Dark Arts: Temperature and Performance Tires

Armchair experts and fresh-faced tuners are drawn to the allure of anodized turbochargers and top-dollar racing shocks; asserting these bits make the greatest leaps in the search for performance. Of course, those pieces play important parts in whittling away at lap times and increasing trap speeds, but if an aspiring driver really wants to chip away at—nay, chop chunks off—their times, they’ve gotta get their head around performance tires.

Unlike some bolt-on modifications, getting the most from a nice set of sticky tires takes a sympathetic touch and a comprehensive understanding of the rubber’s limitations. Though tricky and demanding, they’re the one performance variable which makes the greatest difference in performance in any category—they’re that important.

The Limitations of a Focused Tire

Unlike an all-weather tire, a DOT R-compound tire has a narrow operating range; typically between 160 ˚F to 220 ˚F. On either side of this range, the tire fails to provide much grip or feel, and some fall off more abruptly than others. Finding that sweet spot where the tire works at its best, but before the tire begins to overheat and grow greasy is key.

Overheating track tires can lead to a snappy situation! Photo credit: http://papots-garage.blogspot.com

Temperatures aside, the peak range in performance for a performance tire is usually a bit narrower. As Toyo’s Product Engineer Drew Dayton notes, “A street tire will be much easier to drive at the limit and will be more forgiving if driven above the limit. Street tires usually offer more audible feedback which can help the novice driver hear what is going on with the tire before they may actually feel what is going on with the tire.” Having that level of progression is so helpful; the amateur driver will typically have a hard time reacting to rapid breakaway of a hardcore tire.

Hiding Behind Grip

However, Dayton notes that the DOT-R tire “will have a lot more grip than a street tire which can potentially mask some driver deficiencies.” Since the limit is so much higher, a driver streaking away on DOT-R tires might not be pushing quite as hard as the charger on all-seasons. Dayton says, “A winter tire is designed with a special compound that stays soft at freezing temperature. This has to do with the glass transition temperature of the rubber compound.” Of course, this helps in chilly weather, but when pushed to the limit, these tires generally become useless after a dozen hot laps—they’re reduced to silly putty.

A performance-oriented tire is not as usable in different situations, and, predictably, is best used on a track. “An R-compound will get harder and harder as the temperature gets lower and lower. At some point, the compound will lose enough of its flexibility that it can crack under load or movement,” Dayton warns. That said, there are shades of grey to this black rubber market, and there’s always something that offers an ideal compromise between traction and friendliness in chilly weather.

Harnessing Heat

To begin understanding the racing tire, a driver needs to get out a set of contact thermometers and take some preliminary measurements. Those with a piercing-style thermocouple probe are better than infrared sensors because they get a temperature reading of the tire’s carcass, not just the tread surface. By inserting a thermocouple probe into the meat of the tire roughly 1 mm, the user has a more accurate sense of the general tire temperature since surface temperatures fluctuate much more rapidly.

Probing the carcass of the tire will give a more consistent read of the overall temperature, as the surface temperature fluctuates more rapidly.

First, the user must pierce the tread on the outside tread, in the center tread, and on the inside tread. The outermost measurements are taken roughly one inch from the tire’s shoulder. By assessing tread wear and temperature across these three areas, they’ll get a clear understanding of the contact patch and where the tire is being used. In an ideal world the tire is near-equally spread across the surface of the road, but with surface undulations, incorrect suspension settings, and corners of different radii and speeds, it’s a challenging thing to achieve.

Though that approach will do for most track rats and amateur racers, the boys in the big leagues—GT3 racing and further up the totem pole—have a few other options at their disposal. “There are TPMS-based solutions that measure the air pressure and the internal tire temperature with an infrared thermometer built into the TPMS system. There are also systems which can be tied in to the telemetry and data acquisition to measure the temperature of the tread from the outside,” informs Dayton. A series of infrared sensors or infrared cameras, mounted to the vehicle, can measure tread surface temperatures as the race progresses and give the guys running at Le Mans the extra iota of longevity needed to outrun their rivals.

