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Big Enough?

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Big Enough?

As the diameters of performance tires increase, manufacturers have to walk the fine line between function and fantasy

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You’re walking along the show floor at the International Tire Expo in Las Vegas. You pass a seemingly endless collection of cars and tires. Wheels shimmer on display. Parts and accessories abound. And the people ®ƒ oh, the people.

It’s crowded, it’s busy and you’re thinking you brought the wrong pair of shoes to wear. Then, in the distance, you see it. You’re not exactly sure what it is at first. It’s up on a pedestal, bathed in lights and surrounded by gape-mouthed gawkers. It’s the centerpiece of a tire manufacturer’s booth; a mass of shiny chrome with whisker thin black around the edges.

As you get closer, you realize what you’re looking at – a 26-inch tire/wheel assembly. Your brain takes a few minutes to comprehend the sight as you walk around the display.

"They can’t really make them that size," you mutter, fixated not on the bright wheel but the barely visible tire surrounding it. Your brain snaps back with, "What’s the app?"

Whatever the application, this is the way performance tires are going. Bigger is better. But realistically, how big can they possibly make a performance tire?

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The quick answer is as big as the wheel well allows. However, there’s more to it than that. The tire has to be technically sound. There has to be a wheel to mount it on. And, oh yeah, there has to be a market. No sense making an enormous performance tire without a market.

The genesis for when these larger sizes of tire became popular is subjective. Some say it was the 1960s, some the 70s, and some insist it was within the last two decades. No matter when super-sized rubber took off, its still riding high today.

"Putting on larger tires has been popular for many years, but raising rim diameters began in the late 1980s," said Rick Brennan, Kumho Tire USA’s high performance brand manager. "In the 1970s, putting larger tires on the rear of the ‘57 Chevy was the rage. Today, more than 30% of ultra-high performance tire sales are to fit a plus-sized wheel."

The question of what drove the popularity – OE or aftermarket ®“ is nearly as murky as when it started. There are arguments for both, with OE having done some things the aftermarket copied and vice versa.

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"Larger aftermarket sizes started to become more prevalent with muscle cars because they needed more power to the rear of the vehicle," said Darren Thomas, Falken Tire’s director of marketing. "At the OE level, it started with European sports cars that had 18-, 19- and 20-inch tires. Porsche really made a change, adding an 18-inch tire at OE in the 1980s. That was quite a coup."

Yokohama sees Porsche leading the OE charge for bigger tires, as well. "The increase in sizes really started back in the mid-1970s, when Porsche went from 15- to 16-inch tires at the same overall diameter. That’s really the point where plus-sizing, as we know it, really began," said Art Michalik, Yokohama’s director of marketing communications.

Still others, Mark Jackson, Cooper’s high performance/UHP marketing manager, for one, believe the trend toward bigger tires came out of racing. "The concept to plus-size has always been out there, but it came to the passenger market from motorsports," he said. "In my opinion, plus-sizing became hot in the late 1980s, and it’s gone wild in the last five years."

When and where the popularity of super sizing started is merely a history lesson. At present, these sizes are not only pervading the market, in some cases, they are the market. And there’s little doubt they will continue to grow.

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But really, how big can tiremakers make a performance tire?

Sky’s the Limit?

Right now, the only thing that stands in the way of the size of tires is the vehicle. Or, as Falken’s Thomas put it, "the limitation on how big is up to the imagination."

Just as the tire needs to be able to handle the workload of the vehicle, the vehicle needs to be able to handle the tire. What vehicle is the tire being mounted on, and can it even handle a 24-inch tire, for example.

"The size limitations are dictated by the vehicle," said Brennan. "How big is the fender well? How much clearance is there between the suspension and the tire? Can you make a tire with enough load capacity to fit inside the fender in the dimension that you want? How much unsprung weight can the suspension handle?

"As you make the wheel bigger and bigger, the tire width must increase to make up for lost air volume. You can either keep increasing the size of the container by increasing the width of the tire or you can increase the amount of air you put inside the tire, such as with extra load versions."

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Earl Knoper, Toyo Tire USA’s senior vice president of marketing, concurs. The automakers dictate maximum tire size because the replacement tiremakers must equip vehicles with tires of the same overall diameter as OE tires.

"OE carmakers design their wheel spindle height to be one half of the overall tire diameter," Knoper said. "In most cases, the wheel spindle height of a vehicle is between 11 and 14 inches. If the wheel spindle height is raised substantially, the height of the car is likely to be raised also," he said.

"This means that most car tires will have an overall available diameter of between 22 and 28 inches. When Toyo designs plus fitments for cars, we require no more difference than plus or minus 2% of the OE overall diameter."

