Tire Safety: Don't Ignore the Rubber on the Road


  • Minivans

    Minivans

    Around and around she goes, when she'll slide, nobody knows. | March 18, 2010

When was the last time you checked your tires' air pressure? Don't remember? Or maybe it was done the last time you had an oil change (or at least you hope it was). Most people don't care about tire pressure because they don't notice "anything wrong" with their vehicle. It still seems to go and stop just fine. A similar philosophy seems to apply to winter driving ("I've got four-wheel drive and all-season tires, I'm all set") and worn tires ("They handle fine"). Well it may seem that way, until you're forced to quickly slow down and/or swerve to avoid an accident, be it on dry, wet or icy pavement.

After attending a tire safety event put on by the folks at the Tire Rack, we fully realized that keeping tabs on the condition of your tires and knowing which ones to use for winter driving make a huge difference in how your vehicle will react should you need to make an accident avoidance maneuver. The Tire Rack boasts the largest "one-stop" selection of tires and wheels in the U.S. and it also offers performance accessories such as suspension and brake upgrades. By using its Web site, consumers can select the correct type and size tires for their ride, depending on their needs and driving style. Bumping up your wheel size is a no-brainer as the site's configurator automatically shows only the wheel and tire combinations that will fit a given vehicle. In addition to the considerable amount of products it offers, the Tire Rack boasts an army of tire experts ready to give advice as well as its own on-site tire testing facility where said experts push tires to their limits in a safe environment.

At the Tire Rack event, we sampled braking on ice with Jeep Grand Cherokees equipped with three different tire setups, drove a pair of BMW 3 Series coupes (one with correct tire pressure and one that was low but within proposed government limits) around a track, drove another pair of 330s (one with snow tires all around and another with them just on the back) and lastly compared new versus well-worn tires on a pair of minivans by circling around a wet skid pad.

In the interest of making this piece user-friendly, we've listed the factors that affect tire performance and safety, how the Tire Rack had us test them and what you should do to maximize the safety and performance of your vehicle's link to the road.

Tire pressure

When a tire is underinflated, most of the car's weight is concentrated on the tread that's located just under the sidewalls, rather than being spread out evenly across the full width of the tire. This means that as the tire rolls, the sidewall gets continually flexed (squished, if you will) and heats up. This affects both performance and safety. In addition to degrading the handling of the vehicle (via the mushy steering response courtesy of the flexing sidewalls), a tire that's considerably low on air can blow out due to the stress from the heat buildup and the constant flexing of the sidewall.

For testing the effects that low tire pressure can have on a vehicle's performance, the Tire Rack had us drive two identical BMW 330Ci automatic coupes — one with proper tire pressure and the other with 30 percent less than the recommended amount. Why 30 percent less? Because soon the government is going to require that tire-pressure monitoring systems be standard in all new vehicles, and 30 percent happens to be the maximum amount that it feels is safe to allow for underinflation.

Looking at the two Bimmers side-by-side, it was hard to tell which one was low on air. The short, stiff sidewalls of the 225/50R16 Goodyear Eagle GT-HRs hid the deficit well. But when we drove them back-to-back, it was plainly evident which car's tires were underinflated, as the 330Ci's normally precise handling dynamics were softened, making it seem as if the car had a softer, almost mushy suspension in comparison. In all fairness, the low-air car didn't feel unsafe, but it certainly didn't inspire driver confidence the way the properly aired-up car did with its precise handling and steering feedback.

We can't overstate this: check your tire pressure at least once a month, perhaps on the first of the month to make it easy to remember. And while you're at it, give the tires a once-over and look for uneven wear patterns as well as cut or bulging sidewalls. Irregular wear could signal the need for an alignment or replacement of worn suspension components. A deep cut or a bubble in the sidewall indicates a potential weak spot that could lead to a blowout. Lastly, check the tread depth, either with an inexpensive gauge or by using the penny test. To do the latter, stick a penny upside-down in the middle and outside tread grooves, if you can see Lincoln's whole head then it's time for new tires.

And you don't want to overinflate 'em either, as that won't allow the desired full tread contact (due to the car riding chiefly on just the center of the tread) and will make the ride stiff.

