Apr 23, 2007 update to the Mazda STS2 Miata project car

Bridgestone vs Falken

Some of the contestants begin to line up
Sure glad we bought that cordless impact gun!

As mentioned in an earlier installment, Bridgestone has decided to once again support grassroots motorsports in a big way. In addition to tire purchase discounts to all SCCA members, they will also have on-site service at select events including the Solo Nationals and a contingency program. Most importantly, they are bringing two new sizes of their proven RE-01R tire to the states that will fit our STS2 Miata. These are 205/45-16 and 195/50-15. These two sizes are equal in external dimensions to the 205/40-16 and 205/50-15 Falken> Azenis that we currently run. While retail sale of these new sizes is imminent, we were able to obtain early samples to do some testing.

Since Nationals is now held on asphalt, performance on that surface is the most interesting metric. We arranged to once again use the Mineral Wells Airport where we’ve done previous testing, since we have a standard test course painted down, as well as a large skid pad. We chose to test a fresh set of shaved RE-01R’s in the 205/45-16 size, against some fresh Falken RT615’s in the 205/50-15 size. This removes the gearing change from the test since these tires have the same outside measurements. Two of the Falkens were completely fresh, while the other two had several heat cycles on them. Conventional wisdom is that the Falkens work best after several heat cycles and we’ll test that concept, as well. All of these tires are on 7.5” wide, lightweight SSR rims.

Courtesy of STS2 competitor Neal Bellamy, we also have available a cycled-in set of 205/40-16 Falkens on 16x7.5” SSR’s for some of our testing. And finally, we have some old RT215 Falkens on 7” rims in 205/50-15.

Our test plan involves first using the skid pad to establish a pressure/performance curve for the new Bridgestones. We use tire temps and lap times to figure that out. With that data in hand, we move through each set of tires collecting similar data at the pressures we’ve previously determined to work best. After all of that is done, we move to the practice course to run some comparison laps. Here is all of the nitty-gritty:

Skid Pad (182’ diameter, ambient temps 65-75 degrees)

Data format: start psi, (F temp:out/mid/in) (R temp:out/mid/in) (lap1, 2, 3, 4) => end psi

Bridgestone RE-01R 205/45-16 1: 36 (126/126/106) (101/106/101) (11.013, 11.048, 11.104, 11.130) => 38 2: 41 (123/124/109) (100/102/097) (11.240, 11.271, 11.284, 11.217) => 41 3: 37 (136/139/126) (110/117/104) (10.997, 11.067, 11.074, 11.053) => 39 4: 36 (132/136/118) (103/108/100) (11.066, 11.203, 11.259, 11.337) => 37 5: 33 (127/134/121) (115/117/108) (11.163, 11.553, 11.635, 11.644) => 34 6: 37 (127/134/120) (109/116/106) (10.986, 11.103, 11.245, 11.188) => 38

Comments: Set 1 baseline at 36 psi. Tires were at ambient temp to start.
Set 2 add 4 psi to existing pressure and performance drops by 1-2 tenths. Set 3 drop 4 psi back out to bracket the test and performance is restored. Set 4 drop another 3 psi and performance drops slightly. Tire gets squirmy. Set 5 drop another 4 psi and performance drops even more. Set 6 back to previous best to bracket the test and performance is restored.

Falken Azenis RT615 205/50-15 (F: fresh, R: cycled) 7: 36 (124/123/107) (104/104/095) (11.530, 11.622, 11.494, 11.355) => 37 8: 36 (133/130/116) (120/115/110) (11.323, 11.501, 11.482, 11.478) => 37

Set 7 baseline, tires at ambient. Squirmy feel, slow times. Set 8 redo for test confidence. Slightly better times as tire warms

Falken Azenis RT615 205/50-15 (F: fresh, R: cycled) 9: 36 (135/130/118) (110/116/104) (11.221, 11.434, 11.345, 11.243) => 37

Set 9 swap front and rear tires. Performance improves with cycled tires in front.

Falken Azenis RT615 205/40-16 10: 36 (133/124/113) (104/101/096) (11.284, 11.335, 11.264, 11.310) => 39 11: 36 (143/141/130) (113/113/110) (11.343, 11.488, 11.424, 11.438) => 37

Set 10 baseline, tires somewhat warm from use on another car Set 11 backup for confidence. Performance drops as front tires get too warm.

Falken Azenis RT215 205/50-15 (Front only, rear as above) 12: 36 (128/121/116) (115/105/100) (11.297, 11.420, 11.422, 11.399) => 37 13: 36 (142/138/133) (127/115/110) (11.207, 11.448, 11.397, 11.400) => 37

Set 12 baseline, tires at ambient Set 13 backup for confidence. Similar results.

Skid Pad conclusions: Clearly the Bridgestone generates better numbers for basic lateral grip. Unlike the Falken, the RE01R is quite sensitive to pressures, requiring only a few psi off optimal to affect performance. The temperature pattern seemed unusual, in that lower pressures made the center temps go up and the outers down. Temps indicate that less camber may be needed.

Test course: (sweeper, 7 cone slalom, sweeper, offsets, 3-4 laps)

Falken RT615 205/50-15 @ 36 psi 1: 25.991, 26.208, 25.817, 25.393 2: 25.975, 26.031, 25.799, 25.603 3: 25.579, 25.733, 25.296, 25.535

Sets 1 & 2 are familiarization runs to establish rhythm for the course. Set 3 is the focused test set.

Bridgestone RE-01R 205/45-16 @ 37 psi 4: 25.211, 25.104, 25.293, (timer failure on 4th lap) 5: 25.103, 25.716, 25.744

Set 4 is the test set Set 5 was run immediately after set 4. Tires were extremely hot afterwards.

Falken RT615 205/50-15 @ 36 psi 6: 25.790, 25.694, 25.664, 25.618

Set 6 brackets the test and shows similar performance to the first set

Test Course Conclusions: The Bridgestone was quicker on the test course than the Falken by several tenths of a second. Overheating will hurt the RE01R’s performance, so temperature management will be critical.

We have additional electronic data acquisition information from the test course laps to process which should help us back up our driver’s impressions. In general, though, the Bridgestone’s strength seems to be basic ultimate grip. It is a bit vague in transitional elements, but holds on in a big way through steady state maneuvers. Transitional performance can likely be enhanced through suspension adjustments, though.

Two other interesting things we noticed: 1) The RE01R shows 2-3 pts softer at ambient temps on a durometer vs the fresh RT615. Also, its circumferential grooves are about 2/32nds deeper than the void area in the tread blocks. Shaving or wear to 2/32nds yields a virtual slick tread design.

Bridgestone clearly has winner on their hands and we will be committing to their use for this season for our Miata. We hope to test the 195/50-15 size in the very near future to determine which size is the best for our application.

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