Tires - Nitrogen air loss study

Filling tires with nitrogen rather than air is becoming a common practice in the replacement tire market. This service offers tire dealers another avenue for making money while also promoting safety. The claimed safety benefits often include the potential for reducing air loss compared to an air-filled tire. Maintaining proper inflation can help prevent tire overheating; promote optimum tread life; and reduce rubber aging and wheel corrosion. The use of nitrogen in large truck fleets and the commercial tire industry are well documented and support these claims.
The National Highway Traffic Safety Administration (NHTSA) has seen reduced aging of tires filled with nitrogen. Though the data does support that passenger car tires could benefit by all the claims made for nitrogen, tire manufacturers say that they already design tires to perform well with air inflation. And while nitrogen will do no harm, manufacturers say that they don't see the need to use nitrogen, which generally adds $5 or more per tire charge.

Consumer Reports wanted to find out if nitrogen is worth the price, so we purchased a Nitrogen Inflation System and checked out how well the inflation held up over a one year period. We evaluated pairs of 31 tire models of H- and V-speed rated, all-season tires used in our tread wear test from 2006. We filled one tire per model with air and the other with nitrogen. The test was quite simple: fill and set the inflation pressure at room temperature to 30 psi (pounds per square inch); set the tire outdoors for one year; and then recheck the inflation pressure at room temperature after a one year period.
The tires were filled and deflated three times with nitrogen to purge the air out of the tire cavity. We also used an oxygen analyzer to be sure we had 95-percent nitrogen purity in the tire--the claimed purity limit of our nitrogen system, which generates nitrogen gas from ambient air.

The test started on September 20, 2006 and the final measurements were taken on September 20, 2007. The results show nitrogen does reduce pressure loss over time, but the reduction is only a 1.3 psi difference from air-filled tires. The average loss of air-filled tires was just 3.5 psi from the initial 30 pressure setting. Nitrogen-filled tires lost an average of 2.2 psi from the initial 30 psi setting. More important, all tires lost air pressure regardless of the inflation medium, so consumers should check their tires' air pressure routinely. No evaluation was done to assess the aging claim.
Bottom line: Overall, consumers can use nitrogen and might enjoy the slight improvement in air retention provided, but it's not a substitute for regular inflation checks.

Nitrogen Tire Inflation - The Nitrogen Debate

By Jay Chen


For a while now, I've been debating back and forth on the use of Nitrogen gas for filling tires. Race teams almost exclusively use it for that slight advantage that it brings over air filled tires, which justifies the costs. But that's racing, where the difference between a more aerodynamic hood latch is cause for controversy. Then Nissan announced that the new GTR would come with Nitrogen filled tires to enhance its high speed stability. Maybe that's just supercar hoopla, but the fact that Nitrogen is being considered for use on passenger cars by the OEMs says something. The matter was brought up again as some local BMW dealers are now offering to fill tires with Nitrogen as a free service, which means now it's dirt cheap.
So, does that mean everyone should be running on Nitrogen just because racers use it, and now that it's affordable? I'm still on the fence about this even though there are definite advantages to using Nitrogen gas. We have to get some background on what gas does in order to understand why Nitrogen might be advantageous.
Regardless of what gases we fill our tires with, the purpose of the gas is to both support the weight of the vehicle, add some spring and damping, and help maintain the tire's toroid shape. Anything in a gaseous state does this well since a gas can conform to any shape and will distribute pressure forces uniformly. We could even use liquids to evenly distribute the force, but that would make for a lot of unsprung weight. Liquids are also essentially incompressible, which would be an issue if the tire hits a bump that reduces its internal volume, turning your wheels and tires into water grenades from hell.
Gasses are lighter and can compress, which is why we use them. There is exactly the same pressure pushing against every square inch inside the tire and against the wheels. The pressure pushing out is always uniform, so that the tire is always evenly loaded. Having a consistent pressure distribution is what maintains the even tension on the hoop that the rubber forms around the wheel; it's this hoop stress that really carries the weight of the car, not the sidewalls.
But because gasses can compress, it also means they can expand. Since the gases inside a tire are held under constant volume and mass conditions, it means tire pressure can only go up as temperatures increase, which we see all the time at the track.
This brings us to the physical properties of Nitrogen. For most intents and purposes, we treat Nitrogen as an inert gas, even though it's not a real noble gas. It has a lower molecular weight than air, by about one gram for every 22.4 liters under standard temperature and pressure conditions. It's also not a melting pot of other gasses like air, which is made up of 78.08 percent Nitrogen, 20.95 percent Oxygen, 0.93 percent Argon, and a bunch of other trace elements. Even though air is mostly Nitrogen, the Oxygen in it, which are larger molecules and have more atomic mass, is what makes air just slightly heavier than Nitrogen. In most other aspects, air shares almost identical properties with Nitrogen since that's what it's mostly made of.
So, let's do some back of the envelope calculations just to see what the differences might be between air and Nitrogen. Take a hypothetical 205/50/15 track tire that has a tread surface temperature of 170 F. Without knowing the heat transfer rates of the tire, we'll just assume that the internal gas temperature is about 120 F. A rough calculation of the volume inside comes with about 19.6 liters of gas volume inside the wheel recess and tire. Under the Ideal Gas Law, this comes to about 0.103 pounds of gas if filled with Nitrogen and 0.107 pounds if filled with air, a hair splitting difference of 0.004 pounds or 1.6 grams in unsprung and rotational mass.

And the chart above displays a average pressure loss difference of 1.2 PSI between air and N over one year.

1.2 PSI ladies and gentlemen, that is 0.08 (zero point zero eight) kg or bar for us metric users.

"The benefits of a nitrogen filled tire are huge compared to a traditionally air filled option."

Huge?, yeah right.