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One Hot Lap

“Cool car. Looked fast out there. What kind of lap times were you turning?” It’s the inevitable conversation at track days around the country, where quick lap times are worn like a badge of honor by those who achieve them. This is especially true when the vehicle of choice doesn’t appear to be all that fast.

And while the driver is key to extracting maximum performance from the vehicle at all points around the track, the tires are what connect the car to the road. Maximum grip, response, communication and consistency are the traits drivers need for that one hot lap.

Sensing an opportunity, tire makers have responded to this market by creating a whole class of streetable track day tires, ones that carry DOT approvals and deliver in spades on the road course. The downside of these R-comps is longevity: Their sticky rubber quickly becomes a sacrificial offering to the god of speed.

At the same time, various automakers have engaged in an ever-escalating battle of high-performance street cars equipped with longer-lasting and supergrippy street tires. Those OE fitments have served as the basis of the tire market’s Extreme Performance Summer segment, and they have a collateral effect on the replacement tire business. This is a win-win for track enthusiasts, since these tires approach the performance of streetable track day tires but are easier on the wallet.

Various competition-oriented motorsports organizations have recognized this trend and restricted tire usage to those with UTQG treadwear ratings of 200 or better in some of their classes. LeMons, Redline Time Attack, Ultimate Street Car Association and One Lap of America are examples of these venues , while ChumpCar’s minimum is close at 180. These tires also find a home in NASA Time Trial, where the points-oriented modification system often favors their use.

In our June 2015 issue, we tested the latest crop of these tires around the cones for autocross use. This time, we decided to see how they perform on the road course.

Our format for the test was typical of these events: one warmup lap, three timed hot laps, and then a cooldown. We bracketed each session using the winner of our track test from two years ago, the BFGoodrich g-Force Rival. This allowed us to verify that conditions didn’t change throughout the day and that the driver and car were consistent.

Since running at the limit on unfamiliar tires is risky business, we chose to use familiar equipment in a wellknown environment: the One Lap CRX running on its home track, Harris Hill Raceway. Situated in the rolling Hill Country of Central Texas, the track offers medium-high speeds and a variety of corners and elevations.

We held our test on a typical spring day for the area: sunny, with ambient temps in the 80s. All tires were 205/50R15 mounted on 15x8-inch 6UL wheels by 949 Racing.

BFGoodrich g-Force Rival

HTML Tables
average lap best lap
87.23 87.21

Now going on its third year in the field, the g-Force Rival is a track favorite, especially for low-buck endurance racers and budget-oriented track day enthusiasts. This tire has a reputation for delivering consistently fast laps, long life, and intuitive handling. As a result, BFG will continue to produce this tire, selling it alongside their new Rival-S. (The S is better suited to autocrossing and two- or three-lap time trial events.) The Rival won our last street tire shootout, so it will also serve as our control tire here.

On the track, the Rival performed as expected, with lap times all within a tenth of a second. According to the data logger, it was especially good in the center of the turn and at putting down power early at corner exit while still loaded up laterally.

Dunlop Direzza ZII Star Spec

HTML Tables
average lap best lap
87.25 87.10

Dunlop has been a longtime participant in the Extreme Performance Summer tire group with their Direzza line, and continues to tweak it every year or two to stay near the front. Compared to the original version of the ZII, the Star Spec is a bit more consistent with respect to heat tolerance, delivering quick times right out of the box and staying fast throughout a session. Pushed hard and long, though, it will fall off a bit. The new compound is also a bit less edgy, though still extremely responsive.

Hitting the pavement, we found the new tire eager to dive to an apex, but with a little less mid-corner grip than the Rival. Further, the limit of adhesion fell off a bit more abruptly. That combination made our times less consistent, but the Star Spec’s best single lap slotted just in front of the Rival’s by a tenth.

Bridgestone Potenza RE-71R

HTML Tables
average lap best lap
86.20 86.00

New for this year, the RE-71R has the enthusiast community buzzing. It was the first tire from the class of 2015 to hit the market and represents a massive leap forward in on-track performance. Most of that gain comes from a much grippier tread compound, but at a cost: reduced longevity. Between both of our tests, this tire wore about 20- to 30-percent faster than the earlier tires. The wear rate did slow down after the first heat cycle, though, so it should benefit significantly from a gentle first use to avoid graining.

From the first corner on, we were in love with this tire. It has all the responsiveness of the ZII, but with much more grip–especially mid-corner. Breakaway at the limit is progressive, making it easy to keep right at 100 percent through the whole turn. Power-down off the corner is especially good, as is deep braking. This tire also talks a bit, making it extremely communicative and intuitive with fast driving.

