mx5cartalk.com

Hello all,

My car (2004 SE) is essentially track use only now and I'm thinking of upgrading it's current dual road/track suspension configuration to something more suitable. Based on my car's current suspension setup and my driving ability/style, I'm fairly happy with the car's balance at track speeds (current wheel alignment notwithstanding!) so I'm aiming to aim for a similar front roll couple percentage.

I've been looking at Fat Cat Motorsport's Suspension Design Spreadsheet and working through the numbers. I have a standalone version 7.1 in Excel and have also been comparing the results with the current version online. I've noted that the online version has amended data for spring motion ratios and also has included additional data for leverage ratios.

In the V7.1 spreadsheet I've plugged in my current setup and a couple of previous set ups by way of comparison. The reason I'm using the standalone version is that it allows me to change swaybar arm length, thereby quantifying the changes in FRC due to the effect of adjustable sway bars. The changes to FRC correspond with my notes regarding the car's handling and surprisingly there only appears to be around a 1.5% variation in FRC between "unsettled" and "stable".

For my current setup, the Online spreadsheet gives me an FRC of 54.0%, with roll stiffness of 2708.9 lb-ft/deg and bounce frequencies of 1.78 Hz and 1.8 Hz front and rear respectively. (Note: this is using its fixed values for sway bar arm length).

The V7.1 spreadsheet gives me an FRC of 57.0%, with roll stiffness of 2412.5 lb-ft/deg and bounce frequencies of 1.69Hz and 1.58Hz. (also using the fixed values for sway bar arm length for comparison's sake). From a purely subjective, seat of the pants assessment I feel as though an FRC of 57% is probably closer. From what I'm lead to believe, an FRC of 54% is way too low for what is essentially a fairly neutral handling MX5, but please correct me if I'm wrong.

Based on these variations I'm not looking to either of the FCM models for absolute values, rather I intend to use V7.1 to compare the relative differences between alternative setups and to try and get something close to the indicative V7.1 FRC that I have for my existing setup.

So, here are the the questions:

* Has anyone here used the FCM spreadsheets in a similar manner?
* What results did you get?
* Did the changes in FRC, bounce frequencies etc correspond with a noticeable change to the feel or handling of the car?
* What was your ballpark variation in FRC between "good" and "not so good"?

Any feedback or input would be most welcome. Many thanks in advance... and apologies for the long winded post!

Bump...

Are there no students of the FCM spreadsheet out there?

Sorry I'm not a student of the FCM spreadsheet, however I would not worry about changes between versions rather keep using one version set a baseline and work from there. Make sure you know your corner weights and unsprung weights etc. it may also pay to check your cars motion ratio as well. Verify the measurements being used.

Good luck.

* Has anyone here used the FCM spreadsheets in a similar manner?
The old 7.1 spreadsheets are my bible. I plug in a few new spring rates each night before I go to sleep.

* What results did you get?
Trophies. The car is pretty gutless, so grip = win.

* Did the changes in FRC, bounce frequencies etc correspond with a noticeable change to the feel or handling of the car?
Absolutely. There's a clear correlation between FRC and the understeer/oversteer balance. Bounce is more subtle,

* What was your ballpark variation in FRC between "good" and "not so good"?
I have an ideal FRC value for my driving style. +2/-1 around that is too much.


I use the old FCM 7.1 spreadsheet for probably the same reasons that you do. I've also changed the way it works to accommodate variable length sway arms but basically it's relative change that matters to me, not absolute vales.

In 7.1 a stock NA8 has FRC of 60.1%. My sweet spot is 55.0%, or 5.1% biassed towards oversteer from stock. At 55% I can drive the car neutral but pour on oversteer when I need too. Anything under 54% is a bit of a handful, anything over 57% is a snowplough.

The important thing is not that 55% is sweet but that I have a measure of where the sweet spot is and what the effect of changing will be. This becames a big time and money saver. If I do change springs or sways then I know in advance precisely what the change will do to handling. I can make the choice of how I'll change it and I'll know pretty well how to drive it from the first lap. I haven't wasted any money yet on bad setups, it's all just been forward progression.

Bounce frequency changes are dríven primarily by spring rates and wheel weights, I use 7.1 as the calculator but the theory and target values comes from Dale at Racing Car Technology, a conveniently (for me anyway) local Batemans Bay firm that specialises in race car setups. http://www.racing-car-technology.com.au ... 0index.htm This is the stuff that has taken me from 5/4 to 7/5 to 10/7 and probably in the future to 14/10 spring rates.

Thanks for the replies gents.

Certainly it's my intention is to only use V7.1 to create an arbitrary benchmark FRC based on my current setup and then to fool around with spring rates and sway bar settings etc to get back to roughly the same value. Hopefully that way the setup should be fairly close "out of the box" once changes are made.