Data acquisition is a helpful and, especially nowadays, accessible tool.

Turning Temperatures into Turning Force

“Tire temperatures can be used to analyze the alignment set-up, pressure settings and overall performance of the tire,” notes Dayton. A clever user can take these readings and deduce what the exact issue is with the tire. If the center is significantly hotter than the edges, they can try lowering tire pressure to reduce the ballooning of the tire. Conversely, if the edges are hotter than the center, the tire might be concave and need a few more pounds of air.

If the inner edge is too hot, chances are there’s too much static negative camber, and the outside edge of the tire isn’t coming into contact frequently enough with the asphalt. If the outer edge is too hot, it’s likely there is not enough negative camber, or possibly too much toe-in. If the desired tire temperature isn’t ever reached, it’s possible that the tire is too wide, the tire is over-inflated, or that particular axle isn’t as soft and compliant as it should be.

Here, the overheated sidewall led to the tread chunking. Photo credit: drivermod.ca

If the tire does get too hot, there are some telltale signs. Toyo’s Jay Jones, a longtime instructor and experienced SCCA racer, notes: “you can listen to the tire as it skips over the pavement. If it’s producing a hollow sound, like a thump on a watermelon, it’s probably past its best.” At this point, the tire is now convex in shape due to elevated inflation pressures, and skips across the surface of the road on the center of the narrowed contact patch.

When, without changing technique, the tire begins to give up and slide more frequently, it’s safe to assume the tire is past its peak performance range. As Jones wisely instructs, “Racing is all about resource management. At that point, you have to back off slightly—which requires a lot of discipline!” Caught in the heat of competition, it’s exceptionally challenging to restrain oneself and put a little less energy into the tire. However, this is the only way to bring it back into its ideal range and improve average race pace, which is the real aim at the end of the day—it’s what gave Alain Prost the ability to come from nowhere to win the race.

Pressures in Practice

What an avid track rat can expect to do is to get a sense of the tire and its characteristics as it breaks away, and when it’s at its best. If, after a practice session to get up to speed, a driver can string together fifteen-odd quick and consistent laps, they’re likely within the ideal temperature window. Dayton says, “As with any data collection, taking temperatures systematically will provide consistent tire data that can easily be compared to data from other runs.”

Photo credit: NASA Speed

After they can manage a series of hot laps a few tenths within their personal best, they ought to peel away from the track and whip out the pyrometer. As Dayton suggests, “After a lapping session, pull into hot pits without a cool down lap if your vehicle permits; this will give the most accurate temperature readings. Start with the tire doing the most amount of work around the track, which is usually one of the outside tires. After taking temperature measurements, take all of the hot pressure measurements in the same order around the vehicle, and analyze the results.”

With the temperature and pressure measurements in hand, a driver has a general target when lapping. There might be quicker laps possible on that tire, but a street tire, even a track-oriented one, might degrade rapidly if pushed beyond its ideal temperature range. At least at this pace, vehicle behavior can be assessed thanks to consistency in the tire, and that’s probably more useful for the enthusiast looking to find more speed and not setting pole position.

Getting the Nitty Gritty with Ken Payne

I chatted with Ken Payne, a man who’s been with Michelin for 29 years after starting as a test driver at Michelin’s Research and Development Center in Greenville, SC. Currently, Payne is the Motorsport Technical Director for Michelin North America and leads a team responsible for development of Michelin and BFGoodrich competition tires. He held positions in tire design, Original Equipment account management, and product marketing prior to moving into the competition department. He’s also competed in various forms of road racing and rallying for more than 30 years; once stage rallying vintage Saabs and currently running a Volvo 242 in low-cost endurance races on those rare occasions where time permits.

The questions were, I imagined, somewhat straightforward and self-contained. It turns out that there are no short answers when discussing tires and the factors which determine their performance characteristics.

Parry: What contributes to the operating range of a sport/track tire?

Payne: Tire pressure is a major contributor. It’s as important as suspension setup, since it affects the shape, size and “spring rate” of competition tires. These sort of tires are designed for quick changes of direction, and so they need to have fairly stiff sidewalls and lateral strength to provide the response and predictability drivers want.

