FRC for a track car with some aero.
 

Hello everybody.

I'm currently looking at upgrading my suspension again.

Car: heavy NB 1160kg's wet with driver. Frame rails and roll bar, FM sway bars f+r, 15x9 with r-comps. 4" front splitter and gtc-200. Updating engine with a gtx2863r with a goal of 320-350whp. prioritys are track>street. It is registered and no extreme weight savings has been done.

Tracks: Medium speed mostly, some fast. Auto-x for fun, but this is not a priority.

Question: I'm looking at some custom dampers with 12,5/9kg/mm springs (i.e. about 700/500lb/in). Combined with my sway bars this gives me a FRC of 55.3% usin fat cats spreadsheet. I should be able to change the balance a few % adjusting the sway's, but is this really a good thing for track? My fear is that this will cause excessive wheel spin on corner exit.

Preferences: Neutral to slight oversteer on turn in and corner exit, neutral mid corner.
I will probably start out with zero toe and slightly higher camber up front.

Has anyone used rates close to this and compared it to closer to 60%, thoughts? Xidas have a slightly higher FRC, Öhlins are around 56% and Tein flex is somewhere around 54% according to the FCM spreadsheet.

To summarize, will going for a FRC of 55.3% stick me rear first in the gravel trap / armco / ditch?

700/500 is wrong. 700/400 or 800/500. If you add substantial aero or tire grip, you may find a need for even more front rate, 900/500 or thereabouts.

You do not want to get into the dance of tuning static balance and aero balance against each other. Dial in the static balance without your aero devices, then add aero and tune the aero devices independently. It is easy to get into a situation where you have lots of cross-talk and a car which handles really, really poorly if you try to tune them both at the same time.

Xidas have no FRC, they are just dampers. FRC is dictated by a combination of spring rates, sway bars and rake.

Andrew is spot on. Tune your mechanical grip balance without aero then add your aero bits. Personally, I like a car that's neutral at lower speeds and gradually gets a wee bit tighter with speed. Easily done with aero.

Thanks, Emilio, I should have said, on my car in the spread sheet.

I really should test drive a 800/500 car. Thanks for the other advice as well, wise words.

Given R-comps and aero, going to need more front spring than the 700 if you have any high speed (>120mph) sections that aren't pure straights.

We're running 900 and 1000# front springs on S2 cars now with 225/45 RC-1's @2300#. They have probably zero front and rear lift but probably no net downforce. Our starting point for track cars is 800/500 but that quickly jumps to 1000/500 if any net downforce is added.
Our current S1 build should have in the region of 200-300# net df @ 100mph. It will start with 1100/500.

Educate me, not being from the states I don't really know the spec of an S2 car.

It seems like I should aim for 800/500 with secondary springs, Not too hard to go stiffer if I need to. I'm coming from about half that, riding along on my bump stops and loosing time and grip everywhere :)

There is one particular corner that needs to work, a 110-120mph 15-20 degree left hand that is slightly off camber. It's a scary corner since its also blind, I need to be able to go through it with confidence.

800/500 would be the minimum. We've run those rates on Sean's car (2200 lbs) with GT-250 and 5" splitter and it's on the soft side but works well for low/medium speed tracks. Needs more for high speed sections. Shocks can usually handle spring rate changes of about +/-20% so get the shocks valved for about 900 and you can add spring if necessary. (No need to worry for XIDAs as those can handle very wide spring rate range).

Aero is a funny thing. At first blush, it seems to add confidence. Take a car that is a bit sketchy through a high speed corner and add aero to it and driver's first reaction is wow it made that corner much more drama-free; the car was more stable and there was more grip. But, that's really just because now they're driving through that corner at the same speed as before but there's more grip available. In other words, they need to re-calibrate to the available grip and raise their minimum speed through that corner. Once you do that, it's right back to being sketchy, except now your mind is telling you you're going in faster than feels right ;)

Sentic,

It doesn't seem like you are absorbing the advice you requested.
S2 is an otherwise stock body Miata with flat front air dam and 14" duck tail rear spoiler. None of that matters though. What matters is the lack of lift and corresponding spring rates. Your car will have a a modest amount of downforce, Perhaps you did not know that. No one here with any experience with downforce in fast track Miatas will recommend lower rates. Also, your car is a porker. I'd start with 1000/500, 54106 front sway bar, 14mm rear.

Yikes, I just converted 1160kg to lbs. Add 100 lbs of spring to anything I said for that weight :eek:

Yep, It's heavy as F***, could be slightly lighter after the winter since I'm rebuilding/changing a lot, but will not end up below 1000kg's.
I know I have some downforce, and it has been helping me going faster despite driving around on the bump stops.

