Put tires at top of car "winterizing" list

  • Published
  • By Col. Michael Panarisi
  • Air Armament Center Safety Office
With winter fully upon us, it's already a little late to get our vehicles ready for the rough weather and conditions ahead. Getting this accomplished before you need it is the way to go.

Up north, we call this "winterizing," and we get crazy about anti-freeze, wiper fluid, water grabbing gas additives, and wiper blades.

Some climates are not as severe during the winter, but these are all good things to take care of no matter where you call home this year. At the top of the list is tires.

Maybe some of you are thinking "what's this guy know about tires?" I ran my own garage before I entered the Air Force, and I raced cars for years. In my prep for a degree in mechanical engineering, I took a couple extra courses in automotive applications. One of them spent an entire block on tires. It was fascinating, and I've been a student of tire technology ever since.

Most of us run all season tires, so all we need to do is check the condition, age, and pressure. The condition is the hard part...tread depth, road damage, and sidewall cracks, some of these are easy to miss. Damage can be hard to find, so spend some time looking here.

Don't tolerate sidewall cracks. Sometimes called "dry rot," these deterioration patterns suggest the rubber is nearing the end of its lifespan, and trying to stretch this can leave you stranded or much worse. Get a pro to look at this. They know from experience there's just no way to predict failure when these cracks start appearing.

If your tires are more than five years old, it's time to think about an update. Every tire has a "birthday" stamped on the side, and the Department of Transportation requires tire manufacturers to follow a standard marking scheme. Of course, the tire's birthday is in code. The "magic decoder ring," which displays a tire's birthday, is available on the DOT website.

The "US DOT Tire Identification Number" is stamped on the sidewall near the rim. On some tires it's hidden on the axle side, more commonly on raised white lettered tires. You might have to scoot around under the car a bit to find it. Once you find the code, it contains the tire's birthday. The last four-digits of the DOT number reveal the week and year the tire came out of the factory, so 2809 would be the 28th week of 2009.

The only tire pressure you need to know is the one printed on the vehicle data plate. Most of these are on the driver's side door jam. It displays the manufacturer's recommended tire pressure, as their judgment of the best compromise between traction, handling, noise, wear, etc. They tune the suspension components around this number, and have carefully determined how the tread contacts the road, called the "contact patch," at that pressure. Any deviation makes you the test pilot.

The factory recommended pressure is a "cold" pressure. The engineers know the pressure will rise with heat, and if you are using the same size and brand the car was born with, no worries. But if you change the tires, you need to make sure the maximum allowable pressure for that tire (also printed on the tire sidewall) gives you some headroom as the tire heats up.

The only way to know how much margin you have is to stop and take a reading on a hot day after some time at highway speeds.

That temperature sensitivity (about one psi for every 10 degrees Fahrenheit) means you have to adjust the tire pressure as the seasons change, typically in fall and spring. Now that summer is behind us, many of you are probably seeing some tire pressure warning lights if your vehicle has a tire pressure monitoring system.

If you filled your tires when it was 95 degrees outside last July, when the temps dip into the thirties, you could be almost 10 psi low.

It's best to check first thing in the morning, and in the shade. That will give you a true "cold" reading.

Extra pressure
With gas prices still on the rise, there's a temptation to "add a little extra" with thoughts of decreasing rolling resistance and increasing gas mileage. The extra air consumes your margin, and causes the contact patch to change shape. It mucks with the handling, wet traction, and braking effectiveness, plus it makes the center of the tires wear out faster than the edges.
There are tons of misinformation on the claimed benefits of using nitrogen in vehicle tires. It would take pages to dispute all the rhetoric out there on this subject, so look at the big ones. First, remember that air is around 80 percent nitrogen to begin with, so we aren't talking huge differences to start out with.

There are claims that nitrogen is a good deal because it leaks out more slowly (backed up by pointing out nitrogen's slightly larger molecular size). A consumer magazine took on this myth and found out it's actually true, but on the order of one or two psi a year. Since you have to adjust your tire pressure at least twice a year anyway, that difference isn't going to save you a trip to the air pump.

Bottom line - keeping up with the tire pressure is probably the single most important user-safety and gas savings task you can accomplish, and it does take some intervention as the seasons change. However, this is not the place to get creative. Follow the factory numbers, check it often, and stay safe.