This paper investigates how the passive adaptability of an underactuated robot leg to uneven terrain is affected by variations in design parameters. In particular, the joint torque coupling ratio, segment length ratio, and rest angles are varied to determine configurations that allow for maximum terrain roughness adaptability while minimizing the transmission of disturbance forces to the body. In addition, a series of alternate leg actuation configurations are considered. The results show that a proximal/distal joint torque coupling ratio of 2 with an inverted distal joint, a proximal/distal leg length ratio of 1.25, and an initial proximal joint angle of −53 deg maximize the terrain variability over which the robot can remain stable by exerting a near-constant vertical reaction force while minimizing lateral force and moment disturbances. In addition, the spring stiffness ratio allows for a tradeoff to be made between the different performance metrics. Finally, the robot's stability with respect to its posture is discussed.