Monday, 4 February 2013

Fill your tyres with nitrogen

Did you know that some people claim that there is a special benefit to be gained by filling your tyres with nitrogen instead of air?

Why could that be?

The gas inside a tyre is only there for one purpose - namely to keep the tyre from going flat (and specifically flat at the bottom as it is rare for a tyre to go flat at the top).  The right pressure ensures that the correct area of rubber is in contact with the road and takes up some of the vibration from the uneven surface. 

Air is 78% nitrogen already, and all but 1% of the remainder is oxygen, so what might be the advantage of going for a gas that you have to pay for instead of free air?

Nitrogen is very slightly lighter than air, so the 'moment of inertia' of the whole wheel might be reduced, but only by an unnoticeable amount.  Nitrogen is dry whereas air contains a little moisture which might condense into a tiny droplet or two of water.  So what?  Certainly corrosion of wheels isn't such a massive problem.  When did you last see a wheel rusted away from the inside.  Why could that possibly be worse than the constant attack of the atmosphere on the outside of the wheel?

Some claim that oxygen molecules are smaller than nitrogen so it diffuses more easily through the rubber allowing the pressure of the tyre to vary from optimal more quickly.  This is plain wrong.  Oxygen is bigger and will diffuse more slowly (albeit not much bigger and not much more slowly).

It is also plain wrong to claim that oxygen expands and contracts more if the tyre temperature changes.  Anyone claiming that is unaware of the 'gas laws' -  basic physics taught to 15 year-olds.

There must be only one explanation for the recommendation to use nitrogen in tyres.


Surely it just another way to extract money from the gullible.

Small note: Or have I missed something else?

And yes - we really do spell the word tyre with a y in UK English.  


LadyAtheist said...

I live a block away from a *tire* store that sells nitrogen, and one of my coworkers swears by it. She also is anti-GMO (which GMO product? All of them! Don't ask for specifics! Oh yeah, and you should recycle everything and otherwise live like you're in a monastery)

Thank you for helping me with the sciency stuff. My only question would be whether nitrogen maintains pressure during temperature changes better than nitrogen-oxygen-carbon dioxide - diesel exhaust - pollen + germs would. We have very wide and fast temperature changes here in middle America.

LadyAtheist said...

oops I guess you answered the temperature question -- I was thinking about as it sits rather than as you drive on it.

...and basic physics is not taught to 15-year-olds in the U.S. unless they specifically sign up for it, which is like asking for a daily beating in the locker room, or if you're female, it's like asking never to have a boyfriend even though your class will be 90% male.

Derby Sceptic said...

Looking at the Kwik Fit site it appears even the tyre companies don't really know why they sell the service. They claim that by filling tyres with a mixture of nitrogen and oxygen you can prevent tyres deflating by stopping oxygen seeping IN!
Isn't a mix of nitrogen and oxygen essentially air?

Hilary said...

Yup, definitely a profit thing.I was told this about Nitrogen last year and it baffled me as I couldn't think of one valid reason why it is better for tyres. Glad to know I'm not alone in this :)