Long ago, when Project Z was first introduced, our geek-at-large, Dave Coleman, installed four parts that put the car into the 1g club: wheels, tires, a rear anti-roll bar and a limited-slip differential. As he said, the car drives brilliantly. So when the time came for Project Z to get new shoes, it was hard to make the decision on whether to stick with the Coleman arrangement of 275/35R18 Falken Azenis RT-615 tires all round, or try something different.
Although I'm not typically one to mess with a good thing, I decided on something new. Which, ironically, is the set-up many Z tuners use. Coleman concentrated on maximizing grip through using the widest possible tire, with the same sizes front and rear. This makes the front contact patch proportionally greater than the rear when compared to stock. The sacrifice is high-speed stability, since the front offers more grip. Consequently, the Z oversteers like a drift car at low speed and teeters on a knife's edge at high speed. He calls it balanced, I call it scary. Stock Zs and tuned cars usually use a staggered set-up, which puts more tire in the back, making the car more manageable at low speed and more stable at high speed.
Snooping around on Tire Rack's website found few street rubber options that have as much if not more grip than the BFG KD or Azenis tires we've already tried. I also needed the staggered 255/40R18 front and 275/40R18 rear set-up I felt was best. There are only a few options that fit the criteria (like the Toyo RA1 treaded R-comp or the Kumho MX, both of which we were familiar with).
My mind wasn't set until my recent trip to Japan, where I encountered the Yokohama Advan Neova AD07. Recently brought Stateside, this ber street tire got the best reviews. Tire Rack even gave it a special classification on its site. It was worth a shot and provided a chance to try out some new product. Yokohama just didn't have the exact sizes I wanted.
Here's a crash course in tire and contact patch set-up:
The Neova only comes in 255/40R18 in front-almost an inch skinnier than our current 275/35R18 RT-615 tires, and over half an inch taller at 26 inches. I had two options for the rear, a 265/40 or a 285/30. Do the math and you'll realize that both are smaller in diameter at 25.3 and 24.7 inches respectively. This isn't the best idea in the first place, but particularly for the Z's traction control system, which assumes the rear tire is larger in diameter. Although this wasn't the case with the Coleman set-up anyway, my new plan would make it worse. Which meant two things: we would still have to drive with the TCS off all the time. And, all other things being equal, we would sacrifice ultimate braking distance, because the smaller rear rolling diameter effectively increases rear brake bias. In terms of optimal braking, Zs like smaller front tires, or at best, same-diameter tires front and rear. I wasn't going to give up rear grip, so I took the plunge and chose the 285/30R18, a stretch for a rear wheel that's 9.5 inches wide.
Such a drastic difference in rolling radius also has implications on the contact patch. Remember that front tire width has been reduced, while overall diameter is increased, resulting in a longer but narrower contact patch. The rear contact patch becomes wider but shorter from the smaller rolling radius. The average contact patch area is now somewhere between that of Coleman's set-up and stock-not so Dave-scary, nor as lawyer-friendly.
By Jay Chen
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