## 5 Conclusions  
1. The wheel-side electric drive system is supposed to be applied in different vehicle models. The traditional durability development for the certain kinds of vehicle load spectrum cannot meet the requirements of highly modularized development. To achieve highly efficient development with enough credibility, the comprehensive load spectrum is irreplaceable, while the modeling accuracy is decisive for the simulation and test.
2. The electric motor drives the wheel through two-stage fixed axis helical gears, so the power transmission path is short and acts as the suspension arm at the same time. As a result, the gears are crucial to the system reliability and NVH performance. The gears are manufactured by the hobbing and form grinding process, which ensures high accuracy and supports a free-style tooth shaping modification. In the gear processing, the involute part and root fillet curve are generated by hobbing, and the final tooth shape is generated by grinding. Through geometric theory deduction, the relationship between the hob parameters and the tooth root fillet curve can be described functionally.
3. Based on the rain flow methodology and extrapolation theory, a comprehensive load spectrum with nine stages is formulated, which can cover the working conditions of the basic version, NVH version, and durability version. According to the Miner cumulative damage hypothesis, the equivalent durability mileage of 150,000 km is converted. The prototype machine is simulated and verified on the test bench with the comprehensive load spectrum. The whole test lasts 525 h, with the estimated cumulative damage of the system to be 63.27%. The test result shows the robust performance of the system.
4. The wheel-side electric drive system is applied in three models with different working conditions, which are the basic version, NVH version, and durability version. If tested in the traditional way, the bench test would take over 4500 h, which is totally unacceptable in terms of time cost and financial expense. The modularized development introduced in this paper takes 525 h, which significantly improves the test efficiency. And the modularized development can be even more efficient with more and more application models. The evaluation and conversion of different load spectra and the preciseness of teeth modeling play a vital role in the modularized development, which has been the main novelty of this paper.
5. In the future, more work is needed regarding the technical trend in wire-by-chassis. The wheel-side electric drive system has the potential to be integrated and upgraded with the brake system and active suspension system. In this way, the actuators are all located near the wheel-end, which means they can offer a quick response in safety redundancies. This architecture is especially suitable for autonomous driving vehicles,  
\begin{table}
\begin{tabular}{c c} \hline
**Test Item** & **Test Data** \\ \hline Equivalent damage of a single cycle & 0.527\% \\ Cumulative damage of the total durability test (150,000 km) & 63.27\% \\ Equivalent damage in the base version & 52. 15\% \\ Equivalent damage in the durability version & 60.61\% \\ Equivalent damage in the NVH version & 45.29\% \\ Loading time of a single cycle (min) & 260 \\ Loading time in total (min) & 31,500 \\
2\# stage driving gear loaded times in a single cycle & 217,147 \\
2\# stage driving gear loaded times in the whole test & 26,158,640 \\ \hline \end{tabular}
\end{table}
Table 5: Fatigue information of theoretical calculation.  
where the central controller is responsible for decision-making, and all the actuators at the wheel-end are responsible for rapid response. For the future application of wheel-side electric drive system, the research in this paper provides data support and a development reference for more applications in the future, including gear damage calculation, product lifespan evaluation, system reliability design, lightweight optimization, etc.