Endurance tire testing: part 1

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I had an MRI recently that shows I have a herniated disc. So my back problems are pretty fucking real as well as being debilitatingly painful. As such, I can’t do any performance driving for a while. Good thing I have a twin brother who can step in and drive for me. In this case, it was a tire test day at Thunderhill West (follow link for instructional video). The car was my Yaris in mostly B-Spec trim but I recently upgraded the calipers to increase the brake pad choices. The Yaris has such a huge cargo area that we were able to fit several tool kits, 2 people, and 6 tires inside even with a full cage. I also have a custom tire rack that fits on a mini hitch that increases its capacity to another 4 should the need arise.

Here is a brief list of the tires we were testing. A more thorough description is given below.

• Falken RT615K+ 205/50/15
• Bridgestone RE71R 205/50/15
• Hankook RS4 225/45/15
• Toyo R1R 225/45/15

Five 20-minute sessions isn’t an ideal way to test tires. There aren’t that many runs, the runs are a bit longer than necessary, and there isn’t enough time to make a tire change in the middle of a run with a pit crew composed of exactly one gimpy 51-year old (me). So how do we figure out which is the best tire when the test conditions are so volatile? We have 3 methods.

1. Lap times
2. Feel of the car from the driver’s perspective
3. Telemetry analysis

In part 1 of this post we’ll look at lap times and how the tires feel from the perspective of the driver. Next week I’ll present the details and show why telemetry is so important. Now let’s hear from Mario, whose contributions are in blue text.

I tried to stick with a pace that I would do in an endurance race, and not to find faster lines or experiment with driving style too much. The point was to go for consistent laps, and measure all the tires on a level playing field. Naturally the air and track temperature changed throughout the day, and as this was a HPDE session, I had to throw away some good laps to manage traffic. Also, I don’t know the track that well, and so my driving was certainly going to improve throughout the day.

Let’s talk about the tires in the order they were run.

Falken RT615K+ 205/50/15 15×7

Fast 1:40.9, Median 1:41.6

Our team has used a lot of different performance tires over the years (Bridgestone RE11A, RE71R; Dunlop Z1, Z2; Falken RT615, RT615K, RT615K+; Federal 595 RSRR; Hankook RS3, RS4; Hoosier SM7; Toyo RR, RA1; Nitto NT01, NT05, Yokohama S.Drive). Historically, some version of Falken RT615K has been our go-to tire. The reason for this is that it strikes a nice balance in expense, life, and grip. It’s not the fastest tire, but it is one of the more durable in the 200TW category. Given that we’re more cheap than fast, we like Falkens. We almost always mount these on 15×7 rims even though 15×8 (or even 15×9) are supposed to be faster. Why? No good reason. Possibly because Spec Miata uses a 15×7.

I’ve heard that the RT615K+ is made in the same plant as the Dunlop Z3 Star Spec. The difference is the tread pattern. Falkens are typically $10 cheaper per tire. If they truly are the same thing, the choice comes down to camber wear. Dunlops have a symmetrical tread pattern, which means you can flip the tires on the rims, which may extend tread life quite a bit. If you read reviews on the RT615K+, people say they get greasy if you run them hard. That’s not a bad thing if you ask me. If you’re driving the limit, any tire will get greasy.

The tires we had for the test day were mounted on 15×7 Kosei K1 rims. They had seen quite a bit of track action, but only as rear tires on the Yaris. That means they were hardly worn at all. For the rears, we had a set of old RE71Rs (see below). Let’s hear what Mario has to say about the RT615K+..

I’ve driven the Falken Azenis 615K probably more often than any other tire, and they are the gold standard which I measure everything against. I hadn’t tried the 615K+ yet, so I was happy to go out on the Azenis first and see what all the plus was about.

My experience with them is that they warm up quickly, and usually the fastest lap is the second lap, but then they get a bit greasy when hot. Traction drops off a bit then, but stays at that level forever. And so they felt the same as always, with good audible feedback and a generous traction limit that doesn’t suddenly go away.

However, after trying the other tires, the gold standard is now the old standard. By comparison, the turn in was vague, and they simply don’t have as much stick. Perhaps on a 8” rim they would have worked better, but I doubt an inch of rim width was going to be the night-and-day difference I’d experience with the other tires. Later in the day we’d put these on the rear, and for that, they are my tire of choice.

Hankook RS4 225/45/15 15×8 (first run)

Fast 1:37.4, Median 1:38.4

The RSR has become one of the most popular endurance racing tires. One reason is that much of the competition has reengineered their tires for the larger autocross market where grip is more important than longevity. RS4s are a more traditional 200 TW tire that last a long time.

