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Why are the center wheels of the Curiosity rover not in line with the chassis?

I understand when turning, one side is in line with an arc it will make. Would the other side's wheels not be on the same arc?

For example, if we assume the rover will drive clockwise, the top right wheel will face inward the top left will face outward and the top center will be perpendicular to the radius of the turn length. The top wheels will be in the path of the arc.

The bottom wheels however from what i understand will not be in the path of the arc.

Hope someone can clarify why they did this. TIA

enter image description here

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2 Answers 2

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Why the wheels are placed where they are is probably due to combination of factors, such as weight distribution, foldability, loading forces, etc.

But for the turning it doesn't matter, as the outer wheels on the corners are individually steerable. This allows to place the center point of the rotation arc on the line going through both center wheels. The center wheels direction of travel is then, by definition, tangental to their respective arcs. The outer wheels can each be rotated to be tangental to their respective arcs, so that each of the six wheels are tangental to their own arc.

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Each of Curiosity's wheels have a drive motor, but only the four outer wheels are steerable. The suspension system is based on rocker-bogie system on the Pathfinder rover.

The term "rocker" comes from the design of the differential, which keeps the rover body balanced, enabling it to "rock" up or down depending on the various positions of the multiple wheels.

Looking at the design of the rover, from the picture in the question, the location of the center wheels is dictated by the width of the payload chassis; more particularly, the "axle" beneath the payload chassis. These wheels cannot be any closer to the body of the rover.

The outer wheels could have been placed closer to the body of the chassis by reducing the angle of their support beams. This would have reduced the stability of the rover. It's the same principle as the stability of wide gauge railroads compared to narrow gauge railroads. Additionally, placing the outer wheels closer to the chassis would have increased the disparity between them and the center wheels.

If the outer wheels were placed in line with the center wheels the angle of their support members would have been greater than what it is. This would have increased their lengths slightly, which would also have increased their masses and thus the overall mass of the rover.

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