Knoper also said that another size-determining factor is figuring out which aspect ratio will be suitable for certain applications. "American, European and Japanese carmakers mostly use 55 or higher aspect ratios for OE – with a few exceptions. Even the most exotic cars, such as Porsche and Ferrari, use 40 aspect ratio or higher."

A third thing that may limit the size of wheels is tire/wheel assembly weight. Wheels are heavier than tires, and the bigger the tire diameter, the heavier the wheel. As tire/wheel weight increases, acceleration and braking effectiveness decreases, as the vehicle must do more work to move or stop the tire and wheel. That’s simple physics. A hundred-pound tire/wheel assembly on a Civic wouldn’t be the best thing.

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So, really, is there a limit?

"As long as a band of rubber is at least covering the space between the rim flanges, there’s no limit – especially in the tuner segment," said Cooper’s Jackson.

Issue of Practicality

Realistically, the limitation on tire size is its practicality. A 26-inch tire looks real sharp and can attract attention, but it is practical? Is it able to do what the vehicle needs it do, and what the consumer wants it to do – safely and reliably?

And the best way to judge practicality is sales.

"These large sizes become impractical when you can no longer sell it. That’s the reality," said Thomas. "Two years ago when Yokohama debuted a 26-inch tire at ITE/SEMA, we had to start defining what was practical. As soon as there’s a market and the tire can support the technical requirements of a vehicle, it’s practical."

When it comes to supporting technical requirement, that, of course, varies. The tire/wheel package needs to support the load and allow the vehicle’s brakes to stop the unsprung mass – using an acceptable loss of engine horsepower.

"By definition, a high performance tire should enhance a vehicle’s performance," said Cooper’s Jackson. "So when the tire/wheel package drastically increases a vehicle’s unsprung weight, it becomes a fashion statement, not a performance enhancement.

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"If you want the look, that’s fine. But if you set your vehicle’s tire/wheel package to plus four, realize there are a couple of downsides. The brakes were engineered to stop a much lighter tire/wheel package. The engine was matched to a much lighter tire/wheel package. So you won’t stop or start as well as before.

"The upside? Your handling will feel like you’re on a rail. The only weight transfer you’ll get is shifting your cup of coffee from your right hand to your left."

"Practicality is in the eye of the beholder," as Kumho’s Brennan said. If the consumer wants a large tire for looks, things like ride comfort and noise go out the window.

"But if you define practical as ‘when the tire no longer provides its basic functions to it’s fullest’, we passed the practical limit some time ago," Brennan said.

Design Criteria

Making a 24- or 26-inch tire doesn’t happen without a lot of thought – just like any other tire. However, with these larger performance tires, many of which are installed for purely aesthetic enhancement, they still need to be functionally workable ®“ practical.

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In order to get everything right, a decision has to be made at the OE level. "Is a large rim diameter tire a fashion accessory or a vehicle improvement?" asked Jackson. "Design efforts will be different depending on the answer to that question."

If the tire is to simply be used as a fashion accessory – and the buyer is aware of such ®“ it can be built to perform in that manner. However, if there’s a modicum of chance it could be used for both good looks and real world driving, it has to be able to stand up to the task. Everything still comes back to whether or not the tire is functionally workable.

"First and foremost, when we design tires, our primary concern is the relationship of the concept of the tire and the actual application. Nothing gets to market unless it’s technically sound," said Thomas. After that, he says, Falken evaluates the cost/benefit analysis of building the tire versus the profit potential of the tire.

The load capacity of these performance tires seems to be one of the biggest issues among tiremakers. No matter the application, the tire must be able to handle everything that’s thrown its way.

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"Load capacity is a big issue when you get into plus five, six and seven fitments," said Brennan. "As the air chamber gets smaller, air pressure must be increased to carry the same or more load.

"Reinforced or extra load specifications are necessary in many fitments today. We are now reaching the limits of today’s specifications in some fitments, such as 24-inch and larger rim diameters for SUVs."

Yokohama’s Michalik agrees completely. "Load capacity is the greatest concern, even on small imported cars," he said. "And when you consider large SUVs, those are very large vehicles with tremendous towing and load-carrying capabilities. The tire has to be capable of meeting those demands."

Smooth Ridin’

One fact is generally assumed by tiremakers: the taller the wheel, the smaller the sidewall, the less ride comfort.

Some manufacturers believe that when sidewall height gets below 100 millimeters (3.9 inches), the tire’s ability to absorb bumps begins to decrease rapidly. A 215/35R18 is going to provide pretty low ride comfort. As the market grows – and as the number of non-performance drivers buy these tires ®“ the issue of ride comfort becomes of greater concern.

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"There is no question that ride quality diminishes as the wheel diameter goes up," said Michalik. "Not only is there less sidewall to serve as a springing medium, but typically the construction is heavier to allow for greater inflation to maintain the load-carrying capacity.

