I have heard the theory that if you have driven more than a mile or so, the tyres have heated up to expand the air inside enough to give you a different reading than if they were cold - about 2-3 psi.

i.e. if cold, it would be 30 but if hot 32-33, so giving the impression you don't need to inflate any more, even if you really do.

Is this bs-alert time, or is there something to it?

Or would switching to Nitrogen give a different result - even though I'm not anxious to pay for that... or is the whole Nitrogen thing another pile of bs to get you to buy something else?

Thanks for any advice.

Rick

about air expanding and changing the tire pressure. If you look at drag racing cars, yes, they do under inflate the tires, and they sure do expand, but, you are talking about super high performance cars accelerating much much faster than anything the family hauler will ever do. I have always maintained the tire people know what they are about, if it says 30 psi, then 30 psi it is.

As for nitrogen, my 08 milan HAD nitrogen in the tires. They claimed that nitrogen is a more stable gas (which it is) and would not expand or contract like oxygen (which is why it is used in formula 1 car tires. So, unless they grocery getter is made by McLaren, I really don't see a need for nitrogen in most driving conditions.
On top of all that, nitrogen is going to cost you a few bucks a tire to fill. You will also have to pay a special fee to have the tire "purged" of oxygen. Is it a rip off? I think so. ymmv (especially if you under inflate those tires!!!)

"Bruce Springsteen? I LOVE him! He's like the American version of Bryan Adams!"

_Robin Sherbotsky

Yes, after a few miles the tires heat up and the pressure increases. I bought new a 2010 Malibu with a display where if you are really board while driving watch the change. The most I seen is 3 psi even when driving on the highway during the summer, I don't compensate.
When the OnStar was free, I even got email when the car felt the pressure was too low. You got to be careful when you have the tires rotated because the tires' sensors are key to their original locations. You have to take it to a shop because the resetting computer cost a few hundred bucks, you can't do it yourself, like my old car. If you get a flat and put on a spare, that really sets the warning going.
Its really a shame that the new cars come with this annoying feature. Its my wife's car and yes it is a safety feature but how many wives called their husband or had it taken to a shop because of a warning that implies the car is dangerous to drive? I check my 12 year old car with an old fashion tire gauge and it been around this country with no problems.

as if it was really a major problem we'd all have heard much more about it by now.

As for Nitrogen, the story someone told me was that the N molecule is larger than the O one, so it doesn't leak out of the tyre as quickly.......
(rolleyes)

Cheers

Rick

It's those pesky laws of physics. Tyre (or tire) pressure is always represented as "cold" pressure. So when measuring the pressure after driving to someplace where you can check and pump up your gyres (tires), be sure to add a couple of pounds to the published number to get the correct pressure. Under inflated tires (gyres) run hotter too (due to the friction of flexing), so take that into consideration. What this means is that if your tyres are significantly under inflated you will probably have to give them an extra pound or two when adding air. You should check them cold the next day before driving if you can.

the in the bright sun side a few lbs higher.

I have a home compressor and fill up/check in the morn, in the garage, while everything is cool. Don't forget the spare.

If you don't have such, drive to the nearest gas station (air's prob no longer free!), overfill by 3-4, drive home, next morning burp down to the correct psi as per your car's owner's manual.

Friends who race, have raced, and my Porsche mechanic friend who wrenches for rich boy racers, all use N. More stable, less likely to escape thru the rubber in time like 'air', pressure less effected by temps, is dry whereas gas station air is prob filled w/ water. Water is of course bad for metal wheels and will give false pressure readings. A few lbs may not be a big deal w/ passenger car tires but when setting up a race car for that day's track/conditions, it's important.

I have regular air in all my cars.

Btw, you just can't switch from air to N. To do it right you gotta break the beads, make sure the insides are dry-dry, then fill w/ N.

