There's nothing like new rubber to make the an older vehicle drive like this year's model...unless there's a snag.
The set of new tires were plopped on my doorstep like a pair of double-bound donuts. Fresh from the factory and tied with plastic strips, the Firestone Firehawk Wide-ovals begged to be mounted, balanced, and spun into action on my test vehicle--a 2007 Mazda MX-5 Miata. But what followed became an experiment and an education in tire design and application, and a greater appreciation for the technology in both the car and the tires.
I took the new set of Firestone's to my local Discount Tire outlet, where I have purchased tires and maintained all my cars for years. They mounted, balanced, and set pressures for the tires, and put them on my car in less than an hour's time. The beefy-looking new tread rolled smoothly and soundlessly, and the tires' stiffness made the Miata drive like new--despite its low 6-digit mileage on the odometer.
Driving joy turned to concern, however, when it became apparent that I could steer the car by using the gas pedal. Even at moderate speeds, a gentle push on the accelerator would goose the engine, and cause the car to veer to the left. Reducing fuel to the engine would cause the car to pull abruptly to the right. I headed back to the tire shop.
The technicians took a look at the tires--no obvious blemishes could be seen. They double checked the balance and the tire pressures, and just to be safe, rotated the tires on both sides of the car, front to rear. The problem was even more pronounced at all speeds. Acceleration caused the Miata to pull to the left; de-acceleration sent it into a dip to the right. Freeway acceleration was hair-raising, as the car seemed to want to go in its own direction.When I explained what was happening to the Firestone media rep, she was aghast, and immediately consulted Firestone Engineers. The word came back down the line to get the car to a designated Firestone store, where the manager would swap out the tires for another set. And so I made an appointment to have the problem addressed.
Not all tire stores are created equal.
I've been in some of the great ones, and I've been in some places that I frankly would not trust to patch a bicycle tire. The Firestone store in Tomball, Texas is a beautiful facility--clean and efficient. I was immediately put at ease by the manager, Jason Harris.
His crew un-mounted the Firehawks and inspected them for damage.
When I described what was happening, Harris commented that sometimes tires have imperfections and do strange things. Then he told me Firestone was going to replace the Firehawks with a different tire--one recommended by their engineers for my vehicle.
They mounted a set of Bridgestone Potenza's on the car. Same company, but a slightly different tire architecture, and as it turns out, a different manufacturing process, as well.
T. J. Tennent is Engineering Manager for Bridgestone-Firestone. He knows tires inside and out--especially inside.
"In every tire, there is an element of conicity," he explained. The root of that word, "cone," is the key--it's a slant of the tread relative to the sidewall, which will cause a tire to veer left or right if allowed to roll independently. And as he explained, the Firehawk tires, while an excellent tire for American iron, just don't play well with cars with more sensitively-tuned suspension systems--like Mazda's MX-5.
"You're basically driving a Japanese Lotus Elise," Tennent told me. The suspension on the Lotus two-seater is so finely tuned, factory mechanics must weigh the driver, and tune the car accordingly. The Mazda is similarly peculiar, and even a one-pound difference in tire pressures can cause the car to veer and pull. That was the key to the trouble I was having with Firestone's Firehawk tires--they're just not made for such nuanced suspension harmonics.
"The Firehawks are engineered for big American muscle-cars with lots of horsepower," Tennent said. Those cars are more forgiving of variations in tire manufacturing. Further, Tennent explained, "no tire is truly, perfectly round." During the manufacturing process, the conicity phenomenon comes into play, which is caused by the belts inside the tire not being totally centered. As you accelerate, the conicity pulls the tire in one direction; as you slow down, the center of gravity on that tire shifts, and the tire pulls the other way. "It only takes one tire on a vehicle to be out of tolerance to cause this," Tennent said.
The Bridgestone Potenza's are made for the metric suspension harmonics of the Mazda MX-5. I could immediately tell the difference in the car's handling--straight and true while accelerating through the gears; solid and stable while de-accelerating.
Tennent says it's not too common a mistake, but one that is warrantable, if caught immediately. The bottom line is understanding the suspension of the car you're mounting tires on. Firestone and Bridgestone tires are excellent products, but they're not interchangable.
"Typically, Firestone's are going to go on American muscle cars, like the Dodge Challenger," Tennent says. "The Bridgestone's are going to be found on more high-end, performance vehicles, and the Potenza's are the top of the line for those cars," he says.
One other interesting acknowledgement from Tennent: "Discount Tires by and large do an excellent, excellent job of selling and mounting tires," he said. "I don't always let them know who I am when I come into the store," (being a Firestone engineer). "They always do it right."
That's high praise from the high priest of tire design.
