Tire Pressure questions

[79sunrunner]

Hey guys!
I bought my self a Fiat awhile back and this is my first front engine front wheel drive car. According to the tire placard it suggests the tire pressures to be set to 38psi front and 30psi rear. However when I have the car serviced at the dealer, they raise the rear pressure to 36psi. Hmmm maybe Fiat posted a bullitan to change that. These tires are filled with nitrogen too, so if I notice the tires are low, how much would it cost to fill them up at a tire shop and if I'm in a serious bind, can I fill them up with air for an emergency?

 

[yonyon]

Most common off-the-shelf air (not sure about the fancy stuff) is about 78% nitrogen anyway so it should work in a pinch. The worst issue is that it has some oxygen in there so it becomes possible for the rubber to oxidise. Tires don't oxidise all that fast on the inside anyway if they stay cool so keeping the tire fully inflated helps with the biggest drawback of adding the air.

 

[widman]

I would say the dealer is probably not informed, or has old habits. You'd be amazed at dealer "standardization" Same thing that hurt the Corvair - people putting the same amount of air front and rear (require 10 to 12 psi less in front, since engine is in rear). With 36 in the back those two tires are probably riding only on the center, and will wear very unevenly without enough weight on them.

 

[79sunrunner]

And another question would be, what is the MAX psi meaning on the side of the tire. Is that the max pressure the tire can take (maybe an extra 20 psi for safety regulations) Is that a cold pressure reading, or hot as well. The reason I ask is I used to have a Ford Ranger with tiny wheels and tired (I can't remember size), so I got larger rims and tires, and I couldn't find the best psi range for the new tires. the 35psi placard on the door made them look like the tire was sagging. The max psi range on those tires were 60psi printed on the sidewall, so to prevent hard shocks that would damage it, I set them to 45, hoping it would be correct.

These are just random questions that have been bugging me. Thanks for answering

 

[Cardenio327]

If you adjust the pressures to those on the placard it will probably handle the best. Just remember the pressures on the placard are cold pressures, during the day or while driving the pressures will run up to 8 psi above the cold set pressure.

Air from any regular compressor will be 78% nitrogen. Tire store nitrogen is only 95% pure if the tires have been inflated by the proper nitrogen inflation method, which your tires most likely haven't been. Even if the tech that installs your tires understands how and why it should be inflated differently, most won't do it right anyway. Nitrogen's benefits in automotive use are mostly so inconsequential, even if you drive at 250 mph you won't see a measurable benefit. 99.5% hype and marketing against 0.5% improvement over regular old dry compressor air.

 

[Cardenio327]

Originally Posted By: 79sunrunner
And another question would be, what is the MAX psi meaning on the side of the tire. Is that the max pressure the tire can take (maybe an extra 20 psi for safety regulations) Is that a cold pressure reading, or hot as well. The reason I ask is I used to have a Ford Ranger with tiny wheels and tired (I can't remember size), so I got larger rims and tires, and I couldn't find the best psi range for the new tires. the 35psi placard on the door made them look like the tire was sagging. The max psi range on those tires were 60psi printed on the sidewall, so to prevent hard shocks that would damage it, I set them to 45, hoping it would be correct.

These are just random questions that have been bugging me. Thanks for answering


Looks like Yonyon beat me to the 78% nitrogen argument.

On type LT & type C tires the maximum inflation pressure is also the pressure at which the tire reaches maximum cargo capacity.

On modern type P & Euro tires the maximum pressure is not the maximum cargo pressure, it is a higher pressure for correcting handling in rear engine and other sports cars at speeds in excess of 135 mph. Example: a Euro SL tire reaches maximum cargo at 36 psi, but it could be labeled for a maximum pressure of either 36, 44 or 51 psi.

On your Ranger, a larger tire would most likely require a lower pressure to maintain the same contact patch, perhaps 28 or 30 psi UNLESS the larger tires was a type LT tire, then it would most likely need a higher pressure. The sidewall bulge you observed was probably just a characteristic of that model of radial tire.

 

[Cardenio327]

Originally Posted By: 79sunrunner
Is that a cold pressure reading, or hot as well.


Cold. A tire that reads 51 psi maximum can safely measure 59 psi after for example driving at 80 mph in 95 °F weather.

Most any new 4 ply rated tire (4 ply rated tires are labeled as 35, 36, 40, 41, 44 or 51 psi) will stand around 150 psi before it bursts, but driving at that pressure would likely break the radial cords or split the tire open from bead to bead.