Again, for passenger cars, not worth the bother in my mind. Maybe if you're one of those 'boy racers' and think that tiny lil' bit extra will give you some edge in the next street race up in the twisties. But prob not. Going into a corner at Daytona at 199 mph, yes a lb or two will make a big diff!

and that ten degrees can come from a hot road surface or just running the tires. Tires running on the summer freeway at 80 mph might gain 2 or 3 pounds of pressure. So if the tires read 34 pounds at 70 degrees it would take something like a 200 degree rise in temps to push the tire to its maximum unsafe pressure. It's not really a big concern to most of us. Unless you like tires with 50 PSI in them in the first place.

Nitrogen? That's for the big boys. If you race, and manage to keep a set of tires more than one race, it might be cost effective. It is more stable than Air and doesn't expand as much. Great for your super fuel dragsters, or your Indy cars but not much else.

Yes, the molecules are larger than most other gases, but Air itself is 78% Nitrogen so comparing it to oxygen is silly because we fill our tires with compressed air. The most pressure you could lose through the rubber is 22%, so maybe 8 PSI. And face it, oxygen is explosive. Not a good choice for tires!

Sun and oxygen are the two biggest enemies of tires. But oxygen is in the air around us. Filling the tires with nitrogen doesn't make any real difference in the life of a tire. Unless you just like telling people you fill your tires with nitrogen it really makes no sense. Even a super car can't be run at 200 mph for hours on end in any country in the world. Except maybe the interior of Australia? But considering the roads there it would be suicide.

I've heard of those ritzy tire monitors going bad and showing either 3 PSI or 60 PSI in tires that are fine. And having each tire coded to a certain corner of the car makes the recommended tire rotation a royal pain. If you could just push a button and change the programming that would be fine but most of the time you can't, it's a dealer reprogramming. I'd rather have a digital gauge in my glove box. When that fails it only costs a few bucks to replace.

Illegitimi non carborundum
In memory of Denis Keegan.

Over inflation is not near as bad as under. But the tire is built to "expand" as pressure increases from heat.

Nitrogen seems to be a bit of overkill. It is used as a preservative. I once had nitrogen filled on purchase, not sure they lasted any longer. Also, once you add air it is compromised.

Nitrogen is what is injected into snack food bags to keep fresh and reduce damage. Plump bag for a reason. also hides how little you actually get.



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By just adding air, you are adding 78% nitrogen in any case.

But it is that 20.95% that does the damage. Otherwise air would be used as a preservative, and we know it is not.

So adding air does not perform the same role as "pure nitrogen"




There is no such thing as an unbuildable kit, just some kits one may consider not worth building.

Ive realized that most people ... tend not to be direct when they feel something is shoddy because they want to be liked, "which is actually a vain trait".
[Walter Isaacson's (author of Steve Jobs) recounting of his interview with Jony Ive, Chief Designer at Apple @ page p. 461]



BUY THIS BOOK
http://tinyurl.com/Ididntseeitcoming

oxygen, meaning, it doesn't expand like oxygen will, nor will it contract like oxygen will.

It WILL expand and contract, but not as quickly.

"Bruce Springsteen? I LOVE him! He's like the American version of Bryan Adams!"

_Robin Sherbotsky

But the benefit seems to be minimal.

Had nitrogen used only once with tires I bought at Costco. All other tire dealers use good ol' air.

Then again on one of our cars we have run-flats. Who knows what's in them! Car does not have a spare..



There is no such thing as an unbuildable kit, just some kits one may consider not worth building.

Ive realized that most people ... tend not to be direct when they feel something is shoddy because they want to be liked, "which is actually a vain trait".
[Walter Isaacson's (author of Steve Jobs) recounting of his interview with Jony Ive, Chief Designer at Apple @ page p. 461]



BUY THIS BOOK
http://tinyurl.com/Ididntseeitcoming

and all it really did was set me back 5 bucks every time I had to fill it due to a leak......

"Bruce Springsteen? I LOVE him! He's like the American version of Bryan Adams!"

_Robin Sherbotsky

Tires on the SR-71 were giving tons of problems until they started using nitrogen in them.

Take the tire pressure first thing in the morning, go drive a mile, five miles and measure them at each point.

Or come visit in OKLAHOMA. You can take them in the 80 degree F range in the morning and the 110 degree F in the afternoon.!