It doesn't take very many trips to a road race course before the amateur realizes that it's not what's under the hood that's important, but how much lateral force and braking force his car can develop. Even having all the cool suspension parts on your car, as one soon finds, has little benefit if you're sliding through a gravel trap at 100 mph because you've got hard-compound street tires! Ultimately, even good brakes can't compensate for a lack of grip in the tire department.
We have discovered the importance of good tires in our own 1993 Firebird Formula project car, Thunderchicken. We originally started out with Goodyear F1 Steels (size 275/40ZR17, mounted on Fikse FM/5 9.5x17-inch wheels). These donuts served us fine as our learning progressed and our car control developed. They also performed admirably on the street in all kinds of weather conditions. Yet, they left much to be desired on the track where their grip fell far short of Thunderchicken's capabilities. What we needed was a set of affordable DOT race tires. We're guessing we're not the only ones either.
Our recent trip to Virginia International Raceway was twofold: to get in some additional instruction via Track Time driving school, and to test two budget-oriented tire candidates. Specifically, we tested two competing DOT racing tires from two well-know tire manufacturers--the time-honored Kumho V700 Victoracer and the brand new Nitto Extreme NT555R II road race tire. We had already installed the Kumho V700s (275/40ZR17) on a set of Performance Wheel Outlet Motorsport 280 wheels (size 9x17). These beautiful and inexpensive one-piece castings are actually patterned off the famous 1995 Cobra R wheel. Unlike their Ford counterpart, the mounting pad is milled for a proper fourth-gen wheel offset and the bolt circle is drilled for GMs (5 on 4 3/4 inch). A series of scallops is then machined into the edges to give them further differentiation from the Ford design. We ordered ours with a bare finish and opted to spray them black for a truly mean appearance. Since our original report on these wheels (see "Road Race Ready," March 2001), we have verified that PWO now makes this wheel in a 10.5 x 17 size ($230 each) which easily fits a larger 295/35 or 315/35R17 tire for the rear of the F-body or C4 Corvette.
The new Nitto tires, which we elected to mount on our road-going Fikse FM/5 wheels, are the real story here. In a bold move, Nitto has decided to get into the DOT road race tire arena with its new Extreme R II DOT tire. Patterned very similarly to its popular Extreme R Drag Radial, the road race version has enhancements which make it more in tune with road racing, namely a stiffer sidewall. We found the soft rubber compound to be similar in grip to the Drag Radial (its UTQG treadwear rating is 100).
We think Nitto's newfound interest in amateur road racing is appropriate, given the manufacturer's positioning in the affordable performance market. We've seen Mustang and F-body road racers sneak Nitto drag radials (on all four corners!) into road race courses for a few years so a full-blown DOT road race tire was bound to happen sometime. To test the market, Nitto is offering only one size for now, a GM-favoring 275/40R17. As most of you know, this is the most popular plus-one size for fourth gen F-bodies. It also fits on Mustangs (it's a squeeze) and C4 Corvettes, giving Nitto a good customer base for the investment. Before you carry on about a lack of sizes, keep in mind that Nitto could've started out with a smaller, far more popular import size (Hondas and VWs being far more common in the ranks of autocrossing and amateur road racing). That decision took some nerve, and a lot of faith that the GM market is only going to grow.
In the other corner is a grand old favorite of amateur road racers, the Kumho V700 Victoracer. Kumho's commitment to grassroots motorsports goes back several years and has continued to be sustained-though not so aggressively pursued--by the company. Predominantly a maker of industrial tires (semi trucks, earth movers, forklifts, etc.), Kumho freely admits that its street performance and DOT race tires are sideline businesses which make use of excess plant capacity. The upside to consumers is that the end cost is far lower than competing performance products from BFG, Goodyear, Hoosier, Pirelli or Yokohama. Also, advertising and distribution costs are practically nil, driving retail prices of Kumho tires even lower. For the DOT Victoracer V700, the selection of sizes is excellent, with virtually all of the domestic ponycar and Corvette C4 sizes being well represented. It's no wonder the Kumho V700 is a favorite of amateur autocrossers, time trialers and road racers around the country.
In spite of having our full-tread Kumhos mounted on our PWO Motorsport 280 rims for the past year, we had yet to sample them on a true road course. We didn't know what to expect, but other Kumho users repeatedly told us how consistent the tire was from sticker to slicker. (Fully a third of the cars at VIR that weekend were shod with V700s.) Another observation freely given was that the V700 tends to suffer from accelerated wear, particularly on the outside shoulder. We did not, however, arrive at the same conclusion, perhaps in part because we had set our front-end alignment up with 1.4 degrees of negative camber on both wheels. This had the effect of reducing the wear on the outside edges of the two front tires (both Kumho and Nitto), but how much effect it had we can't say.
