As modern sportscars produce an incredible amount of downforce to keep glued to the road, even the slightest increase in ride height – raising the cars off the ground to allow them clearance over the kerbs – will severely diminish the amount of downforce that can be produced.

Andretti Green Racing claimed the overall victory in the streets of Detroit last year, thanks to double stinting its Michelin ‘Street Soft’ tires. (LAT)

“Street courses tend to have a lot of low-speed, 90-degree corners around city blocks,” Koenigstein explains. “This means a lot of traction events where the rear tires are punished by full-throttle acceleration from very low speeds. So this sometimes leads to rapid wear on the rear and was a key factor to consider when developing our ‘Street Soft’ tires.”

Michelins ‘Street Soft’ tires, the brand’s softest tire compound that’s been raced in the ALMS, provides prototype teams with additional grip without compromising tire life or wear. Specifically developed for the tight and unforgiving street courses, ‘Street Soft’ tires were first introduced at St. Petersburg in 2008, where Michelin technical partner team Audi Sport North America went on to victory.

Audi also used ‘Street Softs’ at Mazda Raceway Laguna Seca - a natural terrain course - and also claimed victory. But Laguna is a track Michelin feels it’s yet to master, as whatever compound they throw at it, it proves to never be soft enough.

While GT teams don’t have the “Street Soft” option, they do prefer a softer compound in ideal conditions. Track temperature, as well as ambient temperature and humidity varies from one street circuit to another. A 90-degree, humidity-free day in Long Beach is vastly different to the water-heavy air on a 90-degree day in St. Petersburg.

“Smooth, un-abrasive tracks typically can use softer tires,” explains Risi Competizione technical director Rick Mayer. “Street tracks are soft tire targets. They usually have short corners that don’t load the tires excessively and therefore don’t hurt them. The most difficult tracks are the low grip tracks such as when we used to race at Portland in the heat.”

Ambient temperatures at Portland reached in excess of 100 degrees Fahrenheit, and that luckily won’t be the case at Miller Motorsports Park this weekend. Compared to the slippery and inconsistent surface at Portland, Miller’s state-of-the-art facility is less than four years old, featuring billiard table smooth asphalt.

However, grip levels are not optimal, and high 4,200-foot Utah altitude provides less power for naturally aspirated cars. Drifting sand can also become an issue.

Sand can also be a problem at Mazda Raceway Laguna Seca, where just a touch of a wheel off the track surface can produce a giant dust cloud. (Marshall Pruett)

“The sand at Miller can be a problem, especially if it’s windy and there's a sand storm, which there invariably is,” Weston says. “The tires are normally very smooth, as though they've been polished by the sand. Grip can be at a premium there, but it's also very smooth, like a European track, so you can take liberties with stiffer springs and lowering the ride height. Aerodynamic grip is compromised there because of the altitude.”

To overcome these challenges, Michelin works closely with its technical partner teams to come up with the best tire package. A Michelin tire engineer is assigned to each of its teams and works with the car’s engineer to outline likely tire solutions. Whether it’s in private testing or at a race weekend, the Michelin tire engineer is a crucial asset to each team’s success.

Teams generally have a choice of two or three different compounds, ranging from ‘Hot Weather’ to ‘Medium’ and ‘Soft’, as well as the aforementioned ‘Street Softs’ for prototypes. It’s also not uncommon for teams to use different tire compounds on the four corners of their cars.

Working in tandem, both the team and tire engineers are in constant communication on the car’s setup, and also the decision on whether to single or double stint a tire during a race. By single stinting, or keeping the same set of tires on for only one full fuel run, a more aggressive approach can be taken, thus opting for a softer compound. But if a team elects to double stint, they may have to run a harder compound and make other chassis adjustments to reduce tire wear.

Michelin tire engineers are an integral part of a team at every ALMS weekend. (Michelin Motorsports)

“All of this gets discussed with our Michelin tire engineer before we ever run a lap,” Mayer explains. “Our Michelin engineer will keep notes and records of starting tire pressures and have opinions on what hot pressures would be optimal. He constantly looks at the aspect of the tires and measures track, ambient and tire temperatures; everything changes with these variables.

“If we’re sliding or spinning the rear tires, the Michelin engineer can see it on the tire. If we grain a tire [the tearing apart of tread rubber], they can see it. We’ll discuss chassis setup changes to solve or help these issues or we’ll possibly choose a different compound for that corner or corners of the car. The tire engineer is pivotal. The only thing touching the ground is the tire. You have to adjust the setup and strategy to the tire.”

At the end of every ALMS race weekend, Michelin takes its tire data and applies it back into its extensive R&D program. With an advanced concept group working a few years ahead of production, development is ongoing in the quest to produce optimal tires for the road and for the racetrack.

After all, its the technical partners like Michelin that make ALMS racing relevant, and an incubator for future road-going technology.


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