Archive for Monday, April 29, 2002

Traction, speed play roles when driving in the rain

April 29, 2002


My windshield wipers were sweeping at full bore. The blurry speed limit sign said 70 mph, but I took it down to 60.

I was driving back from a volleyball tournament in Lee's Summit, Mo., with my daughter Bonnie and her teammate Katie.

As heavy sheets of water challenged my wipers, a red sedan went around me, seeming to ignore the heavy rain.

Up ahead I saw flashing lights. I came upon a state trooper who was checking two cars that had gone down into the median.

I tapped the brakes. But there was a loose feeling in the steering wheel.

And I started to slide Â

Getting a grip

Hydroplaning on rain-slick roadways causes about 13 percent of traffic accidents each year, according to federal highway statistics. That compares with about 2 percent caused by snow or sleet.

The planing effect occurs when a small film of water wedges underneath the tire.

At a certain speed in a certain level of water on the roadway, tires lose all or most of their contact with the surface and ride on that film of water. It's like driving on a sheet of ice.

Because of differences in road surface, tire tread and rainfall amounts, it's difficult to predict when hydroplaning will occur. Variables include the amount of water, tire rotation speed, tire tread and road surface.

NASA has been studying hydroplaning since the 1950s in an effort to reduce plane crashes on wet landing strips. Its research has found that cutting grooves into the landing strips and using tires with deeper treads help to channel the water away and increase traction.

And tire manufacturers have been developing tests and technology to build tires that grip the road in wet conditions.

Rain tires

If you walk into a tire shop, you're likely to find all-season tires or some kind of rain tire. But the big question is, how do you tell which tire is best?

I checked traction ratings for some top tire brands on the Department of Transportation Web site (


The DOT's tire grading system is called Uniform Tire Quality Grading. Ratings are marked on the sidewalls and on a paper tag attached to the tread of each new tire. They include ratings for tread, traction and performance in high temperatures.

The traction ratings represent the tire's ability to stop on wet concrete or asphalt under controlled government testing specifications.

Tires are rated on a scale, from best to worst, of AA, A, B and C.

I found 61 tires that received the AA rating for traction. I decided to check which tires had the top criteria for all three ratings, plus the one for tread wear.

That left three tires: Firestone's Firehawk LH (in size 215/60R16 only), BF Goodrich's G-Force T/A KDWS (all sizes) and Pirelli's Scorpion P-Metric (all sizes).

I also decided to look at Goodyear's newest "rain tire" Â the Aquatred 3 Â which received a AA traction rating, a high "wear" rating and a B for temperature.

Popular Mechanics and Edmunds magazine writers have praised the Aquatred 3's technical advances for driving on wet pavement.

Technicians who developed the Aquatred tire in the 1980s and '90s created it with the idea that a center tread would channel water away. They started by putting two motorcycle tires side by side and experimented from there.

The latest generation of the Aquatred doesn't have that central groove but still has two central channels and a wider tread surface to grip the road. The tread has a series of grooves that angle away from the center in a "V" fashion.

A rule of thumb?

But speed is probably a greater factor than tread on a highway.

"We tell people that a rain-slick road is really no different than an ice-slick road," said Trooper Mark Engholm, public information officer for the Kansas Highway Patrol.

The only rule of thumb in a rainstorm is to lower your speed to make sure your tires maintain traction, he said. And it's a good idea to double the following distance from the car ahead from two seconds to four seconds.

He also tells drivers to beware of "crossover" lanes at the start and end of construction zones on four-lane highways. Water tends to pool in those areas, where drivers are diverted from four-lane to two-lane traffic.

"Kansas weather changes very quickly," Engholm said. "We have to be aware and ready to respond to changes in road conditions."

Back on track

 As I started to slide on the rain-slick highway, I took my foot off the gas and steered in the direction of the slide.

Suddenly the wheels caught and I was in control again.

I looked over at Bonnie. She was asleep with her head resting on a pillow pressed against the window. The rear-view mirror showed me that Katie also had her eyes closed and was listening to her headphones.

I gripped the wheel a little tighter and eased it down to 50 mph. Home was still about 25 miles away.

And there were a lot more volleyball games to dream about.

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