Measuring Up

It’s at the heart of the capitalist ideal: the belief that the best-performing products will be the most successful. When that concept is applied to the motorsports arena, we get a popular marketing strategy: “Win on Sunday, sell on Monday.” In other words, if a product proves its mettle during the race weekend, it should be flying off the shelves by the start of the work week. That mantra has been the force behind just about every product in our scene, from motor oils to complete car lines.
It’s also true in the ultra-high-performance (UHP) tire market, where success in sanctioned competition often directly translates into increased sales of street tires. Even those not competing in autocross’s Street Touring ranks—a great breeding ground for top-tier street tires—still want the fastest tire possible. They’re also willing to pay top dollar for it.
The competition to deliver the best tire has attracted a large field, and in 2009 we needed to do two rounds of testing just to sort through it. As soon as we thought the tire companies were done raising the bar, new models would hit the streets.
The latest crop of these ultra-high-performance tires has simply amazed us, delivering performance unseen just a few short years ago. In fact, the winner bested the Falken Azenis RT-215—the quickest UHP tire from only a couple seasons back—by seven-tenths on a 35-second course. In the world of autocross, that’s nearly an eternity.

This level of improvement has raised a logical question: How much faster can these street tires get before crossing the line into the realm of race-ready, R-compound rubber? In fact, the soft-to-the-touch compounds used by several of the latest UHP tires have some people declaring that they’re really R-compounds in disguise. Internet forums have been buzzing over this very theory, with supporters pulling data from recent event results in an attempt to prove it. The comparisons have not been very scientific, however, often pitting Stock-class autocross cars against their modified Street Touring variants, ignoring the effects of the wheel, suspension and driveline allowances that the ST cars enjoy. For example, comparing an E Stock Miata with its Street Touring equivalent is really an apples and oranges exercise. How about if we do some real head-to-head data-gathering in an attempt to settle the argument? Yay, more tire testing. To reduce the complexity of the exercise, we decided to test a single, strong example of each type of tire. For the UHP street tire, we went with our latest winner, the Kumho ECSTA XS. We even shaved our set to maximize performance, as removing a few millimeters of tread reduces squirm and limits heat buildup. To keep this an intramural affair, our R-compound race tire would also come from Kumho; we ordered a set of ECSTA V710s. All of the tires tested were the same size—205/50R15—and we used the same wheel setup for all of our runs, 15x7.5-inch SSR Competitions up front matched with a pair of 15x7-inch Enkei RPF1s out back. The test vehicle would be our trusty Street Touring 1989 Honda Civic Si, and our experiment went down during a convenient Texas Region SCCA test and tune event on the clean concrete at Pennington Field.

Rule of Thumb

For years, a rule of thumb has circulated among autocross enthusiasts: R-compound race tires are worth about 2 seconds per minute over passenger-friendly street tires. Lately, Internet pundits have proclaimed the difference to be less than a second. As the online community would say, O RLY?
The attributes that most obviously set the R-compound tire apart are its very soft rubber compound and its almost total lack of tread. In theory, these features should produce superior lateral grip, especially on concrete. That’s because the soft rubber—often a product of race slick development—can get down into the pores of the surface and really hook up. Additionally, the R-compund’s tread compound gets tacky as it heats up, yielding even more stick.
Internally, things get even more interesting. The R-compound tire’s belt package has just enough puncture resistance to meet DOT regs, yet it remains strong and lightweight for optimal performance. This is especially apparent in the sidewalls, which are incredibly stiff in order to deliver instant transitional response. This stiffness also counters the mushy feel that the softer compound would otherwise add to the mix.
Not only did we want to produce a true quantitative comparison, but we also wanted to see where the time difference between R-compound and street tires occurred. What does each type of tire do well? What is it about an R-compound tire that makes it faster? Is it the rubber compound, the construction, or some other factor that gives it the edge? A GPS- and accelerometer-based MaxQData system plus our impressions would help answer those questions.

