Locomotion on unstructured terrain poses a significant challenge for wheeled mobile robots lacking reconfigurable mechanisms. Achieving both stability and agile motion in such environments requires a hybrid approach that leverages their adaptable nature to varying surface conditions while ensuring efficient mobility. In this research, we present Improbability Roller 2, a refined iteration of our hybrid mobile robot with variable-diameter wheels, simplifying the design while improving its maneuverability and adaptability on unstructured terrain. The new compliant outer wheel structure and its folding mechanism allow for a higher wheel size change ratio. With the combination of multimode steering, which integrates both differential drive control and steering based on wheel size disparity, the robot can now optimize locomotion on diverse terrain while maintaining traction. The robot was tested across various obstacles and multiple surface conditions to validate the effectiveness of the new wheel design and the dual steering strategy. Experiments, including slope and step climbing, confined space traversal, and locomotion on loose gravel and snow, demonstrated the robot’s improved terrain adaptability, consistent traction, and control across varying surfaces.
