Required vs. Recommended Tire Pressures

Running on underinflated tires can be dangerous.  Underinflated tires they create more friction and more heat, which leads to not only bad handling, but also can produce structural problems.

You may remember the late 1990’s, when Firestone tires had problems with tires blowing out or treads coming off.  The flat tires caused vehicles to roll over and there were more than numerous (Wikipedia says 250, but there’s no source for the stat)  fatalities and many more injuries. The majority (but not all) of the vehicles on which this happened were Ford Explorers. So there’s a rule now that any car made after 2007 has to have tire pressure sensors that warn the driver when a tire is significantly under-inflated. (Significantly means around 25% under pressure.)

Underinflated tires produce high stresses and temperatures. In a correctly inflated tire, the gas inside the tire supports most of the car’s weight. If the tire is underinflated, then there’s not enough gas pressure and the sidewall of the tire has to support the weight.  An underinflated tire flexes a lot as it rolls, which causes two major problems.  One is that it put more stress on the tire, and the second is that it produces more heat. (Graphic from http://www.tirebuyer.com/education/tire-pressure-and-performance.)

Underinflated tires heat up faster. You need some heat in the tire for it to work right; but there’s a Goldilocks situation here. Too cold and they don’t have any grip. Too hot and they fail. The pressure has to be just right.

Which makes you wonder, how hard can it be to just put the right pressure in the tires in the first place?

Street tires are rarely called upon to sustain the extreme conditions race tires endure.  The issue is what we call tire pressure build – the increase in the tire pressure due to the heating of the tire and the gas inside the tire. At it’s most basic, heat is simply the motion of molecules. The faster the molecules are moving, the higher the temperature.

The faster air molecules move (i.e. the hotter they get), the harder they hit the tire walls and the higher the tire pressure.  This is why your car’s owner manual tells you to measure the air pressure in your tires when the tires are cold. The pressure changes a lot with temperature, as explained in the video I did for the National Science Foundation.

For the temperatures passenger car tires reach, a good rule of thumb is that every 10 degrees Fahrenheit corresponds to 1 psi (pounds per square inch) increase in pressure. NASCAR tires routinely change pressure by 20-40 psi from the tire sitting on the pit wall to the time it’s run a couple of laps. So the pressure you put into the tire is no where near the pressure you have three to five laps into the race. For a tire to have the right pressure during a long run, it has to have a much lower pressure when it first goes on the car. It takes a few laps for the tire to heat up, so the tire is really being stressed in the initial part of a run.

It’s a calculated risk teams take in how low they will start their tires.  Goodyear specifies a recommended pressure for each tire. NASCAR officials often check right front tire pressures in the pits.  The idea is that because the tires work in concert, if you the left front is way off, the car won’t be set up well. But there’s plenty of evidence that teams are getting around the recommendations and starting off with very low tire pressures.

Lee Spencer had an article this week suggesting that NASCAR might finally give in and require tire sensors as a way to get around the “shenanigans” (Her word, but one I love using). She uses the example of the left rear tire failures of the 48 car at New Hampshire a couple weeks ago. Johnson swore it wasn’t because of low tire pressure. Goodyear begged to differ.

So why not just put tire sensors on all four tires and let NASCAR officials monitor the tire pressure before the tires go on the car? Teams already use sensors during testing. They’re basically replacement caps for the tires that measure and transmit the tire pressure through an RF (radio frequency) link to a data logger on the car.  Places like McLaren (where the picture at right comes from) already have sensors for both inner and outer liners. The technology is pretty much already there to require sensors on the cars.

So why not just go ahead and put sensors on the cars?

Because NASCAR is very careful when it comes to opening cans of worms. Who will you allow to have access to the data?  The pit official? The team? Just before the tires go on the car, or will you allow access to that data during (or even after) the race? A lot of the same issues that arose when NASCAR made the transition to Electronic Fuel Injection will come into play here. How much technical data do we want the teams to have?

Sure enough, the first thing the husband said when I mentioned the idea to him was “That’d be great because then the teams would know when they had a tire going down.”

Do we want to go there? Are you encouraging crew chiefs to try edgier setups because they expect they’ll have a bit of a warning before they lose a tire? Do you want to take the driver’s ability to sense what’s happening with the car out of the picture?  NASCAR historically has had a de facto ‘no real-time telemetry’ rule for a very long time. The science and engineering in NASCAR go into the developing and preparing the car.  At race time, you turn over the majority of the control to the driver.

The other issue that always comes up is how teams will attempt to game the system. When NASCAR allowed for skew in the rear end of the cars, teams kept pushing the skew until NASCAR had to say “enough”. That we’re even talking about tires and tire sensors is because of teams ignoring the recommended pressures. Pushing the envelope is their job.

The discussions, according to Lee Spencer’s interview of Goodyear’s Greg Stucker, are ongoing, but NASCAR has a tendency to spend quite a bit of time thinking through issues like this before taking action.  The problem needs to be addressed, both to protect Goodyear’s reputation and for driver safety.