Nitrogen myths and realities

Question:

I saw the article on nitrogen for tires in the December 2016 issue. The discussion about high pressures caused by water vapor is not valid. Yes, compressed air may have moisture, and though possible, it’s unlikely that much liquid water would be present. The pressure in a tire, like any closed vessel, follows the “ideal gas law,” and a rough calculation would show that the pressure change is proportional to the change in absolute temperature (Rankine in Fahrenheit). 70F is 530R, and so if you’re out driving fast for an extended period, the tire temperature could get to 130F or 590R, or an 11% increase, say from 30 to 33 psi.

If you were racing in the desert, perhaps it could approach 200F, but then the initial temperature may be 90F, and the pressure increase would be about 20%, say from 35 to 42 psi. If a few drops of water were present in a tire at cold temperatures, they would turn to vapor, which behaves similar to the gas at low concentrations. If you had a lot of liquid water, the tire balance would be way off, but it certainly isn’t going to blow the tire off the rim. Water boils at 212F at normal atmospheric pressure. In a pressure vessel like a tire at 30 psi, the boiling temperature is in the range of 275F. This is why radiators are pressurized, as it raises the boiling point, just as pressure cookers are used to cook food at higher temperatures.

Filling tires on racecars and airplanes with pure nitrogen has some advantages, but for the most part there is only one benefit in regular passenger cars and trucks. Air is 78% nitrogen to begin with, but pure nitrogen has a lower permeation rate, and so the tire loses pressure more slowly, and some studies say maybe a 1/3 slower rate. Tire life and performance is enhanced by consistently maintaining proper tire pressures. Using air suggests one should check tire pressures a little more frequently. Most of the rest of nitrogen arguments are anecdotal or snake oil claims. That said, there is no disadvantage to nitrogen other than cost.

Answer:

Do you have an air compressor? If so you must know that you need to drain the water out of it after each use in order to prevent the tank from rusting out. And if you use compressed air for painting work, you must have at least one, and very likely two, water traps in the line so you won’t get water drops shooting onto your wet, freshly applied finish. As to how much water is in the air, that depends on the humidity of the air going into the compressor. If you fill your tires on a wet cold day there could be quite a bit of moisture, and it can rot tires, rust rims, and will expand when heated.

As for how much tires heat up on the road, that depends on a lot of things. One factor is how much rolling friction the tire experiences, and then add to that how hot the roadway is. It has been my experience that tires can easily get too hot to handle in racing situations, especially in the California desert in the summer. You are correct to say that, depending on the pressure to which it is inflated, a tire is not likely to depart from the rim, though there is plenty of old drag racing video footage showing that it does happen.

One thing for sure though, is that as air in the tire expands with heat, and as the water trapped inside expands along with it, the tire pressure goes up considerably. And that is a problem because high tire pressure can cause a car to experience oversteer and become twitchy. After all, your tires are the most important part of your car’s steering and suspension, and under- or over-inflation can cause major safety problems at speed. That is the primary reason racers use nitrogen exclusively. In fact, tire pressure is so critical to them that it is constantly fiddled with in Indy car racing in order to give the driver optimum handling and safety.

And don’t forget the centrifugal force of a tire spinning rapidly. Racing tires in Formula I and Indy cars are quite thin to minimize that, and that is why drivers have to come in for two to four pit stops per race due to tire wear. Drag racers and others who use heavier tires screw the rims to the beads of the tires to keep them from pulling away or moving on the rims.

It would be foolhardy to take a high-performance car out for a romp at very high speeds without going over the car to optimize its safety, and that applies especially to its tires. The news is full of stories of people killing themselves and others while driving at high speeds without regard to the condition of their car.