Of dubious value was the Kumho's DOT designation. Although we hadn't yet raced the Kumho before VIR, we did have the opportunity to gather some 500 miles of street use, ostensibly to "heat cycle" them and do braking and skidpad testing. During this time on the street, the rubber became so soft that virtually everything the tire came in contact with (which wasn't part of the road) was picked up by the tire and either became embedded in the tire or was ejected into the fenderwells. This is clearly a characteristic of a true racing tire with a no-compromise compound. As you might expect, this is a good news/bad news deal. You want maximum grip on the track, but don't want your street tire to be a magnet for all the detritus on the road. Regardless, it quickly became obvious to us that the V700 is a DOT tire in name only, much like a Mickey Thompson ET Street DOT is to a drag racer. You may also remember that we originally wanted a DOT tire so that we had the option of driving the tires to and from the track. After putting 500 street miles on the Kumhos, we would not recommend this as an option for this particular tire.
The Nittos fared far better on the street, despite their minimal treadwear rating and soft, grippy compound. Perhaps it was due to the deeper voids between tread blocks or the higher treadwear rating, but in 4,000 miles of street driving, the Nittos rarely picked up debris nor did they sling rocks obnoxiously into the sheetmetal (a positive trait both on and off the track). As a side note, since replacing the Goodyear F1s with the Nittos, we now make our daily commute with them as well. We even used them in three major downpours and, although they weren't the best rain tires, they performed acceptably well at highway speeds. With their shallow tread depth, the Kumhos would be simply unusable in the same conditions. As far as driving to and from track events with the Nittos on the car, we experienced no problem and feel this can be safely accomplished on a regular basis.
Going into this test, we thought that perhaps the Nitto wasn't quite as "hardcore" as the Kumho. It certainly didn't exhibit any of the nasty race-tire-built-for-the-street characteristics (R1s anybody?). The expectation was that the Kumho would grip like a beast while the Nitto would noisily drift and skid its way around corners like virtually all the other prosumer ultra-performance tires on the market. What we found shocked and amazed us, but before we get to the results, let's go to some background information on our test.
We tested both tires over two days in six discrete 25-minute test sessions around VIR's 2.3-mile 17-turn road course. After each session we measured all four tire pressures and the temperatures across each tire (outside, middle and inside). Throughout the test, we kept optimizing tire pressures to achieve an even temperature across the tire surface. This ensures the largest possible working contact patch at all four corners. The first two sessions of day one were conducted with the Nittos, however most of our attention was focused on learning the line of the track. Tire evaluation was limited during this time. The third (and last) session of day one was conducted with the Kumhos at full tilt. Day two began with the Kumhos (session one) and ended with the Nittos (sessions 2 and 3). Thus we had two honest-to-goodness full-tilt sessions with each set of tires. In the pit area, we had the staff of Kenny Brown Performance helping us with tire test data and tire changing duties, and while we were on track we were aided with lap timing by Atlanta-area Impala SS nut Paul Troup.
Throughout the series of test sessions, we ran in group "B" which is an advanced group largely numbered with instructors and prepared cars. It should be noted that we rarely found ourselves in a position where our progress was blocked by slower cars. Had this been the case, our lap times might have been more a function of "traffic" than true tire performance.
After getting acquainted with VIR's beautiful course during our two first sessions, we changed from the Nittos over to the Kumhos for some serious testing. We immediately noticed that the car felt grippier--just as we had anticipated. As the laps wore on during our first session, the tires felt consistent from lap to lap. We had noticed even in our first two sessions with the Nittos that the engine severely overpowered the rear tires on corner exit, so by now we were getting pretty adept at feathering the throttle coming out of a turn. Ideally, with a car as powerful and as heavy as Thunderchicken, we needed a wider rear tire--a 315/35ZR17 being optimal--but in fairness, this was not an option for the Nitto (the V700 is in fact manufactured in this size, a plus for the Kumho). With the Kumhos on, we initially were able to come out of the turns hotter, but after a half lap of this, the rears would get hot and oily and we would have to go easy for a half lap or so.
The Kumhos really don't exhibit any audible cues as to where the limit of grip is. Good street tires usually begin "singing" when the grip reaches between 75 and 80 percent of their limit and get louder as the limit approaches. (For the beginner, this can be both startling and welcome!) If you've been doing road race schools, time trialing or autocrossing with street tires for a while, a change to the Kumho will take a few practice laps because there is no tire squeal to speak of. They grip well up to the limit and then slide out in a predictable fashion. Even at the limit when they start to slide, the Kumhos are quite controllable. The only gripe we had was that if you don't know where the limit is and you overheat them, you'll be going slow for a little while so they can cool down. Continued punishment will only net slower lap times, more sliding, blistered tires and possibly sheetmetal damage. There isn't a lot of rubber on the Kumho V700s so you can't really afford to overheat them too often. After getting to know the Kumho's secrets, we were able to push our lap times down significantly, from around 2:11 (typical with the Nittos in our first two sessions) down to a best of 1:53.4 toward the end of our Kumho tests. Thunderchicken was clearly going faster and the tires were a big part of that. On to the Nittos.
