Racing legend Mario Andretti remembers running low on time during a tire test session in California around 1970.
In those days, engineers for tire companies created grooves in Indy racing tires by hand, thought to better handle dust and debris on the track. But time was short this testing day and engineers wanted to try a new tire construction but didn’t have time to create grooves.
“They sent me out in slicks,” Andretti told Seniorhelpline. “They told me to take it careful and I went out there and set a record. It took a couple of years where we were doing less and less grooving and going faster and faster until we said let’s not groove at all. It was that primitive to some degree, but it all had to be proven on the track.”
Proving tires on the track has turned into a mainstay of the racing world. For over 100 years, we’ve seen changes in construction and compounds. But every change traces its history back to the same starting point—the 1911 Indy 500.
Where Rubber Meets the Road
Ever since the first set of 4.5-inch-wide tires rolled into the winner’s circle on Ray Harroun’s car at the Indianapolis Motor Speedway in 1911, tire manufacturers have worked to improve construction to get the cars moving faster. The diameters and widths of the tire have changed, the styles have slickened, construction has morphed, and compounds have evolved.
“The science that goes into tires, when you talk about back in the day to today, it is amazing when you see the chemistry and science that goes into each tire,” says Jay Frye, IndyCar president. “It is very labor-intensive to build.”
The story starts in 1911, with the first tires to win the Indy 500, 4.5-inch-wide Firestone tires with just 2.5 inches of actual tread hitting the road when cornering. Incremental changes took place for the next few decades, even as tire manufactures reduced fabric use in tires in lieu of ply (layered) construction, but still only minutely widening the tire.
It wasn’t until 1965 that the 9.2-inch Firestone tires hit the winner’s circle, roughly doubling the size of the tire used in 1911. Still though, even as Firestone and Goodyear created race-specific models for their teams, widths from both manufacturers hovered around 10 inches for another three decades.
The big changes, says Cara Adams, chief engineer for Bridgestone Americas Motorsports of Firestone Racing, came with the shift to radial construction, slicks, and the eventual widening of the rubber.
Tires were originally crafted by crisscrossing fabric atop a core, but the radial design arranges cords radially from the center of the tire for improved traction and flexibility.
“I remember my very first ride in Trenton, New Jersey,” Andretti says. “I don’t think the tires were more than 9 inches wide in the rear and 6 or 7 inches in the front and I will tell you, I remember even then the hardness of the compound. Oh my God, they were there to last for some time. I’m thankful I was born early enough to experience all of this.”
Andretti says the Firestone-Goodyear competition was key for the sport, creating an aggressive path toward development in the mid- and late-1960s.
“The progress we were making where you didn’t have to have a different car, but the performance you were gaining was increasing lap speeds up to six miles per hour from one year to the next and it was all to do with tire performance,” Andretti says. “All of a sudden we got smarter.”
That smart performance came from tire engineers willing to look at every aspect of a tire construction, from grooves to interior design.
Andretti first credits the slicks, and then changing from a cross-ply construction to radials. “I remember my very first run in Ontario, California, on radials I was told to be careful and on the third lap a whole belt came off on the right rear and I woke up in the hospital,” he says. “That was my first introduction to radials, and they learned a lot from that one. But all of this was progress, tremendous progress.”
In 1972, Indy allowed a bolt-on wing to the car, changing the speeds, requiring manufacturers to adapt to the car,” Adams says. The addition of the bolt-on wing increased average speeds nearly 20 mph overnight, significantly amplifying the downforce placed on the tires. Tire manufacturers needed construction that could improve grip without losing durability and at the same time keeping tires consistent.
That ushered in the use of radial construction. The change to the radial tire allowed for a greater grip and consistency.
“One set to the next is the same,” she says, “and it lowered how much steering you had to put on. It is a lot better for aerodynamics.”
Development also came from the drivers, such as Andretti, who selected a smaller tire for the left rear placement.
“I was going quicker because (the smaller tire) was helping me maintain speed through the tight corner,” Andretti says. “It dawned on me that I think we need to do that.”
Andretti was already setting up his car’s weight unevenly and the stagger of the rear tires furthered this design. He remembers a race where people started taking notice of his setup.
“Al Unser had a better car than me, no question about it, but after three laps there was a huge accident and a red flag and we stopped. I caught them looking in my car and asking what does he have in it. That is how much it paid off for us.”
The Tire for Every Need
And it wasn’t long before tire manufacturers were making different molds for different tire positions. Today, Firestone intentionally creates stagger in the designs. The right-side construction of a tire to run on an oval requires more durability. A track forcing drivers to always turn left requires a transferring of load into the outside tire, requiring a thicker tire with more air pressure and stiffness. Even the compounds are different.
The original small tread width and large rim diameter has, over time, started to change. The increase of width and decrease of diameters changes the aspect ratio of the tire, allowing for more grip with stiffness, Adams says.
Road courses offer a different take, but with over 60 variations of race tire specifications based on surface, conditions, and speeds, each race offers an exacting blend. This weekend 26,000 Firestone Firehawk tires will race for the 2019 season on the 16 oval, road, and street course venues in the NTT IndyCar Series schedule. “The compound or construction is unique to each track,” Adams says.
For the Indy 500 and the 5,000 tires used there, the right side must sustain long duration—turn one can last 35 seconds—of loads over 3,000 pounds.
“The right front is stiff, the right rear is stiff, matched to the front,” Adams says. “They are durable and consistent.”
The right rear tire comes one-third of an inch bigger than the left rear, which is also softer. The left front is where the grip comes to play, so expect to see 11-inch treads in the front and over 15-inch treads in the rear.
Firestone, which has served for the past 20 years as the exclusive provider of racing tires for the IndyCar circuit, will produce one specification for every corner of the car from its factory in Akron, Ohio. Adams says most teams will have two variations of dry tires, one that puts a focus on durability and an alternate that has an initial performance benefit that drops away quickly. Every team will also have rain tires. Every tire includes a mix of fabric, steel, and over 10 different compounds.
Frye says having an exclusive partner allows IndyCar and Firestone to work together on supply and safety. Firestone engineers collaborate with drivers from a variety of teams to make incremental changes to the performance of the tires, all while offering a consistent safety product to all teams, he says.
A New Tire for 2019
For the 2019 Indy 500, Firestone updated the Aerokit design introduced in 2018. The fresh approach offers a tire that changes the shape of where the rubber meets the road to improve load dependency. The goal allows a more consistent level of grip as the tire is unloaded in traffic, Adams says.
With IndyCar regulated so tightly, there isn’t much room for teams to optimize the performance of cars—but tires offer some flexibility. Firestone provides a tire pressure range and teams can tinker within that range. While Adams says that every team at the Indy 500 will pretty much have the same tire, teams can select from different compounds for stiffness and softness.
“I think it is important to have where the drivers have to manage the tire through the course of the race," Frye says.
Even with the evolution of Indy, Andretti says the tires have played a pivotal role in the speeds of the sport.
“There were moments when there were huge, huge changes (in car construction) and the speeds would go up 9 to 12 miles an hour,” Andretti says. “All that gain was through the corners...and guess who has to deal with that: the tires.”