Endurance tire testing: part 2

Before getting to the latest post, I want to remind people that there is a YSAR author contest with the top prize being a Rumblestrip DLT1-GPS lap timer. For details, click the Contest link at the top of the page.

Last week I blogged about how my brother and I tested a few tires to find out which one was the best. What exactly does best mean? Lap time certainly matters, but also consistency. We do endurance racing, not time trials. So our ideal tire is one that gives the driver the confidence to lap consistently fast. I’ll summarize Mario’s impressions from last time.

• RS-4 is the best tire because it feels best and records fastest laps
• RE-71R is a good tire, but hard to drive consistently fast
• RT615K is a good rear tire, but it lacks grip and feel on the front
• R1R is soft and feels weird

This week, let’s take a look at telemetry traces and see if we can get a little more resolution on the differences among the tires.

RS-4 (first run)

The two laps in the graph below have nearly identical lap times (1:37.4). But they are really very different laps. Looking at the time difference in the bottom pane, you can see that one lap gets ahead of the other by about 0.25 seconds. Then it loses all that time and ends up 0.12 seconds behind. While the fastest lap recorded was 1:37.4 it could very easily have been a high 1:36.

R1R

Here are 3 laps on the R1Rs in the same 0.2 second span (1:38.3 – 1:38.5). Again, while that sounds pretty consistent, it isn’t. Mario drives the first and second halves of the course very differently. The first half of Thunderhill West has more compromises and high speed corners. The second half features several hairpins. He drives the hairpins very consistently, but not the high speed corners. Why? Maybe he has better braking markers in the hairpins? In any case, when trying to figure out which tire is best, we have to take into account each corner, not the lap time.

RE71R

The 3 laps graphed are all 1:38.1X, so amazingly close together. I haven’t plotted the fastest lap here. Overall, the laps look more consistent than the R1R laps. Is that because the driver is getting more accustomed to the track or because the tires give better feedback?

RS4 (second run)

These 3 runs are within 0.1 seconds of each other. The fastest lap was not plotted. Overall, consistency is much better.

All runs combined

Overlaying all the runs, you can see just how much variation there is in first half of the track. Mario felt much more confident on RS4s, and this translates into braking much later. This produces a transient time gain that is partially lost by braking a little too deep. These are bumps in the time graph at the bottom relative to the fastest blue lap. The corner where RS4s appear to help the most is T6 through T7. Here, the blue lines pull away from the others (see bottom time lost). Once in the hairpins, RE71Rs appear to be just as good as RS4s despite having narrow tread and wheel widths.

Conclusions

So what did we learn? For one thing, laps that look the same from the perspective of a stop watch can be really different in detail. There’s too much variability in the first half of the lap to say much about the relative grip of the tires. We can say, however, that the feel of the tire matters very much to the driver. In the second half of the track, where comparisons are more robust, RE71Rs may be slightly better than RS4s. Despite having only one session on RE71Rs, he drove them at least as well as the RS4s.

Let’s finish this off with a few bullet points

• Because feel is such an important characteristic, you really should try a few tires rather than settling on what is cheap or convenient.
• It’s probably easier to fit tires to the driver rather than asking your driver to change their style to fit the tire.
• Don’t rely on a stopwatch to tell you which tire is best.
• Doing tire tests on a 5-run HPDE day with a driver who hasn’t driven the track in over a year isn’t going to get the most consistent data.
• It was a great day of driving and data mining, and I can wholeheartedly recommend taking tires and timers to the race track.