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Just wondering, why are unmanned rovers almost always using six wheels (adding more weight) as opposed to four wheels? Is it for added traction, redundancy, balance, or a combination of these factors?

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  • $\begingroup$ By the way Astrobotic proposed four-wheel design using a rocker based suspension and chain-coupled drive wheels on each side. It didn't seem to have the high ground clearance of the six--wheeled JPL designs through. (And it hasn't flown yet, of course.) $\endgroup$ – Andy Sep 19 '16 at 17:36
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Short answer: More wheels results in greater stability, allowing the rover to traverse more varied types of terrain. It also provides redundancy, in case one of the wheels breaks, the rover is not completely crippled.

Long answer:

The Mars rovers all use the same design, called the Rocker-Bogie. This is a special design which allows the rover to traverse many different terrains while keeping the body of the rover stable. Since the wheels are not connected to each other, it also allows the two sides of the rover to go on different slopes simultaneously.

Rocker bogie.gif

(From Wikipedia)

This design allows Curiosity to tilt 45 degrees in any direction without flipping over (though automatic sensors prevent it from going over 30).

In addition, each wheel can be equipped with its own motor. This allows the rover to rotate in place and to continue functioning even if one of the wheels is damaged. For example, when the Mars Exploration Rover, Spirit, broke one of its front wheels, the other wheels were able to provide enough power to keep it going.

There are other designs which differ from Rocker-Bogie suspension, but most of these also use six wheels. (Such as the proposed Russian Marsokhod rover which never actually launched.). Having more wheels increases traction and stability, resulting in a safer rover overall. You could make a design with four wheels, but this would be less stable. Alternatively, you could put 8 or 10 wheels on, but this would drastically increase the cost and weight of the rover. Therefore, 6 wheels is a good middle ground that provides lots of traction and stability without being too heavy.

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    $\begingroup$ Are you thinking of Lunokhod, which had eight wheels each on an independent torsional suspension? Fun fact: the wheels could be explosively jettisoned! $\endgroup$ – pericynthion Sep 19 '16 at 5:39
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    $\begingroup$ Did you mean the "Marsokhod" rover? A six-wheeler using a flexible chassis of a different type. (Doesn't seem to offer the high ground clearance of the rocker-bogie, but interesting anyway.) $\endgroup$ – Andy Sep 19 '16 at 15:14
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    $\begingroup$ Yeah, that one. That design actually offers a lot more power and traction, but at the expense of being heavier and more power-consuming. Thanks for reminding me of the name. $\endgroup$ – Phiteros Sep 19 '16 at 15:26
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    $\begingroup$ This is a special design which allows… Are all the points not possible with 4 wheels? In my opinion, this answer explain what can be done with 6 wheels but not why it's impossible with 4 wheels. $\endgroup$ – A.L Sep 19 '16 at 16:57
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    $\begingroup$ Also worth mentioning: the ATHLETE rover. Not only does it have 6 wheels - on 6 legs - it can also be split into two rovers each with 3 wheels. $\endgroup$ – kim holder Sep 19 '16 at 22:24
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The rocker-bogie suspension has its origins in a 1969 study by M. G. Bekker, 'Introduction to Terrain-Vehicle Systems' which I haven't been able to find online.

By 1994, NASA had designed a series of rover concepts named 'Rocky'. They started with an articulated 6-wheeled rover. By the 4th iteration (yes, Rocky IV), the 6-wheel rocker bogie design was established. The 6th iteration would lead to the Sojourner design which flew in 1997 as part of the Pathfinder mission.

A discussion of rocker-bogie vs other designs can be found in 'Sojourner: An Insider's View of the Mars Pathfinder Mission' by A. Mishkin.

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