Forces acting on a racing bobsleigh in the ice channel at Berchtesgaden Königssee, Germany were systematically measured using a complete HBM measurement chain by the material triboloy group at the Zentralinstitut für Medizintechnik ZIMT of the TU München. Strain gages on the bob carriage and skids gave a first glimpse into force conditions during a bob race through the ice channel. The knowledge gained facilitates the route to systematic optimization of bob skids.
Until now, skids for bob sport have been exclusively developed and produced on the basis of empirical information. Comprehensive experience of the bob pilot and trainer, together with long-term testing, are the main guarantee for top places in international competitions. Hard to reproduce training conditions, changing weather and track conditions make a direct comparison of different skid geometries and materials difficult.
However, verified measurement results are the basis for specific influencing of skid characteristics and are essential as the input data for FEM calculations. In addition, they enable the determination of the forces acting on the bob and skid body, which enables an accurate description of the behavior between the skid and the ice. The knowledge of the location and level of penetration of the sliding body in the ice layer can help optimize the sliding process.
High mechanical load, simultaneous measurement of 48 sensor signals with a sampling rate >2,000 Hz, accelerations up to 5g, low temperatures and battery operation, together with the smallest possible dimensions, all make the MGCplus the optimal partner in the ice channel. The strain on the skids, total load per skid and the accelerations on the skid and bob frame are measured. With regards to skid strain measurements, two skids could be measured simultaneously as the use of the MGCplus means that a total of 48 measurement channels are available.
The use of capacitive sensors for additional accelerometer measurements was possible by simply exchanging the racks on site. Linear strain gages were used to determine the axle deflection. Rosette strain gages were attached to the skids for measuring the principle stresses and their directions.
In total 21 measurement runs were carried out in the ice channel at Königssee with the Bavarian bob team members Karl Angerer, Christoph Gaisreiter and Katrin Dostthaler. In total, a data volume of 43MB per measurement run was recorded, calculated on a signal width of 32 bit, a measurement frequency of 2,400 Hz over 48 individual channels and a recording duration of 100 seconds.
The measurement data generated was stored in MGCplus on a compact flashmemory card and then read and processed on a notebook with the
The data acquisition system was positioned between the pilot and the brakeman when measurement runs were implemented in the twoperson
bob. Particular attention was paid to the installation to ensure that there was no danger of injury to the bob team. The MGCplus was grounded via the bob frame and skids.
As specific optimization of skid geometry is only possible when load, location, time and position of the bob are known, all measurement results generated must be linked with the geometric and construction properties of the ice channel. This was implemented by assigning the measurement results using an event map developed for the bob track. Figure 5 (see PDF file) shows a schematic diagram of the bob track in combination with the recorded axle load of a two-person bob. The various events in the ice channel can be clearly recognized as sensor signals in the diagram.
Because of the centripetal forces acting on the bob, the axle deflection and the sensor signal acquired increases quadratically with increasing speed and linearly with decreasing track radius.
At the last steep curve, the echo wall, the force acting on the pilot and team rises to five times their body weight. The bob is subject to the maximum force at this point.
The position of the bob was precisely determined using the signal for the precise assignment of loads to the corresponding location in the ice channel. This can be implemented by integrating the speed over time. The speed can be determined by comparing the axle signals: The deflections of the front axle were repeated after a slight time delay at the rear axle. The average speed could be determined because the axle distance of the bob was known.
Greater competitive pressure and ever faster equipment requires systematic
procedures in all further developments in this winter sport. Measurement technology holds a key position here as it can provide statements about arising loads and it can be used as the basis for verification of simulations.
Based on the measurement results recorded at the ice channel in Berchtesgaden, an FEM model was created at the Zentralinstitut für Medizintechnik of the TU München which permits the simulation of the measured processes.