Muscle Mustangs & Fast Fords
Race Ready Radials
Testing the Nitto NT-555RII Extreme R DOT-legal road race tires
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Tire manufacturers have a tough challenge. They must build tires that are equal to or better than the stock OE rubber and in turn satisfy the many needs of customers who are already driving on pretty good equipment. Today's new cars handle really well, some can even rip through the turns with race car-like agility.
Nevertheless, the level of grip depends on the four little contact patches between you and the road, and if you're going to push the limits, you'll find the stock stuff just isn't grippy enough. Tire companies know this and many of them offer rubber packing the potential for increased traction, yet without sacrificing ride quality or safety.
When it comes to constructing a street-legal competition tire, the challenge is even greater. In road racing, open track driving or autocrossing, drivers work the tires extremely hard--way past any level you could achieve during normal street driving. A skilled road course or autocross driver will manipulate the sprung weight of the vehicle using the throttle, brakes and steering to get the highest level of grip during acceleration, cornering and braking. Fuel economy, road noise and long life are not a concern, but grip, seat-of-the-pants feel, steering response and safety are.
Nitto Tires of Cypress, California, has risen to the occasion and produced excellent tires for the street and the strip. It's likely you're familiar with the high performance NT-555 Extreme ZR series of tires and also the popular DOT-legal Nitto Extreme Drag tires for street and strip. Recently, though, Nitto released its latest design, the NT-555RII Extreme R that is perfect for road racing, open track events, autocrossing or maximum traction on dry roads.
"We had a big demand from our customers and from the folks at Steeda to make a DOT road-racing tire that could work with the same success as our Extreme Drag tire," says Conrad Galamgam, a staff engineer at Nitto Tires. "Some guys were using the Extreme Drag tires for autocrossing or even road racing [as this author was] because of the soft compound, but those tires have a soft sidewall designed for street use and drag racing, and we knew we could provide a better tire suited for cornering. So we went ahead and developed the 555RII, which is, in fact, a derivative of the Extreme Drag tire.
"The 555RII maintains the same large contact patch, tread pattern and tread compound as the Extreme Drag, but the radius from the sidewall to the tread is slightly different and there is a big difference in the sidewall construction," added Galamgam. "The internal construction of the sidewall on the road race tire is much stiffer. It has unique sidewall compounds that protect the casing plies, improve sidewall rigidity and provide heat resistance for extreme lateral acceleration.
"Additionally, the 555RII has a much higher density compound and that will allow it to hold up under increased side loading."
Road racing or aggressive cornering places extreme loads on the sidewall of the tires (as you can see in the lead photo) so a stiff sidewall is essential to help maintain control and grip. With reduced sidewall deflection comes better feel. It also keeps the tread flatter and more compliant with the road surface. The end result is almost always increased grip. If there is a downside to a stiff sidewall, it is that ride quality will suffer, but that's rarely a concern when trying to achieve maximum adhesion.
In the real world, the NT-555RII design should translate into increased traction and quicker laps times. Currently, the NT-555RII tires are available in three sizes: 275/40ZR17, 285/35ZR18 and 305/35ZR18) and they retail for $161 per tire.
Where The Rubber Meets The Road
While the technical data sounded good, we wanted to know if the tires would perform as advertised in the heat of battle. So, we picked up a set of 275/40ZR17-inch Nitto 555RII tires, wrapped them around Halibrand Cobra III rims we got from Discount Tire Direct in Scottsdale, Arizona, and strapped them to editor Jim Campisano's own '97 Cobra project car, better known as Superfly, Destroyer Of Hideous Camaros.
For comparison's sake, we used his everyday tires, a set of Nitto NT-555 265/40/ZR18s on '00 Cobra R rims, as the baseline. The testing was done on the 10-turn, 1.35-mile road course at Englishtown. Long-time readers may remember that our hopped-up Cobra features HP Motorsports caster/camber plates, Tokico springs, shocks and struts (street/strip versions for our test), with Steeda control arms out back. In addition, the warmed over 283-inch engine is making 11-second horsepower (without a power adder) and is backed by a Tremec six-speed and 4.88 gears. Braking is improved over stock with Baer EradiSpeed rotors and stock replacement pads.
About the struts: We set them to five for this test, their tightest setting. This is roughly equivelent to two on Tokico's normal five-way adjustable struts--exactly where Campy found optimal handling with the Cobra at Watkins Glen. He had done a two-day TrackTime driving school at the Glen with the street/drag struts and found them to be perfectly acceptable in the corners, though they allowed a bit of frontend lift on the long straights. For the small course at E-town, we thought they'd do fine.
Campisano's snake is set up well and that made the driver (me) very comfortable. On this particular day we found quite a bit of loose dirt on the track, so I took a few extra laps to clear the groove and to get myself up to speed. Campy's Mustang is more neutral than a stocker, but understeer or oversteer can be induced quite easily. The standard 555 tires provided good grip, but there was a noticeable push, or understeer just after entry on tight turns, and just enough oversteer on exit to get my attention. As usual, I did my best to be smooth with the application of brake, steering input and throttle. After about six or eight laps we broke out the stopwatch and timed laps of 1:32.31, 1:28.96, and 1:30.01. I felt really comfortable with those times so we went back to the pits and swapped on the chrome Cobra III Halibrand wheels with the "sticker" 555RII's.
Superfly took on a whole new look with the Halibrand wheels and we were expecting an entirely new level of handling with the Extreme RII tires on there. Because the tires were new, right down to the stickers, I ran a few laps at a modest pace to build some heat and to break in the new rubber. Tire pressure was set at 28 psi (rear) and 30 (front), and I was off for some hot laps. The difference in grip was noticeable in the first turn.
I can best describe the improvement as going from driving a Mustang with good handling to now having race-car like cornering in a street Mustang. The soft compound of the 555RIIs allowed me to get really aggressive on entry, and lap after lap I gained more confidence and went quicker and quicker. From a "feel" standpoint, the road-race tires had more bite on initial turn in, which enabled me to maintain higher speeds through the turns. Consequently, I was able to pick up the throttle earlier upon exiting the corners for greater straightaway speed. What fun.
At the limit, the Nitto's were predictable, and that inspires confidence in any driver. In lay terms, I could feel the car start to break traction prior to it sliding out (or pushing badly), and that communication gives the driver the opportunity to make an adjustment to keep the car at the limit, but without going over the edge.
So, exactly how much quicker are the 555RII's? We clocked them as being 3.28 seconds quicker, with our new best lap time being 1:25.75, and the remainder of the laps coming in at 1:25.68, 1:25.77 and 1:26.52. The difference was night and day. While the NT-555RII's are not the best choice for everyday driving (because they'd wear out quicker and the sticky tread pick up all sorts of road debris) they will provide a high level of fun and they are D.O.T.-legal. We highly recommend getting a set.
In fact, I already ordered a set for my '87 LX, along with a set of '95 Cobra R rims from Discount Tire Direct, and I can't wait to get back to the track and have some fun with my own car. See you at the road course.