Answer Man

This is why your tires lose air pressure when it gets cold

Tires 101: Be prepared for winter weather

The difference between all-season tires, snow tires and all-terrain tires is explained by Bill Coleman, owner of Tim's Tire Center in Olathe. And he cautions that none are going to be much help in ice.
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The difference between all-season tires, snow tires and all-terrain tires is explained by Bill Coleman, owner of Tim's Tire Center in Olathe. And he cautions that none are going to be much help in ice.

Q: Something very annoying is happening to my cars that are parked outside in the cold. (It doesn’t seem to affect those in the garage). When I start the car, the “low tire pressure” warning light comes on. To fix the problem, I have to put a few psi of air into the offending tires. So, where did all that air suddenly go overnight? When it warms up, will I now have too much air? What about the tire stores, which offer to put nitrogen into my tires for a hefty fee? I purchased a 1-year-old vehicle with the green valve stem caps, which I think signify “nitrogen only.” What if I add regular air?

S.F., of Edwardsville

A: You’re undoubtedly thinking that if these four tough, rubber balloons are strong enough to carry your car tens of thousands of miles, the least they could do would be to maintain a constant air pressure for weeks or months at a time.

Well, they can’t and don’t. Just as I invariably must add a few psis to my high-pressure bike tires every week or so, you can expect even new, properly mounted car tires to regularly lose pressure for two reasons — normal osmosis and changes in temperature. Allow me to expand on both:

Just as lead cannot block the transmission of all exotic radiation, even those seemingly thick tire walls cannot completely block the escape of air. This is called osmosis (or permeation), and it usually results in the loss of 1-3 psi per month depending on the model and make of tire, according to utires.com.

As a result, you should be vigilant about checking pressure on a regular basis. Just neglecting such checks for two or three months could result in a sizable loss from your normal 30-35 psi level, which could cut into the life of your tire. Remember, too, that neglecting regular tire pressure checks is estimate to cause 75 percent of flat-tire issues, and the last thing you want is to be left stranded, especially in the dead of winter.

This is especially critical this time of the year because of the other reason for pressure loss: temperature changes. It’s an indisputable law of physics that as air cools, the space between its molecules contracts, resulting in a drop in pressure.

The rule of thumb is that your tires will lose about 2 percent of their pressure for every 10-degree drop in temperature, so if the thermometer drops 20 or 30 degrees overnight, you might lose two or three pounds of pressure. That could trigger your warning light, especially if your pressure were close to the lower limit to begin with. It’s not that the air really went anywhere, but it is exerting less pressure because of the morning chill. This could result in poor traction and handling.

So what’s a driver to do? According to the experts, you should always pump up your tires to recommended pressures when they are cold — either after the car has sat out all night or within a very few minutes of starting your drive. If you check your tires after more than 15 minutes of driving, you should subtract a couple of psi from the reading on your gauge and fill accordingly. You might also take that 1-2 psi fudge factor into account if the tires had been sitting in the sun prior to starting, because that, too, could increase the pressure despite the ambient temperature.

So that leaves one question — should you fill the tires with regular (normal air) or premium (nitrogen)? While nitrogen may offer some benefits, the experts I’ve found — including Edmunds and cars.com — say it’s not worth the extra bucks.

The theory is that nitrogen molecules are larger than those of normal air, so they will be slower to leak out of the tire through permeation. As a result, proponents say, tires will maintain a healthy pressure longer, leading to better tire life and fuel economy for those who don’t check their tires regularly. In addition, they argue that nitrogen, unlike air, contains no moisture and therefore is not subject to the temperature fluctuation.

But for average drivers who take the time to check their pressure, experts say this is an expensive gimmick. Remember that regular air is already 78 percent nitrogen while the “pure” nitrogen usually contains only 93 percent to 95 percent of the gas, so the difference isn’t huge. Moreover, they say the advantages of cooler running temperatures and dry rot prevention usually are negligible.

What isn’t negligible is the difference in price. Compressed air machines might cost you a few quarters per year per tire while at least one estimate says you might spend $50-75 or more by keeping a tire properly inflated with nitrogen. So while the green caps indicate the tires came with nitrogen, you might save some green by going with good old-fashioned air.

Today’s trivia

Who founded the Goodyear Tire and Rubber Co.?

Answer to Monday’s trivia: It’s often thought that the ENIAC, dedicated in 1946 at the University of Pennsylvania, was the first electronic digital computer. But in 1973, a U.S. District Court ruled that the ENIAC was built on discoveries made by math/physics professor John Vincent Atanasoff and his graduate student Clifford Berry when they developed the Atanasoff-Berry Computer, which they successfully tested in 1942 at Iowa State College in Ames. The machine was largely forgotten for 30 years because its paper card writer/reader was never perfected, but in 1990, the Institute of Electrical and Electronics Engineers bestowed its Milestone status on it.

Roger Schlueter: 618-239-2465, @RogerAnswer

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