD RX-7 and Fiat Abarth.

The second is a special bonus - a Damping Evaluation spreadsheet where you can more clearly understand the behavior of any shock you have data for and how well-optimized it is. You enter the basic vehicle parameters such as sprung weight, spring rates and see the calculated critical damping factor. Once you enter the shock dyno data you'll see the damping ratios at those damper speeds, plus some QC (quality control) checks I've come up with as to how well-optimized those damping forces are to my Elite standards.

Do note that the motion ratios I'm using are different than the earlier sheet (which are no longer available). I felt the .72 / 0.88 was too high to reflect real-world ride frequencies. You can debate whether or not you agree with the new values but in my experience they are more accurate.

===

In terms of a visual 'walk through', I've created two 'how to' videos. They're still in raw form and I will comment on them when I have time but should be complete enough. For best viewing, use 720p and full screen.

How to use the FCM Elite FRC and Setup Evaluation spreadsheet

How to use the FCM Elite Damping Evaluation spreadsheet

===
In more detail about capabilities, for the first online worksheet, you have three sections.

The first is to play with spring rates and sway bar sizes, plus the ability to include bump stops into the FRC % (front roll couple %) / roll stiffness calculations. Note for the sway bar inputs that the calculator requires you to enter outside diameter and wall thickness. For a solid bar, enter the wall thickness as 1/2 of the outside diameter (intuitive but worth stating). This programming arrangement lets you switch between tubular or solid bars without having to go to another area of the spreadsheet as with the original version. You'll see a few new metrics including the (corrected, x2 factor removed) total roll stiffness, front roll stiffness, rear roll stiffness, % contribution to roll stiffness from sway bar or springs. As before the front and rear bounce frequencies are indicated but now a new column explicitly stating whether the suspension frequencies create Pitch or Flat Ride. To clarify, Flat Ride exists when the rear bounce frequency is greater than the front bounce frequency. I've made several videos on the importance of Flat Ride and have seen this play out in real life. If it's good enough for Porsches, BMWs, and Corvettes, and Mazda has it on the OE Miata suspensions (since the NA at least), it's good enough for your tuned suspensions! In all possible cases for the Miata, perhaps excepting 400+whp monsters for autocross (not track), I've found that about 3% Flat Ride (for Race/Track applications) up to 15% Flat Ride (for Touring / Comfort applications) to work well. You get the best ride, best handling, and least damping required to control the suspension oscillations when Flat Ride is working for you. Many aftermarket setups don't indicate ride frequencies or vendors don't seem to care or pretend they have the 'best spring rates' but decades of engineering knowledge and years of my direct experience and customer feedback prove otherwise. Flat Ride is the way to go. Design for that and choose sways / bump stops to keep your car neutral in cornering. This is an important 'open secret' to why we get such great results.

The second section gives you spring rate and bump stop rate as inputs, plus free reign on sway bar dimensions, from arm length, bar length, hole position and of course outside diameter and wall thickness. I don't have all the possible sway bar combinations listed so someone may want to post those values in this thread (or another one more on-topic) for others to reference. I can also add those values to later updates at the very bottom of the worksheet. If you've got a particular bar you're looking at, odds are you'll be able to measure the values yourself and compare changes in FRC with hole position, etc.

The third section is what I call 'Racer's Edge', where you input sway bar OD and wall thickness, corner weights, and spring rates to see the individual corner bounce frequencies. For a car with asymmetric static weight load, changing springs to the individual corner works very well. Again, I've done this for builds when it's needed. You also get a new calculations of 'max bump force' which was inspired by JasonC_SBB's quarter car modeling and observations that there is a 'launching threshold' based on the sprung weight. The higher the weight, the more bump force you can use without causing launching and loss of tire contact patch (and reduced grip). I've seen that on some applications (typically BMWs) Bilstein's compression tuning seems to reflect an understanding of this relationship, but the rebound unfortunately tends to be woefully overdamped.

Below the Racer's Edge section are the general vehicle inputs, curb and occupant weight, tire / wheel / unsprung weights as a best estimate (adjust as needed per your direct measurements), weight distribution, front and rear track, and default sway bar inputs for the 1st and 3rd sections. Motion ratios for spring, shock, and sway bar are not user adjustable for this free online version.

There is an area at the bottom of the sheet for unit converters, as with the previous sheet, so you don't have to run for your phone or calc program to go between kg/mm and lb/in, etc.

