The answer, it would appear, depends on who you ask. You see, Nitrogen is used in the space shuttle and in commercial airliners mostly because it is non-combustible. The likely hood of the tires on your Porsche getting hot enough, during normal driving, to explode is fairly slim . However, there are other reasons to consider Nitrogen.
Potential Problems from Using Compressed Air in Your Tires
Most tires are inflated with compressed air, (a combination of gasses made up of nearly 78% nitrogen (N2), 21% oxygen (O2) and 1% argon (Ar) and a few other trace gasses). There are a number of possible problems with this mixture and reasons to think about Nitrogen.
- Air Leakage: The molecular makeup of rubber is such that regular compressed air can permeate through the rubber allowing the pressure in your tires to drop at a rate of 1 to 2 PSI per month (assuming constant temperatures).
- Pressure Changes: Along with the normal components of compressed air comes moisture/water and water isn’t good for anything tire related. This moisture is usually a result of humidity in the air and the act of compressing it greatly increases the concentration of water by volume. Don’t believe me? Try this, next time you’re at a gas station and you plan on topping off your tires, depress the chuck on the hose for a few seconds with your thumb. Chance are your thumb will feel moist and you may even see water. It’s this water/moisture that gets into your tire, reacts to temperature changes and can change the pressure of your tires. This is why it is always suggest you check your tire pressure “cold” before you’ve driven the car and heated the tires and the moisture within.
- Oxidation: Oxygen corrodes aluminum and steel possibly weakening your wheels. Additionally, oxygen reacts with rubber, in a sense,“corroding” it too (possibly making your tires unsafe). Rust and dust created from this oxidation can clog valve stems, causing them to leak. Lastly, rough surfaces on wheel flanges and tire beads (due to corrosion) may not seal properly, causing additional leaks. Combine this with the moisture mentioned above and your expensive tires can possibly be damaged
Why Nitrogen May be Better in Your Tires
For some, especially those that drive race cars or who regularly participate in Driver’s Education and other high speed events, Nitrogen provides a number of benefits:
- Air Leakage: Nitrogen molecules are actually larger than the Oxygen molecules found in compressed air. So much so, that while they can still “permeate” the rubber or leak through, they do it at a much slower rate. If you fill a balloon with air by blowing into it, it will shrivel up in a short period of time (that’s the compressed air escaping through the skin of the balloon). Fill that same balloon with Nitrogen and it will remain filled for a much longer period of time. Compressed air leaks at a rate of 1 to 2 psi per month. Nitrogen leaks at a rate of 1 to 2 psi every six months.
- Pressure Changes: Unlike compressed air, Nitrogen is dry. No moisture means fewer, if any, pressure fluctuations. For those of you who race and want to dial in your suspension, you already know this. For those of you spending more time on the track in High Speed Driver’s Ed events, it’s something to keep in mind.
- Oxidation: Nitrogen is far less reactive than compressed air. It won’t cause rust and corrosion on steel or aluminum. More importantly, it won’t degrade the rubber.
Add the three factors above together and you have a recipe for constant pressure and better wheel/tire wear. More consistent pressure translates into better gas mileage and longer life for your expensive tires (not to mention a fine tuned suspension dialed in to your track needs for that particular day).
If Nitrogen provides such great benefits why is there even a question about using it?
- Nitrogen isn’t anywhere near as available as compressed air, yet. While you can purchase cylinders of it from various suppliers, filling stations are few and far between.
- Compressed air is inexpensive compared to Nitrogen. Most gas stations have compressed air available for free or a nominal fee of fifty cents or so. Shops may charge as much as $30 per tire for a nitrogen fill (although $10 is a more reasonable fee and some places will even fill for free with a tire purchase).
Lastly, and most importantly, you can get pretty much the same results (for street use) from compressed air by simply checking your tire pressure more frequently. Checking and adjusting your tires as little as once per month can provide the same benefit(s) that you get from Nitrogen for most drivers. If you’re a racer, then most likely you’re already using nitrogen and rightfully so. If you’re a Porsche enthusiast who enjoys a spirited drive now and then, simply purchase a nice gauge like this Porsche one or this one available from Moroso.
Related Posts
Understanding N-Spec Approval When Choosing Tires For Your Porsche
Choosing the Right Tires for your Porsche
Sumitomo HTR Z III Tire Review
Porsche Wheels and Rims
[Source: Popular Mechanics, Bridgestone, 993C4S]
View Comments (17)
Your assertion that nitrogen is “larger than compressed air” is completely wrong and you should be ashamed of yourself for perpetrating this myth. You statement could only be true if the oxygen in compressed air were smaller than nitrogen. It is easily verified that this is not the case. The molecular weight of an oxygen molecule (dioxygen, O2) is 32.0 atomic units compared to a nitrogen molecule (dinitrogen, N2) at 28.00au. The bond length of an oxygen molecule is 1.208 angstroms compared to 1.098A for a nitrogen molecule. So nitrogen is both smaller and lighter than oxygen and diffused faster than through rubber (and any other material in a tire). This is not just theoretical, but are well established experimentally.
