One of the most fundamental models of biped locomotion is a planar rimless wheel which captures the stance leg motion and the heel strike. In numerical simulations, it is usually assumed that the impact between the leg and the ground is instantaneous and the impact results in no rebound and no slipping. This paper presents an experimental study on motions of a planar rimless wheel to validate the assumptions used to model rimless wheels. We investigate mainly the impact phenomenon with the ground. We built experimental setup to capture the motion of a planar rimless wheel. In our experimental setup, the rimless wheel moves on a slope. The motion is captured by motion capture cameras and a high speed camera. The locations of markers at a spoke are calculated by the images of the motion capture cameras. The motion analysis clarifies the effects of the impact phenomenon.