Stiff sway bar to compensate for soft springs - a dummy check
 

To start, I know the only way to know is to test. However, I'm hoping this dummy check might see if there is something obvious I'm missing before I waste one of just a few runs trying to dial it in.

My Setup:
'99 Miata
Budget Bilsteins with 550 lb/in front and 400 lb/in rear
1.125" Hollow Racing Beat Front Sway Bar
14mm Supermiata Rear Sway Bar
205 Section Maxxis RC-1 on 15x8 wheels
Alignment will probably start out at the more conservative end of the Supermiata Race Alignment (4.75" height, 3.6 camber, whatever caster I can get)

My Intended Use:
HPDE for now, hopefully Time Trials by the end of the season. Probably just at CMP and Road Atlanta. Also street driving to the track and once or twice a week but I'd rather be fast on the track than comfortable on the street though.

My Questions:
I know I dont have what is considered to be enough spring for my tire. I also know 375 lb/in (not 400) is usually recommended for to pair with 550 lb/in. I have set my front sway bar to full stiff and the rear to medium. That way I have some resistance to body roll in the corners and can dial up the sway bar stiffness if needed. Is this a reasonable setup to start with or am I probably going to get a lot of understeer? Would full stiff on both be better with the option to back off of the front to reduce understeer be a better starting point?

Thanks for looking.

It's easy enough to change that it doesn't really matter. Choose one, if you like it, keep it. If you don't, change it.

I run the 1.25" bigboi bar on both my cars with an MSM rear. I find balance to be fine. :)

I am not familiar with the different settings on the SPM rear bar, I've always ran the OEM 14mm MSM bar. I think you'll be able to drive around whatever the car is doing in terms of over/under-steer. Ideally it would be neutral with one being able to induce one or the other, but if I had to choose.... I'd opt for a loose setup at CMP since you need it to rotate on turns like T1, carousel, T11, and T14. A tighter setup can work at RA, but you'll gain quite a bit of tire pressure on the front left after leaning on that front axle a bunch.

Do you have a brake prop valve? If not, you need to install one. I ran into rear lock-up issues in T1 and T14 at cmp on 800/500 and R7s. So it's likely you'll see the same or worse on your springs and RC1s.

The supermiata bar is the same diameter as the MSM bar, but I have no idea how the length of the torque arms compares. I guess I will leave it at medium for CMP in February and adjust from there.

I do not have a prop valve atm, but I plan to add one. Maybe sooner rather than later. I've locked up the rear on star specs and wimpy stock brake pads and I've since put better pads on.

Mis-matched springs do not "fix" mismatched sway bars
 

While the springs and sway bars interact, they also serve different and sometimes disparate functions. Sway bars have no effect on pitch stiffness in a straight line, and only a slight effect when turning. Springs effect wheel rate in a straight line, while sway bars affect wheel rate when turning.
You should not try to compensate for mismatched bounce frequencies with mismatched sway bars. Conversely, you should not compensate for poorly designed/ chosen sway bars with spring rates.

We see this often. Folks call us for a recommendation on a new Xida setup. We get their info which includes a generic sway bar kit with too small a front and too large a rear bar. We tell them to swap those bars out for a set that matches their needs. Customer usually asks if they can keep the bars
and just have us compensate with spring rates. That's like starting with a bowl of soup that has way too much salt in it. No amount of other spices is going to cur the fundamental imbalance.

WRT to just fitting bars that are matched but too stiff for the grip level provided by the tires, it's the same effect. If the main springs are too soft, you still have excess pitch. American performance cars from the 70's through 90's were often tuns this way. Big sway bars, soft springs and no damping. Cornered sorta flat, nosedive under even gentle braking and wallowed everywhere.

So to the OP, if your set up does not work, you know which way to go next :)

Any good resources or books to learn what that means / how it actually effects grip?

Carroll Smith's Tune to Win is a good one.

Herb Adam's Suspension Engineering

And a PDF is available for free online. Thanks for the tip.

I will also add Sixshooters to the reading list.