With Aspect Ratios, How Low is Really Too Low?

In my lifetime tire aspect ratios have ranged from an 85-series to a 25-series. Did you ever clean the whitewalls on a 1952 Chrysler Imperial? Be thankful you didn’t. That’s why I like the new lower aspect blackwall radials. Not only are they easier to clean, they are radials, they handle astoundingly well, they give me a lot more grins per mile, as well as a low-rolling resistance gain in miles-per-gallon.

But many of us wonder how low a tire’s aspect ratio can go before we hit the point of diminishing return. To Michelin’s way of thinking, 25% of a tire’s section width is going to yield an 83 millimeter sidewall. “For everyday street-use tires an 80 millimeter sidewall height is our threshold,” says Mark Ludlow, a tire engineer for Michelin.

“Anything under that size is built only for racing or show cars. But we feel any sidewall that falls under 65 millimeters is more about show than go,” he says. “Think about the tire as an elastic, deformable subject working hard during cornering with virtually no slip angle available for it to do its job. A tire built to that size is simply too rigid.

“When the height of a sidewall is under 80 millimeters it just can’t deliver a normal regime of a slip angle because there is no elasticity or deformity available in a severe handling environment. Put another way, there is simply too little sidewall real estate for the work required of the tire.

“A tire’s job is to hold the car up, help it brake, help it change direction. More recently, the industry has added aesthetics to the list. In the L.A. market we see 22-inch wheels with low aspect tires fitted to a Bentley, which results in an awful ride,” he says.

With the exception of specially-built tires for car shows, car collectors and racing, Michelin has reached a sidewall height threshold beyond which it hesitates to go. “We cannot move much lower, if at all, than where we are at present,” says Ludlow.

With the new CAFE laws set to go into effect over the next decade when carmakers must reach 36 mpg, everything is going to have to get lighter. Tires will be more exotic and vehicle makers will have to turn to such things as carbon fiber and forged aluminum among other materials. But this doesn’t translate into horsepower loss, nor will it take the fun out of driving.

We will see more 2,000 lb., four cylinder vehicles with 150 horsepower and up. Today, four tires make up 5% of the weight of the vehicle and that too will probably change. Gas prices are expected to continue to climb, a fact of life that will give flight to a host of new technologies.

At Michelin, they are reviewing the advantages of the Tweel, a spoked plastic-based wheel and rubber non-pneumatic tire built as a single unit. In effect, it is a spoked tire that deforms like a pneumatic tire, offers low rolling resistance and weighs less than today’s conventional low aspect tire/wheel assembly.

No matter, these changes will not change the fun factor of driving. When you can push well past 200 horsepower with superchargers or turbochargers from an inline four, the fun of driving will be alive and well. Already, we see vehicles offering 36 mpg and we will see plenty more complete with the kind of freeway entrance performance all of us desire.

Aspect Ratio Trends
To make sure we’re all on the same page, let’s digress for moment and look at the math. A tire that has a 50 aspect ratio is half as tall as it is wide. But it would be a mistake to confine the aspect ratio of a tire to the performance relationship, often called the performance envelope.

A simplified analysis of a tire bending under the influence of a side force (as in cornering) shows that the deflection is proportional to the cube of the sidewall height. Put another way, if the height of the sidewall is decreased just a little the stiffness of the sidewall increases a lot.

At Bridgestone/Firestone North American Tire (BFNAT), tire engineer Kurt Berger says the average driver will never discover the total performance envelope of a tire. No matter what the horsepower rating or the lateral g-force ratings built into the tire’s performance envelope, it will not be realized by most drivers,” he says. “Virtually 97% of all drivers will stay within a 5-degree plus or minus steering angle.

“When the tire begins to go to work and squeals a bit, most drivers lift their foot off the accelerator. That’s why we spend a great deal of time focusing within this 5-degree performance envelope,” he says. “It’s no longer about how fast you go, it’s about how you go fast and that is all about handling.”

Like other tire engineers, Berger believes that we are near or already at the bottom of the low tire aspect ratio war. Even though tires are safer today than they were 15 years ago, the Tire & Rim Association lists a 25-series tire at the bottom of its listings for aspect ratios. Why? “As the tire sidewall shrinks and the width of the tire grows larger we lose load carrying capability,” says Berger.

“Although we make a large number of 30-series and 25-series tires for exotic fitments, this is about all-out performance and cosmetics. In effect, these are appearance-oriented tires unless you are a professional race car driver.”

Back in 1980 Berger recalls that the average horsepower per average family sedan was 100. Today that number has doubled to 210 horsepower.

Still there is more on the way in the immediate future, ala the Pontiac GT (referred to as the sports car for the family man), complete with 402 horsepower riding on P245/40R19 summer tires as standard, along with three all-season tire options.

At the same time, entry-level sport utes like the Honda CR-V, Nissan Rogue, Saturn Vue, Subaru Forester and Toyota RAV4 are being delivered with nothing lower than a 55-series aspect ratio and EPA fuel mileage ratings ranging from 21 to 32 mpg (the latter honor belonging to the Saturn Vue). The tire size on the Vue is P255/60R17. The 55-series tire belongs to the Subaru Forester, which rides on P225/55R17 tires.

Is this an emerging aspect trend? It sure looks like it, but it may be too early to call. Take a look at some more brand new cars being snapped up in droves. The Toyota Camry SE V-6 offers 19/28 city-highway mpg and comes equipped with 215/55R17 93V tires.

The new Chevy Malibu LTZ EPA numbers shows17/26 mpg and is equipped with 225/50R18 94T tires, while the Honda Accord EX-L V-6 has EPA numbers of 19/29 and comes OE with 225/50R17 93V tires.

Things to Remember
It appears we have stopped moving to lower and lower aspect ratios in favor of higher sidewall heights for many cars. Still, we should expect lower aspect ratio tires on very high-performance vehicles. Your customers who buy high performance cars generally don’t care about vehicle price or the cost of fuel.

Even so, the majority of your customers don’t have nearly the discretionary income they used to enjoy and will be shopping for top quality tires they can afford even if it means a 55- or 60-series tire. Of course, make sure you install what the vehicle placard calls for. Due to the economy, customers probably won’t be looking to change wheels or tires sizes as often this year.

“Fuel economy will remain a concern for everyone,” says Berger. “A tire’s aspect ratio plays a role in fuel economy but only when the tire gets a lot wider,” he said. “We are concerned about fuel economy, grip, adhesion and treadwear and there are challenges to getting that done.

“A hybrid Toyota Prius owner will call to tell us his tires are bald at 15,000 miles. Instead of buying an OE replacement tire (read: expensive) they purchase a 50,000-mile treadwear warranty tire that will cost them 10% to 12% in fuel economy, but deliver 50,000 miles of treadlife.

“Another trend we see is conversion of summer tires to winter tires as the seasons dictate. We’ve seen this number increase by 60% in the last four years. This keeps alloy wheels from rusting in the winter and a driver can gain in overall longer treadlife for all eight tires if maintained properly.

“The bottom line for us is to match the right tire to the right vehicle and customer,” says Berger. “There are so many tire choices, with so many performance categories and load indexes, it takes time and hard work to do a good job.”

Two obvious issues we haven’t really touched on here: ride quality and load carrying capacity. Keep in mind that the lower the aspect ratio, the harsher the ride. Also, a tire’s ability to support a given load depends on its air volume. It’s important to follow the tiremaker’s recommendations regarding load capacities when going to a larger wheel and a lower aspect ratio tire size and that will never change.