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In urban areas, electric cargo bikes are becoming an increasingly attractive means of transport in many respects. They combine the advantages of easy navigation through city traffic with the ability to transport goods or people comfortably, whilst offering a carbon-neutral mode of travel. The newly developed eCargo-Bike from Mittweida University of Applied Sciences, created at the Intelligent Machine Systems Chair headed by Professor Jörg Hübler, is intended to facilitate the transport of goods and people on campus as a sustainable mobility solution. When transporting larger loads, the frame structure is often subjected to considerable stresses. For the production of a small batch and its approval for use, a comprehensive structural durability test (SDT) is therefore required to prevent material failure in the frame structure during the product’s service life and thus ensure safety. The department has a roller test rig available for this purpose, which can simulate the stresses encountered during the product’s lifetime in a time-compressed manner (see Fig. 1). Two electrically driven rollers generate the driving motion, whilst impact bars mounted on them simulate the passage over uneven ground. The payload, driving speeds and impact bar geometries, as well as the number of passes over the bars, are specified in accordance with DIN EN 17860-3.
To ensure the test is as realistic as possible, it is necessary to compare the loads imposed by the standardised test programme on the roller dynamometer with the loads encountered in real-world conditions. To this end, load data from field tests is being collected as part of the investigations; this data reflects the typical use of the eCargo bike in everyday operation and beyond. To this end, strain gauges have been fitted to high-stress areas of the bike frame, and acceleration sensors have been fitted to the front and rear axles. With a payload identical to that in the test programme, a defined test route of 10 km in length was then traversed across varying ground conditions (see Fig. 2). In addition, loads resulting from special events, such as riding over kerbs or railway sleepers, were factored in. From the stochastic load-time curves (see Fig. 3), evaluable load collectives were determined using methods of load data reduction, Rainflow classification and the analysis of positive stress pairs and class boundary violations. These were scaled up to the total service life of 30,000 km in order to represent the loads over the entire product life cycle.
On the test rig, random measurements of the load condition were carried out at all six standardised test stages and were also analysed using load data reduction methods (see Fig. 4). In accordance with the standard, the rider’s weight was simulated by attaching several ballast bags, each weighing 10 kg. The investigations revealed that the loads induced on the test rig are significantly higher and that the test programme has a markedly more damaging effect on the frame of the cargo bike compared with the data from the field trials. To remedy this, the test programme was gradually scaled back whilst aligning the load spectra until the load level was similar to that of the scaled test ride. The revised test programme is now being used for fatigue testing on further prototypes of the HSMW eCargo bike, with the aim of optimising the frame geometry and ensuring safety throughout the product’s entire service life.
Incidentally, you can experience the eCargo bike first-hand, together with the research group from the Chair of Intelligent Machine Systems, at the EUROBIKE trade fair next week, from 25 to 29 June in Frankfurt am Main, at stand 8.K014
, Mittweida University of Applied Sciences at EUROBIKE
About the person
Jim Köcher has been studying Mechanical Engineering at Mittweida University of Applied Sciences since 2019, specialising in Digital Product Development/Micromobility, and his Master’s thesis focused on a structural integrity analysis of the HSMW eCargo-Bike. Since then, he has been working at the Chair of Intelligent Machine Systems at HSMW and continues to be active in the field of micromobility.
