A structured engineering methodology for multi-phenomenon motorcycle stability assessment

Motorcycle stability encompasses multiple physical phenomena, including straight-running divergence, cornering feasibility, oscillatory modes, and sensitivity to external disturbances. While these phenomena have been extensively studied in isolation, the absence of a transparent integration approach limits comparative assessment across vehicle configurations and operating conditions. This paper presents a structured Motorcycle Stability Index (MSI) that decomposes stability into five physically distinct and non-overlapping sub-indices, each representing a single stability phenomenon: straight-running non-oscillatory stability, steady-state cornering feasibility, high-frequency wobble stability, low-frequency weave stability, and crosswind response sensitivity. Each sub-index is evaluated using a formulation appropriate to its underlying physics, including eigenvalue analysis of the full linearized multibody model, quasi-static equilibrium analysis, and time-domain forced-response simulation, and is mapped to a bounded, dimensionless measure that preserves physical ordering while avoiding double counting by evaluation definition. The sub-indices are combined using a deliberately linear aggregation to maintain transparency and traceability rather than optimization or prediction. Application to representative motorcycle classes demonstrates physically consistent comparative trade-offs across stability phenomena, while robustness analysis confirms that relative rankings remain stable under moderate perturbations of sub-index weighting. Uncertainty is treated explicitly for forced-response behavior and propagated during integration. The proposed index is intended as a decision-support tool for comparative stability assessment within a declared linearized scope and passive rider assumption, providing relative stability indicators rather than direct prediction of real-world vehicle behavior, and complementing rather than replacing established multibody stability analyses. By explicitly separating stability phenomena and defining bounded evaluation measures, the proposed approach provides a transparent basis for engineering trade-off analysis in motorcycle development.