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A letter from Stew


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Newsletter No 45
November 2008

Partial first draft....

NITROGEN IN YOUR TIRES-JUST A BUNCH OF HOT AIR? (Forewarned is forearmed)(When wheel it ever end...)(Drawing a bead on excesses)

Recently I have fielded a number of questions about filling tires with nitrogen.  It is the latest fad in tires, so you might have heard of it.  Typically you go to buy tires or have you car in for some other service and you are told about the benefits of replacing the air in you tires with pure nitrogen.  Should you do it?  Is it a good idea?  I have the answer in two forms, the short one and the long one. 

The short answer is "no, not worth the cost".  The long answer delves into why it is a waste of money for most of us, but why it may work for some people..  If you don't want to know why, you can skip to the next heading.

So, if race cars and some airplanes use nitrogen in their tires, why shouldn't I use it in my car?  Good question.  It will not harm you car, only your wallet.  The real harm I see in switching to pure nitrogen instead of air is that it will give drivers a false sense of security that their tires don't need to be checked.  And, one of the biggest problems I see with car maintenance is neglected tire pressures, whether they are filled with nitrogen or air.  Low tire pressure can be expensive and dangerous.  As the pressures gets lower, the load carrying capacity of the tire diminishes.  It can drop below the point at which it cannot support the car any longer.  This may or may not be noticeable to the driver!  It can lead to loss of stability and hence, control.   Low pressure can also lead to sudden tire failure,  I think we can all agree that correct tire pressure is an important safety issue with cars. 

So, what is the deal with using nitrogen?  Why use it?  The best reason for using pure nitrogen is that it holds a more stable pressure no matter what the weather or how hot the tires get from driving.  Remember that you normally need to check tire pressures when the tires are "cold", that is, not having been run for a while.  That is less important with a pure nitrogen fill.  This leads me to another reason that it may be harmful to your wallet to buy nitrogen. 

It is a well known fact that if you raise tire pressures you get better fuel economy, due to lowering the rolling resistance of the tires.  For example, to optimize fuel economy on my Prius, I run them at 44 psi.  Toyota recommends a much lower pressure around 30 psi.  I can feel the difference in how the car rolls, and can see higher fuel numbers on the dash.  Now, if nitrogen is more stable and the pressure does not rise in a hot tire, the rolling resistance will be higher.  Thus, you will get WORSE fuel economy with a pure nitrogen fill!  You can compensate for this by filling the tires even higher, but how much?

When I was a young man studying aerospace sciences, I ran around with nitrogen in my tires (This was back in the seventies.)  Did it make any difference?  You bet!  I was COOL!  I could make all of my pilot buddies feel left out, because they just had plain old air in their tires.


    Nitrogen filled tires don't have moisture in them.  Air systems used for filling tires professionally also use dried air. 
    Nitrogen filled tires have no oxygen inside them and therefore the wheel can't corrode and the rubber tire cannot rot.  Wheels and tires don't corrode from the inside out.  When dismounting an old cracked tire, the insides are always clean and fresh, unlike the exterior.  I have never seen a tire replaced because of corrosion or rubber deterioration from the inside out.
    TIres hold their pressure longer than with plain air.  The loss of pressure is similar, and pressures should be checked periodically regardless.  See this test:
    Nitrogen filled tires last longer.  There is no evidence this is true.  TIres wear out at the same rate regardless of what is in them.
In summary, there is no practical benefit to filling the tires with nitrogen.  The benefits are theoretical and very unlikely to justify the cost.

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