So, if you represent yourself as an authority on a topic, your should get your facts right. Presenting false information only undermines your credibility, especially when the facts are so easily checked.
@Chem,
I want to thank you for commenting above. It brings up a great point and allows me to kill two birds with one stone.
1. While we always love comments and criticism on our posts, it would be great if people would actually stand behind their comments by using a real name and real email address. Without these two things you are: a.) unlikely to get published; and b.) simply hiding behind and flexing your "internet muscles".
2. We don't claim to be experts and only try to help people by informing them on information we think would be of interest. We take great care in researching our articles and if we make mistakes, we're happy to fix them. After reading your comment and doing a bit of additional research, I think we're correct in our statement. Here's what I found:
"It is often mistakenly assumed that "molecular size" correlates directly with "molecular weight". O2 does have a greater molecular weight (32) than N2 (28) (as you point out above), but O2 is actually smaller in size. Thus, O2 fits through the relatively tight passage ways between polymer chains in the rubber more easily than does N2. The difference is size between O2 and N2 is very small, only about 0.3 times 10 to the -10th meters (0.00000000003 meters).
O2 "permeates" approximately 3-4 times faster than does N2 through a typical rubber, as is used in tires, primarily because O2 has a slightly smaller effective molecular size than does N2."
I did however update/edit "larger than compressed air" to "larger than the Oxygen molecules found in compressed air" so thanks again for that.
Lastly, while I'm no chemist, I always thought molecular size was measured in pico meters and angstroms were used to measure length?
The air and moisture in the tires will corrode the rubber and aluminum in the wheels? How about the air and moisture in the atmosphere that completely surrounds our cars and bodies?
It seems that using nitrogen in a street car is a great profit center for a shop that can get $120 from a very gullible customer! While it may be a safety feature in airplanes, it seems to be a very silly idea in a street car.
@George,
I think you hit on the point of the article. :-) Nitrogen makes great sense for those serious about racing and high speed driving events. However, for those of us that simply enjoy a "spirited drive" in our favorite Porsche we can achieve similar results by simply checking the pressure more often.
I agree with the points you made in the article on the benefits of nitrogen, except for the first point. A nitrogen molecule is smaller (0.10975 mm) than an oxygen molecule (0.1208 mm). The reason why a nitrogen-filled tire does not lose air is primarily because nitrogen gas is 50% less soluble in natural rubber than oxygen gas, which means that it is less able to be dissolved into the rubber than oxygen. Hence, nitrogen is less permeable and has less pressure loss than oxygen. In addition, by making some not-too-expensive design changes to the inner liner, a tire manufacturer can reduce oxygen pressure loss even more and make it close to the loss experienced with nitrogen gas.
@George,
It seems that there is significant conflicting information regarding the actual "size" of a nitrogen molecules and as many ways to get that measurement as Porsche has options. :-)
Can you point us to any definitive resource? If so, it would be greatly appreciated.
However, I think the one thing that most people agree on, and what we were trying to convey with the part of the article that covers "air leakage", is exactly what you said above.
Quoting from our post: "[While Nitrogen] can still “permeate” the rubber or leak through, they do it at a much slower rate"
Without evacuating the air in a tire (vacuum pump) there will always be some oxygen in a tire. If you fill and purge the tires several times that will still leave some oxygen. I have no idea if racers vacuum the air out or not but as I see they must or they are not eliminating all oxygen.
About 3-4 inflations/flushes of nitrogen gets the effect of washing out most of the oxygen and gets the nitrogen closer to 98% instead of ambient 78%. It is true that most of us "racers" use nitrogen in our tires because of the reduction in pressure changes when the tire gets hot on the track. The only trouble is less expense and ease to use a small air compressor for tools and tires at the track than the very large and heavy nitrogen tanks.
I'm of the opinion that Nitrogen for a tire fill is a waste of money even in my race car. The idea that pressure changes are smaller with Nitrogen is also false. The other gases in air, like oxygen, obey the same expansion gas laws that nitrogen obey's. The only time there could be a greater change would be if there was actual liquid water in the tire. How often have you found water in your tire when you remove it from the rim? Also compare the condition of the rubber on the inside vs the outside the next time you have the tire off the rim. You'll find the condtion is much better inside even though there was oxygen in there to degrade the rubber. In short there are no advantages to N2 except for the possibility of slower leaking (I doubt this also but don't have facts to back it up).
-Andy Simpkinson
Spec 911 racer
it's not the gases that create the pressure changes as much as the ambient humidity in compressed air vs dry N2. Racing a 3600 pound car creates pressure changes of compressed air about 15# + whereas with DRY N2, about a 6# increase in hot pressures. Those are facts related to humid compressed air. There is nothing special about N2 as a gas except it is dry from the company that put it in the tank and is less flamable than O2 or we might be talking about compressed dry O2. Anyway, it depends how much variation in tire pressure you are willing to put up with when racing and how much you want to spend to reach the goal (like anything in life), but there is no debate that dry N2 changes pressure under increasing temperature than humid compressed air. As for the inside vs the outside of a tire, the inside is not exposed to UV, wind, sand, dirt, gravel, etc. and should look perfect compared to the outside of the tire.