Wheeled mobile robots (WMR) have been deployed in Mars/Lunar exploration, military missions, and geological investigations, and they must handle rough and deformable terrains. The dynamics at the wheel-soil interface, including the wheel-terrain contact angle, wheel sinkage, and wheel-soil interaction forces and torque, are major factors in the entrapment of NASA SPIRIT MER and the subsequent mission failure. This paper presents an experimental apparatus that is developed to measure the wheel-terrain contact angle, wheel sinkage, and wheel-soil interaction forces and torque in real time to improve our understanding of the WMR mobility on soft and deformable terrains. A method for wheel sinkage evaluation is presented based on measuring the wheel-terrain contact angle. The wheel-soil interaction forces and torque are measured by installing a force/torque sensor along the axle of the apparatus. The proposed approach is implemented on a testbed, and the experimental results are used to evaluate it; they demonstrate the reliable detection of the wheel-terrain contact angle, wheel sinkage, and wheel-soil interaction forces and torque. Moreover, based on the measured wheel-terrain contact angle, the relative errors in the predicted wheel-soil interaction forces and torque are less than 8% compared with the experimental data.