Going from 800/500 to 1000/500 seems like a huge jump and seems like It would add low speed understeer? I'm running the FM bars f+r, that a bit softer up front but stiffer rear from the 54106+msm as I understand it? I guess I just feel like 800 is already a very stiff spring, raising the front rate more, wouldn't it cost me on <90mph sections? Most of the tracks I run are in the 45-100mph bracket except for the straights/almost straights, most of those are 130mph-isch with my former 252whp setup. i.e, there are no really long sweeping curves on them.

1000 sounds like a lot when your reference points are only things like generic coilovers with too much spring rate for their valving, but a really good shock that is actually set up for that spring rate will change your mind about that.

Shit dude my ls3 miata weighs under 1000kg, what do you have in there that's so heavy????

The interior, a/c, possibly power steering. He might even have the spare and jack in the back. When I weighed my MSM with the stock turbo setup, a/c, p/s, half tank, and me in the driver seat it weighed 2550 lbs which is ~1160kg.

Interior but with 2xsparco sprint and schroth 6-points, electric windows (yeah), ps, airbags removed, other steering wheel, front bumper support removed, large 10m plywood splitter reaching back to crossmember, flat lower airdam, gtc200 with risers, roll bar in thick (local racing spec) piping, 3mm stainless framerails, turbostuff including big rad, 6-spd, oil cooler, stainless 3" with one massive silencer and metallic cat etc, rebuilt fuel system with large bore hose. hardtop, no soft top +2kg battery to get slightly better f/r corner weights. still have 3-p in (legal req), wilwoods with 280/276mm discs (should be about the same weight as stock)... err... carpet in trunk when I had it on the scales. I'm fairly light @ 70kgs, so I don't really know where it all is, 2000nb. No metal removed from car. This weight is all fluids in + me, full tank.

I was surprised how heavy it is.

I know that you guys know your race miatas, so I will try and overcome my feelings for 1000lb/in (just so I get this right, that is 17.9kg/mm) springs on your recommendation, but still, the 500 rear? shouldn't 550 or 600 make me faster in the slower bits? Even with my current rear roll bar I get over 60% in FRC. Or course, I can overcome this with the throttle, but only going stiffer up front? the gtc-200 has risers as I wrote above and it also has the gurney flap. Oh, a lot of the tracks I run has quite large height differences, so there is that also, does it matter? Will a non-caged miata be stiff enough to utilize the high rates? Again, I'm not doubting you here, just trying to get as informed as I can.

Asks for advice
Gets advice from the 3 of the 6 people on the US west coast that have direct personal experience
Argues with that advice
Gets explanation
Argues some more

Oh yes, get rid of both the FM bars. Way too small front, too large rear. Or argue with us about why you should keep them.

SuperMiata Sway Bar Kit Miata

I'm out

Emilio - I'm sorry that you take it that way. I suppose that means that the answer is "no, it will not add lower speed understeer". The FM bars are very replaceable.
Why I'm asking about the rear rate is as ThePass said above, its a bit off the 1.6:1-mark and my gut reaction is that it would induce low-medium speed understeer. But as you are winning, and building and selling this stuff, I understand that you have a bunch of r&d supporting this, I take it it is down to the alignment to take care of this possibly imaginary understeer.

Edit: Watching savingtons and your youtube race vids, I am convinced.

With a 14mm rear bar, the 1.6:1 rule of thumb applies up to about 800-850 in the front and then beyond that you begin adding more front spring only. With a smaller rear bar, that ratio continues to apply to higher rates. As always, there is never only one factor at play.

Emilio

im asking this strictly for educational purposes and trust your setup experience.

Why do you increase front rate when majority of the df comes from the rear?

d

I do not use this 1.6:1 "rule of thumb" nor know of of its origin, thus it does not influence my tuning decisions.

Assuming one adds a balanced amount of front and rear downforce, the car will require more front spring.
If you do not add any downforce but add a ton more grip with big Hoosiers, it will need more front spring and bar but not more rear.

As you add grip on corner entry, the car develops a pitching moment due to its roll center axis being sloped downward in front. Normal vehicle dynamics here.
This pitching moment compresses the front end. A lot. Thus moar roll and pitch stiffness needed, but only in front. This is my observation and conclusion but I am not an engineer. Just a designer, tuner, driver guy.
YMMV


Sorry I can't give you a detailed technical dissertation or laymans terms breakdown. It just works.