For a few years, the tire and wheel industries resembled the Cold War nuclear arms race – every year, someone rolled out a bigger weapon. It started with eye-popping, wheel-well-stretching 20-inch wheels and tires and progressed to the 2004 SEMA Show, where gigantic 28-inchers were every where.

A sort of dÉtente has settled in, if only because the cost of missile development has spiraled exponentially. Maybe practicality set in to slow the size race. Perhaps safety concerns raised eyebrows. For whatever the reason, production sizes of ultra-high performance tires and wheels appear to have maxed out at 28 inches.

There is no doubt the replacement market tail has shaken the OE dog; over the last six years, the list of the most popular OE tire sizes has progressed from 14- and 15-inch diameters to 16-, 17- and even 18-inchers. A number of new cars – not pickups or SUVs or exotic sports cars – sport 20-inch tires as standard or options.

OE will be OE, and as long as automakers see the replacement market waist deep in big tires and wheels, they will follow. We’re dealing in the replacement world, where consumers drive sales, and small independent business people must tackle the realities and ramifications of those buyer desires.

Magazine racks are packed with titles playing up the “fashion” of today’s monster wheels and tires – the “bling-bling” factor that makes cash registers ring-ring. And, the SEMA Show feeds that consumer fascination, as each year, newer, shinier and bigger models are unveiled.

Everyone wants to make a profit-filled sale, especially tire dealers and professional installers. But when is big too big? We asked the experts for their thoughts about the real mechanical concerns dealers and shops need to consider, how they can explain these performance-related concerns to customers more caught up in looks rather than safety, and what precautions retailers should take to keep themselves out of legal trouble should problems arise.

Fashion or Faux Pas?

First, and perhaps most importantly, with 28-inch tires and wheels already on the market and rumors floating of 30-inchers, have we reached a practical size limit? Has fashion overtaken safety?

“‘Fashion’ is an excellent word to describe the latest trends because they are certainly not performance or safety driven,” according to Kevin Rohlwing, TIA’s senior vice president of education and technical services.

“As far as safety is concerned, there are definite issues that need to be resolved with 20-inch-plus fitments,” he says. “The increased weight alone can create problems for the braking system, and the changes in handling can affect the drivability of the vehicle.”

“This really depends upon the individual vehicle design and vehicle capabilities,” says Dewey Beach, director of product management for Cooper Tire & Rubber Co. “If the plus-sizing guidelines of matching tire overall diameter, equaling or increasing load-carrying capacity and ensuring there are no tire/vehicle clearance issues are achieved, then the benefits of plus sizing – braking, handling and look – could be experienced.”

“OEMs can design vehicles where 24-inch-plus tire/wheel sizes can be safely applied,” says Phil Pasci, Bridgestone/Firestone North American Tire’s (BFNAT) executive director of consumer tire marketing, “but retro-fitting to existing vehicles designed for smaller tires can lead to issues with load capacity, handling, braking, ride comfort and road hazard.

“I don’t believe there is any question that larger tires/wheels that better fill the wheel well look better on light trucks, but the whole vehicle – brakes, suspension, etc. – must be adjusted when approaching the larger wheel sizes, such as plus-six fitments,” says Pasci.

“The wheel diameter has grown purely as a fashion statement,” says Matt Edmonds, vice president of marketing for The Tire Rack, one of the country’s largest distributors of large tires and wheels. “There is no discernable performance benefit from these applications.”

As far as safety is concerned, though, Edmonds says, “If proper steps are taken in choosing tires and wheels – from load capacity to technical specifications, as well as upgrades to other systems like brakes and suspensions, etc. – then these can be done properly. The consumer needs to be made aware of any safety compromises he or she is making in the name of look.”

“It’s difficult to say whether tire and wheel combinations can get any bigger, because that depends on the size of the vehicle. If they keep getting larger, then it’s safe to say that tires/wheels can follow,” Rohlwing says.

No doubt. As consumer tastes go, so go the OEMs. But is there a maximum diameter limit for cars, pickups and SUVs?

“The fundamental issue is the tire/wheel assembly and whether or not it is within ‘traditional’ deviation ranges,” suggests Darren Thomas, Falken Tire Corp.’s marketing manager. “While overall assembly weight is certainly an issue, when it comes to ride comfort and subjective measures, there are no OEM guidelines or standards for objective metrics. This is also the case for tire compounds, construction and profile when deviating from the OEM-specified tire.”

Wheel diameters, Thomas suggests, “can and will increase as OEM tire assembly ODs increase.”

BFNAT’s Pasci thinks passenger car wheel diameters will “stabilize at a practical maximum” of 20 to 21 inches. “Styling of passenger cars tends to emphasize the ‘low’ look, and tires fit rather tightly into existing wheel wells. Larger wheel diameters will require more sheet metal and more vehicle weight, items that are generally counter to the car styling trends.”

Light trucks and SUVs, on the other hand, are designed for more of a “high” look, with greater ground clearances that leave more room in the wheel well for bigger tires and wheels. Still, Pasci sees this segment stabilizing at around 24 inches. “Even with SUVs, the trends with the larger wheel diameters combined with lower aspect ratios can impact ride comfort and increase the chances for road hazard damage.”

“I don’t think you can set a maximum tire/wheel diameter limit for any particular type of vehicle,” says Rohlwing. Tiremakers recommend that the overall diameter of any replacement tire should be within 3% of the OE tire, he reminds. But that is for typical plus-sizing that minimizes the impact on speedometer operation and brake and suspension system functionality. “It would be helpful if the tire and vehicle manufacturers could agree on the 3% rule across the board, but that is unlikely.”