Switching back to the Nittos was somewhat anti-climactic. We headed back on the track and for the first time we really had a chance to focus on them. Although we had run the Nittos for the first two sessions, we really hadn't concentrated much on the tires--learning the line and shift points had been our primary concern. Now that the track was becoming second nature, we really worked the tires. It's interesting to note that our immediate impression was that the Nittos were slower. Feeding throttle on corner exit was again the big concern (with TC's big 396 putting all that torque on-line), but there was just enough audible tire haze to signal a loss of grip before the tires got too loose or too hot. Even though the Nitto has considerably more tread depth than the Kumho, at 6/32nds depth it's far more shallow than a street tire (typically around 11/32nds). This translates into much less tire sing at the limit. In fact, you really have to listen hard for it over the engine and wind noise. What noise the Nitto does make, however, is just enough to make the novice comfortable at the limit.
In comparing the tires under both hard braking and turn-in, the Nittos and the Kumhos are quite similar. High-speed braking zones produced no lock-up or squirm in either tire and turn-in was sharp and controllable. We did have the benefit of a full Kenny Brown suspension system, Baer GT-P brakes and Performance Friction carbon race pads all around but, as we said earlier, all this won't make an ounce of difference if the tires can't hold the line. At the apex when we're beginning to feed in throttle to balance around the turn, the Nittos gave us more feedback for better driver control, but at the same time felt a tad less grippy and the car tended to drift out a bit more, but not enough to impact lap times negatively. In fact, we clocked our best lap times with the Nittos on the car, with four of the laps in our final Nitto session besting our top Kumho lap!
Behind the wheel, the Kumhos felt like they provided more grip, and normally this will produce better lap times. But we kept coming back to those corners where the tires would be a little hot and right on the edge of traction. As you near the apex, you realize that the tires aren't going to cover the speed and they start sliding away. Not that we were in any danger (the Kumhos are exceptional in their controllability), it's just that the corner (and the next few after) were a total loss as far as making good time. For our car at least, the Kumhos were too soft, or too small or both. Yet the Nittos were the same size and were forgiving enough to keep us rolling when the temps got up there. At the limit when the Nittos got hot and started to slide, they came back sooner and lasted longer before going away again. They also communicated better before they got too hot. Advantage: Nitto.
While all this might sound somewhat subjective, we did attempt to monitor and control the conditions as much as possible. One important contributing element was in monitoring the tire temperatures throughout the day. It is tempting to think that because the Kumhos were more slippery when pushed to the limit, that they tended to get hotter as a result. That just wasn't the case. We saw nearly the same peak temperatures immediately following the Nitto sessions as we did after the Kumho sessions (as much as 200 degrees for the rear tires!). The only thing we can deduce from this is that the rubber compound used for the Nittos is more resilient under adverse conditions than the Kumho's. We would even hazard a guess that if the Nitto tires were shaved to the same tread depth as the Kumho, it would stick just as hard in the turns. We might even suggest that Nitto consider offering its road race tire in shaved form (as Tire Rack currently does for the Kumho) for its most hard-core customers.
From a cost standpoint, we think the Nittos are the best deal of the two, but that's a little complicated and justifies some explanation. Lap times notwithstanding, the Nitto Extreme R II can be used as a street tire in a pinch, while the Kumho is restricted to the race track. The Nitto's better lap times make them the clear-cut winner on track-unless you look at the fact that the Kumhos are slightly less expensive and are available in other sizes. Obviously, if you don't take a 275/40ZR17 (which are available for $155 each through Discount Tire), you'll be looking for the Kumhos, or something far more expensive in a DOT road race tire. If you do take the 275/40ZR17, you'll also want to consider the added life expectancy of the Nittos over the Kumhos. We think they're definitely worth the extra $5 per tire.
Nitto should do well in the GM market with its new NT555RII, but-and that's a big but-it will all depend heavily on the sizes they ultimately decide to offer. Kumho has done its research and offers the V700 in most of the sizes required by Camaro, Firebird and C4 Corvette enthusiasts. Fortunately, Nitto is in the planning stages and encourages input from potential customers, so if you have suggestions for size, contact Nitto at firstname.lastname@example.org. Our guidance is that Nitto manufacture a bare minimum of four sizes--the existing 275/40ZR17 (fourth-gen F-body, front and rear; third-gen F-body, rear; C4 Corvette, front and rear; C5 Corvette, front), a 315/35ZR17 (fourth-gen F-body, rear; ZR1 and C4, rear), a 305/35ZR18 or 315/35ZR18 (C5 Corvette and Z06, rear), and a 245/45ZR17 (third-gen F-body, front; Mustang, front and rear). These sizes would cover 90 percent of popular fitments for late-model GM performance cars and would also fit a variety of other cars including the Mustang. If Nitto elects to move forward with more sizes, they will take into account the non-GM market as well, so feedback from potential customers will undoubtedly influence things.
So which tire is best? The short answer is the Nitto. However, when you look at the availability of a 315/35R17 with the Kumho (for only $10 more each tire) it gets complicated. If we had availed ourselves of the Kumho's larger size on the rear with a 10.5-inch PWO rim, we might very well have a different outcome with the lap times. With over 400 lb.-ft. of torque on hand at 4000 rpm and with 3,800 lbs. of car to support, the 275/40ZR17 was toast--both for the Kumho and the Nitto. One thing's for sure, if we can get our hands on the wider rear meats from both manufacturers, we'd love to do a rematch!