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As the Mars Exploration rovers have reaffirmed, some of the most interesting sites for scientists to explore on planetary surfaces lie in terrains that are currently inaccessible to state-of-the art rovers. We have been developing the Axel rover as a robotic platform to access steep and challenging terrain. We will summarize the recent mechanical upgrades since we introduced the tethered Axel concept last year. In this paper, we will also investigate how different wheel parameters affect Axel's performance on various terrain types. We will begin with a theoretical analysis and then present our experimental results from testing with different wheel designs on flat, sloped, and rocky terrain. Tether tension management is essential for reliable and safe operation of the Axel rover. We will demonstrate a design that uses Axel's three actuators and its body for reeling and unreeling the tether. We will then present theoretical tension calculations and compare them to experimental results in which we implemented a tension sensor to directly measure the forces on the tether.