Jin Yan Sunil Kumar Agrawal Rimless Wheel with Radially Expanding Spokes: Dynamics, Impact, and Stable Gait. 3240-3244 2004 conf/icra/2004 ICRA http://dx.doi.org/10.1109/ROBOT.2004.1308753 db/conf/icra/icra2004-4.html#YanA04