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For front-to-back stability, the maximum height of an obstacle which an unarticulated wheeled vehicle can climb is a function of the wheelbase and the horizontal and vertical position of the center of mass (CM).
The critical angle is the angle at which the center of mass of the vehicle begins to pass outside of the contact points of the wheels. Past the critical angle, the reaction forces at the wheels can no longer counteract the moment created by the vehicle's weight, and the vehicle will tip over. At the critical angle, the vehicle is marginally stable.