Slim tires too cool to be safe?

Times Staff Writer

More and more motorists seem to be driving around in cars sitting atop what look like rubber bands: low profile tires on giant alloy wheels.

Those “rubber bands” are actually high performance or touring tires, and they’re wildly popular among consumers who like a muscular, sporty look.

For the record:

12:00 a.m. Aug. 30, 2006 For The Record
Los Angeles Times Wednesday August 30, 2006 Home Edition Highway 1 Part G Page 2 Features Desk 2 inches; 77 words Type of Material: Correction
Tire size: A column on high-performance tires in last week’s Highway 1 section gave an incomplete equation for figuring the aspect ratio of a tire, a figure generally used to describe the size of a tire’s sidewall. The story suggested taking the diameter of a tire, subtracting the diameter of the rim and then dividing by the width. It failed to include the last two steps: converting diameter size from inches to centimeters and dividing by two.

But like an injury-prone star athlete, the buff body disguises some weaknesses. If you don’t think so, just ask Los Angeles County Superior Court Judge Lance Ito.

Ito wrote to me recently about his two Audi A6s, whose low aspect ratio tires have had nine flats in the last 40,000 miles. He has gone through three sets of tires in that time. “I will never again buy a car with these low-profile tires,” Ito said. You can almost hear his gavel bang.


In fact, engineers and safety experts say low aspect ratio tires -- which have shorter sidewalls -- are more vulnerable to road hazards, such as potholes and other obstructions that can test a tire’s ability to flex at high speed, than their standard counterparts.

Officials from Goodyear, Michelin and Bridgestone -- the three largest tire makers -- all acknowledged in interviews that their low aspect ratio tires are more likely to be damaged by impacts in normal driving.

“They are more susceptible to pothole damage,” said Bill VandeWater, Bridgestone’s consumer tire products manager, in Nashville. “They don’t have as much give before the tread contacts the rim. There isn’t as much deflection capability as a taller tire.”

What can you do about it? Well, advice from manufacturers can be kind of silly. Take, for example, Volkswagen’s suggestion. “Avoid driving on roads with potholes, deep gouges or ridges,” it tells owners of cars equipped with these tires. In other words, keep the car in the garage.

Nonetheless, consumers like the look. At Michelin, 20% of the tires it sells are high performance, and that figure is growing by about 10% annually, while sales of the standard “mass market tires” for cars are shrinking, said Lynn Mann, director of public relations for the tire maker.

Although consumers like the low-profile tires, safety experts are dubious about their practicality.

“My observation, other than styling, is why have them?” asks Clarence Ditlow, executive director of the Center for Auto Safety in Washington, D.C. “My advice to consumers is don’t buy them.”

Perhaps. But many don’t have a choice. Manufacturers are offering autos with wheel wells designed to fit only a low aspect ratio tire, especially luxury cars with a sporty image.

Purchasing tires has always been a mystery to many motorists. When you buy a tire, there are three keys to its size: tread width, rim size and aspect ratio.

For example, a 205-65-15 tire has a 205 millimeter tread width and fits on a 15-inch rim. The 65 is the aspect ratio, a measure of a tire’s profile. It is measured by taking the diameter of the tire, subtracting the diameter of the rim and then dividing by the width. The actual ratio in this example is 0.65, but typically described as 65.

To put it simply, the lower the aspect ratio, the shorter the tire. At one time, most tires had ratios of about 70 and rim sizes of 13 to 15 inches. Then, aspect ratios began to drop and wheel rim sizes began to grow.

Now there are rim sizes up to 24 inches and aspect ratios down to 25.

“It almost does look like a rubber band around the wheel,” VandeWater said.

What’s the point of this madness, other than sexy appearance? Ultra high performance tires almost always have low aspect ratios. These tires have speed ratings up to 186 miles per hour, super soft rubber that grips the road and construction that gives positive steering.

“It is meant for those people who see the freeway exit sign that says 40 mph, and they get that gleam in their eye and think: I bet I can do it at 80 mph,” VandeWater said.

On their highway to heaven, however, these drivers are finding they have to change tires every 20,000 miles. And the ultra high performance tires are like ice skates on cold road surfaces, plus they cost a fortune to replace.

To get the same cool look but greater durability, manufacturers offer what they call touring tires. They have the low aspect ratios, but harder rubber that lasts longer. They also have a little more forgiving ride that doesn’t report every pebble on the roadway.

Unfortunately, all low aspect ratio tires, both ultra performance and touring, are vulnerable to road hazards. And they are more vulnerable to under-inflation, many experts say.

“The lower aspect ratio makes it more sensitive to low inflation,” said Max Nonnamaker, a tire expert and former chief engineer for a tire maker. “When the tire is taller or higher, you can lose more air. Any drop in pressure is more critical, because there is a shorter sidewall and you generate more heat.”

An under-inflated tire flexes more as it goes down the road, generating more heat in the sidewall, which eventually can cause internal damage. Thus, keeping an eye on inflation is critical. But Ito said he checked the pressure in his tires two or three times per week and still experienced problems.

Tire inflation is always a controversial subject. Every tire has embossed on its side a maximum inflation rating. That’s different than the the car manufacturer’s recommended inflation, which is always less.

Nonnamaker believes that tire manufacturers should be required to also list a minimum inflation rating, below which tire damage occurs. But no tire maker wants to list such a rating. They could certainly do so voluntarily.

VandeWater said he believes that damage can occur if inflation drops 5 pounds below the level recommended by the carmaker. Typically, recommended inflation is 32 pounds, so you would be causing damage at 27 pounds. It is a tiny margin that very few consumers appreciate.

In a future column, I’ll examine how the promise of tire pressure monitoring systems, which would warn drivers of low air, have failed to deliver useful technology to consumers.


Contact Ralph Vartabedian at ralph.vartabedian@latimes .com.