An up-close look at Pirelli® tire development and testing for the 2013 SRT® Viper.
When Chris Winkler powered out of the turn at Chrysler's Chelsea Proving Grounds he thought the flock of geese resting on the track ahead certainly had time to get out of the way, so he kept his foot in it. The geese apparently didn't factor in the new SRT® Viper's power improvement and the greater traction of the new Pirelli® tires, because they had to rush to get out of the way. Most of them anyway.
Winkler rolled into the staging area with a big hole in the upper passenger-side windshield. Not the planned result, but maybe Winkler could stuff a pillow with all the goose down blowing around the cockpit of his Viper. Luckily the goose wasn't in there, too. Rack up another story attributed to tire testing for Viper.
Despite that one exception, tire test results went just as planned as the change to Pirelli is a resounding success with two grippy new tire choices in the bill of material for the 2013 Viper. The standard tire is now Pirelli P Zero and the track tire is the Pirelli P Zero Corsa when choosing the Track Pack. Here's how it all happened:
Tire engineer David Colletti is Pirelli's Director – Original Equipment, based in Southfield, Michigan. He's also a race driver—on hiatus now—and a former SRT vehicle dynamics engineer concentrating on tires. There he worked (and raced) with Erich Heuschele and Chris Winkler on such cool things as the Caliber SRT4. Before that, when he was with Goodyear, first as a tire engineer then moving into customer account management, he worked with SRT on the original SRT 300C, Dodge Magnum/Charger and Jeep® Grand Cherokee. After he went to Pirelli, he helped SRT develop the Grand Cherokee SRT tires, including the last-minute summer tire option.
Colletti has good feelings about his time as an SRT engineer. "Good people at SRT," he says. "It makes things easier - it was fun to go to work," he added.
In early 2010, Pirelli got the call from SRT for proposals to develop custom versions of ultra-high-performance tires for the new SRT Viper. You would think he had the inside track, with his close ties to SRT, but, as it goes in the OEM business, there were no guarantees. A supplier has to meet all the criteria—performance, price, etc., to get the business. Pirelli made the grade.
Pirelli's proposal to supply a P Zero street tire and a P Zero Corsa for track driving was accepted, and SRT gave the go-ahead in March 2011 to start a development program.
Pirelli's Ultra High Performance group in Milan, Italy, did initial designs with input from SRT engineers. Pirelli then created tooling for the company's innovative automated tire manufacturing process, called "MIRS," meaning Modular Integrated Robotized System, introduced by Pirelli in 1999. MIRS would be used on the standard Viper tire, the ultra-performance P Zero, and a modified system, called "Next MIRS," would be employed for the higher-performance Corsa.
The Corsa tires are promoted as a system featuring "directional" front tires—Pirelli calls them "Direzionale"—for better wet-weather handling. They have angled grooves that take water from the tread out to the sidewall to maintain tire contact in the wet. Asymmetrical (Asimmetrico) rears are specified for better traction. The combination works very well in most applications, but the engineers found an even better solution as a result of the Viper tire-testing program. More on this later in the story.
In the MIRS system, robots build each tire from beginning to end, with humans only observing from the sidelines. Next, MIRS is a modification that allows custom hand-building of the lower sidewall with textile materials (nylon, steel, Kevlar®) for optimum stiffness to achieve maximum performance. Viper's Corsa tire uses Kevlar.
The MIRS Tire-building robots are arranged in a compact loop using as little as 4,000 square feet on the factory floor. A custom-tooled metal drum, each one created for a particular tire size, is the base that receives specified rubber, fabric and steel, which is laid down by a succession of robots as the drum turns. The tire is then "vulcanized," a heating and curing process that includes molding the tread and sidewall designs. The drum is then removed and the tire is delivered outside the MIRS pod for its first contact with a human, who inspects the tire for flaws before sending it to shipping.
The first prototypes, called "Submission Tires" were built in Pirelli's Turin, Italy, factory and delivered to Chrysler for testing in July 2011. Andrea Arcari, Pirelli's test driver and Pirelli design engineer Matteo Savini traveled to Michigan to work with Chris Winkler, SRT's Vehicle Dynamics Engineer and Heuschele, SRT Vehicle Integration Responsible. They tested the tires on a prototype Viper at the Chrysler Proving Grounds in Chelsea, Michigan.
"Arcari and Winkler rode together," said Colletti. "They would take turns in the driver's seat and discuss their impressions as they went. I would also drive the tires with Erich, so we could add our impressions." While not the official test drivers, the two had lots of experience on the racetrack, so they could contribute meaningfully to the discussions on how to maximize tire performance on the new Viper. This team evaluated all the feedback, came to conclusions, and the Pirelli engineers took the consensus back to Milan for further development.
Another round of testing took place at Chelsea in October, 2011, and subsequently, in January, 2012, at test tracks in Chandler, Arizona, and Pearsall, Texas, where they carried out standardized on-track performance tests on the steadily-improving tires.
During this process, the engineers found that the Asimmetrico tires of the Corsa tire set worked very well on the Viper's front wheels in wet-weather testing. So they decided to use them instead of Direzionales. To make the tire even better, they consulted with the Pirelli Racing Group, which is just down the hall at headquarters.
The Racing Group gave them an even stickier front tire compound that makes the Corsa tire better in wet weather than the previous base all-season tire up to 60 mph. This is a real bonus because these track tires are also good on the street, which makes it possible to have one set of tires on the car instead of switching back and forth from road to race course. Of course, if drivers want to go to the extra effort, they can have optimum performance in both arenas by changing tire types to match road types.
All this work and technology came together by July, 2012, when the tires were certified for production, less than 18 months after program start.
Pirelli is supplying tires to the Connor Assembly plant as we write this. The new Viper, engineered and built in Detroit, has an athletic Italian flair where the rubber meets the road.