 

[Traction]

Everybody always seems to be quick to put down nitrogen when they think it would be better to add water and oxygen to their tires. I guess they should use compressed air in fire extinguishers, shock absorbers, lift cylinders,aircraft tires, etc. Believe it or not it does leak down slower, and cuts way down on bead seat,and valve stem, tpms core corrosion. So use it when available. I have been using nitrogen for 8 years in every tire and have seen the difference. Oxygen is a very powerful gas.

 

[AVB]

Some fire extinguishers and shocks do use compressed air.

 

[y_p_w]

Originally Posted By: Traction
Everybody always seems to be quick to put down nitrogen when they think it would be better to add water and oxygen to their tires. I guess they should use compressed air in fire extinguishers, shock absorbers, lift cylinders,aircraft tires, etc. Believe it or not it does leak down slower, and cuts way down on bead seat,and valve stem, tpms core corrosion. So use it when available. I have been using nitrogen for 8 years in every tire and have seen the difference. Oxygen is a very powerful gas.

It's way oversold. The benefits are minimal and certainly not worth it if it's going to cost extra. The typical membrane N2 generators produce maybe a 95% N2 content. About the best thing they do is remove water vapor, although those effects are minimal and dry air would be as good.

Saying that aircraft engines use nitrogen is like saying that pro golfers use pro clubs or balls. It doesn't really mean much to the average person because its a far different environment.

 

[y_p_w]

Originally Posted By: AVB
Some fire extinguishers and shocks do use compressed air.

A lot of gas-only extinguishers use CO2 or halon. CO2 displaces oxygen, but halon bonds to flammable gas or vapors to inactivate it.

 

[Traction]

Adding oxygen would not be a good idea though.

 

[Traction]

Well I don't charge for it. I just do it because it is the right thing to do. And I said aircraft tires not engines. They would run much better on oxygen and not at all on nitrogen.

 

[widman]

Max is way too high for normal driving. But great for autocross.

The correct way to determine the pressure is to cover them with chalk and drive over a flat surface, or put the chalk powder on the flat surface and drive over it. Then examine for even contact across the tire.

Add or reduce pressure until you have even contact.

 

[63Marauder]

You use the pressure number that is on the tire. Cold.

 

[AVB]

Originally Posted By: y_p_w
Originally Posted By: AVB
Some fire extinguishers and shocks do use compressed air.

A lot of gas-only extinguishers use CO2 or halon. CO2 displaces oxygen, but halon bonds to flammable gas or vapors to inactivate it.


You're correct, the reason I was pointing out that compressed air is used in some was because of the different applications where nitrogen isn't much of an improvement. Like using compressed air as a propellant in a water type fire extinguisher, or in air shocks or air bags.

 

[Traction]

Don't try and re-invent the wheel, just use the pressure the engineers determined, that is on the placard on the door jamb. Never max. pressure on the sidewall. I have seen one guy that was so bad he had 90 psi in the rf and 6 in the lr,did not own a tire pressure gauge, and he thought they were fine.

 

[Falken]

I always add an extra 2 psi to the recommended tire pressures front and back.

Gives me a bit of time before I have to top them up, and it doesn't affect performance too much, just a slightly rougher ride.

I always use an electric compressor, and avoid shop air as it may have a lot of water in it if nobody services the compressor.

The only time I'd use Nitrogen is if it is me that puts it in the tires.

You have to remove the valve core and inflate / deflate 3 cycles, install the valve core, then fill it up a final time and install green caps that have washers in them.

Otherwise, there is no real benefit to adding Nitrogen to your tires if you do it improperly.

And, most people that pay extra to have Nitrogen added usually just get shop air with a green cap put on the valve stem.

 

[LotI]

Originally Posted By: 63Marauder
You use the pressure number that is on the tire. Cold.

Wrong!

The car manufacturer sets the pressure, not the tire manufacturer.

 

[NMBurb02]

Here in the People's Republic of California (not sure if elsewhere as well) shops are required to check tire pressure whenever they service a car and adjust the pressure as necessary. The GP calls for 30 psi front and back, but the independent shop I use fills to 40 psi every time. It makes for a harsh ride but is below the max psi on the tires (50 IIRC) so I'll typically drive around with it at 40 for a while and then bleed to 33-35 when I get around to it.

The Burb's tires are slightly oversized (1 inch/3% taller, same width) and I've never been able to figure out how I need to adjust the placard pressure of 35 psi to account for the difference in volume, despite reading advice on multiple sources (some say higher pressure, some say lower). I tried the chalk method but messed it up somehow and plan on trying it again.

I believe Costco fills with Nitrogen and the one time I purchased tires there is probably the only time I have used nitrogen.