Tire pressure myths, facts, observations, and improvements (canned post)

[theholycow]

Regular air is already 78% nitrogen. I'm not paying for air...

[ShadowWorks]

Originally Posted by theholycow

Regular air is already 78% nitrogen. I'm not paying for air...

Me neither but I would like to try this if it were free, the F1 cars run on pure Nitrogen and Bridgestone test the tires with nitrogen and then one with regular air, they claim better mileage and longer tire life, it makes me wonder if any other gas could be used, something with larger molecules and no corrosive elements, as we know its not that regular air has 78% nitrogen, its the 21% oxygen that is not so good it would appear.

[Greyg]

SWEET! A questions I can help with! I have nitrogen in my 2007 Chrysler Town and Country. I work at a dealership and the store owner let every employee have free nitrogen in one vehicle so we can talk up how great it is and sell it to our customers for $39 for all 5 tires (he even throws in the spare, what a guy). You will get no increase in FE from the nitrogen itself. The benefit is for people who never check their pressure and usually run on low tires because of lack of maintenance. I've read articles and seen interviews where people talk about a 25% increase in fuel economy, my bet is this is because the tires were terribly underinflated to begin with. Nitrogen is less prone to expansion and contraction and will keep your tire pressure more consistant, thereby "increasing" your FE if you never check your tire pressure because it will keep it a more consistant level. I have not noticed any significant increase from air to nitrogen, but it's not necessary to check my pressure as often. Example, filled with Nitrogen, 35 lbs in March. I just checked them today and they are dead on 35 lbs. I usually run them at 40 to 45 lbs, but everytime I get my oil changed the guys take them back down to 35 even though I tell them not to. Bottom line from what I noticed:

If you don't check your tire pressure get the nitrogen, the $40 or whatever will be worth it.

If you check your tire pressure regularly save the $40.

I did get a pretty cool set of metal valve stem covers though. They were stolen once so far, but I know where to get more.

[ziddey]

The last few tanks, I've been running around with 45psi on my 35max tires. fuel econ has soared for me. I used to get 20mpg pure city, 24 hybrid, 30 highway, roughly. The last few tanks, I got 35mpg full highway and 30mpg hybrid. I've been getting heavier on the pedal even. Sure it's a little more unstable, but still completely manageable. Woowoo! My coolant temp sensor is bonkers, so I have it unplugged (programmed fuel trim only), so I'm hoping to get a few more mpg on my cross country drive once I get the part and put it in.

edit: sorry for not having a pure city number. probably won't be doing that for quite some time.

[theholycow]

Congrats on your success!

If it's "a little unstable", you've inflated your tires too much and should really back them off a little. I suspect you will get most or all of the fuel economy advantage at a lower, safer pressure.

[KU40]

I would have to disagree with the statement about tires not ballooning out in the center because of a steel belt. If tires did not do this, they wouldn't wear down the center line if the pressure is too high. Also, the steel belt is not a solid piece of steel as that would seem to imply. It is actually made up of a lot of steel wires, and if you've ever held a wire in your hand (even a steel one), you know that it can bend.

Also, trucks, buses, RV's, etc., all fill their tires up to maximum sidewall pressure.

[theholycow]

The steel belt prevents the tire from ballooning visibly. The wires can bend, but they can't stretch, which would be required for it to balloon visibly. It doesn't prevent the pressure from changing the shape of the pressure of the contact patch.

That said, the only time I've ever seen center wear is on my dad's rear tires, and that happens regardless of the pressure that's in them. I don't know how he does it.

There are charts for trucks, buses, and RVs to figure the appropriate pressure based on load. They're somewhat hard to find, although the RV ones (which are fine for pickups, SUVs, and light trucks wearing the same size tires as listed in the document) are easy:
http://www.google.com/search?q=rv+lo...t+filetype:pdf
If you REALLY want a futile neverending debate, go to rec.outdoors.rv-travel and ask Will "The Curmudgeon" Sill or DJ Osborn about tire pressure. They are both stubborn oldsters who insist on arguing with eachother, though when push comes to shove and you really read what they write, it ends up being the same. One writes in such a way as to imply that it may occasionally be necessary to inflate beyond the tire's maximum inflation rating, while the other insists that his opponent is an idiot.

[KU40]

Actually I would argue that they can stretch. This is why builders put metal inside concrete- to combine the tensional/stretchy ability of the metal with the compressional (or lack thereof) quality of the concrete. This way the building/bridge/etc. can sway in the wind, bend but not break. Or the road can shrink and stretch in heat/cold, and resist cracking (well, it usually works, haha).

If you've ever pulled the valve core out of a stem and let all the air out, you can see that the tire actually shrinks. This usually only happens in the last 5-10 psi however.

But I had to laugh about the story of the two guys. There is almost always a pair like that on every forum, isn't there?

[GasSavers_JoeBob]

I've been running about 42 psi in the tires on my Geo all this year. Worked fine until the last few days, during which I've had to slog through snow and ice (Welcome to Sunny Southern California...land of 5 seasons: fire, flood, earthquake, riot and now SNOW!) Anyway, the car started feeling skittish in the ice and snow. Reduced the pressure to the spec on the doorjamb, 32 psi, and the car handles much better.

When the nice weather returns, I'll probably push the pressure back up, but for now I want a better contact patch on the road.

[theholycow]

I thought that might be the case, and posted about it here:
http://www.gassavers.org/showthread.php?t=9444