I’m pretty green when it comes to car setup, and my understanding on things like front roll couple is notsomuch. But my teammate Alyssa sent me a link to the FatCat Suspension Calculator, and after playing with the tutorial for a while, I learned a thing or two.
Terms
- Bounce frequency – Lower values are comfortable, higher values less so. A 1990 Miata has a bounce frequency of 1.15 front, 1.01 rear. Fat Cat suggests that higher than 1.7 hz is not suitable for the street. That’s about 360 lbs front and 275 rear.
- Roll stiffness – Determines how much body roll the car has in a corner. You can change roll stiffness with sway bars, and it won’t affect bounce frequency.
- Front roll couple (FRC) – A low FRC makes a car easier to steer, but stable at speed, and susceptible to oversteer. A high FRC makes the car more stable at speed, but harder to steer, and tends to understeer. Mazda engineers felt that about 59% FRC was a good balance. Fat Cat says Miatas are biased a bit towards oversteer, and they use a higher FRC.
| Front Roll Couple | Notes |
| Less than 58% | Oversteer |
| 59% | 1990 Mazda Miata |
| 63% | Track and high speed setup |
| 65-68% | Autocross |
My Street Car
My street car has a Racing Beat hollow front sway bar, stock rear bar, and Tein Street Advance shocks with 7k/6 springs, which are the same shocks the calculator uses as an example. Luckily I did these all at the same time, but if I had done the shocks or sway bars individually, it would have ruined the handing.
| Configuration | Bounce | Stiffness | FRC | Result |
| Stock sways and shocks | 1.14, 1.01 | 872 | 58.9% | Stock |
| RB front sway only | Stock | 1295 | 72.5% | Understeer |
| Tein Street Advance only | 1.77, 1.88 | 1910 | 50.5% | Oversteer |
| RB front, stock rear, Tein shocks | 1.77, 1.88 | 2334 | 59.6% | Stock-ish |
Adding just the front sway bar would have made the car boring and push everywhere. Adding just the shocks, the car would have been loose mess, wanting to swap ends. But put all the parts on at the same time, and the car’s balance is very much like a stock Miata.
My Race Car
My 1994 race car is on Spec Miata suspension. When I bought it, the previous owner had disconnected the rear sway bar. That’s a front roll couple of 71.2%, which is a lot, but I didn’t know any better and anyway the car seemed to handle well like that.
But then I added a wing, which resulted in way too much understeer. This was how the car was set up during aero testing at WGI, and if you read about that, you know I also had some unfavorable negative chassis rake. All things combined, the car was an understeering pig.
Before racing at Mid Ohio last year, I reconnected the rear sway bar. This has brought the FRC back to 63.1%, which makes the car steer easier at low speed. And the wing adds progressively more rear bias, so the car transitions to understeer at high speed. The end result is a car that handles really well at all speeds. And that’s one of the cool things about aero, you tune a car so that it’s loose at low speed, and tight at high speed, just like you want it (or at least how I want it).
Getting Back 20 Points in Champcar
Champcar charges 20 points per aftermarket sway bar. I’m using a Spec Miata front sway bar. Is it worth getting 20 points back and using the stock sway bars?
Reverting the car to stock sway bars front and rear would reduce the roll stiffness by about 13%, but the FRC would be only 1.5% more. I don’t know if those changes are things I’d feel or not. However, there’s another option on stock sway bars: changing the spring rate to 850 lb front and 400 lb rear would bring the roll stiffness and FRC very close to where they are now. The only drawback is that bounce frequency would go up in this configuration, from 2.36/1.85 hz to 2.60/2.05 hz. Finally, I could leave the front spring as is, increase the rear spring to 400 lb and remove the rear sway bar, which would have the same FRC as a Spec Miata, but 9% less roll stiffness. In any case, there are a lot of options, and I can get 20 points back if I want to.
| Configuration | Stiffness | FRC |
| Spec Miata 750/325, 24mm/15mm sways | 2952 | 63.1% |
| As above, no rear sway bar | 2750 | 71.2% |
| Spec Miata, stock sway bars | 2581 | 64.6% |
| Stock sway bars, 850/400# springs | 2963 | 62.9% |
| No rear sway, 400# rear spring | 2645 | 63.1% |
NB Miata Rear Suspension
I was reading one of Keith Tanner’s books on Miatas, and he states that NB Miatas have slightly different suspension geometry than an NA, and consequently different motion ratios. On equal springs, an NB has softer rear suspension than an NA. So if you have an NB, multiply your rear spring rate by .87 to get the equivalent to an NA spring on the FatCat calculator.
With that knowledge I ran some numbers on my team mechanic Clayton’s 1999 NB. It has 318 lb front, 233 lb rear springs, and Flyin Miata sway bars 1″ front and 5/8″ rear. The previous owner had disconnected the rear sway bar for some reason, giving a FRC of 71.4%. That’s a lot of front bias. Connecting the rear sway bar brings FRC to 58.8%, which is almost identical to a 1990 Miata. With this suspension setup, bounce frequency is just below the 1.7 hz comfort threshold, and roll stiffness is 2.4x more than a 1990 Miata. Sweet.
Occam's Racer 