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

STS2 Tire Testing

One of the nice things about living in central Texas is that you can run year ‘round. Also, we have a bunch of available asphalt lots that provide similar levels of grip (low-to-medium) to Heartland Park Topeka, though most are much bumpier. This gives us a chance to do some valuable off-season testing in a low-key environment and this past weekend we did just that . Running with the San Antonio Sports Car Club we took the opportunity to try out a tire combination that we saw fast guy Jason Frank run so successfully on his STS Civic at Nats this year. That would be the 205/50-15 Falken RT-215 on the front of the car with the 205/40-16 Falken RT-615 on the rear. If you go back to the tire testing write-up we did early last season on our Civic, you’ll see that we were more consistently fast with the 16” 615’s tire all around, yet were able to occasionally crank out a spectacular run on the 15” 215’s. The 215’s had lightning quick responses in the transitions, but were like an on-off switch at the back of the car. So we opted for the 16’s all last season. After watching Jason, we came to the realization that we could potentially have the best of both worlds with a staggered setup. We could have the responsiveness of the RT215 on the front with the RT615’s progressive breakaway at the rear. What’s more, on the RWD Miata we could still have the gearing advantage of the 16’s as well.

We’ve still got several sets of RT215’s in the garage that we’ve been carting around as rain tires all season. We also have a pair of the tires we ran on our Civic at Nats in 2005 which were shaved and had only a few runs on them. Since the 16’s we currently have mounted have a bunch of runs on them, we figured this would be a reasonably fair comparison. Not exactly a scientific test, but enough to get a feel for the situation.

The other big variable would be the course. Good testing requires a consistent surface and a course that is easy to drive consistently. Again, we’d have to compromise here. At SASCA’s site, the lot is sealed and the grip level gets significantly better over the first couple of heats as the sealer is worn off. Also, a competition course is usually designed to be challenging and may or may not contain a good mix of elements. As such, we arrive at the event early to help setup and were able to give some input on the course design to improve flow and mix. In the end, the course is a good mix of challenging elements, but very hard to drive consistently quick.

By double-entering the event, we are able to run in each of the four heats (normally, you run two and work two, but we did our work assignments during setup/teardown). That gives us three runs in each of the morning heats and four in each of the afternoon sessions. We also run our MaxQData plug & play GPS data logger to gather info on the runs. We are big fans of this unit, as it is versatile and the software can provide helpful information even between runs. Look for a print write-up on this in a future GRM. Now, on to the testing …

We start with our normal 16” setup as a baseline. First run out on a dirty course is a 56.2. It’s pretty much a throwaway as the tires are cold, and grip is down due to the sealer. By the time our second pass comes around, another 30 cars have run and times are beginning to fall. We run 54.8 with a minor bobble near the beginning of the course. Then on the third run we drop to a 54.4 by fixing the bobble and running everything else the same (verified on the MaxQData).

Swapping fronts to the old 15” tires, we notice that the surface of them is a bit “crusty” from oxidation. Heat-cycling a tire and then letting it sit a long time will do this. We should have scuffed them at home but there was no time now. Sure enough, our first run out on these tires the car is skatey for the first couple of maneuvers. But then the grip comes in big-time halfway through the run and we start to feel that old “do no wrong” RT-215 responsiveness. The time is a 55.4 because of the bad first half. Next run we go right down to a 54.4 matching our best first-heat time for the 16’s. More importantly, the RT-215 is a dream through the tight transitional sections (tight Chicago box, double lane change, bent 6-cone 63’ slalom). With our confidence bolstered we go out on the final morning run and just hammer the car through the transitions dropping a full second to 53.3!

During the lunch break we have a look at the data logs and see that the car really isn’t going that much faster in the slaloms, but is instead carving better in the sweepers. Some of this is attributed to being able to more accurately place the car near the inside cones and reduce distance. It is also unclear just how much of the improvement is due to surface grip improvement on the sealed lot. The plan for the afternoon is to leave the 15’s on the car for the 3rd heat, and then swap back to the 16’s for the final heat. This way, the 16’s will have the best opportunity for “last say” and it will validate our initial conclusions. Also, our past experience has been that most of the surface grip improvement comes in the morning.

With cool tires on our first run in the third heat, we fall back to a 54.0. Then with everything now warm, the next run is a 53.7 with a bobble. After that a 53.5 and 53.4. The datalogger shows these runs all to be similar to the 53.3 we did in the morning, so we are confident that the lot has not changed substantially since then. That said, if we were to combine the best parts of each run’s log, we could imagine a high 52.x run.

For the fourth heat, we bracket the test by putting the 16’s back on the car. Our secret hope is to go slower, but we give it our best shot anyway. Right out of the box we can feel that the car is tending a bit more towards oversteer. That would indicate that the 16” front tires were gripping a bit more than the 15’s. Sure enough, we put down a 53.3. The feel is certainly worse, though, as the car has to be turned a bit earlier and is not crisp in transitions. Still, the clock doesn’t lie! Second run is slower at 53.5 as we get late with too much speed into the Chicago box. For the third run we focus on slowing down our entry speed in certain key sections and finally snag a 52.8. We back it up, though not perfectly, with a 53.0 on our final attempt.

Conclusions? Well, the test was not even close to scientific and we are not sure how much to attribute to overall grip changes, as well as driver improvement over the day. But it was pretty clear that there is some potential here. We were basically able to drive about the same times with both setups, with the edge to the 16” RT615’s. We’ll have to repeat this test with fresh-shaved tires on a more straightforward test course and controlled environment sometime early next year.

Happy Holidays!

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