Feb 28, 2007 update to the Mazda STS2 Miata project car

STS2 goes BIG at Dixie Tour

As the largest class with 23 entrants, STS2 made a huge statement this past weekend at the Dixie National Tour. Over the winter many people committed to the class and built cars, some of them completed just days before this event. All of this bodes well for the class earning national status in the very near future.

We picked up right where we left off last season with a solid 9-tenths win in our project Miata. But it wasn’t as lopsided as the results might indicate. Several other drivers had very quick scratch times that were marred by cones, and others just could not put together two fast days. And we also had at least one strong contender go out with a motor problem. As these drivers get their cars developed and put in the seat time, we expect STS2 to be deeply competitive by year-end.

Thursday before the event, there was a practice held on the site to work out the kinks in some new Pro Solo software. This gave us the chance to further test some tire combos that we worked on recently. In particular, we tried swapping in a pair of 205/50-15 Falken RT-615’s on the front of the car, in place of our usual 205/40-16 RT-615’s. The test course was very difficult to drive consistently, but we noticed an immediate pick-up in time of about 2-3 tenths. That is, right up until we put a hefty flat-spot on one of the fronts locking up the front-right tire entering a tight pin turn under threshold braking. Looks like we still need more work on our braking system.

During the event itself, we went back to the tried and true 16” tire setup, but also brought the 15’s to grid when we saw how fast the courses would be. The taller 15” tires give us another 3 MPH which can oftentimes be useful in higher speed courses. We also invited competitor Mark Pilson to co-drive with us when his CRX developed engine gremlins. Mark would be the guinea pig each day, taking the first run and judging the gearing situation. The first day we put the car into the rev limiter a couple of times on course, but nothing that would suggest a gearing change.

Our first-day driving was good , but we did leave some on the table when our third run was not as good as the second. A quick analysis of our last two runs with our MaxQData data logger showed that we had picked up 4 tenths in one section (turnaround), 2 tenths in another (the box), but lost almost 8 tenths in an early section of the course (the “hooks”) . We love our MaxQ as we never leave an event wondering why we went slower!

The second day speeds were much higher entering the turnaround section of the course. Our co-driver came in from his first run with the news that he was on the limiter and coasting for about 200 ft. After a quick rear tire change to the 15’s, we then put down a reasonable time but it was quickly eclipsed by other drivers. The car just did not feel right (unstable rear), and but we attributed that to cold rear tires. Next run is a better time, but the car’s still nervous at the limit. Also, while the gearing helps in the one section, it hurts the acceleration everywhere else. A decision is made to swap back to the 16’s for the final run.

Once on course, it is clear that the right decision has been made. The car feels normal again, and we hammer out a much faster time. With the shorter effective gearing, we instead utilize third gear this time, and then do a tricky 3->1 downshift after threshold braking to pull out of the turnaround. This works well, though we end up a bit too deep into the turnaround due to the lack of engine braking in 3rd gear. Later analysis with MaxQData shows we lost some 4 tenths in the turnaround due to driver error, but gained 2 tenths back accelerating out of that turn (better gearing), 4 tenths accelerating off the start, and a whopping 8 tenths hammering harder through the first-half slaloms. It was good enough to put us out of reach for the day and the weekend and a good way to start the season.

Pondering the tire situation later, we come to the conclusion that the 15” tires are generating larger slip angles compared to the 16” tires in equal load situations. If the car’s handling was neutral with the 16’s and we switch to 15’s at the larger slip angle, we have just upset the balance towards oversteer. Its one thing to do so at the front of the car where steering input can be used to overcome some of this, but in the rear it made the car unstable. We could probably tune this out to allow the mixed setup, but the gearing issue cannot be fixed and the data analysis shows the acceleration value there. Call it a failed experiment.

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