Tyres "perishing"

Jamie_Carter

Ultrasonic wrote: »

That simply isn't true. As above, moisture is the significant difference, NOT the gas.

If that were the case, then they would simply dry the air before using it.

colino

I don't actually have to provide any misguided internet enquirer my engineering qualifications, and I wont, but if you a) believe nitrogen doesn't expand and b) they use inert gases (you know, non-explosive) to fill aircraft tyres because of temperature differences, you really haven't listened, or understood a word any engineer has told you.

Jamie_Carter

colino wrote: »

I don't actually have to provide any misguided internet enquirer my engineering qualifications, and I wont, but if you a) believe nitrogen doesn't expand and b) they use inert gases (you know, non-explosive) to fill aircraft tyres because of temperature differences, you really haven't listened, or understood a word any engineer has told you.

So you aren't an aircraft engineer then??

Or a motor racing engineer??

colino

No neither, though one of my post-school first job offers was with BA and one of my previous businesses did a lot of work on rally cars and worked with a chassis designer to make the then new Zetecs fit FF in the early 90's.
No closer to understanding inert gases then?

Paradigm

The pressure in nitrogen filled tires will change when the temperature changes, just as it does with air filled tires, because nitrogen and oxygen respond to changes in ambient temperature in a similar manner. For example, when your vehicle is parked it will lose a similar amount of pressure for every 10 degree change in temperature, whether the tires are filled with nitrogen or air.
The calculations for this change are based on the Ideal Gas Law. A good rule of thumb is this: For every 10 F degree change in temperature, the pressure will change by 1.9%. If a tire is filled to 32 psi at a temperature of 75 F degrees and the temperature drops 10 degrees, the tire pressure will drop to 31.4 psi; a difference of .6 psi

These fluctuations will occur as the temperature rises and falls no matter what the inflation gas.

Now for a bit of balance

However, nitrogen does not contain the moisture and other contaminants found in compressed air so, as you drive and the tires heat up, nitrogen filled tires will fluctuate less in temperature and pressure than air filled tires while driving. The bottom line is, you will still see pressure changes with nitrogen but, overall, your tires will run cooler and at a more consistent pressure than if they were filled with air.

Taken from http://www.getnitrogen.org/sub.php?view=getTheFacts&subpage=temperatureeffects

For the very small differences it appears to make I'll keep nipping to my local forecourt

GolfBravo

Jamie_Carter wrote: »

Moisture does come into it, but what I described is the main reason.

Your info is wrong.

Calculations: assume aircraft tyre pressure 15 bar

Oxygen isobar coefficient of thermal expansion at 30ºC 3.45, at -40ºC 4.72

Nitrogen isobar coefficient of thermal expansion at 30ºC 3.41, at -40ºC 4.65

So negligible difference between the two, they both expand in hot temperature at a very similar rate. The only difference is less moisture in ≈95% nitrogen aircraft tyre (no ice build up inside tyre at high altitudes/low temperatures).

Standard car tyre normal air (78% nitrogen) vs nitrogen (plus air already present in the tyre, 18% oxygen) difference = totally irrelevant.

Tyre shop profit margin = fantastic!

Ultrasonic

I'm a physicist not an aircraft engineer, but as Paradigm and GolfBravo have explained any differences in thermal expansion of nitrogen, oxygen or dry air are very small. Ideal gas behaviour is a pretty decent approximation for all.

bigjl

I read once that tyres filled with nitrogen will lose less pressure over a period of time due to molecule size. Whereas those filled with normal air will lose more.

Can't remember if it was o2 that would seep out to be honest.

But it did seem to make sense.

I did also see a test on tv where they looked into wether tyre pressues stay more constant with nitrogen and they found there was little difference even though in theory they thought it would be.

They put it down to contamination in the nitrogen they were using.

On the other point have a mate that is an airline pilot and will ask him aboht the tyres on aircraft and the reasoning. Handily he is also a Mechanical Engineering (B Eng) so he may be able to shed some light.

Or he may just kick the tyres on a walk round!

adonis

Ultrasonic wrote: »

As a side note, I always top up the pressures in my tyres with a manual pump. Am I potentially doing long term damage to my wheels due to the moisture content of the air, compared to fuel station forecourt compressors that I guess dry the air?

I always use a footpump on my car, it is perfectly fine to do it.

As a bonus when I get petrol it is amusing watching able bodied people waiting ages to use the garage airline.

knightstyle

Nitrogen in tyres is available in France for 1€ a tyre when you buy new tyres. Our friends have it as they are getting on and with nitrogen you do not need to check the tyre pressures, just go in once a year to the tyre place. They have it in their caravan tyres as well and say that their mpg is better since having it.