Are steel rims bad for fuel consumption

The right wheels for the car : Rarely a round thing

Glossy, matt or two-tone? Big or small Filigree or massive? The wheels can be decisive for the look of a car. It is often overlooked that the wheel arches are responsible for around a quarter of the aerodynamic drag of a vehicle. In addition, the weight of the wheels has a significant influence on driving behavior. The motivation to optimize the rim design is correspondingly high.

The wheels, like the brakes, belong to the unsprung masses. The lower these are, the lower the moment of inertia and the more sensitively the suspension of a car responds. Accordingly, rim manufacturers use materials that are as light as possible. Most of it is aluminum, but magnesium is also used or even carbon. Hybrid rims made of carbon and aluminum are now also available.

Steel and aluminum wheels are the most common

Most cars, however, roll on steel or aluminum wheels. Sheet steel wheels are used in inexpensive vehicles and often for winter tires. “You can't say in general that aluminum wheels are better than steel wheels,” explains Ruprecht Müller, technology expert at ADAC. Steel wheels are above all more robust and can withstand curb bumps.

Manufacturers such as BMW, Mercedes or Volkswagen mainly rely on aluminum wheels. Florian Schek, Head of Lightweight Construction at BMW, says: “Aluminum in different alloy variants tends to offer better rigidity than steel and at the same time less weight.” However, steel rims do not necessarily have weight disadvantages because the material thickness can be thinner. Stefan Gies, Head of Chassis Development at Volkswagen, highlights not only the lower weight but also the greater design possibilities of aluminum. Another advantage is the better dissipation of brake heat.

Special bikes are still rare

This is taken into account in the standard equipment of sporty vehicles whose brakes are heavily used. The engineers need around 30 percent free space for good brake cooling. This is done, among other things, through an open design with comparatively filigree spokes. “The bigger the wheels, the bigger the brakes can be chosen. This significantly influences the overall driving dynamics of the vehicle, ”says BMW engineer Florian Schek

Aluminum wheels can be cast or forged, although casting is cheaper to produce and offers more design freedom. Due to the thick material used in production, cast aluminum wheels are not necessarily lighter than steel wheels. “Forged aluminum wheels weigh less in comparison,” says Jörg Ludwig from the rim manufacturer Otto Fuchs. Because of the low unsprung mass, they offer more comfort. But they also cost more than cast aluminum wheels.

Wheels made of particularly light and expensive materials such as magnesium or carbon eke out a niche existence. Both are mainly used in racing today. Carbon wheels require complex and expensive production, and they do not dissipate heat as well. The carbon hybrid wheel is still in the test stage. It consists of a forged wheel spider and a rim base made of carbon, which keeps the production costs within reasonable limits.

The latest design trends

When it comes to aluminum rims, experts believe that the trend is towards large, sporty wheels that are predominantly forged. The advantage: Larger wheels are usually wider and have flatter tire contours, which means that acceleration, braking and steering forces are better transferred. However, consumption also increases and suspension comfort suffers. Drivers who value comfort should therefore opt for smaller wheels with a maximum size of 16 or 17 inches. "They represent a good compromise between comfort and sportiness," says Ruprecht Müller from ADAC.

When designing a rim, increasing attention is being paid to reducing air resistance. Not least because of the “stricter requirements with regard to CO2 classification”, as Jörg Ludwig from Otto Fuchs says. “The shape of the rim has a significant influence on aerodynamics and thus consumption in practice, especially at high speeds,” explains BMW man Schek. This is also what Manfred Ebetshuber, technical manager at Borbet Austria says. "Wheels should generate as little turbulence as possible and thereby reduce the drag coefficient." A flat, closed design is therefore preferable. (dpa)

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