Winter driving

I have an SUV; do I really need snow tires?
A trio of Jeep Grand Cherokee Limited V8s awaited us on the ice of an indoor hockey rink. One was fitted with the all-season Bridgestone Dueler H/T D689 tires that came as standard equipment, another had studless Bridgestone Winter Dueler DM-Z2 tires (essentially Bridgestone's superb Blizzaks that serve light-truck duty under a different name) and the third was shod with studded snow tires (WinterMaster Plus).

The test consisted of accelerating up to and braking at a set of cones. As expected, the Jeep with the "all-season" rubber had the most trouble starting and stopping as it scrabbled for grip when the throttle was squeezed and took the longest distance to come to rest. Although the Jeeps all had traction control and ABS, this exercise emphasized the point that those technologies can only make the most of the available traction — the systems spastically pulsed the brakes in an attempt to gain traction to move out and slow down. In contrast to the dicey performance of the stock all-season tires, the studded snows felt grippy as they bit into the ice under acceleration and braking. The big surprise was when we sampled the studless Dueler DM-Z2 snow tires. We knew they'd have good grip (well, as good as you can have on ice), and they did. They were just as tenacious as the studded snows. We highly recommend these for folks who deal with serious winter driving.

A word of caution to those who drive SUVs. Just because you have four-wheel drive doesn't mean you can stop or turn any better than a two-wheel-drive car. Four-wheel drive allows better motive traction, but when you're off the gas (such as when you need to turn quickly or stop) it doesn't matter; the laws of physics still apply. If you're tempting fate by driving too fast for the road conditions and need to turn or stop quickly, you might as well be driving a Mustang. This explains why, during slippery driving, we see so many SUVs "off road," meaning on the highway median or spun out on the shoulder. Two words of advice to you would-be hotshots: Slow down.

Don't I just need snow tires on the drive wheels for traction?
This myth was laid to rest when we went out on the dry test track and drove a pair of BMW 330Ci coupes, one with a full set of snow tires (Michelin Artic-Alpins) and the other with those snows on the back (the 330Ci is rear-drive) and high-performance rubber in front (Michelin Pilot XGT H4s).

This was definitely an eye-opener. Before we ran each car, we shut off the stability control system and were warned that one of the cars might end up biting us (car-guy talk for a car sliding out of control). Taking the 330 with the four snows through the course, the car felt solid and predictable. Running the 330 with the Pilots in front and Alpins in the back, the tail wagged increasingly as we zigzagged through the cones and then the car spun 180 degrees. Whoa! We drove home the point of the test with an exclamation point made in rubber marks on the track.

Mismatched tires can (and will, if pushed) adversely affect a car's handling; with the ends of the car having differing levels of grip, it is more apt to slide the tail or even spin out. Had we been driving in the snow, the rear tires probably would've had no problem propelling the car but steering and braking wouldn't fare as well — the high-performance Pilots would've washed out as the snow would easily pack their small grooves, essentially turning them into slicks. Not good on snow, slush or ice.

How do worn-out tires affect wet weather driving safety?
In order to have traction in the rain, tires need to be able to channel the water away; that's why there are grooves in the tread. Rain tires, such as the Michelin HydroEdge, have one or two large grooves running around the tire for that purpose. By channeling the water away, the tread can make contact with the road (this is good, as it allows steering and braking). In contrast, a well-worn tire doesn't have deep enough grooves to do this, and so can ride up on the water. This is called aquaplaning or hydroplaning (not good as you won't be able to steer or brake).

To demonstrate this, the Tire Rack had us drive a pair of Ford Windstar minivans — one with new Michelin HydroEdge tires and the other with the same model of tire only with minimal tread left. We piloted the Windstars around a large circle that was being "rained" on by sprinklers. By gradually increasing the Windstar's speed, we witnessed how the van with the good tires could stick to our intended line at around 40 mph while the almost bald ones allowed that Windstar to slide out.

In short, it's not worth the risk of getting in an accident by driving around on worn-out tires. Sure, you may not notice it if you mostly drive on dry roads, but when the rain comes and you're sliding all over the place, you're gonna wish you replaced those baloney skins.

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