Straddling monorail vehicles are a new rail transportation mode in which the bearing, running, and guiding rely on rubber tyre running gears. Since the lateral span of the running wheel is small, stabilizing wheels are added to enhance the anti-overturning capacity, which is influenced by the contact between the stability wheels and track beam. If one stabilizing wheel loses contact with the track beam, the tendency to overturn increases significantly. To ensure the straddling monorail vehicle has a stable anti-overturning capacity, an initial preload pressure is applied to the all stabilizing wheels and guiding wheels. The specific preload pressure of the stabilizing and guiding wheels is a unique issue of straddling monorail vehicles. This paper deduced a flexible coefficient formula of straddling monorail vehicles, validated the accuracy of the flexible coefficient formula based on dynamics simulations, and discussed the correlation between the flexibility coefficient and stabilizing wheel contact conditions. According to the variation regularity of the anti-overturning capacity of straddling monorail vehicles, the concept of vehicle critical roll angle was proposed.