OEMs face challenges when designing the larger wheels customers desire

wheels tires high vent steel wheels low-aspect ratio tires rolling resistance tire pressure monitoring systems TPMS Although a car's wheels perform the critical function of keeping it on the road, car buyers tend to be more interested in their appearance. Americans want big wheels. The challenge here, says Erich Heuschele, director of vehicle dynamics engineering for street and race technology at Chrysler, is that "they look great, but cause compromise." Big wheels add weight. They also tend to have shorter sidewalls, which helps steering responsiveness but also makes them more likely to bend in a pothole. Shorter sidewalls also have less air volume, so higher tire pressures are required to carry the load, which can result in a stiffer ride.

"To counteract the weight gain of big wheels, steel is becoming less and less common as a wheel material," comments Heuschele. OEMs also are opting for forging, rather than casting, in their wheel construction, especially on more expensive models.

"This process costs more but can save more than five pounds per wheel on a typical 20-inch rim and still meet strength and durability requirements," says Heuschele.

As Honda spokesman Chris Martin explains, aluminum wheels tend to be lighter than steel wheels and to have stronger aesthetic appeal, but aluminum is a more expensive material. Honda uses aluminum for its higher-end models, while opting for steel wheels with plastic wheel covers for lower-end vehicles.

Ford also has pursued plastic covers over steel wheels for some models. The 2010 Fusion, for example, has steel wheels with plastic covers that look like aluminum. "If the customer damages the wheel, all they have to do is replace the cover," says David Rohweder, global manager of tire, wheels and jack engineering for Ford.

General Motors also uses a mixture of steel wheels with decorative covers and aluminum wheels on its vehicles. Additionally, the company uses two different types of construction for its steel wheels. "Standard stamped steel disc wheels with decorative covers are still used in many applications," comments Terry Connolly, GM's director of tire and wheel systems. "High vent steel wheels with matching covers are also used on some vehicles, typically providing more expressive styling."

GM also has tackled the challenges of today's low sidewalls by flow forming cast aluminum rims on some of its models. That approach, according to Connolly, "has been very beneficial in reducing pothole rim damage customers may see with low-aspect ratio tires."

Another important development involving wheels and tires is a move away from using lead for wheel balancing. "From a materials standpoint, we worry about chemicals that aren't environmentally friendly," says Rohweder. Within the last 12 months, Ford stopped using lead balance weights, replacing them with steel or zinc.

Environmental concerns also are driving tire development efforts, which are typically the result of a close collaboration between OEMs and their key suppliers. The amount of carbon dioxide (C02) and other pollutants that vehicles emit are directly related to their fuel economy, which in turn is impacted by rolling resistance. As Mark Cherveny, brand manager for passenger tires for Goodyear, defines it, rolling resistance is "the energy force that a tire consumes while rolling under the influence of a load."

Rolling resistance improved by 25 percent when tire makers made the switch from bias-belted to radial tires in the 1970s, Cherveny notes. Today, Goodyear has high hopes for a new tread compound using functionalized polymer, which the company claims provides a 27 percent improvement in rolling resistance.

"It used to be that there was a tradeoff," comments Cherveny. "To improve rolling resistance, you had to sacrifice tread wear but with our new compound there is no decrease in the miles warranted." Tires based on Goodyear's new tread compound will be used on GM's Volt hybrid electric plug-in vehicle beginning with the 2011 model year and are one element of a broader effort to provide good handling and traction while also contributing to the car's energy goals.

Tire pressure monitoring systems (TPMS) also have significantly impacted wheels and tires. Heuschele says the systems are "really great" and have helped to reduce what used to be the biggest cause of tire failure — under-inflation. A typical tire's sidewall carries only 10 percent of the load, with the inflation air carrying the other 90 percent, Heuschele notes. "You are asking for trouble running around on under-inflated tires."

TPMS systems sensors may be attached directly to the wheel or to the valve stem. "We're seeing a move toward sensors mounted to the rubber stem," comments Scot Holloway, general manager for Bartec USA, which manufactures tools to help technicians reprogram TPMS systems after servicing. That trend, he says, is driven by a desire to move to more of a universal replacement sensor, minimizing the number of spare parts a shop must stock.

One of the downsides to today's TPMS is that each sensor is battery powered. Batteries go bad. Manufacturers hope to eventually eliminate the need for batteries by using energy scavenging and piezoelectric technology to power the sensors.