NASCAR Physics 101: Why Tires on Sprint Cup Cars Keep Failing

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NASCAR Physics 101: Why Tires on Sprint Cup Cars Keep Failing
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If there was a secondary and even more important story to come out of last weekend’s Sprint Cup race in New Hampshire—besides the sweep of the weekend by Brad Keselowski—it was the early retirement, for the second weekend in a row, of six-time Cup champion Jimmie Johnson.

Before the race had gone past a dozen laps, Johnson’s blue and white Chevrolet had not one, but two left rear tires fail in succession. The second failure caused a spin that put the No. 48 Lowe’s machine backwards into the wall. The damage was severe enough to force Johnson out of the race.

He was scored 42nd for the second weekend in a row. (For the record, the previous week’s early exit was due to an early, multi-car pileup in Daytona.)

Johnson was visibly disappointed by his early demise. While doing a postmortem television interview, he was very careful not to point blame for the rapid-fire tire failures—just in case it may have been the result of some trick set up by his inventive crew chief, Chad Knaus.

“Well, there are many ways to speculate and think what could have caused it, and certainly after the first we could have had damage that allowed something to rub on the tire and blow the second one so quickly,” said Johnson to a TNT television pit reporter.

Johnson knew that one or both of the tire failures may have been caused by an overly aggressive rear axle and suspension setup.

The engineering box that NASCAR has placed over the new Gen 6 car allows for very few visible modifications to the body and the chassis—the kind of modifications done in the past that made a car go faster, slow down quicker or turn left more easily.

As a result, crew chiefs and engineers have now gotten very creative with how they build a car and its suspension. And with the removal of ride height restrictions by NASCAR this season, they now work at making the car hug the ground, especially while entering the corner at a high speed.

Tire stagger, which is the use of tires of a different circumference at different corners of the car, helps in making the car turn. Tweaking the tire pressures offers another tool to make the car do what you want and need it to do.

Lower air pressures in the left rear tire, to help the car exit off of the corner, may have contributed to the tire issues on Johnson’s car, according to Goodyear spokesperson Greg Stucker. Similar issues were seen on Joey Logano’s, Ryan Newman’s and Casey Mears' race cars during practice on Friday.

Race engineers and crew chiefs want to be able to make as much grip entering and exiting a corner as is physically possible. On a short track, like New Hampshire Motor Speedway, this is done mechanically, through various adjustments and tweaks to the front and rear suspension setups.

These setups are pre-tested on very expensive computer simulations and then placed on the car at the race shop with the hope that once the driver, like Johnson, gets into the car and drives it onto the race track, it will react pretty much the way the car did in the computer simulation.

It might help to know that as big and wide as Goodyear’s Racing Eagles are on a Sprint Cup car, the contact patch—the part of the tire that touches the ground at any time—is quite small compared to the size of the tires.

The contact patch, the area of the tire that actually touches the surface of the race track as a Cup car enters or exits the corner at a track like NHMS is approximately 36 square inches, or the size of a man’s size 11 standard-width shoe. Simply put, you’ve got four men’s shoes holding up a nearly 3,500-pound race car (with driver and fuel). That's according to The Physics of Nascar: The Science Behind the Speed by noted author Diandra Leslie-Pelecky, who does a great job of explaining the science behind what makes Cup cars go fast and do what they do, and how the crew chiefs make them do what they do.

The lower the air pressure in the tire, the more of the tire’s surface touches the track’s surface. So, we see crew chiefs setting the air pressure (it's actually not just plain air, but nitrogen, which is a more stable gas) at the recommended minimum or even below, which to some might constitute abuse.

But abuse, in this instance, is in the eye of the beholder. And if setting the air pressure a few pounds (or more) below the recommended minimum helps your race car turn left better than it did before, you walk that fine line that crew chiefs walk when they’re looking for even the slimmest of advantages. And you hope your tire won’t fail because of your actions.

Is it abuse or is it just doing what you have to do to win?

