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Michelin GTLM Insider: The Pressures of Competition

A look at the importance of tire pressures and temperatures…

Photo: Michelin

Photo: Michelin

When drivers hit the doctor’s office, the first two things likely to be checked are their temperature and blood pressure.

When they pull into the pits in testing, practice, qualifying or the race, the team may well be more interested in the tires.

Good temperatures and blood pressures are a sign of good health. Something similar is true for tires. And the Michelin engineers serve as tire diagnostic physicians.

Why are pressures so important?

“Tire pressures impact spring rates and how the car transfers loads in braking, cornering and acceleration,” said Ken Payne, technical director motorsports, Michelin North America.

“The tire is designed to work around a certain targeted pressure and shape for optimal performance and the correct pressures are critical to optimizing performance.”

The trick is that pressures must be set on cold tires, but then increase to a targeted hot pressure on track.

“Setting cold tire pressures is like trying to hit a moving target,” said Robbie Holley, race operations manager for Michelin North America.

“You start with the current track and ambient temperatures and look at where you want the hot pressures to end up.

“We are trying to hit a hot pressure target within a range of no more than 0.5 psi and ideally a 0.2 to 0.3 psi range.”

Photo: IMSA

Photo: IMSA

The Wall of Fame

The toughest test for a driver is the period of time between leaving the pit lane on a fresh set of cold tires and the time the tires build to the operating pressures.

“Tire pressures on the out lap are low, so there is more movement of the tire,” said Ford Chip Ganassi Racing’s Dirk Mueller.

“Everyone tells you to be careful. You have to be patient and not use so much curb until the pressures come up.

“At Daytona, leaving the pit lane on an out lap, especially at night, you have to be really careful not to end up in the ‘Wall of Fame’,” referring to the pit lane exit barrier that has claimed series champions, veterans, rookies and even Indy 500 winners.

His co-driver and co-winner of the GTE-Pro class at the 2016 Le Mans 24 Hour race, Joey Hand, agrees.

“Leaving the pits at Daytona at night on a new set of tires is a challenge,” Hand said. “The pit out at Daytona is unique as it bends around inside the track. It’s easy to make a mistake and end your race.

“The out-lap is critical in terms of performance. You are on cold tires and at a low pressure.

“If you push in the first two Horseshoes, you get big understeer than snap oversteer. You actually want to do that to get temperatures and pressures up, but it is risky.”

So why don’t drivers just take it easy on the out lap?

Dan Binks, crew chief of the GTLM class No. 3 Corvette C7.R driven by Jan Magnussen, Antonio Garcia said: “You can lose as much as five seconds on the out-lap if the driver isn’t comfortable and confident enough to push.”

Binks takes new drivers at Daytona down to the Wall of Fame and says, “This is where egos’ crash. You have to be careful, but the track is cold and the tires will lose temperature if you aren’t aggressive.”

“The driver has to push to build the pressures. It’s a grin-and-bear-it deal, man, but that’s why they get the big bucks,” he added.

Photo: Michelin

Photo: Michelin

Taking Temperatures

If 98.6 degrees is a driver’s personal target temperature, what are the targets for race tires?

“Our target hot tire temperatures are 180-220 degrees Fahrenheit,” said Payne.

To determine race tire temperatures, Michelin engineers typically use a pyrometer needle to stick each tire in three locations to measure the inside tread edge, middle, and outside edge.

“We want to see how evenly the temperatures read across the tread surface,” Payne said. “We aren’t necessarily focused on validating pressure through the tire temperatures as much as we are validating the car set up.

“How even is the temperature distribution across the tire tread? Is the tire working across the full surface or is one part of the tire being over or underworked?”

Photo: IMSA

Photo: IMSA

Staying on Target

While the ambient and track temperatures change from morning cool to mid-day hot and evening cool on a typical race weekend, the challenge for the Michelin engineers is to keep the car and tires in harmony with those changes.

“If you start out Friday morning at 80 degrees of ambient and track temperatures of 100 degrees, you may set your starting pressures at a certain number and end up right on target,” said Holley.

“But at four o’clock you head out for the next session and it is now 90 degrees and the track temperatures is now at 120 degrees, you will be way off target if you use the same cold pressures.”

The race day warmup, typically held in cool morning conditions, can throw teams off if they don’t make the right adjustments for the race.

Just a 10-degree change in ambient temperature can require a tire pressure adjustment of 0.7 psi. Likewise, a 10-degree change in track temperature can also require an adjustment.

Holley notes that tire pressures can change as the ambient temperature progresses even as the tires sit in pit lane.

Photo: Michelin

Photo: Michelin

Ready For the Next Stop

“In the race, our Michelin engineers with each WeatherTech Championship GTLM team are monitoring the tire performance throughout each stint,” said Holley.

“They can see the tire pressures through the team’s tire pressure monitoring system, and changing tire pressure is one of the quickest ways to change the handling of the car during the race.”

Temperatures aren’t the only factor is setting cold pressures, Payne said. “You may have two drivers on a team who run equal lap times, but have different techniques,” he added.

“Here in the IMSA Weathertech Championship, the GTLM class is a pure Pro class, but in WEC we also work with several GTE-Am teams.

“In the Pro-Am classes, you also need to consider the gentleman driver. They may not be not as aggressive as the Pros on cold tires and it will take them longer to build pressures.

“About five laps before the next scheduled pit stop, our engineer will check with the team engineer on driver feedback. Then he or she will advise the team engineer of the proposed pressure setting for the next set of tires.”

Once that is decided, the engineer will set the pressures two or three laps before the car pits.

After the pit stop, the Michelin engineer will inspect the used tires and check temperatures, pressures and wear patterns.

“If the pressures and temperatures are good and the tire wear is good, then the tires are healthy and the drivers are happy,” said Payne.

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