Archive for Sunday, February 4, 2007

Toplikar: Nitrogen for tires - just hot air?

February 4, 2007


"Lauren and I are thinking about driving up to see Bonnie and Kate in Iowa in a couple of weeks," Julie told me at my kitchen table.

"Are you driving?"

She nodded. "Probably."

The thought of my daughter, a Kansas University student, making a five-hour trip in the dead of winter to see her twin sister in Iowa City made me worry about the status of her car.

As Julie went downstairs to do another load of laundry, I headed outside.

An icy wind bit into my bare fingers as I checked the pressure on her left front tire. Hmm. Only 28 pounds per square inch. The printing on the tire said the maximum was 44 psi.

Did they leak?

I'd been hearing that putting nitrogen in your tires instead of air could keep them from leaking.

Here's the deal: Nitrogen molecules are larger than the mix of stuff in the normal atmosphere. That means they won't seep out through the tire rubber as quickly as regular air, said Steve Aldrich, store manager at Gregg Tire Co., in northwest Lawrence.

Aldrich told me nitrogen has been used for years for race cars and several tire shops around Lawrence offer nitrogen for tires. He installed his own nitrogen machine last September.

"It's a great idea. I put it in my wife's tires almost immediately," he said.

The machine "vacuums" out all of the air in a tire, then refills it with nitrogen. The process takes a few minutes per tire. He charges $5 a tire.

A nitrogen-filled tire maintains its pressure "three to four times longer" than regular air pressure, Aldrich said.

"By maintaining pressure, it gives you better tire life and better fuel efficiency," he said.

There are other benefits, too, he said.

For example, a lot of newer wheels are made of aluminum alloys. If water vapor gets inside a tire, it can tend to corrode those aluminum alloy wheels from the inside out, he said.

Another advantage of nitrogen-filled tires is they run cooler in the summer, which reduces the chance of a blowout, he said. That's the main reason it's used in race cars. (See more about nitrogen.)

Leaking burritos

All of what Aldrich said sounded logical, but I was a little unsure.

After all, the atmosphere already has plenty of nitrogen in it - 78 percent. Oxygen makes up 21 percent, with about 0.93 percent argon, 0.04 percent carbon dioxide and the rest trace gases. I also found some skeptics online, including the Car Talk radio guys.

I asked Joe Heppert, professor and chairman of Kansas University's chemistry department, about the benefits of nitrogen in tires.

Heppert was skeptical. There's not really that much difference in the size of a nitrogen and an oxygen molecule, he said.

"You mean like the difference between a baseball and a tennis ball?" I asked.

"No," he said with a laugh, "it's like the difference between a Chipotle burrito with lettuce and one without lettuce."

He explained that each of the two molecules is rod-shaped, within 6 percent of one another in size.

But the difference is their reactivity to metal, he said.

Oxygen could corrode the metal on the inside of the tire, which could cause deterioration over time, he said.

But he pointed out that even if you had nitrogen on the inside of the tire, the outside of the tire still would be surrounded by normal atmosphere, which also includes oxygen.

"Chemically, I don't see a lot of rationale with filling tires with pure nitrogen," he said.

I told him I also had another reason for not using pure nitrogen, remembering a scene from the "A View to a Kill" James Bond movie.

If your car goes underwater because Christopher Walken and Grace Jones are trying to kill you, you can always get a few breaths out of the tire stems until they go away.

Pumping up

I drove Julie's car to the gas station, popped three quarters into the air pressure machine and hurriedly went around to all four tires and made sure they all had at least 40 psi. (Update: A few people have written to me telling me this was a bad idea and the maximum pressure is not the same as the optimum pressure, which is usually listed on the inside sticker on the inside door. I've been told that 28 to 36 psi are typical values for passenger vehicles and going over that can result in "excessive wear in the central region of the tread, reduced traction on slippery surfaces and an unnecessarily harsh ride." Thanks to those safety-conscious folks. )

The sub-freezing cold had reddened my fingers, so I went back home to thaw out a bit.

