Nov 9, 2006 update to the Mazda STS2 Miata project car

Keeping the “Street” in ST

One of the main tenets of the Street Touring category is that it embraces common street tuner mods that enthusiasts typically do to their street-driven cars. With these mods your car will handle better than as delivered and will have more power on tap. That said, its easy when setting the car up for serious autocrossing to do things in a way that is detrimental to the ride quality making it no fun on anything but a smooth road. In this installment, we’ll examine the causes and find good solutions that don’t require much compromise.

One place where folks first place the blame is on stiff springs. In reality, this is not typically the main issue. High spring rates require high shock damping rates to properly control them or the car will “pogo” over big bumps. If you don’t have enough shock for the spring (underdamped), you will ruin your ride quality. In the case of a Miata, this would be similar to running OE shocks with anything over about a 250# spring.

The opposite is true, as well. You can have too much shock for the spring (overdamped) to where the shock becomes the spring on momentary inputs like bumps. An example would be a pair of regular Sport Konis cranked to full stiff on OE springs. The good news is that there is a fairly wide range of usable combinations of spring/shock rates. With adjustable performance shocks well-matched to our springs, we can put this to good use by running on the stiff end of that range for competition and running as soft as we can for the street. This is especially true for those of you with hi-dollar double adjustable shocks. Strong compression can severely hurt ride quality for street use.

The next thing to consider is bushings. Stiff bushings reduce compliance in the suspension and help keep our intended alignment specs intact as we stress them with g-forces. But the factory put those soft rubber bushings in there to reduce road vibrations. Hard bushings like urethane and Delrin do a good job of reducing compliance, but transmit a lot of road noise. This is also true of drivetrain bushings. Hard rubber bushings, like those from Mazdaspeed are a better compromise for a dual-use vehicle.

You car also has another set of springs on it that are not typically considered, yet which have a huge impact on ride quality and those are the tires. One of the first things most street tuners do to improve cornering performance is to replace the spongy factory tires with some low-profile, stiff-sidewalled ultimate performance tires. Well, that’s great on the stock springs and shocks, but not so on our stiff performance units as the combined spring rate is now very high. There are two ways to help out here. One is to lower air pressure for the street down to factory specs, rather than the higher competition specs. This increases compliance of the sidewall thus reducing ride harshness. The other method is to swap out those competition tires for something a little more “average” for the street. This will overcome an amazing amount of suspension stiffness.

Seats are yet another spring that affects ride quality, as far as the driver is concerned. Most factory seats have a fair bit of give in the bottom area which absorbs vibration and especially bumps whereas racing seats, especially inexpensive lightweight aluminum ones, have almost no give. If your seats are mounted to the factory adjuster rails, you can easily swap between the race seats and factory seats for long road trips. And with the new weight rules for seats in ST for 2007, there is no longer a significant weight savings to be had by moving to racing seats in a Miata.

Do you run a lot of toe-out in the front of the car to get that super-quick turn-in? We do. And on the street, it can cause the car to wander on crowned roads. This one is easy to fix, though, as it takes just a single turn of one of the tie-rod arms to reduce the toe to a more streetable number. (one full turn = .25”of toe). This will also pay large dividends in reduced tire wear.

Ride height is another place where folks sometimes go too far. For one, you need to make sure you are not running on the bump stops too often. A shorter-than-stock shock body length like comes on the Koni RACE Sports, will pay off here. Alternatively, a Ground Control or Flying Miata shock top will allow for extra travel, or use the inexpensive late-model M2 Miata shock top as described in our suspension build-up article. Also, if you are extremely low, you might actually scrape the ground over speed bumps and such if you are not careful. With most threaded coilovers, you can adjust the ride height up easily for street use to avoid this problem. This will also take some of the tire-wearing camber out of the car and reduce front toe-out all in one easy adjustment.

Exhaust noise can also make a car a headache on the street. An adjustable exhaust like a Supertrapp is a wonderful way to bring the sound under control for street driving, while you can uncork it for maximum performance at an autocross. We’ve found that running nine plates makes our Miata quiet enough to drive through a National Park with no odd looks from bystanders.

And a final note on creature comforts. A base model Miata with no radio and no a/c is very light, but for a dual-use car in a hot climate, it may not be worth it. Consider that when going for those last few pounds. Of course, an IPod or Walkman does wonders on long road trips, and just putting the top up with the windows down and rear window unzipped will keep the sun off your head and a cool breeze blowing through.

All of this comes from hands-on experience with both our STS2 Miata and STS Civic. When traveling the country, we always use the race car to make side trips through the mountains, along the coast line, or just to the store. We also use the cars for daily drives when at home and drive an hour or so to local autocross events. For competition, the Miata is very low, sits on short, stiff 16” Falken tires, has very stiff springs, hard Delrin bushings and an unforgiving Kirkey aluminum racing seats with little padding. Sounds like it would shake your fillings loose, right? Not so. The Koni 2812 shocks have a large adjustment range and when turned down to soft, the car deals with road bumps very nicely. Along the same lines, we also typically run the tires down to about 26 psi for street driving. With just these two changes, the ride is taught but supple over the bumps. When at home, we swap on some 185/60-14 Goodyear Eagle GT’s and run them at about 26 psi and the ride gets even better. In the off-season, we crank the ride height up about 1” and swap out the racing seats. It’s a great car, why not enjoy it as much as possible?

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