In its latest Ford GT video, The Cutting Edge, Ford executive Raj Nair, who heads the company’s global product development, said something that may surprise many fans.
In describing the design and development process for the Ford GT, Nair said: “With the race car, we start from the tire and what does the tire want?”
While designers and engineers diligently work to optimize powertrains, aerodynamics and mechanical elements, tires have an enormous impact as both the limiting and the enabling factor in race car design.
That is why Michelin has had a seat at the table in those early discussions for many years.
“Back in the day, a team getting close to finishing up a new car would call and line up tires for the initial tests,” said Chris Baker, director of motorsport, Michelin North America. “Those days have long since passed.”
“Manufacturers typically come to us very early in the ideation process,” said Baker. “They give us a good sense of what they have in mind and we work to see what is possible and agree on the targets.
“At Le Mans, the Audi, Porsche and Toyota LMP1 Prototypes all have powerful hybrid systems that can combine with their internal engines to produce the equivalent of over 1,000 horsepower.
“We also must consider the torque when the front wheels kick in for four wheel drive.
“To win Le Mans today you need to deliver four stints for Prototype tires and three stints for GT tires; especially given the changes in the rules for the 2016 season, you can’t just grab something off the shelf and expect to deliver the appropriate performance.”
Sometimes a radical solution is required. For example, the DeltaWing had four inch wide front tires.
When the ACO rules for Le Mans reduced torque and tried to slow the powerful LMP1 cars, Michelin developed the ultra-wide/tall front tire to permit cars to brake later and carry more speed through the corners.
The first place that Michelin explores is tire sizing. “At its simplest, a tire is an air chamber designed to manage both the static weight and the dynamic loads imposed by the vehicle,” said Baker.
“Most of our sports car racing is done on 18-inch bead diameter tires like we use in the IMSA WeatherTech SportsCar Championship GTLM class.
“We also use 18’s in the World Endurance Championship and in the FIA Formula E World Championship.
“By contrast, F1 still runs on 13-inch seat diameter tires and IndyCar and NASCAR are on 15-inch seats. Those 13-inch and 15-inch tires could not be less relevant to the consumer market, particularly when even modest trim levels of mainstream cars are frequently equipped with 17-inch, 18-inch or larger seat diameter tires.
“As manufacturers have sought to optimize overall performance – particularly ride, handling and braking – the seat diameters of tires have gotten bigger.”
Same Size, But Different
There are over 250 million vehicles, plus motorcycles, in the U.S. today and well over one billion vehicles worldwide. They don’t all use the same tires. Neither do many of the race cars.
In the WeatherTech Championship, the five manufacturers competing in the GTLM class with Michelin have very different race cars.
The Corvette C7.R and BMW M6 are front engine cars. The Ford GT and Ferrari 488 are mid-engine and, of course, the Porsche 911 is a rear engine car.
“Determining the optimum tire solution is not just a matter of engine location, we also look at the powertrain, aerodynamics and the body mass,” said Baker.
“And, with the Corvette and the Ford GT, we have our respective embedded engineers help develop their production car tires as well as the race cars, so it is a big commitment.”
Michelin currently has three different dry tire constructions to meet the front, mid-engine and rear engine needs and two dry compound options for each construction.
Carry That Weight (and Load)
To illustrate how a tire works in competition, Michelin engineers paint the following word pictures.
A current IMSA GTLM car weighs approximately 2,600 pounds, plus driver and fuel.
At speed on a flat surface, that weight is distributed approximately 40 percent to the front and 60 percent on the rear tires. But that balance is constantly changing, especially on a circuit like Mazda Raceway Laguna Seca.
“The elevation changes here at Mazda Raceway mean that we are dealing in three dimensions several times each lap as the car completes a circuit,” said Ken Payne, technical director, Michelin North America.
“Imagine that the car is climbing to the top of The Corkscrew. As the car runs uphill, more weight and load is transferred to the rear of the car.
“Then, as the car arrives at its braking point; the weight shifts to the front. At virtually the same time, the driver begins to turn in.
“The outside tire has an instantly increased load from the terrain change, a sudden load from the braking action and an isolated load related to the turn in.
“For a brief time nearly all of the loads are on that one front tire. If the tire can’t handle the simultaneous braking, turning and elevation change load, it will limit the turn in or lock up.
“The car then accelerates down the hill, the weight shifts to the rear, the car brakes and turns in at Turn 11 and the dance resumes for another lap.”
Getting in shape
The key to a race tire’s performance is the size and shape of the contact patch. However, it is not simply the size of the contact patch.
“A long, narrow contact patch is fine if you are going straight, but when you turn, the load will shift to a very small part of the contact patch,” said Payne.
“The load can easily overwhelm the capability of the contact patch or lead it to develop excessive heat and a resulting loss of grip.”
“Creating the largest possible contact patch with the right perimeter shape – which is a powerful, but little-known key to increased performance – helps to spread the load across a larger surface area, which contributes to increased dry grip and also helps improve tread life.”
To optimize performance, Michelin engineers have developed the “Variable Contact Patch”, a tire architecture that shifts as the car moves and the loads change.
“The whole idea is to make the best use of the tire,” said Payne. “The more fully integrated the tire is to the car’s design and performance characteristics, the happier the car, driver and tires will be.”