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Loosening The Coleman Grip - Project Z

Appendix J

Jay Chen
Aug 16, 2007
0706_sccp_01_z+project_z+side_view Photo 1/1   |   Loosening The Coleman Grip - Project Z

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.

However, having different-shaped contact patches front and rear doesn't lend itself to predictability. Long narrow contact patches have a marginal advantage in wet traction (like a bicycle tire, which is less prone to hydroplaning) compared to the wide short contact patch of a go-kart tire (which is good for dry traction). It's not that big a deal for dry Southern California streets, but can play havoc for those who drive in changing weather or surface conditions, as different ends of the car will break away differently. Even in the dry, the behavior from different contact patch profiles will vary over different surfaces.

We've also changed the dynamic feel of Project Z with this choice of tire sizes, and not for the better. Putting so much sidewall up front inevitably takes away from the instant steering feel of the old set-up. More sidewall means more flex, so less steering precision, but more ultimate grip. No one else on staff has seemed to notice except for me. Maybe it's the placebo effect. The 255 fronts are also more square and the perfect fit for the front nine-inch-wide wheel, with proper sidewall angles and a symmetrical contact patch. The rears, on the other hand, are slightly puffed out and have little sidewall compliance, so they pick up more high-speed suspension nuances.

So did we make it better than Coleman? Probably not in terms of the small sacrifices. But the big picture is a much stickier and more forgiving car, mostly on account of the Neovas. Driving stupidity in the Z has definitely escalated among the staff. If we had the option, would we choose this set-up again? Definitely not. We just got lucky that the tire made up for all the things we were willing to compromise. Next time, we'd stick with the Neova, but beg and plead for Yokohama to make a 275/40R18 size (hint, hint) and solve all our problems.

Why did I waste two pages talking about the wrong set of tires? To shine some light on how much tire selection matters and convince people to put some thought and money into their tire selection, something we've been preaching all along. Picking the penny-pincher deal at Wal-Mart is tantamount to shooting yourself in the foot. Nothing says more than rolling up with a brand new set of forged Volks alloys mounted with Nangkang grocery-getter tires.Jay ChenEngineering Editor

By Jay Chen
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