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Better dynamics for all-wheel-drives

Friction Shim improve power transfer

In designing their new racing car, the s3racing team at the University of Siegen became aware of the dangers that securing the connection between the shaft and wheel hub with a simple bolt could pose in the event of sudden spikes in power. They solved this problem by introducing friction shim.

Formula 1 may steal the limelight, but ...

... the truly brilliant ideas often find their origin next to the big race tracks. The Formula Student Germany (FSG) racing series, which has been organised by the Formula Student Germany association since 2006, is as much a design competition as a race. And it costs just a fraction of the annual budget of one of the large Formula 1 racing teams. Every year in the late summer, university teams from all over Europe meet at the Hockenheim race track to compete against each another with racing cars from the DIY brand. The actual work – the design and construction of the single-seater racing cars – takes place long before the main event itself.  

Just how demanding this racing series is can be seen from the fact that ... 

... in addition to the tests on the race track, in which the driving characteristics of the cars are assessed in the racing categories of acceleration, endurance, skidpad and autocross, results in the so-called “static” disciplines are also evaluated. This means that the technical construction of the cars as well as the cost and business plans of each team are judged. The participants are able to gain experience in the area of design and construction, but they also have to bring these in line with the business skills that are required in the field of vehicle construction. And besides, at home and abroad, participants all agree that taking part in this racing series is, quite simply, tremendous fun for the members of the student teams!

Dominik Kucia, Formula Student Germany participant …

… is 21 years old and in his fourth semester of studying vehicle design. He has been a member of the s3racing team of the University of Siegen since his first semester at the university. In the course of his two-year career, he has risen through the ranks to become group leader of the drive technology division. The Siegen racing team currently consists of about 25 active members from the fields of mechanical engineering, vehicle construction, electrical engineering, computer science and business administration.

“Our task in the series,” says Kucia, “is first and foremost to design our own racing car, then have the necessary parts manufactured and then assemble it ourselves.” To ensure that teamwork functions smoothly, the crew has divided its tasks into different departments: electronics, chassis, powertrain and suspension. Kucia’s division is powertrain. “My job is mainly to ensure suitable design of the individual components of the drive technology, such as electric motors, gear units, drive shafts and their flange points. With the vehicle we are currently working on, we would for the first time like to use an all-wheel-drive design with internal drives. An important factor in implementing this concept is the positioning of the individual components, especially with regard to optimal power transmission to the wheel.”

A potential point of danger turned out to be...

... the screws that connect the constant velocity joint to the hub in the area of power transmission from the shaft to the wheel hub. Kucia says: “In the event of sudden spikes in power, there’s a high risk that the screws will shear and the connections will separate. In order to ensure better safety at this point, we were looking for a solution that would prevent slipping between the joint and wheel hub. It was important to us that this solution should be as simple as possible. Developing new parts or modifying existing components would have meant additional work for us, for which we had neither the time nor the money.”

The team then came up with an idea that pointed them in the right direction ...

… namely increasing the friction while maintaining the same preload between the two components. Other approaches, such as redesigning the bolt connection to be stronger, had already been rejected: too little freedom of movement at the joint, too great the demands on the material. Alternative solutions such as interlocking of the constant velocity joint were furthermore problematic to manufacture, while cost constraints excluded electric discharge machining on the component.

Putting the final solution into practice was ultimately ...

… surprisingly simple – inserting friction shims solved the problem! Before assembly, the surfaces adjacent to the friction shim were cleaned and made free of any grease. Kucia says: “Then we placed the shim in the wheel carrier at the back of the wheel hub and inserted the constant velocity joint. The hub was then screwed to the joint and the friction foil from the outside – and then the same was done on the opposite side of the axle. His conclusion: “Given their extraordinary effect, the friction shim have proven to be an amazingly uncomplicated solution!”

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