===

The second online worksheet is the damping evaluation spreadsheet. Partly it was deliberating on how much I wanted to share that pushed out launch of the revised version (oh, and work itself!). I definitely wanted to see enthusiasts and racers with more tools that aided in understanding how I saw 'ideal damping' and damper design, and that's what this sheet helps with. It's probably more tech-heavy and if you don't have all the data available for all the speeds it won't reflect the most useful QC results.

Here, your inputs at the top of the sheet are gas force at the shaft (not charge pressure), bump force and rebound force for speeds 0.5, 1, 2, 3, 5, 10, 15, and 19 in/sec. The more data you have the more complete the QC checks will be. The outputs are damping ratio at that damper speed and the rebound-to-bump ratio (including the lifting influence of gas force which effective shifts the entire damping curve upward in real-life). You can refer to the video for each of the metrics as there are more than I want to type out. I've set up the sheet so you're visualizing all four corners of the vehicle from above, top left grouping is front left (driver's front on LHD cars), bottom right grouping is rear right (passenger rear on LHD cars).

I've also put in a graph function for each damper input to visualize what's going on. A picture's worth a thousand words. Watch the scaling or zoom level of the webpage to make sure the graph isn't too big or too small. In an ideal world, I've want to have a few of these overlap each other, but if you download the Roehrig software and have the actual PVP files you can see all the data with their slick interface. At some point I might put up an archive of the PVP and CVP files I have for some of the many dampers I've tested.

At the bottom of the sheet are the inputs which you can take from your FRC / Setup evaluation sheet. You'll need sprung weight, spring rates and motion ratio. Here you can adjust the motion ratio which may apply to people with modified control arms. No, the free FRC sheet will retain fixed motion ratios. If for some reason you have a non co-axial setup (not really typically for Miatas but relevant for vehicles like BMWs, Mazdaspeed 6, etc.), you have to first calculate the critical damping factor at the wheel from the spring's leverage then reflect it that critical damping factor back to the damper to determine how much actual damping force is needed. If someone has a damper they want me to test, I can do that and create a new sheet with those damper forces, plus a sheet shot of the dyno graph. I can do that testing at my discretion and for $25 per dyno to make sure it's not a flood of free requests.

===

I've spent a good bit of time creating, checking, and having a few Elite customers evaluate these spreadsheets however if you find any problems let me know. Make sure to refresh the sheet (bottom left) in case the values seems to hang. You can print the sheet (naturally) but can't save values, so everything is done on the webpage which I believe you can also use offline once the page is loaded. If someone wants their own editable version, that is now available with purchase of an Elite damper upgrade or coilover package. If you want assistance with creating a setup, even if it's not specifically an FCM Elite Bilstein-revalve, I am available for consultation, as per the website contact page.

I'll be monitoring this thread for a few days to get your feedback but otherwise don't plan to post here again (for a while!). You can contact me through the FCM website for build-specific question (with consult fee to apply) and also comment on the Youtube channel for the videos above. I don't want to violate any forum rules on sponsorship although my feeling is that the high value I'm providing for free to the community in terms of these sheets is worth one post every four years. Mods, feel free to edit as needed if you disagree.

Enjoy!

appreciate the vids!

Way to go Shaikh!

Most of what he says goes over my head and he's incredibly verbose for an engineer. I've always really enjoyed reading and watching his stuff because you can tell that he's genuinely interested in it and it's boyishly excited to get other people to share his appreciation for these things.

Looking forward to most of this video going over my head too!

😅

Thanks guys (what's up R-man!). I'm really a philosopher / physicist who 'daylights' as an engineer. My mother was an English teacher and loved telling us stories. I know my verbosity can be a pro or con depending upon the listener and I'm okay with that. I think a life well-lived is one where we enjoy creating and sharing experiences, which means one full of stories. Yes, this whole realm of suspension design and optimization is endlessly fascinating to me, especially since I never imagined I'd leave the semi-comfortable world of semi-conductor engineering to pursue a hobby full-time and turn it into a business. I look forward to seeing what others suspension engineers come up with: each new car or aftermarket option represents an opportunity to see how close they come to my ideal of perfection, and there's always more for me to learn.

The FRC and Setup Comparison spreadsheet is pretty easy to work with without needing the video. The Damping Evaluation spreadsheet has much more going on and won't be for everyone, but it's a great tool for anyone who has a very thorough dyno of a damper and wants to know what I'd think of it (a question I do get asked often enough).

I'm sure your ex-bosses were relieved to not have to read your quarterly reports!

I'm a med student that plays mechanic. Thanks for making some of this stuff more accessible to folks. I've learned a lot from miata turbo and you. It's a pretty cool community.