June 16, 2007 Lightweight wheels are a must-have for automotive enthusiasts as they significantly decrease unsprung weight and improve handling characteristics accordingly. Though fiber-composite materials are used in elite motorsport, and promise better long-term performance than their metal counterparts, they have been unable to make their way into the mainstream due to a lack of suitable testing methods. Standardized testing methods exist for conventional rims made of steel or aluminium, but there is doubt as to whether the testing methods for metal wheels can be applied to composite materials. Now German researchers have created a new method which can reliably simulate how an individual composite wheel will cope with the stresses of driving on public roads, including hitting the curb and potholes, and the meeting of quality standards, without destroying the wheel.

The advantage of plastic wheels is that the material combines high specific strength with low weight, not to mention good looks – the weave of carbon fiber is one of the vogue finishes of the current era. A composite wheel consists of two main components – a matrix and reinforcing fibers. Through their interaction the two components achieve better properties in the composite material than they do separately.

The legal situation for plastic wheels could change soon, giving the designers of lightweight rims more latitude in their choice of material.

Working in conjunction with colleagues from four other Fraunhofer institutes, research engineers at the Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt are developing a simulation method which reliably predicts the quality of the rims. “First we produce a computed tomography image of the wheel,” explains Dr. Andreas Büter, Head of Department at the LBF. “The image enables us to ascertain the length, alignment, curvature and density of the fibers. These parameters are crucial for the strength and load capacity of the material.”

On the basis of these results the research scientists simulate the microstructure of the material, a virtual unitary cell in which they can for the first time depict arbitrary material configuration.

Applying the results from the unitary cell, they use a numerical component model to simulate how the wheel will handle bends in the road or hitting the curb and how it would behave in a crash. “We calculate the stress and elongation occurring in the material under various loadings,” says Büter. “We know from experiments what stresses the material can withstand without being damaged and what elongations will damage it. This enables us to make a reliable assessment of plastic wheels.”

No time frames are yet available for when we might see carbon fiber wheels on roadgoing cars and motorcycles, but rest assured, the time is coming.

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