However, it’s not just stiffness, but tire shape which strongly determines the shape of the tire’s footprint and the breakaway characteristics. For instance, a tire could be designed to have very high traction and a sharp reactions, resulting in a the tire that breaks away less progressively. Such a tire might be considered “peaky”, where the maximum grip offered by the tire is reached quickly but also declines quickly once that peak is surpassed. In practice, this describes a tire which requires a deft touch to get the most of that leads to snappy handling. A progressive tire offers a broader plateau where peak grip remains available over wider range with a more gentle drop-off.

It’s important to consider the “spring rate” of a tire when tuning the setup. Photo credit: Rennlist

Parry: On that note—does a wider tire necessarily break away more abruptly than a narrower tire?

Payne: No, not really. Breakaway characteristics are, as I said, better determined by the tire compound and the shape of the tire. People have that misconception because wider tires tend to be more performance-oriented in nature; with stickier compounds and smaller tread blocks—if any—but the abrupt breakaway is more a function to the aforementioned traits.

Parry: Why exactly does performance fall off on either side of the tire’s ideal temperature range?

Payne: A cold tire isn’t pliable enough—it skates over the surface of the road. As tire temperature increases, it becomes more compliant, and can conform to small pockmarks in the microtexture of the road. As it softens—to a certain point—its surface area expands.

Payne: When you overheat a tire, the surfaces becomes too flexible, cannot hold its shape, and is overly pliable. As we discussed earlier, a certain degree of rigidity is necessary to keep a tire from becoming a marshmallow, where it simply moves around the rim it’s mounted to and fails to deliver any direct feedback. This applies to both the contact patch and the sidewall, and so not only will the tire begin to fall on its side, but the footprint as well.

Look at that tire flex! Photo credit: forum.Miata.net

If you wear through the tread depth of an R-compound tire, you generally lose grip with the inner layer. However, as you wear away at the tire’s mass and the tread get thinner, the tire won’t retain heat as well. This means it won’t deliver as consistently; the tire’s temperature fluctuates more rapidly. We refer to this as the tire getting “peakier.”

Parry: If a reduced tread depth has these negative effects, why do some people choose to shave their tires then?

Payne: It’s not all bad. Shaving tires reduces tread depth, and taller tread blocks flex more. Too much block movement overheats the tire. This also means greater block rigidity, which means a more direct connection with the road due to less deflection from the tire.

Parry: What about shaving tires to save weight?

Payne: That’s a second-order priority.

Parry: Let me propose a hypothetical. If there was a clear example of tire temperature indicating a flaw in suspension setup, what would it be?

Payne: Since the temperatures of the inner shoulder should be warmer than the center, which should be warmer than the outer shoulder, keep an eye out for the outside shoulder becoming too hot. This is a clear indication of too little negative camber, or too much toe-in.  

Parry: Any advice you’d like to leave with our club racing readership?

Payne: Keep track of your pressures. Know the ambient temperature, and assess tire temperature when you’re fresh off the track. Keep records. Experiment, and try to find out what the ideal hot pressure is.

Clearly, getting the most from a set of tires is no simple feat. There are too many variables to consider in one or two track days, but the important idea to take away from this is that there’s a feel, a character, and an ideal for each and every tire. Learning where they work best, and how to keep them in their sweet spot is one of the skills that every driver must learn if they want to be competitive. More than controlling oversteer or braking late, the art of tire conservation is learned last, and it’s usually the drivers who are best at sympathy for those little black bits of rubber underneath the car that finish most often up front.

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About the author

Tommy Parry

Tommy Parry has been racing and writing about racing cars for the past seven years. As an automotive enthusiast from a young age, he worked jobs revolving around cars throughout high school, and tried his hand on the race track on his 20th birthday. After winning his first outdoor kart race, Tommy began working as an apprentice mechanic to amateur racers in the Bay Area to sharpen his mechanical understanding. He has worked as a track day instructor and automotive writer since 2012, and continues to race karts, formula cars, sedans, and rally cars in the San Francisco region.
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