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The Japanese government is planning to send a rover to investigate the moon's surface in the near future, and it has been carrying out a considerable amount of research and making significant developments to make this possible. Toward that end, we have been developing and proposing expandable three-wheeled rovers. This paper proposes a new concept for a three-wheeled rover, namely the Tri-Star IV, and its mechanical design is described herein. Tri-Star IV has two arms that can rotate around horizontal shoulder axes and connect the rover's main body, and two rear wheels with off-centered crank-type axles that can expand from a storage posture. The two arms can be adjusted to adapt to traversing over sloped terrain, including recovery from rollover. The new wheels for traveling on the moon's surface, which is covered with regolith, are composed of a spring spoke and canvas cloth. In this project, two types of Tri-Star IV were developed and field-tested. The first was designed for mobility over rough sloped terrain. The other rover had a new front wheel installed that was designed for tethered descent/climbing over steep slopes. This paper reports on the mobilities of Tri-Star IV, which were verified in experiments conducted on real sand and a test bed and in a field test at the Nakatajima sand dune in Japan.