My current setup runs 403 lb (7.2kg) front springs, 300 lb (5.4kg) rear springs with 24mm (full hard) and 16mm (full soft) adjustable bars. This gives an FRC of 58.5% which works for my car (NB) and my driving style. (According to the spreadsheet a standard MSM/SE FRC is 61.2%).

The reason I'm looking at the FCM spreadsheet is because it's been recommended that I move up to 490 lb (8.7kg) front and move the old front 403 lb (7.2kg) springs to the rear. According to the spreadsheet the highest FRC I could get with this combination is 56.2%, which is along way short of my 58.5% sweet spot.

Phil, out of curiosity what bumpstops are you using?

I'm not even sure if the bump stops that are in there are in any sort of usable condition. They look totally perished. They're pretty much irrelevant to me anyway because the springs are so hard I never hit them.

manga_blue wrote:I'm not even sure if the bump stops that are in there are in any sort of usable condition. They look totally perished. They're pretty much irrelevant to me anyway because the springs are so hard I never hit them.

what tyres and allignment do you run with that setup?

track? street?

cheers in advance

It's a daily driver/track pig. Street tyres daily, R-specs on the weekend. Check out alignment specs in the tech guide section.

Tony wrote:The reason I'm looking at the FCM spreadsheet is because it's been recommended that I move up to 490 lb (8.7kg) front and move the old front 403 lb (7.2kg) springs to the rear.

I'll have to give you a drive of mine on the highway to see how you feel about high spring rates. Ride is very subjective.

Thanks Phil.

It's not really the high spring rate that concerns me as my car doesn't see the road now; rather it's the apparent "unbalanced" FRC for the proposed setup that I'm questioning.

Unbalanced from what? The previous calculation or from feel on the road? Set a baseline, drive, make changes in the calculation, makes changes in the car, drive, rinse repeat. As I have been told it can be over thought. Now before I make changes to the car I reset to my current baseline (which I keep in a notebook) and make small changes from there. Key, at least for me is to record the changes and note what it feels like.

Yes, that's what I'm trying to say; perhaps I wasn't clear.

Using the spreadsheet I have determined a benchmark FRC using the existing setup that I'm reasonably happy with.

Another setup has been suggested to me, however the calculated FRC compared to my existing setup indicates that it will have much more oversteer (i.e. "unbalanced", or not neutral if you'd prefer) and that the oversteer cannot be dialled out with the sway bar adjustments I have.

Hence my original questions regarding the ability of the spreadsheet to accurately predict changes and how these changes translate to actual on track behaviour.

Hopefully I'm not over thinking it; my sole aim is to do is pick a heavier spring combination that will yield a result close to my current FRC percentage as a starting point. I'm prepared to fiddle and adjust from there, but I don't want to waste time and money fitting up a combination that was never going to work anyway.

Tony wrote: and that the oversteer cannot be dialled out with the sway bar adjustments I have.

I feel the pain of your overthinking However sway bars are not the only thing that can be changed to 'dial' out oversteer, the following can be done to increase understeer:

Increase front weight, decrease rear (do you know what your corner weights are as this may not be possible),
Front ride height increase, rear decrease (again this should not be done in isolation of the corner weights),
Reduce front tyre pressure, increase rear,
Narrower front wheel, wider rear,
More positive camber front, more negative camber rear,
More front rear toe-in,
Sway bar stiffer front, softer rear,
Spring rates siffer front, softer rear,
Bump stiffer front, softer rear,
Rebound front increase, rear decrease,
Reduce front downforce, increase rear downforce.

I can tell you from personal experience that my car without its rear wing oversteers at a moments notice. I need to do some more adjustments as I detailed above to dial out the oversteer on public roads as the big wing is no longer used on public roads. However I can't change the alignment nor the spring rates however I can play around with the corner weights. If I change the corner weights (and ride heights) I will just note the changes and adjust them back just before a track day. It helps that I have a set of scales at home

With scales you can also dial out ant sway bar pre-load (if you have adjustable end links) or add some preload.

Your input is appreciated.

I am well aware of all the adjustments that can be made, and yes my car has been corner balanced and yes I do know the weights.

The whole point of benchmarking the FRC percentage is so that a new spring combination, when matched with the sway bar adjustment and damper valving I have, will (hopefully) be in the ballpark from the start.

Fine tuning the setup will then be completed using the range of tools and techniques that you listed.

The aim of this thread was to determine the ability of the FCM spreadsheet to accurately predict changes in behaviour and how the values on the spreadsheet translate to actual behaviour on the track.

I think I have my answer; thanks.

Tony wrote:The aim of this thread was to determine the ability of the FCM spreadsheet to accurately predict changes in behaviour and how the values on the spreadsheet translate to actual behaviour on the track.

Point noted

No worries, thanks again for your input.