Going in Circles

Before hitting the pavement, we picked our exercises and set up our test matrix: skidpad followed by actual autocross testing. We also first cleaned both courses with some non-test tires.
To isolate and measure lateral grip, we set up a 125-foot-diameter skidpad and fired up the timer. We also used this process to establish optimal air pressures for the race tires—we did the same for the street tires during our previous tests.
We first mounted the UHP Kumho ECSTA XS tires and pushed them to the limit for baselines. The times were very consistent, yielding a four-lap total of 36.66 seconds.
Next, we put the race-ready V710 through its paces, starting out at higher pressures and completing four-lap runs to find its optimal setting. At 38 psi, the tire felt “skatey,” and its 36.10-second time wasn’t much of an advantage over the street tires. Dropping 4 psi produced nearly the same times.
Were the pundits right? Had the gap between the UHP street tires and the competition-destined R-compound tires been narrowed that much?
Well, maybe not. We dropped the pressures of the race tires another 4 psi—down to 30 psi—and were rewarded with that monstrous “hand of God” grip that R-compound tires are known to produce. This time we stopped the clocks at 35.65 seconds, a full-second improvement over the street tire baseline.
Since we seemed to be narrowing in on the best pressures for the R-compound tires, we only dropped the pressures by 3 psi for our next run. We slowed a tad to 35.76 seconds. To verify our findings, we removed another 3 psi of pressure—down to 24 psi—and slowed even more to 35.91 seconds.
To bracket our pressure testing, we then reset the tires to 30 psi—our previous best—and did another run. A 35.56-second time showed up on the clock, comparing nicely with the earlier time at that pressure.
Finally, after 36 laps with the V710 race tire, we remounted the XS street tire. We wanted to see if the surface conditions had changed, and our 36.40-second time favorably compared with our starting baseline.
Bottom line? The race tires beat the street tires by 0.84 second, which equates to 1.4 seconds on a 60-second autocross course full of nothing but steady-state sweepers.

Weaving and Winding

We saw a great deal more roll on the stickier tires. It was a dead giveaway that the race-oriented Kumho V710s were generating higher lateral g-loads than the Street Touring-legal ECSTA XS rubber.

Next, we moved to our full course to get a more typical comparison. This test would add in some transitional handling plus the usual braking and acceleration zones found in autocross.
After several familiarization laps, we ran our test plan of three double laps through the course on each set of tires, starting with the XS. After baselining with an average time of 44.9 seconds, we bolted on the V710 race tires and took to the course—then promptly put the car on two wheels!
Certainly the Civic is known for three-wheeling through the cones, but the immense grip offered by the V710 had clearly found the limits of our Street Touring-based suspension; it pulled the inside-front wheel just barely off the ground, allowing it to freewheel as we exited an uphill offset transition. No harm, no foul, as we were beginning to unwind the steering wheel on corner exit. However, the incident managed to grab our attention in a way that no street tire ever has.
Numerically, the results speak for themselves, as the race tires outpaced the street rubber by about 1.3 seconds on the 44-second course. Some quick math shows a 1.8-second advantage on a minute-long, national-caliber course. That’s certainly an improvement over the 2-second “standard” of old, but nowhere near as big a difference as some have claimed.

Digesting Data

With lap times and data from our MaxQData MQGPS-Quantum data logger in hand, we set out to uncover the cause of the time differential between the two tires. Common wisdom says that the R-compound’s biggest benefit is its massive sustained lateral grip, and our skidpad showed this to be true.
The R-compound tires also delivered notably better braking and acceleration in our test. Since our Civic doesn’t have a limited-slip differential and is often traction-limited on corner exit, the V710 race tires allowed for extremely early power-on and much deeper braking. However, the UHP street tires didn’t totally concede defeat; they spiked high peak g-loads in transitions, narrowing the gap in the slaloms.
As with all tests, this was just one data point using one course, one surface, one car and one setup under one day’s conditions. Experience tells us that the time differential would be a few tenths less on asphalt, and also that R-compound tires wouldn’t work as well in cold ambient temps. Plus, cars that aren’t traction-limited—like those with all-wheel drive—may not benefit as much from the additional grip.
And finally, some suspension tuning for the R-comps would have added to the differential. If we had kept both front tires planted, the car would have been even faster on the race rubber.
Regardless of the details, we clearly landed in the ballpark of that popular 2-seconds-per-minute rule of thumb. We are also not likely to see any new UHP models any time soon, while the rumor mill is working overtime regarding fresh entries in the DOT R-compound world; Goodyear and Pirelli are expected to release new tires soon, while BFGoodrich and Hoosier have updates on the way. Either way, the bar just keeps moving higher, and racers everywhere will continue to reap the benefits.

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Comments
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bmwohio
bmwohio None
7/4/10 10:50 p.m.

Great article, would love to see more research on R-compund tires only. Much like you do with the street tire shootout, do the same with R-comp tires. Thanks!

viper_racer
viper_racer New Reader
10/3/10 10:01 a.m.

save money by buying used racing tires like the dot R compounds and full out slicks at www.johnbtires.com

stickcarlson
stickcarlson
12/29/12 11:11 a.m.

Great read. It would have been nice to have all three tires be the same width. I'd think that'd have accounted for most of the gains over the 16" wheels

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