Getting out of Julie's car, I looked at my van.

Was that front tire a little low again?

I had the tire gauge handy, so I checked it.

Geez. Only 22 psi?

I was out of quarters. So I went to the garage and got the bike pump and got ready for some exercise.

Maybe I was just hungry.

For some reason, as I pumped, all I could think about were super-tiny Chipotle burritos.


LogicMan 11 years, 3 months ago


You likely did a boo-boo by inflating the tires to 40 psi. Assuming you have factory-size tires on that car, check the tire inflation sticker that should be on the driver's door frame, or the frame of the vehicle where the door meets it (possibly down low).

The factory inflations help give the engineered performance for braking and tire life, for example. By likely over inflating the tires, you reduced traction for braking especially on wet/icy/snowy roads. Also possibly tire life and ride quality, but slightly increased MPG.

The inflation pressures are also for cold tires -- like after sitting in the garage over night.

Also be sure to check the pressure seasonally as warm temperatures increase the pressure, and reverse for cold weather.

This is an important safety issue (remember the Ford SUV rollovers due to improper inflation ...), so you need to print a correction in the paper ASAP.

FYI -- Sears et al. sell little air compressors with hoses and tire chucks for $99 or less (often on sale). They are a good investment!

LogicMan 11 years, 3 months ago

P.S. The maximum tire air pressure molded on to the tire sidewall is just that. Is is not the air pressure that the tire is supposed to be inflated.

P.S.#2 Web sites such as have excellent tools for selecting proper replacement tires for vehicles, even if you end up buying them locally.

Janet Lowther 11 years, 3 months ago

Previous comments have covered the pressure matter.

However, as I understand it, Nitrogen is very slightly LESS dense than air: Air being composed predominantly of Nitrogen and Oxygen, with atomic weights of 14 and 16 respectively.

That would lead me to expect the average density of air to be very slightly more than that of nitrogen.

Now, if you wanted to use something denser and less reactive than air, how about carbon dioxide? CO2 is significantly denser than air, and on top of that you get a lot more in a given cylinder since it's liquefied. A lot of serious off-roaders carry a cylinder of CO2 to inflate tires and run air tools.

compmd 11 years, 3 months ago

carbon dioxide does not exist as a liquid. it undergoes sublimation, a phase change from a solid directly to a gas. any cylinder of co2 has gas in it. the drivers that carry co2 do so because it is cheap, not because it has special powers.

also, cold air compresses. so when the pressure in your tires drops a bit from autumn weather to the 16 degree crap weve had lately, that's probably just physics at work. reinflating your tires when its cold will cause them to be overpressure when it warms back up.

who says using something more dense is a good idea? you increase the mass of the gas within the tires, thus increasing inertia, thus decreasing your fuel economy.

nitrogen for tires is more or less a novelty for the average driver. this article just goes to show that a little knowledge is a dangerous thing.

Tristan Moody 11 years, 3 months ago

Point of clarification for something compmd said: CO2 can exist as a liquid at pressures above 5.1 atmospheres, a pressure easily obtained in compressed gas cylinders, which typically hold pressures between 200 and 400 atm. One only needs to gently shake a reasonably full CO2 cylinder and listen to the sloshing to recognize that a significant portion of the contents contained within are liquified.

Michael Stanclift 11 years, 3 months ago

Hey Dave, the QuikTrip at 23rd and Haskell has free air. No quarters required.

Also, would you please stop using the LJWorld as your blog site?

Crossfire 11 years, 1 month ago

Inside the door of in the glove box or in your owners manual are only the "recommended" pressures. Realize that your tires will hold alot more than the maximum listed pressure. If you dismount your tires and fill them with sand, remount them and then air them up to 80 or 100 psi you should never have to check them again, ever. Sand never leaks out and the extra weight will prevent rollovers.

50YearResident 11 years, 1 month ago

Crossfire, You have to be careful about smooking weed, drinking and shooting up at the same time. It gives you strange ideas.

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