Apr 21, 2006 update to the Mazda STS2 Miata project car

Dialing In the Suspension - Pt 3

Here’s the final installment of our Dial In session at Mineral Wells for the STS2 Miata.

Two days after the main test session, we got a chance to do some more testing. We were holding the actual Evolution Dial In class, and had a small number of students. So there was ample downtime to try a few things here and there in our Miata. Also, one of the participants had brought out an STS Civic that was similar to ours so we’d get a good benchmark on how we were doing in our STS2 Miata.

Here’s a sample skid pad run (me driving) in the Civic:

10.979, 11.194, 11.112, 11.073

And in the Miata:

11.405, 11.382, 11.285, 11.368

The bad news was that the Miata was several tenths slower in basic steady-state cornering on an 11-second course! Simply maddening! And the tire temps on the Civic showed a much higher spread front to rear, showing that it was not as well-balanced as the Miata. But the Civic just went round and round with very little steering effort, while the Miata felt heavy.

To eliminate variables, and because the other driver wanted to try out my 16” 615’s, we swapped tires and I bolted on his 15” 215’s.

11.429, 11.467, 11.254, 11.276, 11.368

No significant change. Problem was in the car, not the tires. Let’s try that bigger rear bar again and put it on full stiff this time.

11.418, 11.255, 11.275, 11.233, 11.320

In the afternoon, we get some time to do slalom runs and work on shock settings. Adding compression on both ends makes the car much more predictable and it starts to come alive in transition. Some of the mushiness of the 615’s goes away. We play with the rebound settings at various extremes but find no improvement.

Finally, it hits me. We’ve been driving around the real problem. When pressed hard, the car is loose in the slalom because we are forcing it. It does not want to dive in on the cones. In short, we don’t have enough front toe-out!

Front toe can be a wondrous thing. It controls the initial turn-in response by using the inside front tire to steer inside the turn before enough weight transfer occurs to reduce its influence by loading up the outside tire. A little bit of toe-out can go a long way, so we had set it to .125” total toe-out. In our Civic we run .25” total, so we adjust the Miata to that same setting. Instantly, the slalom performance is improved and the car dives in on each cone.

We then take some full course runs and are consistently within three tenths of the Civic. Happy with that, we take the car back over to the skid pad and turn this:

11.210, 11.046, 11.166, 11.141, 11.200

Finally! Now the car goes around just as fast as the Civic. And it has a much better steering feel, easily making the turns.

Unfortunately, our test weekend is now over. In the end, we learned a lot about things that made little difference, and found a few that made a big difference. Now that we are in the ballpark on those latter items, we’ll need another session to try some more stuff. We didn’t really get enough time to play with shocks after we fixed the front toe, so we’ll have to find time to do that. But we are definitely faster now than before the weekend began.

In the future, we still need to work some on ride height, rake, and perhaps some rear alignment settings (toe and camber).

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