Data logger analysis revealed the magic: Braking zones were shortened, deceleration rates were 15 percent faster, and midcorner speeds were up about 3 mph. However, the magic doesn’t last long. By the third lap, the tire’s performance started to suffer– not so much in the middle of the turn, but in the multitasking zones of corner entry and exit. The result: a total lap time deficit of about half a second.

BFGoodrich g-Force Rival-S

HTML Tables
average lap best lap
86.50 86.30

The Rival-S echoed its performance from our autocross test on the track, pushing the bar forward by a solid second per lap over the original Rival. It has similar track manners but way more grip. That means deeper braking, higher mid-corner speeds and better forward bite. Unlike the earlier version, however, this one is virtually silent, making it a bit more of a challenge to explore all of its performance. This showed up in testing in the form of a slower, more uncertain first lap followed by two consistently fast hot laps.

Compared to the RE-71R, driving on the Rival-S is less intuitive. The limits of adhesion are quite high, but breakaway is so mild that the edge is not as easy to find. The large slip-angle plateau allows for overdriving with little penalty, but you need to be familiar with this tire to derive maximum performance–not unlike the Toyo R1R of the previous generation. This characteristic may contribute to reduced tread life as drivers push too hard and grind away at the tire.

Our data logger shows that the Rival-S and RE-71R are well matched mid-corner, with the big differences at entry and exit. Braking on the BFG is an astounding 1.05g, which is 10 percent better than the Bridgestone. However, the Bridgestone puts down power way earlier coming off the turn. Although the Rival-S’s braking advantage can extend the highest-speed sections of the straights, it can’t make up for the RE-71R’s increased entry speed onto the straight, which then extends all the way down. Advantage: Bridgestone.

Second Opinion: Testing with a Heavier Car

For a second opinion, we handed off our sets of our top two test tires to David Whitener. His mission was to put them through their paces on track using a much heavier car: a 2002 Honda Civic Si prepared for SCCA STF autocross and NASA TTF time trials. Unlike the Miata or CRX, this car has a less efficient strut suspension, a minimum competition weight of more than 2800 pounds, and an open differential.

Would the results be different on this car? We headed to MotorSport Ranch in Cresson, Texas, to find out.

Testing, Testing

First up was the Bridgestone RE-71 R, and right off the bat this tire felt perfectly at home on the big Civic. Its speciality is putting down power, and that really seemed to pay off. Trap speeds at the ends of long straights felt much faster than before. After the first three-lap stint, a quick glance at the lap timer confirmed the butt dyno: 1 full second faster than the TT class record our driver, David Whitener, set a month earlier on Federal 595 RS-R tires.

Next up was the BFGoodrich Rival-S, which had incredible mid-corner grip that kept the car super planted and easy to modulate. The BFG became harder to drive the hotter it got, but the lap times never fell off in any of the three sessions.

The Bridgestone RE-71R was the definitive winner on this car, and the data analysis shows where it made the lap time gains. Although the BFG could sustain higher lateral g-loads, the Bridgestone was able to put down power earlier on corner exit. Typically this was worth 2 to 3 mph all the way down the long straight.– David Whitener

Does Size Matter?

Both the BFGoodrich g-Force Rival-S and the Bridgestone Potenz a RE-71R are available in limited sizes, so we decided to see how much of the lap time delta we could make up for with a larger tire. Bridgestone’s largest 15-incher is the 205, while BFG also makes a 225/45R15. We matched that to a 15x9-inch rim for optimal performance and added it to our test rotation. The results were spectacular: Lap times dropped another full second.

Reviewing the data logs, we found performance increases everywhere, but especially at corner exit. That’s where the big footprint was able to put down all the available power as soon as we applied it. Most of the lap time benefits came from those increased straightaway speeds.

Sources

949 Racing
wheels
949racing.com
(949) 716-3111

Auto-Spec
mounting
automotive-specialist.com
(512) 472-4977

BFGoodrich Tires
tires
bfgoodrichtires.com
(877) 788-8899

Bridgestone Americas Tire Operations
tires
bridgestonetire.com
(866) 775-6480

Dunlop Tires
tires
dunloptires.com
(800) 321-2136

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Comments

View comments on the GRM forums
mazdeuce
mazdeuce PowerDork
2/10/16 3:36 p.m.

Were both of the Civics David tested his cars? And did you see times drop a second for the CRX and the Civic Si?
That full second is a pretty big deal for TT and might make a difference in how points are spent.

LuxInterior
LuxInterior HalfDork
2/10/16 3:51 p.m.

It's great to have the data and driving impressions. I wish the RE-71R came in 225/45 15.

brian6speed
brian6speed New Reader
2/11/16 7:50 p.m.

Been using star spec II, really wanna try RE-71R. Autocross and track my 03 Acura CL. There is a PDK Porsche Boxster Spyder who seems to always beat me by 1 second running RE-71R tires. Maybe that is all I need to compete.

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