World’s first production-ready thermoplastic wheel rim

In September 2011, smart and BASF have exhibited a world first at the International Motor Show (IAA) in Frankfurt/Main: The visionary “smart forvision”, the new concept vehicle from smart, is the world’s first automobile to ride on thermoplastic wheel rims. However, they are not just a concept, but their performance has already been tested for high-volume production. The wheel rims will also be on display at the BASF stand (hall B4, stand 4306) during the Fakuma plastics trade fair that takes place from October 18 to 22 in Friedrichshafen, Germany.

The smart all-plastic wheel rim is over 30 percent lighter than a standard production aluminum wheel. The plastic rim weighs only six kilos and is made of the new specialty polyamide Ultramid® Structure from BASF. This contributes to a noticeable reduction in the vehicle’s weight by a total of 12 kilos and translates into lower fuel consumption when the vehicle is equipped with a conventional engine. In the case of a battery-powered auto such as the “smart forvision”, the result is greater range. The complete rim (to be technically correct: the wheel) – consists of two parts: the actual supporting component and a wheel cover that is also manufactured from BASF Ultramid®. Both parts have a supporting function and withstand high loads under driving conditions. The wheel cover can be painted in any color used for the vehicle. In Frankfurt, it was shown – like the complete “smart forvision” - in white and copper.

“Thermoset rims are already being used in racing and for short production runs”, explained Heiko Hess, who is responsible for wheel development at BASF.“ However, compared to thermoplastic rims, which can be injection molded, they are considerably more complex, more expensive to manufacture and thus unsuitable for high-volume production”.  

Ultramid Structure: Specialty plastic for high loads

The new Ultramid® Structure from BASF’s polyamide line is a composite plastic that can withstand high loads, because it is reinforced with long fibers and was introduced at the K 2010 plastics trade fair in Düsseldorf. The long-fiber reinforcement makes it especially strong – it can replace metal wherever energy absorption is a major requirement, e.g. in crash absorbers, seat structures, battery holders, engine mounts and other structural components. The Ultramid Structure grade used here is particularly rigid, impact-resistant and dimensionally stable.

Designed on the computer

As was the case with numerous Ultramid® parts in recent years, the universally applicable simulation tool ULTRASIM™ from BASF, was used when designing this high-strength Ultramid rim. With the aid of a topology optimization the shape and position of the ribs in the plastic rim can be predicted and optimized on the computer without compromising the design. The form of the wheel was already defined in an early design phase and had to be modified only slightly on the actual part. As a result, it was possible to develop this highly safety-relevant part, which previously existed only in metal, in close cooperation with the Daimler wheel rim experts in a very short period of time. The extensive load tests that the wheel had to pass at smart and BASF included the tolerance test on the wheel measuring machine, the fatigue test on the biaxial wheel test stand (ZWARP test), the biaxial wheel test, the impact / radial impact test when dropped from a tower and the burst test. Coupling to the axle, and thus load transfer, is accomplished by means of metal inserts and bolts.

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