Tires may be among the last items put on a vehicle at the assembly, plant, but they certainly aren't an afterthought in the development process. In fact, tire suppliers are brought into platform discussions two to five years before launch, the same as other component vendors.

The reason? Tires have as much of an impact on how a car handles and rides as nearly any vehicle system. They are, after all, the only part of a vehicle that comes in contact with the road.

"Handling is the first criterion (OEMs) use to accept or reject a tire for a new vehicle," says Phil Pacsi, product information and education specialist at Bridgestone/ Firestone Inc.

Engineers working on original-equipment (OE) rubber also concentrate on fuel economy and rolling resistance, noise, wet handling and ride comfort. Wear is almost a secondary automaker requirement these days, say tire manufacturers; radials simply last longer.

One might expect a tire engineer's job to be easier with such a narrow focus. Not so, say those involved in the process. Of an average tire's 15 basic parts, sidewall rigidity, most influences handling performance. Then again, so does the crown angle of the belts under the tread and the reinforcements in the bead area (where the tire contacts the wheel).

"It's a very complicated balance," says Mr. Pacsi, noting that if a tire has the stiff sidewalls required for high-performance handling it inevitably gives passengers a rough ride over bumpy pavement. Conversely, if a tire, adheres well in hot cornering maneuvers, it's not likely to last long.

"There are a lot of tradeoffs," says Tim Roy, Goodyear Tire & Rubber Co. senior tire test engineer. "One may be willing to trade comfort for handling," adds John Kullman, chief Goodyear engineer for passenger car tire programs.

"There are a series of meetings to set objectives and negotiate with automakers," says Mr. Kullman, describing the beginning of the tire development process, which can cost between $500,000 and $3 million.

Chris Baker, Michelin North America's marketing manager for high-performance tires, says: "The process is pretty similar, whether it's a mom-and-pop car or a Viper. The biggest challenge is to get plugged into the persona of the car."

With OEM load and speed-rating parameters in hand and persona in mind, tire manufacturers begin their work in the laboratory. There, several off-the-shelf tires are speed-tested and evaluated for cornering power and rolling resistance. "Frequently on a high-volume car, we can take one of our existing tires and start with it, rather than designing an entirely new tire," says Mr. Baker.

Two recent advances have changed the face of tire development. One is the partnering between automakers and suppliers. The other is computer design.

"It's more of a team effort now," says Goodyear's Mr. Roy. "It used to be: `Here's our tire, be happy.' The team concept is very important now, and has been for the last five years."

Like the rest of the automotive industry, computers play a huge role in tire development. Engineers use computer models to evaluate stress between a tire's belts, cords and rubber. Computers also factor in road forces, pressure distribution and heat, all of which contribute to rolling resistance and fuel economy.

The ability to predict the cornering properties of an actual rolling tire in hours rather than the weeks it used to take with prototypes, has big benefits.

In hopes of landing more automaker business, each tire manufacturer boasts advantages in its development process. Michelin, Goodyear and Bridgestone/ Firestone all say they transfer technology from racing programs to passenger tires.

Michelin, for example, had approximately a year's worth of testing days over a three-year period for the original Viper. Bridgestone/firestone says it has a technological edge -- that its long-link carbon black (LL Carbon), using a longer-chained carbon structure, is more effective in reconciling conflicting performance objectives of wear resistance and rolling resistance.

It also boasts of its five-pitch noise reduction technology (5 [degrees] NR) that uses alternating tread block designs to create specific sound phases that cancel each other out, resulting in a quieter tire.

Goodyear touts its Pressure Analysis System (GPAS), which measures pressure changes across a tire footprint to identify where tread slip could occur. "We can't make stress disappear completely, so we're learning new ways to manage it," says Goodyear Engineer Mike Trinko. We're designing it into today's tires."