Custom-made plastic bearings for Trabant wheel suspension

It is very difficult to find spare parts for the Trabi. The plain bearings required also have a special geometry, which means that procuring newly manufactured spare parts is expensive. The bearings must also guarantee smooth running, be wear-resistant and withstand the load in the steering knuckle.

With plain bearings made from the material iglidur J, the Trabant enthusiasts have found a maintenance-free and lubrication-free solution that also offers an excellent coefficient of friction. Thanks to a special part, which is manufactured with injection moulding, it is possible to produce the plain bearings cost-effectively. This means they can be customised precisely to the application.

Hofmann has hit the bull's eye. Trabi enthusiasts can be found all over the world - 30,000 vehicles are registered in Germany alone. And all owners sing the painful song of spare-parts supply. "I started out with three boxes in the cellar," says Hofmann. Today, 13 years later, Trabantwelt.de is the market leader in Germany. "The spare parts fill three warehouses - from individual screws for trim strips to hinges for boot lids to entire chassis and tuning engines with 35HP." Twelve employees take care of the dispatch of the 1,800 parts and ensure replenishment.
 
The supply of spare parts has its pitfalls. Take steering, for example. Pivot bearing bushings for the steering knuckles, the swivelling wheel carriers, are in short supply. "After the fall of the Berlin Wall, there was a warehouse in Werdau that was cleared. We removed the original bushings by the shovelful and were happy," says Hofmann. But then came the shock in the warehouse when the blue box with the bushings was empty. What now? Imported goods made of simple plastic were out of the question, the quality was too poor and the service life too short. "We therefore wanted to have the components manufactured ourselves." And so Hofmann searched the Internet for a partner to manufacture the pivot bearing bushings.

This was the beginning of the collaboration. An original bushing was sent by post to Cologne. A design drawing followed. A lathe produced a prototype. Hofmann wanted to test it before high-volume production. The eight-hour stop-start field race in Pausa, Saxony, was just what was needed. The racing Trabis spend hours racing over uneven terrain - a stress test for the steering. A friendly racing team was able to convince Hofmann to take part in an A-B comparison. The original GDR bushings were used in one vehicle and the bushings made of iglidur high-performance plastic in the other. Which version would win? After the cross-race, Hofmann got down under the cars, eager to see how the bushings fared. The result was surprising. The GDR bushing was worn out whereas the competitor from Cologne had hardly suffered any damage. "That was a phenomenal baptism of fire," says Hofmann happily. "From that moment on, it was clear to us that the products were worth their weight in gold as spare parts." And so, series production began.

The injection-moulding process was used, in which molten plastic is injected through a nozzle into a closed mould and cured. The injection-moulded plain bearings are even more robust than the machined versions. "As a dealer, we're almost shooting ourselves in the foot with this quality," says Hofmann, laughing. "The bearings are so robust, they last an incredibly long time. A customer who buys the bearing won't need another one any time soon."

But what makes the difference in quality? The original bushings consist of textile waste pressed with resin. The counterparts from Cologne, on the other hand, are made of iglidur J, a high-performance plastic that igus developed in the laboratory. "The material has a density of 1.49g/cm3 and a compressive strength of 35MPa - roughly equivalent to the pressure of two small cars bearing down on an index finger," explains Kevin Buettner, Technical Sales Consultant at igus. The material is so robust and wear-resistant that it has become a popular alternative to metal plain bearings in the industry.

"The high-performance plastic is a mixture of a base polymer, fibres, fillers and as solid lubricants," says Büttner. "The base polymers are decisive for the wear resistance. The fibres and fillers reinforce the bearing so that they can also absorb high forces and edge loads." Another advantage over the original bushings is that the DDR product is hygroscopic. It absorbs water via its porous surface and swells up like a sponge. "Many Trabi owners therefore struggle with stiff steering systems," explains Hofmann. "A problem that the plain bearings made of high-performance plastic have solved." The moisture absorption of the bearings made from the polymer iglidur J amounts to around 0.3% in a normal climate. "The steering therefore remains smoother in the long term."