May 18, 2006 update to the Mazda STS2 Miata project car

STS2 Comes of Age

Newsflash: STS2 car takes the Super Challenge win at Atwater Pro Solo!

In the time since the last update, we’ve done some major dyno testing of various components (details in another article) and competed in several national events with very good results. We’ve also had a chance to see how STS2 has continued to evolve as a class across the country. Finally, we’ve started making event-day adjustments to optimize for various course type and surfaces.

After taking a break to compete successfully in our STS Civic at Walnut Ridge (Tour and Pro), we moved on to Houston and Atwater for the National Tour events in the STS2 Miata. Talk about a contrast! In Houston, the surface was low grip asphalt that punished you for being slightly off-line due to the gravel build-up, and the course featured a good mix of slaloms and sweepers. Atwater, on the other hand, was almost entirely fast sweeping turns on super high grip concrete. The results, though, were the same in one regard. We took a second place at each. In Houston it was a result of several small driving errors that cost us. In Atwater, though, it was failing to get up to speed quick enough on the amazing grip that took its toll. The first day we were 6 tenths out, but only 2 tenths on the second day. Too bad it wasn’t a three-day event!

The Atwater Tour was a watershed event for the STS2 cars. With a total of 10 entries, all of the major target cars were represented (CRX, Miata, MR2 and RX7). More importantly, it was the first time that STS2 beat STS at a national event. Class winner Mark Scroggs did it both days, and I did it the second day. Woo-hoo!

We’ve also started to make adjustments on the car during these events. With increased grip comes increased body roll and the need for more static negative camber to compensate. Due to the big sweeping turns, the rear of the car was coming loose before the front so we wanted to get camber in the rear to stabilize it. The easiest way to do this at the track is to adjust the ride height. A number of years ago we spent time on an alignment rack plotting out the rear camber and toe change curves on our Miata. From our notes, we knew that dropping the ride height would add about a half-degree of negative camber to each side of the car for each .25” lowering of the spring perch. By lowering the car .5” at the shock (about .75” actual ride height) we went from -1.75 camber to -2.75 in the rear.

Dropping the rear height also has two other effects. It has a small effect on toe, moving .05” towards toe-out (total). Normally, this would be in the opposite direction of what we wanted, but we already had .125” total toe-in, so it would not be a problem. Finally, the drop also changed the rake of the car. Rake is the relative ride heights of the car from rear to front. Miatas tend to work best about .5” higher in the rear than in the front. Less than that and the car will not rotate properly. Of course, in our case we were looking for less rotation so less rake worked in our favor as well as the camber.

At the Atwater Pro Solo, the courses were a bit tighter so the car was pushy at the limit. With the improved camber setting of the previous week we decided that the rear could now handle more of the weight transfer. To compensate we went to a larger rear sway bar for the Sunday runs and it really paid off. We had been running the stock 12mm bar set in rubber bushings. Instead we opted for the Racing Beat 5/8” bar, set on the softer of the two settings. This made an immediately noticeable difference with the car now carving the turns instead of floating around. We were able to get on the gas much sooner on corner exit and picked up boatloads of time doing so.

Speaking of the Atwater Pro Solo, it was also a turning point for our project car: its first win! And what a win it was. After once again finishing second in the class competition, we were starting to wonder if this would only ever be a second-place car? But with increased confidence in our demon tweak, we knew there was more time out there. Starting off with a “grudge match” win against the class winner (Mark Scroggs) in the first round, we continued to drop time throughout the Super Challenge, eventually running times that were a half second faster on both courses! The quickness and consistency paid off with an overall victory. Furthermore, those times were once again faster than the STS winner.

STS2 has now shown that it is for real. Next up: Mineral Wells.

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