## 1 Introduction
For example, the UNECE R100 (uniform provisions concerning the approval of vehicles about specific requirements for the electric power train) [1] is the safety requirements concerning the electric drive of road vehicles of categories M and N1, the maximum design speed of which exceeds 25 km/h and which are equipped with an electric drive, exceptvehicles with a permanent connection to the network. Retrofitted buses, though mechanically sound, must now be evaluated through the same lens as newly manufactured electric vehicles. The development of an applicable simulation methodology in accordance with R100 is the subject of the current research. Establishing such methodologies is a vital step in streamlining certification, reducing development time, and accelerating fleet-wide transformation. While electric vehicle safety has been broadly studied, the unique challenges posed by retrofitting diesel buses to electric powertrains remain underexplored. While numerous studies address frontal crash behaviour in electric vehicles, few tackle the nuanced structural risks introduced by converting existing diesel platforms. The UNECE R100 regulations are related to the impulse impact and foresee the transverse 10 g and longitudinal 12 g (frontal impact) accelerations. This focus on specific acceleration parameters highlights the strict safety standards that converted buses must meet to be viable on the road. It is evident that the re-equipment of diesel buses with electric ones is just beginning its trend; therefore, there is a lack of scientific research on the impacts of R100. On the other hand, the exact nature of the frontal impact of buses is particularly presented in relevant scientific publications dedicated to the UNECE R29 and NCAPs (New Car Assessment Programs) regulations (in the case of R100, the analysis of the results is performed mostly locally--on the battery blocks). These studies provide valuable insights but do not fully address the structural complexities of retrofitted vehicles as a whole. Thus, in works presented by [PERSON] et al. [2], the validation of frontal crashworthiness simulation for low-entry type buses is investigated according to UNECE R29 requirements. Key findings include the observation that city buses, unlike suburban and intercity models, have lower stiffness in frontal collisions. This necessitates the inclusion of deformation zones in new models, which are currently lacking in most city buses, and signalise that low-entry and low-floor buses have a significantly lighter front end compared to traditional designs. [PERSON] et al. [3] evaluated the structural safety of a newly developed hydrogen fuel cell city bus body frame using finite element analysis under four conditions: horizontal bending, ultimate torsion, emergency cornering, and braking. While the initial design met the stiffness and vibration criteria, stress levels exceeded the material's yield strength under torsion and cornering. Size optimisation of frame components reduced peak stresses by up to 20.13%, ensuring compliance with strength requirements and achieving a 106 kg weight reduction. This demonstrates the importance of structural optimisation to balance safety and efficiency in next-generation bus designs. Frontal impact on the bus superstructure, as per UNECE R29 and NCAP, is a subject of [4]. The study [5] investigates the crashworthiness of crash box components using finite element models sourced from NHTSA (National Highway Traffic Safety Administration) vehicle data. Key parameters--energy absorption, crush force efficiency, and specific energy absorption--were analysed and optimised through RSM (Response Surface Methodology) design of experiments. Reduced-order models using the sPGD (sparse Proper Generalised Decomposition) method achieved 90% accuracy in capturing nonlinear behaviour. Results showed a 34.33% improvement in energy absorption and an 18.91% mass reduction in heavier variants without compromising performance.