Nitrogen...

[Duke W.]

Re: Nitrogen...

Those must be radials.

In vintage, where we race on bias ply tires, I get my best grip with cold pressures of 23 psi front, 21 psi rear. I have no idea what the hot pressure becomes

Yes, radials as I stated. The Toyo Proxes RA-1s I run on the Cosworth Vega work best with hot pressure of about 44 psi and a tread temperature in the range of 180-200F, and the Yokohama A008s on the MR-2 are similar. Under these conditions they will develop cornering force in the range of 1g on a level surface.

Radials like higher pressure than low cord angle, stiff sidewall bias plys.

Also of note is that too much pressure can be detrimental to handling. The Yokohama A509s (wear rating 160) summer high performance tires will oversteer more if I use more than 32psi, cold. The A008s (wear rating 80) work better with less oversteer at 36, cold either on the road or track. Tire sizes are size-biased (195/60R-14 front, 225/60R-14 rear), and with this setup the respective load capacity distribution of the tires is equal to vehicle static weight distribution. about 44/56 percent.

The OE rear tire size was 205/60R-14, and it oversteered like a '65 911, even with 5 mm rear toe in, The 225/60s added 200 pounds more load capacity at each rear corner, and the car was neutral even with rear toe reduced to 1.5 mm and brought the wildly optimistic speedometer reading within one percent of true.

Tire load capacity is very important, but few pay any attention to it. One of the problems with going to larger wheels with a tire of the same revs/miles is loss of load capacity, which is approximately proportional to the tire's contained air volume. That's why guys ran "oversize" tires back before there were dedicated racing tires. All other things equal, the less load on the tire relative to its maximum load capacity, the greater cornering force it will generate.

That non-linear behavior is also why anti-roll diameter changes will alter dynamic response.

Duke

[Dick W.]

Re: Nitrogen...

Drove Yukon about 50 miles yesterday evening at 55-70 mph. Tire pressure was the same as it was when the tires were installed five months ago, 33# cold. At the end of the first leg the tp was #35 same as it has always been since they were installed. Even on a one day drive back from Kenner LA last year.

I was not a believer the nitrogen before this set, but I have become a believer.

[Michael W.]

Re: Nitrogen...

Drove Yukon about 50 miles yesterday evening at 55-70 mph. Tire pressure was the same as it was when the tires were installed five months ago, 33# cold. At the end of the first leg the tp was #35 same as it has always been since they were installed. Even on a one day drive back from Kenner LA last year.

I was not a believer the nitrogen before this set, but I have become a believer.

Dick-

for your experiment to prove a point, substitute air for the nitrogen and re-do the drive. If you get the same pressure rise of 2 PSI, this will indicate that the laws of physics apply as stated above. If there is a greater pressure differential, let's say you see 40# instead of 35#, this will mean that you're consistently running the tires under inflated by 5 PSI and need to up the cold pressure by the same amount.

[Dick W.]

Re: Nitrogen...

Dick-

for your experiment to prove a point, substitute air for the nitrogen and re-do the drive. If you get the same pressure rise of 2 PSI, this will indicate that the laws of physics apply as stated above. If there is a greater pressure differential, let's say you see 40# instead of 35#, this will mean that you're consistently running the tires under inflated by 5 PSI and need to up the cold pressure by the same amount.

I saw pressure variations greater than these on the OEM times and just plain old air.3-5# to be exact.

[Duke W.]

Re: Nitrogen...

If you fill tires in the summer in warm to hot humid conditions, there will likely be condensation of water vapor when winter air temperature drops below the dew point of the air in the tire. This condensation will add to the pressure loss resulting from the lower temperature.

Then when you drive and the air in the tire rises above the dew point, the condensed moisture will evaporate and will cause more pressure rise than if the air was dry.

This is a point that had been previously discussed. In fact, it's been emphasized. Commercial nitrogen is nearly devoid of water vapor, so condensation or ice formation is not an issue. Condensation/ice formation can be an issue with compressed air, especially if the system doesn't have any kind of drier or desiccant, but this may not remove enough moisture to prevent condensation when ambient temperature drops below the dew point of the air in the tire.

If you fill your tires in Florida with compressed air in the winter and then drive it to North Dakota, condensation/ice formation will likely be an issue.

Oxygen, nitrogen, and water vapor all behave as ideal gases (follow Boyle's Law, pV=RT), individually or in any combination, but once water vapor, which is water in the gas phase condenses to a liquid or solid things change.

The important issue is to fill tires with a gas that has the least possible water vapor. Commercial dry nitrogen certainly meets that requirement, but if you use air, have a good drier system on the compressor tank outlet, and try to fire up the compressor and fill the tires when relative humidity is low.

I have been known to "change the air" in my tires in the fall and winter when we get a Santa Ana condition and the dew point drops to well below freezing, which is well below the coldest winter temperature along the Southern California coast.

"Hey, Duke, what are ya up to?"

"Ah, just changin' the air in the tires."

My neighbors think I'm crazy. I've tried to explain why I do this, but just get a blank stare back.

Duke

[Jim D.]

Re: Nitrogen...

Because of this thread, I took note on a recently completed trip with my GMC truck. Starting tire pressure with POA (plain old air), 35 PSI, which is the recommended cold inflated pressure. After 140 non-stop miles at 75 MPH the pressure in all 4 tires was 37 PSI.

[Duke W.]

Re: Nitrogen...

What was the ambient temperature range. The hotter the outside air, the more TP will increase at speed.

Even on a cool day on a car sitting outside, the sunny side will see more increase than the shaddy side.

Duke

[Jim D.]

[Duke W.]

Re: Nitrogen...

The answer I was looking for was about what was ambient. Tires will see less pressure rise in the fifties than in the eighties.

Duke

[Michael J.]

Re: Nitrogen...

The important issue is to fill tires with a gas that has the least possible water vapor. Commercial dry nitrogen certainly meets that requirement, but if you use air, have a good drier system on the compressor tank outlet, and try to fire up the compressor and fill the tires when relative humidity is low.


Duke

[John M.]

Re: Nitrogen...

Why not use hydrogen? The car would be lighter and has
mileage would improve.......

[John M.]

Re: Nitrogen...

its mileage

[Bill H.]

Re: Nitrogen...

What was the ambient temperature range. The hotter the outside air, the more TP will increase at speed.

Even on a cool day on a car sitting outside, the sunny side will see more increase than the shaddy side.

Duke

We get that a the track, open wheel car under the trailer awning with the sun at an angle will heat those tries up by 40-50 degrees easily. I roll the cars out into full sun a half hour before setting final tire press to go out on track.

[Michael G.]

Re: Nitrogen...

Next, somebody will be selling an additive that you put in your tires to allow the evil oxygen from the air to pass right through the rubber of the tire, leaving only nitrogen in the tire. Its a perfect scam, nobody can tell if the oxygen really passed through and the pressure will perform exactly like it would if you had put in pure nitrogen...

As a matter of fact I think I'll put the new "OXI-Suck" product on eBAy right away (I think a small squirt of helium would work great)... It'll be just like lead additive, an undetectable non-benefit, I'll make a fortune overnight...

[Don H.]

Re: Nitrogen...

this all kind of makes you wonder how people managed since about 1902 filling their car tires with that nasty old air from the compressor at the filling station.
I have been putting good old compressed air in car tires for about 50 years, and never once checked outside humidity level before doing it. Just lucky I guess.