Wheels are becoming cheaper, lighter, brighter and bigger. As one auto executive of a wheel maker said: We’re re-inventing the wheel every day here! Given that aluminium wheels are typically 20% lighter than the steel variety, alloy wheels are becoming commonplace. In North America, more than half of all cars have alloy wheels while one-third of new cars in Europe now come equipped with them.
Although aluminium wheels are generally lighter in weight, more readily styled and more expensive than steel wheels, material and manufacturing costs associated with alloy wheels are considerably higher, making them at least three times more expensive than steel wheels. For this reason, the steel wheel market will remain significant as vehicle makers continue to specify cheaper steel wheels for more moderately priced passenger cars and light trucks and for most spare wheels.
Steel wheels get lighter
New technologies now allow the manufacture of steel wheels that are lighter and more highly styled. The world’s largest wheel maker, Hayes Lemmerz, worked with the American Iron and Steel Institute (AISI) to develop a lightweight microalloy steel wheel that shaves some 5lbs off each wheel, or 25% in weight savings.
Other trends in wheel design include larger diameter wheels. There are two main reasons for that. The first is appearance; people like the highly styled wheels with low profile tyres. The other follows the trend toward more powerful engines. These require more powerful and bigger brakes which, in turn, demand a larger diameter wheel.
Steel and aluminium offer vehicle makers a range of options. Steel wheels, which are heavier than aluminium wheels, are typically low-cost, high-volume production items that consist of two separate pieces – a rim and a centre – that are welded together. Aluminium wheels are generally lighter in weight, more readily styled and more expensive than steel wheels. Aluminium wheels are around 20% lighter than steel wheels, but the material and manufacturing costs are considerably higher, making them at least three times more expensive than steel wheels.
But not all aluminium wheels are lighter than steel wheels. For its Mondeo model, Ford has developed a lightweight 6 ½ J x 16 steel wheel weighing only 8.25 kilograms, some 2.5 kilos lighter than a conventional steel wheel and 800 grams lighter than a cast aluminium wheel. The breakthrough was made possible by using DP-W 600 and FB-W 45 high strength steel grades from ThyssenKrupp Stahl. The five wheels of a car account for around 3% of its curb weight. ThyssenKrupp Stahl offers dual-phase steels with a soft ferritic microstructure interspersed with hard martensitic islands, along with ferritic steels. The company claims that these new grades of steel mean that lightweight steel wheels are generally lighter than comparable cast aluminium wheels.
GM’s four-spoke lightweight steel wheel
GM is working with a number of suppliers to develop a new four-spoke lightweight steel wheel that will look just like an alloy wheel but will cost a great deal less. It is thought that the new wheels could first be used on GM’s next generation of standard-size pick-up trucks, due for 2005, which are now being designed as part of the GMT900 programme. If the new designs work, costs could be cut by at least $175 per vehicle.
Hayes Lemmerz’s full-face fabricated (FFF) aluminium
New technologies now allow manufacture of wheels that are lighter, stronger, more highly styled and may be made faster and more cost effectively. Hayes Lemmerz points out that to achieve a lighter wheel, full-face fabricated (FFF) aluminium, which achieves weight reduction through spinning rims during manufacture and produces wheels that are half the weight of equivalent steel wheels. Using 16 x 7 FFF rims on a truck, for example, could achieve weight reduction of 60-70lbs per vehicle.
The company believes that full-face cast (FFC) aluminium wheels offer cost and weight savings as well as greater design freedom. FFC uses a cast centre disc and spun aluminium rim that are welded together. Hayes Lemmerz claims that FFC offers greater flexibility because one rim can be used on a variety of centre discs, allowing a vehicle maker to use one rim for an entire family of wheels. The real benefit is that because of the spinning process, the larger the wheel, the larger the weight savings. Hayes Lemmerz developed a lightweight microalloy steel wheel in a technology partnership with the American Iron and Steel Institute. This achieved a reduction of 5lbs per wheel (or 25lbs per vehicle, including the spare wheel), by using ultra high-strength materials and a spin-thinning process. This varies the thickness of sections of the wheel relative to the strength required. The company claims that its microalloy wheel also allows for different cladding (hub caps) to be mounted to the wheel. It means a vehicle maker can select one wheel for an entire family of vehicles and choose appropriate cladding depending on the type of vehicle.
Sumitomo Metal Industries Ltd has started to mass-produce steel wheels, which help in reducing the auto vibrations, for Honda’s Fit minicar. Sumitomo has also reduced the weight of the wheel by 10% compared to other similar products. The company has used computer models to simulate vibrations and improve the wheel design in order to prevent resonance, which is caused when the wheel of the car vibrates in the same frequency as the car body and tyres.