The tires used in the test were mounted on 15×8 TR C1 rims. The tires had been used in a previous Lemons race and had about half of their tread remaining. The rear tires were the same as the test above (old RE71R).

The second time I turned the wheel I knew I was on a totally different tire. I was initially a little bit nervous because the steering was so different than the Azenis, but the tires warmed up quickly, and I to them: super accurate turn in, great feedback, and you can hear them working. I put down a few consistent laps and was surprised to see they were over 3 seconds faster than the Azenis!

That’s pretty astounding considering these were back-to-back sessions an hour apart. The track and weather conditions were probably as similar as they were going to be, and I don’t think my driving line or technique changed much from the previous session.

The RS4s seemed to take a bit longer to come in than RT615K+, and were fastest on the 4th or 5th lap. After that they seemed to fall off about a quarter second.

Toyo R1R 225/45/15 15×9

Fast 1:38.3, Median 1:39.3

Originally a 140 TW tire, Toyo later rebranded the R1R as a 200 (probably to get more sales). Magazine tire reviews consistently report that the R1R is a pretty soft tire that wears quickly. It’s supposedly really good in the rain. It has one of the more interesting tread patterns. The brand new tires in the test were mounted on 15×9 Konig Dekagrams. Rear tires were as above.

I thought these were going to be the fastest, because they were the only ones on 9” rims, and they are basically rebadged 140 TW tires. But I just didn’t have much confidence under braking. They didn’t have as much audible feedback, and even through the wheel and pedals I never knew what they were doing. Any corner which required some initial braking cost me time, and while I thought these were the fastest tires around T2 (I could floor it all the way around), it was simply because I didn’t have the confidence to go through T1 faster. The run up to T7 requires the longest and hardest braking, and I felt like I was going to flat-spot them every time.

When we pulled the tires off, they looked totally different. The way the rubber was melting off the tire made them look like they were much softer than the other tires. I’d worry about the longevity of these in an endurance race. However, they were also brand new tires with full tread, and probably got the hottest because of that. I hear that R1Rs are good rain tires, and that’s probably where these will be used now.

Bridgestone RE71R 205/50/15 15×7

Fast 1:37.1, Median 1:38.2

The RE71R is well known as a cheater tire because it’s more like a 100TW than a 200TW in grip and longevity. I’ve heard some stories of them lasting only a few hours. My experience is that they are actually more durable than other tires on my Yaris. Despite its low power, my Yaris has caused blistering and chunking on most of the tires it has seen. It’s pretty frustrating to see a tire with very little wear except the shoulder has been completely chewed away. RE71Rs don’t do that because they can handle the heat.

We had planned to use these on 8” rims, but due to some fitment problems, we had to use the ones that we’d been using as rear tires, which were on 7” rims. That meant we had to move the Azenis to the rear, and so this wouldn’t be an apples-to-apples comparison with the other rubber.

But that turned out to be not such a bad thing, as the RE71R front and RT615K+ rear made a balanced combination with neutral handling. I was able to rotate the car much easier and play with balance more effectively.

As such, the RE71Rs set down the fastest time of the day (so far), but I didn’t feel they were as consistent. I felt like the one fast lap was an outlier, and that I couldn’t drive them that way lap after lap. These were also the oldest tires, and I’m not sure the effect of that.

Hankook RS4 225/45/15 15×8 (second run)

Fast 1:36.7, Median 1:37.3

We went back to RS4 front leaving the 615K+ on the rear, and again the handling was very neutral, similar to the RE71Rs. Taking some traction away from the rear definitely helps me.

Compared to the RE71R, the RS4s instilled more confidence, and this might be down to simply the sounds they make. A better driver might go fastest on the RE71R (or maybe even the R1Rs), but I felt better on the RS4s. And they were more fun.

So much so that on the last two laps I decided to screw consistency and up my pace. I immediately dropped half a second and did a 1:36.4. On my second flying lap I looked at the RumbleStrip and saw I had another .3 seconds in hand and thought I had a sub 36 lap in there… But they threw the checker on me in T8, and so I didn’t get a chance to find out.

Conclusions Part 1

Although we had come to the track to determine which tire was fastest, one of the most important lessons we learned was how much feedback is important to the driver. A simple tire swap can make a huge difference in the way a car feels and consequently what performance a driver can extract from a car.

Mario drove the RS4s faster and more consistently than any of the other tires. He also felt more confident with RT615Ks on the rear rather than the stickier RE71Rs. Moving forward, we’re now considering new combinations of tires. Perhaps 245 width RS4s in the front? Maybe something in 195 width for the rears? More tests will follow, but not until my back heals or my brother visits again.

Next week we’ll take a higher resolution look at the data using telemetry and see why you should always run telemetry.