Crosswind stability and critical safe speed of medium-duty box trucks on expressways under strong-wind conditions

Introduction Crosswind-induced rollover is a major safety concern for medium-duty box trucks operating on expressways in high–wind regions. Existing crosswind studies have substantially improved the understanding of aerodynamic loading and vehicle dynamic responses of high–sided vehicles, especially on bridges, in gusty winds, or under selected vehicle configurations. However, relatively limited attention has been paid to the expressway operation of medium–duty box trucks on long, straight, wind–exposed road sections, where loading state, wind yaw angle, wind intensity, and vehicle speed jointly determine rollover risk. Methods To address this gap, this study develops a TruckSim model of a two–axle medium–duty box truck and establishes a mechanism–oriented rollover risk assessment framework considering wind speed, wind yaw angle, loading condition, and vehicle speed. Three loading conditions, namely unladen, half-laden, and fully laden, are examined. Crosswind scenarios are defined with yaw angles of 30 ° to 150 ° and wind intensities corresponding to Beaufort scale levels 6 to 12. A dual–indicator method combining the minimum tyre vertical load and Load Transfer Ratio is adopted. The minimum tyre vertical load is used to identify wheel lift-off and incipient rollover, while the Load Transfer Ratio is used to evaluate lateral load transfer and rollover risk evolution. For the dual–tyre rear axle, same–side inner and outer tyre loads are aggregated into wheel–assembly loads. The front–axle, rear–axle, and whole–vehicle Load Transfer Ratios are then calculated separately, and their maximum value is used as the comprehensive rollover risk indicator. Results The results show that wind yaw angle has a significant effect on rollover risk. The range of 90 ° to 120 ° is the most critical, with 120 ° being the most adverse wind direction. Rollover risk increases nonlinearly with vehicle speed and wind speed, and the unladen condition shows the highest crosswind sensitivity. Discussion Compared with previous single-factor or scenario–specific evaluations, the proposed framework advances existing studies by linking multi-factor simulation results to critical safe speeds and differentiated engineering control thresholds for medium–duty box trucks. The findings provide a basis for loading–sensitive speed limits, real–time risk warnings, and traffic management of medium-duty box trucks in high–wind expressway environments.