Seller Beware

When dealing with extra-large fitments, dealers and shops need to heed the guidelines offered by tiremakers, of course. Experts agree that the first thing retailers must do is find out if the customer’s dream can become a reality by referencing the tire/wheel fitment guidelines provided by both tire and vehicle manufacturers. This includes making sure the load-carrying capacities of the proposed new tires and wheels meet or exceed those of the OE assembly.

Next, training is vital, especially when handling expensive tires and wheels. “Extra care should be taken in mounting and dismounting not to damage the tire beads or to scratch the costly wheels,” says Bob Toth, performance tire marketing manager for Goodyear Tire & Rubber Co. “Make sure tire technicians are well-trained on their tire mounting and balancing equipment. Using the correct tire changing equipment and related adaptors is key.”

Cooper’s Beach concurs with that, and adds, “Both the size and weight of the assemblies could result in ergonomic issues. From a performance standpoint, larger wheel diameters mean lower tire aspect ratios, which increase the susceptibility to impact (pothole) damage and increased ride harshness.”

“Make sure that the vehicle and the tire and wheel combination work together technically in fit – bolt pattern, offset, center bore, diameter, width, etc. – and in respect to load capacity,” says Edmonds. “If a set of tires and wheels is installed that is increasing unsprung and rotational weight, the shop has the opportunity to upgrade brake systems with aftermarket brake pads, rotors, plus-sized rotors or, in the extreme, a complete brake system.”

Consumers can be easily fooled into thinking that bigger means more, especially when it comes to load capacity. The opposite is fact, though, says Pasci. With larger wheel diameter tires, “the tire air chamber is typically smaller, meaning the tire load capacity is less.”

And just because it fits in the wheel well doesn’t mean it fits, warns Pasci, who says dealers and shops need to consider the whole “corner,” not just the tire/wheel package.

“Tire fit can be checked relatively easily in the shop, but the ramifications to vehicle electronic systems are harder to determine,” Pasci says. “Another concern is that consumers will frequently not want to change to larger brakes once they move up to 20-inch or larger wheels. The vehicle might still have a brake package designed around a 16-inch wheel. But, because the larger tires are capable of providing stronger braking power, they can overpower the brakes and result in brake slippage and longer stopping distances.”

“Many wheel manufacturers have technical support people that can help a dealer or shop make informed decisions that won’t create any safety or performance concerns,” says TIA’s Rohlwing. “The difficulty arises when a consumer insists on installing tires and wheels that are not within that 3% tolerance.” In some states, getting the customer to sign a waiver may help in those instances when the customer is extra insistent, he suggests.

Unsprung Weight Impact

When it comes to larger diameter wheels and tires, one of the issues most often mentioned is the question of unsprung weight, or rather, the impact of heavier tires and wheels on vehicle systems – brakes, suspension, transmission, engine – and on-road performance.

But the impact of greater unsprung weight can be more difficult to assess and may be more far reaching than simple mechanics.

“Higher unsprung weight can mean more energy is required to accelerate and decelerate the vehicle,” says Cooper’s Beach. “And, it can increase the vehicle’s overall rolling resistance, decreasing fuel economy. Suspension and braking upgrades, as well as maintaining proper inflation pressure, will help counter some of the possible tradeoffs.”

“It is more than just additional unsprung weight,” insists The Tire Rack’s Edmonds. “Additional rotational weight – acting like a flywheel – has an impact on systems within the vehicle. These work very much like putting ankle weights on before you go for a run. It is harder to sprint quickly and stop quickly, all the time doing it smoothly.”

Falken’s Thomas feels that, to establish the real impact of added unsprung weight, “it is imperative that we first define performance – braking, handling, stability, etc. – and measure the OE performance standards, and then measure the deviation to those standards once the aftermarket tire/wheel assembly has been installed.

“Although the popular assertion is that an increase in unsprung weight negatively affects a vehicle’s ‘performance,’ the industry has yet to evaluate the acceptable range of performance of all vehicles, considering the multitude of driving conditions a single vehicle model may be exposed to,” he says.

As an example, Thomas says to consider two identical SUVs. One is driven locally at minimal sustained speeds and rarely loaded, while the other hauls heavy loads while pulling a trailer on the freeway. “Which case is defined as the performance benchmark? Which case defines the OE standard?” he asks. “It is not logical to assume that the light-duty application with an increase in unsprung weight will perform worse than the same vehicle with OE equipment under commercial use.

“The fundamental point is that ‘OE performance’ is not a static measure,” he says. “It is very dynamic as the performance target moves depending upon the actual use of each vehicle.”

Grave Concerns

There are a lot of issues to consider when a customer wants monster-sized tires and wheels, as you can see. It is not as cut and dried as find ‘em, order ‘em, install ‘em and bill ‘em. In fact, sometimes, dealers and shop owners might best be served by simply refusing the sale.

In general, experts agree that retailers face the same legal and liability issues whether they are selling big wheels and tires or servicing the family minivan. The smart first step is to pay attention to the recommendations of the vehicle manufacturer and tiremaker.

“Dealers cannot be expected to evaluate all possible combinations of tire size, types and vehicles,” reminds Pasci. “We direct our dealers always to check in the vehicle owner’s manual for any restrictions the OEM may have regarding replacement tire selection – and to follow those vehicle manufacturer recommendations.”

“As long as the replacement tires are within 3% of the OE outside diameter, and the speed rating and load index are equal to or greater than the OE tires, then a retailer’s liability should be minimal,” said TIA’s Rohlwing. “If those guidelines aren’t followed, it’s simply a roll of dice.”