18th-April-08.
 
Some e-mails I received this week, point out that many new cars now seem to be delivered with tires that are filled with nitrogen.
This may well have something to do with the fact that all new cars must now be equipped with tire pressure monitors.
And the basic question is " Do I really need nitrogen in my tires and why is it there in the first place?

So to repeat a blog I wrote about a year ago, here's the skinny on the subject of nitrogen in tires:

The air we breathe is 78% nitrogen and the other 22% is oxygen, so your tires already have a lot of nitrogen in them.
Garages (and dealers) that have invested as much as $5000 in a nitrogen generator, which extracts the gas from the air
and dries it,  will tell you that racing cars use it, aeroplanes use it, that your tire pressures will remain much more stable
and that the pure nitrogen doesn't support oxidation so the tire doesn't rot from the inside.

Oh and by the way, there's a $10 charge for filling each tire.

On the other hand, there appears to be absolutely no harm in using nitrogen to fill tires, so as long as the shop doesn't
hit you with any of these extra charges.    

I've done some research on the subject and almost everything I found was anecdotal. 

There are statements made like "Nitrogen  doesn't attack the inner lining of tire"

In our experience, tire treads always wear out long before the casing gives up, so I don't believe that oxidation of the interior
of car tires exists. I believe we're into another snake oil situation, but one that's a bit more scientific and therefore more difficult
to nail down.

Here's why: 

Dry air or dry nitrogen will follow the combined gas laws more accurately than air containing moisture,
but the difference really is academic.

Oxidation of the rubber inside a tyre has never been of concern to me, nor anyone else I know of.
I have only ever seen tyres fail from normal wear, cuts from impact or sharp objects or by oxidation or ozone attack of the
outer carcass. The cracks in the sidewall, always start on the outside, never on the inside.

Race car teams will often use nitrogen to fill tires, but largely out of convenience rather than due to any performance benefit. 
No ancillary equipment is needed to fill a tire if you have a bottle of nitrogen on hand. 
If you use an air compressor, you either need an electrical generator, or you need to find a place to plug in,
or else you need a gasoline powered compressor.

In racing applications the best option is nitrogen, the next is really dry compressed air and the last is wet air from a compressor.

100% nitrogen will carry much less moisture than that which is in the atmosphere.
Condensation inside the pressure tank of a compressor can be corrosive and may have unpredictable thermal properties.
Formula1 teams now make tire pressure adjustments in 1/4 psi increments to correct handling so the thermal effects are
significant. I don't think there will be any benefit for a Cadillac Escalade.

When a tire heats up, the air and any moisture inside it heats up as well. When the moisture inside the tire heats up, the water
molecules move further apart, increasing the tire pressure. By removing this moisture, the pressure stays more consistent
over the entire heat cycle of the tire.

On a race car, a 1/4 psi difference can change the handling of a car significantly. The humidity inside a tire does not have to
be zero, but if it is not at least kept consistent from one set of tires to another, to equal a 1/4 psi increase in the current set,
a 1 psi change may need to be made in another set because of the differing humidities.

Is nitrogen completely necessary?
No. Air can be dried using inline air dryers and such. However, it's much easier to keep
tire humidity consistent when all the tires can be filled from the same 1500 psi bottle of nitrogen delivered to the shop.

As far as race tires go, this is the reason to use nitrogen. Because of the short life of a race tire, how much air permeates
out of a tire over a year, or even two hours, is of no concern.

Commercial aircraft tires are routinely nitrogen filled.

Apparently, one of the reasons behind using nitrogen in aircraft tyres is that they fly up to 40,000 ft where the temperature
can be as low as -60C.

Any water in the tyres will freeze and is not likely to thaw completely before landing.

If it settles in one spot, then the tyre has to break it free when it flexes and there is the possibility of causing an unbalance
as the tyre spins up from zero to 130 mph. I can see the cold and condensation freezing in the tire as a concern in aircraft. 
Aircraft tires are pressurized to something like 250 - 300 psi.  It is far easier to do that with a nitrogen bottle at 1,500
psi than maintaining a multi-stage air compressor and high pressure drier.

The logic behind the statement that because the nitrogen molecule is larger and therefore is less likely to permeate
through the tire than oxygen needs to be verified. In fact, air leaking past valve seats and bead to rim joints is a
much greater problem in car tires.

Fighter jet brake fires used to be common place, even after the fire had been covered with powder, the pads would 
smoulder for some time. If the tyre failed it was better that nitrogen was released over the smouldering pads than air.

A large amount of heat is generated by the brakes when stopping a fast jet, the aircraft then trundles at a walking pace
back to the gate so very little cooling air is available to dissipate the heat. The aircraft is then left to stand and the heat soaks
into the tyres. Allegedly nitrogen is also much better able to withstand this heat soak than air.

Some large trucking fleets use nitrogen too.  It increases the life span of the tire carcass which can be pretty long, since
they retread them forever.  This is not a factor on passenger cars.

I have seen tyres that need a regular top up, but I have also seen tyres that hold pressure for a year or more.
Surely with a matched set of tyres, the difference in ability to hold pressure is due to leaks, not the difference in diffusion rates.
The loss due to diffusion is in my opinion, negligible, and I would expect in the order of less than 1% per year.

For a street car, I can see little benefit in using nitrogen. But you should check your tire pressures just about every week,
and you should not worry if pressures increase 1 or 3 psi during a daily commute in summer.

As far as corrosion resistance is concerned, rusting will only occur when the oxygen in the air can reach bare metal.
Unless, for some reason, your steel wheels didn't come painted on the inside, or the surface was damaged before the
tire was mounted, the side of the wheel open to the ambient air will corrode much, much faster.
Alloy wheels, of course, will not corrode from moisture or oxygen exposure

The bottom line becomes one of special applications and so what's sauce for the goose (race cars and aeroplanes) is certainly
not sauce for the gander (car and SUV owners).

If you can get it for free - take it. If there's a charge, refuse the offer.