"We’ve actually seen the start of a new trend where street tuners are backing off some of the larger diameters, as the tradeoffs are too great."

The tradeoff between looks and ride comfort is a big sacrifice. But then, performance tuning is about compromise and drivers must determine what is important to them. How important are looks to comfort to power?

"For every point of aspect ratio they reduce, there is a related loss of ride smoothness," said Jackson. "For every two pounds of added unsprung weight on the drive axle, the driver looses on horsepower. That’s traditional motorsports logic. So, if you increase your tire and wheel weight on the drive axle by 30 pounds, you subtract 15 horsepower at the wheels. If you have a Ferrari, you might not miss the ponies. If you drive a slammed Civic, the loss is noticeable."

Dealers also have to be concerned about driver comfort complaints. Some buyers may simply not understand what they’re getting into comfort- and control-wise. Vibration, harshness and road surface feedback are also magnified. It would be wise to warn buyers of every potential downside, including any driving limitations, before they commit to purchasing.

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Bright, Shiny Objects

Obviously, a tire doesn’t mean a thing without a wheel to go along with it.

For a variety of reasons, tiremakers need to have a very good working relationship with a wheel manufacturer. Not only is a good wheel needed for testing purposes, but market demand for larger rim diameter tires won’t be created without the right wheels.

"As a tire manufacturer, we have to think that when we test a tire, we have to have a wheels," said Thomas. "Without them, the tire is lost."

And let’s not forget about secrecy. The top-secret nature of creating a 25- or 26-inch tires and wheels mandates tight-lipped assistance. "You need a joint relationship that’s confidential because you don’t want a wheel company to release a concept of a tire and wheel that may never hit the market," Thomas said.

Many times, making one of these larger tires is "uncharted territory," as Kumho’s Brennan says. "A close relationship with a wheel maker is mandatory. Simply put, you cannot make and sell a tire without a wheel, and a wheel is of little use without a tire."

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Now, while Falken, Kumho, Toyo and Yokohama agree the tire and wheel must almost be manufactured concurrently to create market demand, Cooper takes a slightly different approach.

Cooper feels it would rather wait for market demand to occur and then fill the need with product. "We’re technologically advanced, but not a product innovator when it comes to (escalating wheel size)," said Jackson. "We’ll quickly produce the tire when the market warrants it."

Monkey See, Monkey Do

There’s no doubt tiremakers are trying to top each other when it comes to tire sizes. And it’s especially easy to reach that conclusion if you’ve been to the last three ITE/SEMA Shows where tiremakers have been one-up-sizing each other. Prestige is a big factor when it comes to the size of one’s performance tire.

Company A rolls out a large performance tire and six months later Company B trots out a tire one-inch larger. One tiremaker refers to it as the "Rim Wars."

"In the automotive industry, heck, in any industry, there is a constant one-upsmanship struggle," said Cooper’s Jackson. "The innovators knock themselves out staying on top, then sprint playing catch-up once a competitor bests them.

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According to Thomas, tiremakers do participate in these "Rim Wars." But, he says, there’s a gamble involved – companies need to be sure there’s a reason to escalate tire size.

"Yes, tire companies do throw down the gauntlet, but there’s risk," he said. "When Toyo unveiled its 23-inch Proxes, they immediately had a market for it because the preparation on the wheels had already been done. When Yokohama launched its 26-inch tire, there wasn’t a market and there wasn’t a wheel to support it."

For Thomas, increasing tire rim diameters is an example of the "law of diminishing return."

"At some point the category struggles because as you go taller and taller, the market gets smaller. One company introduces a big size and then gets trumped by another company who makes a bigger size. Then companies have to work to beat the new size, and all the while the market is shrinking."

It’s easy to get caught up in having the bragging rights to the newest sizes, as Michalik of Yokohama says. But tiremakers need to keep the current state of the market in mind. What patterns are customers buying, and in what sizes?

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"With cosmetics heavily driving the tuner market, the growth of very large rim diameters has exploded," Brennan said. "A certain portion of the market will put the largest wheel and tire combination they can on their vehicles.

"But for companies driving their image and marketshare by staying at the leading edge of what is new, they must stay the course to maintain their current strategy. With smaller and smaller total market size potential, tiremakers must decide whether it’s worth the money and effort to develop these tires."

So how big can manufacturers make a performance tire? As you can see the answer varies. But are manufacturers still going to develop 26-, 28- even 30-inch sizes, if only just for show? You bet.

Will we see 26-, 28- and even 30-inch tires become common place? Fickle consumer preference, the economy, OE input, vehicle design and practicality will dictate the course.

Dealers know that market demand for larger sizes is still growing. And even if there isn’t a clear answer as to the viability of larger rim diameter tires, dealers know that selling a plus-sized tire means lots of profit – and the more the plus-sizing, the bigger the profit.

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