Remember, the target to setting up a Cup car at a place like NHMS is to make it turn left as easily as possible and then grip the track as best it can when it's time for the driver to mash his right foot back on the gas pedal as he exits the corner.

Do you recall the tight engineering box? A crew chief also has to make sure that, when his driver hits the gas pedal coming out of the corner, the car is pointed in the direction it's supposed to go. Sometimes that doesn’t happen with this new car, and so drivers complain of “no forward bite.” The solution? A front suspension setup that puts an extraordinary level of stress on the right front tire.

If you tilt the right front tire inward so that the top surface faces out as far as you can, then you’re probably doing it right. According to Goodyear, when a car is driving into a corner at full speed, there is more than 4,000 pounds of load (weight), just on that one tire.

This too can cause a tire to fail, like we've seen at other banked race tracks. At NHMS, crew chiefs will set a radical angle of camber (tilt of the tire) on the front tires, especially the right front. When the right front tire has a lot of camber in it, it flattens to the ground as the race car barrels into the turn at full speed.

OK, enough with all the technical stuff. I know that you've learned far more about tires and suspension setups than you cared to know, but as a NASCAR fan—whether hardcore or casual—you should know this stuff, since it will explain a lot about why your favorite driver is either doing great or struggling and have a better understanding about the job that Goodyear does. It is pretty remarkable.

Admittedly, they do miss the mark sometimes. We remember the 2008 Brickyard 400 when the tires failed to lay rubber down and instead produced a powdery substance and a slippery surface that produced little to no traction and wore tires out in about 12 laps instead of the normal 30 to 35.

While some of the blame was directed toward the grinding of the Speedway’s surface, the bulk of the blame fell on Goodyear—and rightfully so.

A similar problem occurred during practice for the spring race at Bristol in 2011, after the track had been reconfigured. It forced NASCAR and Goodyear to truck more than a thousand tires to the short track in an effort to come up with the proper tire combination.

It’s not like Goodyear just throws darts at the wall when it comes to figuring out how to build tires that they know will be abused and pushed beyond their limits. Yet they're still expected to deliver a total of 400 to 500 miles of high speed wear during a race.

Goodyear continues to test their tires. But all their testing won’t eliminate the kind of tire failures we saw last weekend at NHMS. It doesn't mean we won’t see the same thing happen during any of the next six races of the regular season, when crew chiefs are still in an aggressive mode trying to make the Chase.

The new Gen 6 car is a pretty cool vehicle that crew chiefs are just now starting to figure out how to make go fast. Tremendous sums are spent to develop even the smallest of parts that make up the suspension and its associated pieces.

And the same and more is spent in developing the software programs that tell race engineers how to put those parts and pieces together so that the car will be comfortable to drive no matter what the size track.

In the middle of a pack of Cup cars going 200 miles per hour, like at Daytona or driving into a corner at even higher speeds at tracks like Texas Motor Speedway and Michigan International Speedway, having big horsepower isn't the only advantage you will need.

Ask a Cup driver what is more important, and he’ll likely say "both," but it’s more important to be able to drive your race car deeper into the corner than the next guy.

By Chase time, the engineers and crew chiefs have to get it right. You can be sure that Chad Knaus and his race engineer know this, as analyst David Caraviello points out in his column on NASCAR.com about the risk-reward factor with tires. So will the other 14 crew chiefs and race engineers in the Chase.

When the Sprint Cup circus returns to Loudon for the second race of the Chase—a race that, in the past, has been a wild card that defined the Chase field—there’ll be no excuses for failed tires early in the race.

So before you cuss out Goodyear, NASCAR or your favorite driver’s crew chief the next time a tire fails, or there’s talk of cords showing through the tread (which is a whole other column), think about everything that went into the failure.

Aggressive engineering, aggressive driving and the heavy responsibility placed upon the one part of the car that teams don’t produce or have in their possession—until they get to the race track.

It’s all kind of crazy when you think about it.

*Unless otherwise noted, all quotes were obtained first-hand.

*Information obtained from official media guides and other official sources and NASCAR, team and manufacturer releases.

Follow me on Twitter: @BobMargolis

 

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